## The sauropod neck mass project: an experiment in open science

### March 4, 2013

Brachiosaurus sp. BYU 12866 c5? in left lateral view with CT slices, some corrected for distortion.

Last Tuesday Mike popped up in Gchat to ask me about sauropod neck masses.  We started throwing around some numbers, derived from volumetric estimates and some off-the-cuff guessing. Rather than tell you more about it, I should just paste our conversation, minimally edited for clarity and with a few hopefully helpful links thrown in.

Mike: Dud. Neck masses.
Mike: Taylor (2009:803) measured the neck of Giraffatitan by GDI as 4117 liters.
Matt: k
Mike: I didn’t convert that to a mass, but I guess density of 0.5 is as good as any, which gives us (say) 2 tonnes.
Matt: That works for me.
Mike: That’s for an 8.5 m neck. So Supersaurus at 15 …
Matt: Yep. Almost twice as long, and not much more slender, and from what I’ve seen, ASP about the same.
Mike: Is 1.76 times as long. If it was isometric with the G. neck, it would be 5.5 times as heavy, which is 11 tonnes.
Matt: Oh.
Mike: So first: yeesh. Like, that is the mass of a whole freaking Diplo. Now we surely have to say isometry is unlikely.
Matt: Prolly.
Mike: But just multiplying out by length is unrealistic too. So maybe I should guess at mass =~ l^2? If I went with that, I’d get 6410 kg, which is elephant mass.
Matt: Something just occurred to me. Like, just now. For my 2006 poster, I calculated the mass of the cervical series in Giraffatitan, by summing over the CT slices from Brachiosaurus sp. BYU 12866 and multiplying by appropriate scale factors for the rest of the verts. We could “skin” that in muscle, and actually figure this out, for various muscle thicknesses, for one sauropod.
Mike: We should totally do that … if we had some idea how heavily muscled it was.
Matt: Well, obviously the thing to do is what Hutch et al. did for the tyrannosaurs, and put on several soft tissue envelopes. Crazy skinny, our best guess, markedly unfit, OMG, etc. It’s not that much more work. In fact, that could be my SVPCA talk this year.
Mike: Sure, but that’s just how to mitigate our ignorance. All we’d be doing at this point is taking n guesses instead of one. But, yeah, we should do it. Or you should if you prefer.
Matt: Let’s make it a Wedel and Taylor. I’ll crunch the numbers, but I want your input.
Mike: Works for me!
Matt: Good. Now let’s file it until April at least.

BYU 12613, a posterior cervical probably referable to Diplodocus, in dorsal (top), left lateral (left), and posterior (right) views. It most closely resembles C14 of D. carnegii CM 84/94 (Hatcher 1901: plate 3) despite being less than half as large, with a centrum length of 270 mm compared to 642 mm for C14 of D. carnegii. From Wedel and Taylor (in press).

Matt: Oh!
Matt: Also.
Matt: You know that little Diplo cervical from BYU that we figure in our in-press paper?
Mike: I think I know the one, yeah.
Matt: I am SUCH a moron. I have CT scans of the whole thing.
Mike: Good.
Matt: I forgot that Kent and I scanned it back in 2008. Even blogged about it, fer cryin’ out loud.  So I can do the sum-over-slices, scale-for-other-verts thing for Diplodocus, too. Which is at least closer to Supes than JANGO is.
Mike: Remind me, is it from a juvenile?
Matt: Maybe, maybe not. It IS tiny, but the neural spine is fused, the internal structure is crazy complex, and it doesn’t have any obvious juvenile characters other than just being small. The ASP is about as high as it gets in diplodocids. Which, as you may remember, is not nearly as high as it gets in titanosauriforms–that’s another paper that needs writing. Damn it. To know all this stuff and not have told it yet is killing me.
Mike: PeerJ!
Matt: I know!
Mike: Bottom line, it’s nuts that no-one has ever even tried to weigh a sauropod neck.* We should definitely do it, even if we do a really crappy job, if only so that others feel obliged to rebut.
Matt: Quite. Let’s do it. For reals.
Mike: In April. Done.

* R. McNeill Alexander (1985, 1989) did estimate the mass of the neck of Diplodocus, based on the old Invicta model and assuming a specific gravity of 1.0. Which was a start, and waaay better than no estimate at all. Still, let’s pretend that Mike meant “tried based on the actual fossils and what we know now about pneumaticity”.

The stuff about putting everything off until April is in there because we have a March 31 deadline to get a couple of major manuscripts submitted for an edited thingy. And we’ve made a pact to put off all other sciencing until we get those babies in. But I want to blog about this now, so I am.

Another thing Mike and I have been talking a lot about lately is the relation between blogging and paper-writing. The mode we’ve seen most often is to blog about something and then repurpose or rewrite the blog posts as a paper. Darren paved the way on this (at least in our scientific circle–people we don’t know probably did it earlier), with “Why azhdarchids were giant storks“, which became Witton and Naish (2008). Then last year our string of posts (starting here) on neural spine bifurcation in Morrison sauropods became the guts–and most of the muscles and skin, too–of our in-press paper on the same topic.

But there’s another way, which is to blog parts of the science as you’re doing them, which is what Mike was doing with Tutorial 20–that’s a piece of one of our papers due on March 31.

Along the way, we’ve talked about John Hawks’ model of using his blog as a place to keep his notes. We could, and should, do more of that, instead of mostly keeping our science out of the public eye until it’s ready to deploy (which I will always favor for certain projects, such as anything containing formal taxonomic acts).

And I’ve been thinking that maybe it’s time for me–for us–to take a step that others have already taken, and do the obvious thing. Which is not to write a series of blog posts and then decide later to turn it into a paper (I wasn’t certain that I’d be writing a paper on neural spine bifurcation until I had written the second post in that series), but to write the paper as a series of blog posts, deliberately and from the outset, and get community feedback along the way. And I think that the sauropod neck mass project is perfect for that.

Don’t expect this to become the most common topic of our posts, or even a frequent one. We still have to get those manuscripts done by the end of March, and we have no shortage of other projects waiting in the wings. And we’ll still post on goofy stuff, and on open access, and on sauropod stuff that has nothing to do with this–probably on that stuff a lot more often than on this. But every now and then there will be a post in this series, possibly written in my discretionary blogging time, that will hopefully move the paper along incrementally.

References

## Diplodocus carnegii cervicals, from Hatcher (1901)

### February 7, 2013

Here are cervical vertebrae 2-15 of Diplodocus carnegii in right lateral view, from Hatcher (1901: plate 3). Click to embiggen, and then just gaze in wonder for a while.

Wouldn’t that look smashing, printed, framed, and hanging on the wall?

I wonder if I will ever stop finding new interesting things to think about in this image. I doubt it.

(For a bit o’ fair-and-balanced, remember that this neck may not be complete, and that some of the neural spines are sculptures.)

Thanks to Mike for the scan.

Reference

Hatcher, John Bell. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.

## Hot sauropod news, part 1: rampant pneumaticity in saltasaurines

### June 28, 2012

Caudal pneumaticity in saltasaurines. Cerda et al. (2012: fig. 1).

Earlier this month I was amazed to see the new paper by Cerda et al. (2012), “Extreme postcranial pneumaticity in sauropod dinosaurs from South America.” The title is dramatic, but the paper delivers the promised extremeness in spades. Almost every figure in the paper is a gobsmacker, starting with Figure 1, which shows pneumatic foramina and cavities in the middle and even distal caudals of Rocasaurus, Neuquensaurus, and Saltasaurus. This is most welcome. Since the 1990s there have been reports of saltasaurs with “spongy bone” in their tail vertebrae, but it hasn’t been clear until now whether that “spongy bone” meant pneumatic air cells or just normal marrow-filled trabecular bone. The answer is air cells, loads of ‘em, way farther down the tail than I expected.

Caudal pneumaticity in diplodocines. Top, transverse cross-section through an anterior caudal of Tornieria, from Janensch (1947: fig. 9). Bottom, caudals of Diplodocus, from Osborn (1899: fig. 13).

Here’s why this is awesome. Lateral fossae occur in the proximal caudals of lots of neosauropods, maybe most, but only a few taxa go in for really invasive caudal pneumaticity with big internal chambers. In fact, the only other sauropod clade with such extensive pneumaticity so far down the tail are the diplodocines, including Diplodocus, Barosaurus, and Tornieria. But they do things differently, with BIG, “pleurocoel”-type foramina on the lateral surfaces of the centra, leading to BIG–but simple–camerae inside, and vertebral cross-sections that look like I-beams. In contrast, the saltasaurines have numerous small foramina on the centrum and neural arch that lead to complexes of small pneumatic camellae, giving their vertebrae honeycomb cross-sections. So caudal pneumaticity in diplodocines and saltsaurines is convergent in its presence and extent but clade-specific in its development. Pneumaticity doesn’t get much cooler than that.

Pneumatic ilia in saltasaurines. Cerda et al. (2012: fig. 3).

But it does get a little cooler. Because the stuff in the rest of the paper is even more mind-blowing. Cerda et al. (2012) go on to describe and illustrate–compellingly, with photos–pneumatic cavities in the ilia, scapulae, and coracoids of saltasaurines. And, crucially, these cavities are connected to the outside by pneumatic foramina. This is important. Chambers have been reported in the ilia of several sauropods, mostly somphospondyls but also in the diplodocoid Amazonsaurus. But it hasn’t been clear until now whether those chambers connected to the outside. No patent foramen, no pneumaticity. It seemed unlikely that these sauropods had big marrow-filled vacuities in their ilia–as far as I know, all of the non-pneumatic ilia out there in Tetrapoda are filled with trabecular bone, and big open marrow spaces only occur in the long bones of the limbs. And, as I noted in my 2009 paper, the phylogenetic distribution of iliac chambers is consistent with pneumaticity, in that the chambers are only found in those sauropods that already have sacral pneumaticity (showing that pneumatic diverticula were already loose in their rear ends). But it’s nice to have confirmation.

So, the pneumatic ilia in Rocasaurus, Neuquensaurus, and Saltasaurus are cool because they suggest that all the other big chambers in sauropod ilia were pneumatic as well. And for those of you keeping score at home, that’s another parallel acquisition in Diplodocoidea and Somphospondyli (given the apparent absence of iliac chambers in Camarasaurus and the brachiosaurids, although maybe we should bust open a few brachiosaur ilia just to be sure*).

* I kid, I kid.**

** Seriously, though, if you “drop” one and find some chambers, call me!

Pectoral pneumaticity in saltasaurines. Cerda et al. (2012: fig. 2).

But that’s not all. The possibility of pneumatic ilia has been floating around for a while now, and most of us who were aware of the iliac chambers in sauropods probably assumed that eventually someone would find the specimens that would show that they were pneumatic. At least, that was my assumption, and as far as I know no-one ever floated an alternative hypothesis to explain the chambers. But I certainly did not expect pneumaticity in the shoulder girdle. And yet there they are: chambers with associated foramina in the scap and coracoid of Saltasaurus and in the coracoid of Neuquensaurus. Wacky. And extremely important, because this is the first evidence that sauropods had clavicular air sacs like those of theropods and pterosaurs. So either all three clades evolved a shedload of air sacs independently, or the basic layout of the avian respiratory system was already present in the ancestral ornithodiran. I know where I’d put my money.

There’s loads more interesting stuff to talk about, like the fact that the ultra-pneumatic saltasaurines are among the smallest sauropods, or the way that fossae and camerae are evolutionary antecedent to camellae in the vertebrae of sauropods, so maybe we should start looking for fossae and camerae in the girdle bones of other sauropods, or further macroevolutionary parallels in the evolution of pneumaticity in pterosaurs, sauropods, and theropods. Each one of those things could be a blog post or maybe a whole dissertation. But my mind is already thoroughly blown. I’m going to go lie down for a while. Congratulations to Cerda et al. on what is probably the most important paper ever written on sauropod pneumaticity.

References

• Cerda, I.A., Salgado, L., and Powell, J.E. 2012. Extreme postcranial pneumaticity in sauropod dinosaurs from South America. Palaeontologische Zeitschrift. DOI 10.1007/s12542-012-0140-6
• Janensch, W. 1947. Pneumatizitat bei Wirbeln von Sauropoden und anderen Saurischien. Palaeontographica, Supplement 7, 3:1–25.
• Osborn, H. F. 1899. A skeleton of Diplodocus. Memoirs of the American Museum of Natural History 1:191–214.

## Neural spine bifurcation in sauropods, Part 6: more reasons why Haplocanthosaurus is not a juvenile of a known diplodocid

### April 15, 2012

Last time, we saw why Haplocanthosaurus couldn’t be a juvenile of Apatosaurus or Diplodocus, based on osteology alone.  But there’s more:

# Ontogenetic status of Haplocanthosaurus

Here is where is gets really surreal.  Woodruff and Fowler (2012) blithely assume that Haplocanthosaurus is a juvenile of something, but the type specimen of the type species — H. priscus CM 572 — is an adult.  As Hatcher (1903:3) explains:

The type No. 572 of the present genus consists of the two posterior cervicals, ten dorsals, five sacrals, nineteen caudals, both ilia, ischia and pubes, two chevrons, a femur and a nearly complete series of ribs, all in an excellent state of preservation and pertaining to an individual fully adult as is shown by the coössified neural spines and centra.

So far as I can see, Woodruff and Fowler are confused because the second species that Hatcher describes, H. utterbacki, is based on the subadult specimen CM 879.  Where possible in the previous post, I have used illustrations of the adult H. priscus, so that the comparisons are of adult with adult.  The exceptions are the two anterior cervicals and the first dorsal, which are known only from H. utterbacki.  And sure enough, if you look closely at the illustrations, you can see that in these vertebrae and only these vertebrae, Hatcher had the neurocentral junction illustrated — because it wasn’t yet fused.

Haplocanthosaurus posterior, mid and anterior cervical vertebrae, C14, C9 and C4, in right lateral view. C14 of adult H. priscus (from Hatcher 1903:plate I); C9 and C4 of H. utterbacki (from plate II). Red ellipses highlight neurocentral sutures.

As it happens, the difference in ontogenetic status between these two specimens is nicely illustrated by Wedel (2009), although he was only in it for the pneumaticity:

Neurocentral fusion in Haplocanthosaurus. A, B. Posterior cervical vertebra C?12 of sub-adult H. utterbacki holotype CM 879: A, X-ray in right lateral view; B, coronal CT slice showing separate ossificaton of centrum and neural arch. C, D. Mid-dorsal vertebra D6 of adult H. priscus holotype CM 572: X-rays in (A) right lateral and (B) anterior view, showing fully fused neural arch. Wedel (2009:fig. 6)

So H. utterbacki CM 879 certainly is an immature form of something; and that something is Haplocanthosaurus, most likely H. priscus.  (The characters which Hatcher used to separate the two species are not particularly convincing.)

With that out the way, we can move on to …

# Phylogenetic analysis

A simple way to evaluate the parsimony or otherwise of a synonymy is to use a phylogenetic analysis. In their abstract, Woodruff and Fowler claim that “On the basis of shallow bifurcation of its cervical and dorsal neural spines, the small diplodocid Suuwassea is more parsimoniously interpreted as an immature specimen of an already recognized diplodocid taxon”.  Without getting into the subject of Suuwassea again — Matt pretty much wrapped that up in part 4 — the point here is that the word “parsimony” has a particular meaning in studies of evolution: it refers to minimising the number of character-state changes.  And we have tools for measuring those.

So let’s use parsimony to evaluate the hypothesis that Haplocanthosaurus is one of the previously known diplodocids.  Pretending for the moment that Haplocanthosaurus really was known only from subadults, how many additional steps would we need to account for if ontogeny were to change its position to make it group with one of the diplodocids?

You don’t need to be a cladistics wizard to do this.  (Which is handy, since I am not one.)  Here’s the method:

• Start with an existing matrix, add constraints, re-run it, and see how the tree-length changes.  Since I am familiar with it, I started with the matrix from my 2009 paper on brachiosaurs.
• Re-run the matrix to verify that you get the same result as in the published paper based on it.  This gives you confidence that you’re running it right.  In this case, I got a minimum tree length of 791 steps, just as in Taylor (2009).
• Add extra instructions to the run-script defining and imposing constraints.  Note that you do not have to mess with the characters, taxa or codings to do this.
• Run the matrix again, with the constraint in place, and see how the tree-length changes.
• Repeat as needed with other constraints to evaluate other phylogenetric hypotheses.

(This is how we produced the part of the Brontomerus paper (Taylor et al. 2011:89) where we said “One further step is sufficient to place Brontomerus as a brachiosaurid, a basal (non−camarasauromorph) macronarian, a basal (non−diplodocid) diplodocoid or even a non−neosauropod. Three further steps are required for Brontomerus to be recovered as a saltasaurid, specifically an opisthocoelicaudiine”.  And that’s why we weren’t at all dogmatic about its position.)

Anyway, going through this exercise with Haplocanthosaurus constrained in turn to be the sister taxon to Apatosaurus, Diplodocus, etc., yielded the following results:

• (no constraint) –  791 steps
• Apatosaurus — 817 (26 extra)
• Diplodocus — 825 (34 extra)
• Barosaurus — 815 (24 extra)
• Camarasaurus — 793 (2 extra)
• Brachiosaurus — 797 (6 extra)

(I threw in the other well-known Morrisson-Formation sauropods Camarasaurus and Brachiosaurus, even though Woodruff and Fowler don’t mention them, just because it was easy to do and I was interested to see what would happen.  And when I say Brachiosaurus, I mean B. altithorax, not Giraffatitan.)

I hope you’re as shocked as I am to see that for Haplocanthosaurus to emerge as the sister taxon of any diplodocid needs a minimum of 24 additional steps — or an incredible 34 for it to be sister to Diplodocus.  In other words, the hypothesis is grossly unparsimonious.  Of course, that doesn’t in itself mean that it’s false: but it does render it an extraordinary claim, which means that it needs extraordinary evidence.  And while “the simple spines of Haplocanthosaurus might bifurcate when it grows up” is extraordinary evidence, it’s not in the way that Carl Sagan meant it.

In short, running this simple exercise — it took me about a hour, mostly to remember how to do constraints in PAUP* — would have given Woodruff and Fowler pause for thought before dragging Haplocanthosaurus into their paper.

Oh, and it’s ironic that placing Haplo as sister to Brachiosaurus requires only a quarter as many steps as the closest diplodocid, and as sister to Camarasaurus requires only two steps.  If you really want to synonymise Haplocanthosaurus, Camarasaurus is the place to start.  (But don’t get excited, it’s not Camarasaurus either.  It’s Haplocanthosaurus.)

[By the way, anyone who'd like to replicate this experiment for themselves is welcome: all the files are available on my web-site.  You only really need the .nex file, which you can feed to PAUP*, but I threw in the log-file, the generated tree files and the summary file, too.  Extra Credit: run this same exercise to evaluate the parsimony of Suuwassea as a subadult of one of these other genera.  Report back here when you're done to earn SV-POW! points.]

# Conclusion

It’s a truism that we stand on the shoulders of giants.  In the case of sauropod studies, those giants are people like J. B. Hatcher, Charles Gilmore, Osborn and Mook and — bringing it up to date — John McIntosh, Paul Upchurch, Jeff Wilson and Jerry Harris.  When Hatcher described Haplocanthosaurus as a new genus rather than a subadult Diplodocus, he wasn’t naive.  He recognised the effects of ontogeny, and he was aware that one of his two specimens was adult and the other subadult.  He was also probably more familiar with Diplodocus osteology than anyone else has ever been before or since, having written the definitive monograph on that animal just two years previously (Hatcher 1901).

By the same token, people like Upchurch and Wilson have done us all a huge favour by making the hard yards in sauropod phylogenetics.  If we’re going to go challenging the standard consensus phylogeny, it’s just good sense to go back to their work (or the more recent work of others, such as Whitlock 2011), re-run the analyses with our pet hypotheses encoded as constraints, and see what they tell us.

So in the end, my point is this: let’s not waste our giants.  Let’s take the time to get up on their shoulders and survey the landscape from up there, rather than staying down at ground level and seeing how high we can jump from a standing start.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## References

• Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
• Hatcher, J.B. 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds; additional remarks on Diplodocus. Memoirs of the Carnegie Museum 2:1-75.
• Taylor, M.P. 2009. A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of Vertebrate Paleontology 29(3):787-806.
• Taylor, M.P., Wedel, M.J. and Cifelli, R.L. 2011. A new sauropod dinosaur from the Lower Cretaceous Cedar Mountain Formation, Utah, USA. Acta Palaeontologica Polonica 56(1):75-98. doi:10.4202/app.2010.0073
• Wedel, M.J. 2009. Evidence for bird-like air sacs in saurischian dinosaurs. Journal of Experimental Zoology 311A:611-628.
• Whitlock, J.A. 2011. A phylogenetic analysis of Diplodocoidea (Saurischia: Sauropoda). Zoological Journal of the Linnean Society 161(4):872-915. doi: 10.1111/j.1096-3642.2010.00665.x
• Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.

## Introduction

Last time around, Matt walked through a lot of the detailed cervical morphology of Suuwassea and known diplodocids to show that, contra the suggestion of Woodruff and Fowler (2012), Suuwassea is distinct and can’t be explained away as an ontogenomorph of a previously known genus.

Although Suuwassea is singled out for special treatment in this paper, other genera do not escape unscathed.  From the Conclusions section on page 9:

Just as particularly large diplodocid specimens (e.g., Seismosaurus; Gillette, 1991) have been more recently recognized as large and potentially older individuals of already recognized taxa (Diplodocus; Lucas et al., 2006; Lovelace et al., 2007), taxa deﬁned on small specimens (such as Suuwassea, but also potentially Barosaurus, Haplocanthosaurus, and ‘‘Brontodiplodocus’’), might represent immature forms of Diplodocus or Apatosaurus.

I have to admit I more or less fell out of my chair when I saw the suggestion that poor old Haplocanthosaurus might be Diplodocus or Apatosaurus.  I think this idea comes from a misstatement in the very first sentence of the abstract:

Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of the three subclades Rebbachisauridae, Dicraeosauridae, and Diplodocidae is strongly influenced by a relatively small number of characters.

As a statement of fact, this is simply the opposite of the truth: in all the major phylogenetic analyses, the arrangement of subclades with Diplodocoidea is the most stable part of the tree, supported by more characters than all the other clades.

For example, in the analysis of Upchurch et al. (2004) in The Dinosauria II, fig. 13.18 shows that the nodes with the highest bootstrap percentages are Diplodocinae (96%), Dicraeosauridae (95%) and Diplodocidae (93%).

Or consider the analysis of Wilson (2002).  While it’s getting on a bit, it still scores highly by being the most explicit published sauropod analysis, with comprehensive lists of apomorphies.  Table 12 lists the decay indexes for the 24 nodes in the strict consensus tree.  Apart from the three very basal nodes separating sauropods from their outgroups, the two highest-scoring clades are Diplodocidae and Diplodocinae (DI=7), followed by four clades all with DI=5 of which two are Dicraeosauridae and Flagellicaudata (which Wilson just called “Dicraeosauridae + Diplodocidae” as it had not yet been named).  (It’s well worth reading Wilson’s Appendix 3 to see the synapomorphies supporting these nodes in the MPTs: he lists 14 separating Diplodocimorpha from the node it shares with Haplocanthosaurus, 18 separating Flagellicaudata from the node it shares with Rebbachisauridae, 16 separating Diplodocidae from the node it shares with Dicareosauridae, and seven separating Diplodocinae from the node it shares with Apatosaurus).*

* Why are the lists of apomorphies longer than the decay indexes?  Because they list the apomorphies as they occur in the specific topology of the consensus tree.  Nodes within that tree can be made to collapse without wiping out all the apomorphies by rejuggling other parts of the tree to move character-state transitions around.  So although (for example) 26 characters separate Flagellicaudata from Rebbachisauridae (18 + 8 synapomorphies respectively) you can rejuggle the whole tree to break the monophyly of Flagellicaudata while making the entire tree only five steps longer.

Anyway, for whatever reason, Woodruff and Fowler felt that the stability of the diplodocoid clades was in question, and this presumably influenced their hypothesis that Haplocanthosaurus could be easily moved down into one of the diplodocid genera.

Next time we’ll be considering the implications for the tree.  But today, let’s take a moment to do this the old-fashioned way, by looking at …

## Osteology

### Pelvis

Hatcher (1903), ever helpful, included a comparative plate in his monograph which should help us to evaluate the idea that Haplo is a known diplodocid:

Pelves of diplodocids and Haplocanthosaurus. 1. Pelvis of Brontosaurus excelsus (No. 568); 2. Pelvis of Diplodocus carnegii (No. 94); 3. Pelvis of Haplocanthosaurus priscus (No. 572).  All seen from left side.  1, 2, 3, 4, 5 indicate neural spines of respective sacral vertebra.  Presumably to scale.  Direct from Hatcher (1903:plate IV).

Based on this, the pelvis of Haplocanthosaurus differs from those of the diplodocids in having a proportionally lower ilium, in the absence of the laterally facing rugosity on the posterodorsal margin of the ilium, in the very small distal expansion of the pubis and in the almost non-existent distal expansion of the ischium.  These are all characters of the limb-girdle elements, which do not change greatly through ontogeny in sauropods.

But the evidence from the sacral vertebrae is just as significant: the neural spines in the sacral area are less than half as tall as in the diplodocids — and this in an animal whose dorsal neural spines are conspicuously tall.  The spines are also more anteroposteriorly elongate and plate-like.  What’s more, sacral spines 1, 2 and 3 have fused into a single plate in Haplocanthosaurus, while the spine of S1 remains well separated from 2 and 3 in the diplodocids.  So the ontogenetic hypothesis would have to say that the spine of S1 unfuses through ontogeny.  Which is not something I’ve heard of happening in any sauropod, or indeed any animal.

So the pelvis and sacrum seem distinct.  But Woodruff and Fowler’s (2012) notion of ontogenetic synonymy is built on the idea that the differences in the cervical and dorsal vertebrae are ontogenetic.  So let’s take a look at them.

### Cervical vertebrae

Posterior, mid and anterior cervical vertebrae, in right lateral view, of (top to bottom), Haplocanthosaurus, Apatosaurus louisae CM 3018 (from Gilmore 1936:plate XXIV, reversed for ease of comparison) and Diplodocus carnegii CM 84 (from Hatcher 1901:plate III), scaled to roughly the same size.  For the diplodocids, we illustrate C13, C9 and C4.  For Haplocanthosaurus, we illustrate C14 of H. priscus (from Hatcher 1903:plate I) and C9 and C4 of H. utterbacki (from plate II).

It should be immediately apparent that the Haplocanthosaurus cervicals have less extensive pneumatic features than those of the diplodocids, but that is one feature which we know does vary ontogenetically.  There are other differences: for example, the cervical ribs in Haplocanthosaurus are level with the bottom centrum rather than hanging below.  Still, if you kind of squint a bit, you could probably persuade yourself that the Haplocanthus vertebrae look like possible juveniles of Diplodocus.

Unless you look at them from behind:

Posterior cervical vertebrae C15 and C14, in posterior view, of (top to bottom), Haplocanthosaurus priscus CM 572 (from Hatcher 1903:plate I), Apatosaurus louisae CM 3018 (from Gilmore 1936:plate XXIV) and Diplodocus carnegii CM 84 (from Hatcher 1901:plate III), scaled to the same centrum-to-neural-spine height.

(Unfortunately, these are the only Haplocanthosaurus cervical vertebrae that Hatcher had illustrated in posterior view, so we can’t compare more anterior ones.)

From this perspective, we can immediately significant differences:

• First, that unsplit spine.  Yes, we know that Woodruff and Fowler (2012) have argued that it could be ontogenetic, but these are vertebrae from the most deeply bifurcated region of a diplodocid neck, in a decent sized animal, and there is nothing that so much as hints at bifurcation.
• That whacking great ligament scar running right down the back (and also the front, not pictured) of the neural spine.  There is nothing like this in any diplodocid — neither on the metapophyses nor running though the trough.  And remember, scars like these tend to become more prominent through ontogeny.
• The neural arch (i.e. the region between the postzygapophyses and the centrum) is taller in Haplocanthosaurusmuch taller in the case of C15.
• The plates running out to the diapophyses are less dorsoventrally expanded in Haplocanthosaurus.
• The centrum is smaller as a proportion of total height — especially, much smaller than in Diplodocus.
• The parapophyses extend directly laterally rather than ventrolaterally (hence the position of the cervical ribs level with the bottom of the centrum).

So it doesn’t look good for the juvenile-diplodocid hypothesis.  But let’s take a look at the …

### Dorsal vertebrae

Posterior, mid and anterior dorsal vertebrae, in right lateral view, of (top to bottom), Haplocanthosaurus, Apatosaurus louisae CM 3018 (from Gilmore 1936:plate XXV, reversed for ease of comparison) and Diplodocus carnegii CM 84 (from Hatcher 1901:plate VII), scaled to roughly the same size.  For the diplodocids, we illustrate D9, D5 and D2.  For Haplocanthosaurus, which has four more dorsals, we illustrate D13 and D7 of H. priscus (from Hatcher 1903:plate I) and D2 of H. utterbacki (from plate II).

Here we see that Haplocanthosaurus has dorsolaterally inclined diapophyses (which we’ll see more clearly in a minute), a prominent spinodiapohyseal lamina in posterior dorsals, and no infraparapophyseal lamination.  Also, the dorsal vertebrae have reached their full height by the middle of the series (in fact the last nine dorsals are startlingly similar in proportions), whereas in diplodocids, total height continues to increase posteriorly.

Now let’s see those vertebrae in posterior view:

Posterior, mid and anterior dorsal vertebrae, in posterior view, of (top to bottom), Haplocanthosaurus priscus CM 572 (From Hatcher 1903:plate I), Apatosaurus louisae CM 3018 (from Gilmore 1936:plate XXV) and Diplodocus carnegii CM 84 (from Hatcher 1901:plate VII), scaled to the same height of the mid dorsal.  For the diplodocids, we illustrate D9, D5 and D1.  For Haplocanthosaurus, which has four more dorsals, we illustrate D13, D6 and D1.

Here is where it all falls apart.  The Haplocanthosaurus dorsals differ from those of the diplodocids in almost every respect:

• Of course we have the non-bifid spine in again, in the anterior dorsal, but let’s not keep flogging that dead horse.
• In the mid and posterior dorsals, the neurapophysis is rounded in posterior view rather than square.
• In the posterior dorsal, the neural spine has laterally directed triangular processes near the top.
• All three Haplocanthosaurus neural spines have broad, rugose ligament scars, whereas those of the diplodocids have narrow postspinal laminae.
• The neural spines (measured from the diapophyses upwards) are much shorter than in the diplodocids; but
• The neural arches (measured from the centrum up to the diapophyses) are much taller.
• The diapophyses have distinct club-like rugosities at their tips.
• the diapophyses of the mid and posterior dorsals are inclined strongly upwards
• The hyposphenes of mid and posterior dorsals have very long centropostzygapophyseal laminae curving up in a gentle arch.
• The centra are smaller than those of Apatosaurus, and much smaller than those of Diplodocus.

(By the way, it’s interesting how very different the D5s of Apatosaurus and Diplodocus are.  Since both are from uncontroversially adult specimens, bifurcation was evidently very different between these genera.)

So based on the vertebrae alone, the case of Haplocanthosaurus as an immature form of Diplodocus or Apatosaurus is blown right out of the water.  And this is without even looking at the appendicular material — for example, the scapula and coracoid illustrated by Hatcher (1903:figs 17-19), which are so completely different from those of diplodocids.

But there’s more.  Tune in next time for the rest.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## References

• Gillette, D.D. 1991. Seismosaurus halli, gen. et sp. nov., a new sauropod dinosaur from the Morrison Formation (Upper Jurassic/Lower Cretaceous) of New Mexico, USA. Journal of Vertebrate Paleontology 11(4):417-433.
• Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
• Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
• Hatcher, J.B. 1903. Osteology of Haplocanthosaurus with description of a new species, and remarks on the probable habits of the Sauropoda and the age and origin of the Atlantosaurus beds; additional remarks on Diplodocus. Memoirs of the Carnegie Museum 2:1-75.
• Lovelace, D.M., Hartman, S.A., Wahl, W.R. 2008. Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, Rio de Janeiro 65(4):527-544.
• Lucas, S.G., Spielmann, J.A., Rinehart, L.F., Heckert, A.B., Herne, M.C., Hunt, A.P., Foster, J.R., Sullivan, R.M. 2006, Taxonomic status of Seismosaurus hallorum, a Late Jurassic sauropod dinosaur from New Mexico. New Mexico Museum of Natural History and Science Bulletin 36:149-162.
• Upchurch, P. Barrett, P.M., Dodson, P. 2004. Sauropoda. pp. 259-322 in D.B. Weishampel, P. Dodson and H. Osmólska (eds.), The Dinosauria, 2nd edition. University of California Press, Berkeley and Los Angeles. 861 pp.
• Wilson, J.A. 2002. Sauropod dinosaur phylogeny: critique and cladistic analysis. Zoological Journal of the Linnean Society 136:217-276.
• Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.

# Special bonus illustrations

I composited the cervical and dorsal series above into the following compound illustrations.  As always, click through for full resolution.

Lateral view:

Posterior, mid and anterior dorsal vertebrae and cervical vertebrae, in right lateral view, of (top to bottom), Haplocanthosaurus, Apatosaurus louisae CM 3018 (from Gilmore 1936:plates XXIV and XXV, reversed for ease of comparison) and Diplodocus carnegii CM 84 (from Hatcher 1901:plates III and VII), scaled to roughly the same size. For the diplodocids, we illustrate D9, D5, D2, C13, C9 and C4. For Haplocanthosaurus, which has four more dorsals, we illustrate D13, D7 and C14 of H. priscus (from Hatcher 1903:plate I) and D2, C9 and C4 of H. utterbacki (from plate II).

Posterior view:

Posterior, mid and anterior dorsal vertebrae and posterior cervical vertebrae C15 and C14, in posterior view, of (top to bottom), Haplocanthosaurus priscus CM 572 (From Hatcher 1903:plate I), Apatosaurus louisae CM 3018 (from Gilmore 1936:plates XXIV and XXV) and Diplodocus carnegii CM 84 (from Hatcher 1901:plates III and VII), scaled to the same height of the mid dorsal. For the diplodocids, we illustrate D9, D5 and D1. For Haplocanthosaurus, which has four more dorsals, we illustrate D13, D6 and D1.

## Neural spine bifurcation in sauropods, Part 4: is Suuwassea a juvenile of a known diplodocid?

### April 12, 2012

I don’t intend to write a comprehensive treatise on the morphology and phylogeny of Suuwassea. Jerry Harris has already done that, several times over (Harris 2006a, b, c, 2007, Whitlock and Harris 2010). Rather, I want to address the contention of Woodruff and Fowler (2012) that Suuwassea is a juvenile of a known diplodocid, building on the information presented in the first three posts in this series (Part 1, Part 2, Part 3).

In the abstract, Woodruff and Fowler (2012:1) wrote:

On the basis of shallow bifurcation of its cervical and dorsal neural spines, the small diplodocid Suuwassea is more parsimoniously interpreted as an immature specimen of an already recognized diplodocid taxon.

First of all, that’s not what ‘parsimoniously’ means. It’s just not. In a phylogenetic analysis using unweighted characters, there is no such thing as a ‘key’ character — which by the way means that the subtitle of the paper, “a critical phylogenetic character”, is wrong. All characters are equal. Even if the characters were weighted, neural spine bifurcation would have to be weighted pretty darned heavily for it to outweigh all the other characters combined, which is what the sentence quoted above suggests.

## Comparisons with known diplodocids

Next problem: if Suuwassea is a juvenile of an already recognized diplodocid, it shouldn’t take long to figure out which one. There aren’t all that many candidates, and we can consider them in turn.  There are loads of characters, especially cranial and appendicular, separating Suuwassea from Apatosaurus, Diplodocus and the rest, and anyone who wants to keep track of all of them is welcome to do so. I care about vertebrae, and I’m prepared to argue that Suuwassea is a distinct taxon based on cervical morphology alone.

### Diplodocus

Here are the sixth cervical vertebrae of Suuwassea emiliae ANS 21122 (Harris 2006c:Text-fig. 7B) and Diplodocus carnegii CM 84/94 (Hatcher 1901:pl. 3, flipped left-to-right for ease of comparison). They are not to scale–I made the images the same cotyle diameter for ease of comparison.

Elongation first. C6 of S. emilieae has a centrum length of 257 mm, a cotyle diameter of 75 mm, and so an EI of 3.4. C6 of D. carnegii has a centrum length of 442 mm, a cotyle diameter of 99 mm, and an EI of 4.5. So Diplodocus is one third more elongate than Suuwassea. It is true that sauropod cervicals elongate through ontogeny, but the Suuwassea holotype is a decent-sized animal, and would be expected to have attained adult proportions even if it was not fully adult (also, ANS 21122 has more cervical ribs fused than CM 84/94). We know from the juvenile ?Sauroposeidon vertebra YPM 5294 (Wedel et al. 2000:372) that that subadult sauropod cervicals attained great elongation: this element is from an animal young enough to have had an unfused neural arch but it has an EI exceeding 5.0.

Then there’s neural spine shape. Yes, it is variable in sauropods, but this is ridiculous. I strongly doubt that any non-pathological Diplodocus cervical anywhere ever has had a neural spine shaped like that of the Suuwassea vertebra.

Also note that the prezygapophyses of the D. carnegii C6 strongly overhang the condyle but are only slightly elevated, whereas those of S. emilieae are right above the condyle but strongly elevated, so that the prezygapophyseal rami might fairly be called pedestals. Such pedestaling of the prezygapophyses is present in some cervicals of Apatosaurus, although perhaps not to the same extreme. Some Apatosaurus cervicals have pretty funky, smokestack-looking neural spines, although–again–not to the same extreme as in Suuwassea. Still, from the mid-centrum on up, S. emiliae looks a bit apatosaur-ish. So let’s try that next.

### Apatosaurus

Here we have C6 of Suuwassea as before, this time with Apatosaurus louisae CM 3018 (Gilmore 1936:pl. 24), again scaled to the same cotyle diameter.

C6 of A. louisae has a centrum length of 440 mm, a cotyle diameter of 150 mm, and an EI of 2.9 (I know it doesn’t look that short, but I’m going off Gilmore’s data, and I trust the measuring tape more than the drafting pen, no matter how skillfully the latter is wielded.)  So this is not a bad match with the value of 3.4 for Suuwassea.

Of course, the glaring problem with suggesting that Suuwassea is a juvenile Apatosaurus is that it has normal-sized cervical ribs, not the insane scythes of doom that hang below the centrum of every post-axial Apatosaurus cervical (see these posts [#1, #2, #3] for some crazy examples, and this post for more pictures and discussion). The giant cervical ribs are present even in very juvenile Apatosaurus cervicals, such as the large collection of juvenile apatosaurs in the BYU collection from Cactus Park (albeit unfused; the immense parapophyses still point the way even if the ribs themselves are missing).

I know, I know, I just said that there is no such thing as a key character. But all of the known species of Apatosaurus have giant cervical ribs, and indeed are often identified in the field as Apatosaurus on that basis alone. I suppose it’s not impossible that Suuwassea is nested within the other Apatosaurus species, based on some bizarre combination of as-yet undiscovered characters and intermediate specimens, and lost the giant cervical ribs along the way, but now we’re into angels dancing on the heads of non-existent pins. If Suuwassea is an apatosaurine but outside the clade of giant-cervical-rib-bearing Apatosaurus, then whether we call it a species of Apatosaurus or a separate genus–say, Suuwassea–is more a matter of taste than anything else.  Note that Lovelace et al. (2008) recovered Suuwassea as an apatosaurine, but not as Apatosaurus.

Lest anyone without access to the paper think I’m cheating by hiding serial variation, here are the other well-preserved cervicals of Suuwassea, to scale:

Suuwassea emilieae cervicals 3, 5, and 6 in left lateral view, from Harris (2006c:Text-figs. 5, 6, and 7)

So, if Suuwassea is a juvenile of a known diplodocid but it’s not Diplodocus or Apatosaurus, what’s left?

### Barosaurus

Barosaurus lentus AMNH 6341 cervicals 8-16 in left lateral view, from McIntosh (2005:fig. 2.1)

Probably not.

Unconvincing.

### “Amphicoelias brontodiplodocus”

"Amphicoelias brontodiplodocus" cervicals 7-10 in left lateral view, from Galiano and Albersdorfer (2010:fig 10a)

Okay, now I’m just messing with you.

## Is Suuwassea even a juvenile?

By now it is probably obvious, even from cervical morphology alone, that if ANS 21122 is a juvenile of anything, it’s a juvenile Suuwassea. But is it in fact a juvenile?

We-ell. The cervical neural arches are all fused, but not all of the cervical ribs are. Jerry did a fine job of describing exactly what was going on at each serial position (Harris 2006c). In C3, the left cervical rib is not attached, and the right one is attached at the parapophysis but not fused. In C5, the ribs are attached, not fused at the parapophyses, and fused at the diapophyses*. In C6, the ribs are fused at both attachment points. C7 lacks the ribs, but their absence appears to be caused by breakage rather than lack of fusion. One fragmentary posterior cervical of uncertain position is missing the diapophyses but has one rib fused at the parapophysis.

* This is cool because it is the first time that I know of that anyone has documented which of the two attachment points fused first within a single cervical rib. I wonder if other sauropods did it the same way?

So based on cervicals alone, we would infer that Suuwassea was not fully mature. However–and this is absolutely crucial for the synonymization hypothesis–the Suuwassea holotype ANS 21122 already has a greater degree of cervical element fusion than Diplodocus carnegii holotype CM 84/94 (which has unfused ribs back to C5) and Apatosaurus CM 555 (which has unfused arches back to C8 and unfused ribs throughout), both of which have attained essentially ‘adult’ morphology. So if Woodruff and Fowler (2012) are correct, the ontogenetic clock has to run forward from CM 555 and CM 84/94, through a Suuwassea-like stage, and then back to normal Apatosaurus or Diplodocus morphology.

But we don’t have to rely on cervicals alone, because ANS 21122 also includes some dorsals and caudals. And the caudals are very interesting in that the neural arches are not fused through most of the series. Harris (2006c:1107):

Of all the caudal vertebrae preserved in ANS 21122, only the distal, ‘whiplash’ caudals are complete. All the remaining vertebrae consist only of vertebral bodies that lack all phylogenetically informative portions of their respective arches. On the proximal and middle caudals, this absence is due to lack of fusion as evidenced by the deeply fluted articular surfaces for the arches on the bodies. In contrast, the arches on the most distal vertebrae that retain them are seamlessly fused, but everything dorsal to the bases of the corporozygapophyseal laminae are broken.

Now this is pretty darned interesting, because it shows that neural arch fusion in Suuwassea was not a simple zipper that ran from back to front (as in crocs [Brochu 1996] and phytosaurs [Irmis 2007*]) or front to back. We can’t really say, based on this one specimen, what the sequence was, but we can say for certain that the anterior and middle caudals came last. Oh, and for what it’s worth, the scap-coracoid joint is also unfused (Harris 2007), but we know that that’s often the case for substantially “adult” sauropods such as the mounted Berlin Giraffatitan.

* Relevant to this entire post series are the wise words of my homeboy and former Padian labmate Randy Irmis, who wrote in the abstract of his 2007 neurocentral fusion paper:

A preliminary survey indicates that there is considerable variation of both the sequence and timing of neurocentral suture closure in other archosaur clades. Therefore, it is unwise to apply a priori the crocodylian pattern to other archosaur groups to determine ontogenetic stage. Currently, apart from histological data, there are few if any reliable independent criteria for determining ontogenetic stage. I propose that histology be integrated with independent ontogenetic criteria (such as neurocentral suture closure) and morphometric data to provide a better understanding of archosaur ontogeny.

The unfused arches in the Suuwassea caudals are especially interesting because, for the first time that I know of, we have a sauropod with cervical neural arches and at least some cervical ribs fused, but with unfused neural arches elsewhere in the body. This is in contrast to D. carnegii CM 84/94, in which all the neural arches are fused but the anterior cervical ribs are not. So the developmental timing in Suuwassea is dramatically different than in D. carnegii, at least, which is one more problem for the synonymization hypothesis. Two more problems, actually, in that (1) Suuwassea probably isn’t Diplodocus, and (2) it doesn’t belong in the same ontogenetic series as Diplodocus, contra Woodruff and Fowler (2012:Figs. 3 and 9)–if the timing of the various fusions differs between the taxa, there is no basis for assuming that the hypothetical ontogenetic bifurcation would follow the same rules.

And speaking of ontogenetic bifurcation, a final point about the ‘bifurcations’ in Suuwassea.

Woodruff and Fowler (2012:Fig. 9)

The first line of the caption is misleading. Two of these vertebrae have weakly bifurcated neural spines because they are sixth cervicals (Suuwassea in B, Apatosaurus in D), and that’s what you expect in C6 in adult diplodocids. One of them, the C5 of Suuwassea in C, isn’t bifurcated at all: it’s broken. Harris (2006c:1099):

The spinous process expands mediolaterally toward its apex, attaining maximal width just proximal to its terminus. A long, narrow crack at the distal end gives the appearance of bifurcation, but the collinear dorsal margin indicates that no true split was present.

As for the final vertebra, MOR 592 in A, who knows? Woodruff and Fowler (2012) do not say what serial position it is from. Based on the shallow notch in the spine, I’ll bet it’s either a C6 or very close to it–and if so, no deeper split is expected.

So the entire rationale for the taxonomic side of Woodruff and Fowler (2012)–that Suuwassea has incompletely bifurcated neural spines because it is a juvenile –turns out be an illusion caused by not taking serial variation into account. Suuwassea ANS 21122 probably is a subadult, based on the unfused caudal neural arches, but its cervical vertebrae already show the expected adult morphology in neural arch fusion, cervical rib fusion, and–most importantly–neural spine bifurcation.

## Envoi

The evidence that Suuwassea is not a juvenile of a known diplodocid is not in this post. It’s in the hard work, comprehensive descriptions, and detailed, thoughtful comparisons by Harris and Dodson (2004), Harris (2006a, b, c, 2007), Lovelace et al. (2008), Whitlock and Harris (2010), and Whitlock (2011). This post is just an arrow scratched in the dirt. Please, go read those papers. And then read all the monographs I cited in the first post in this series (and am too lazy to cite again here). Give those people their due by taking their work seriously and learning from it.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## Neural spine bifurcation in sauropods, Part 3: the evidence from ontogenetic series

### April 10, 2012

This is the third post in a series on neural spine bifurcation in sauropods, inspired by Woodruff and Fowler (2012). In the first post, I looked at neural spine bifurcation in Morrison sauropod genera based on the classic monographic descriptions. In the second post, I showed that size is an unreliable criterion for assessing age and that serial variation can mimic ontogenetic change in sauropod cervicals. In this post I look at the evidence for ontogenetic changes in neural spine bifurcation presented by Woodruff and Fowler (2012). This posts builds on the last two, so please refer back to them as needed.

## Another opening digression, on the OMNH baby sauropod material this time

Nearly all of the Morrison Formation material in the OMNH collections comes from Black Mesa in the Oklahoma panhandle. It was collected in the 1930s by WPA crews working under the direction of J. Willis Stovall. Adequate tools and training for fossil preparation were in short supply. A lot of the prep was done by unskilled laborers using hammers, chisels, pen-knives, and sandpaper (apologies if you have experience with fossil preparation and are now feeling a bit ill). Uncommonly for the Morrison, the bones are very similar in color to the rock matrix, and the prep guys sometimes didn’t realize that they were sanding through bone until they got through the cortex and  into the trabeculae. Consequently, a lot of interesting morphology on the OMNH Morrison material has been sanded right off, especially some of the more delicate processes on the vertebrae. This will become important later on.

## Do the ‘ontogenetic’ series in Woodruff and Fowler (2012) actually show increasing bifurcation through development?

In the Materials and Methods, Woodruff and Fowler (2012:2) stated:

Study specimens comprise 38 cervical, eight dorsal, and two caudal vertebrae from 18 immature and one adult diplodocid (Diplodocus sp., Apatosaurus sp., and Barosaurus sp.), and two immature macronarians (both Camarasaurus sp.).

However, their Table 1 and Supplementary Information list only 15 specimens, not 18. Of the 15, one is probably not a diplodocid (SMA 0009 ‘Baby Toni’) — a fact that, oddly, the authors knew, as stated in the Supplementary Information.  Of the remaining 14 specimens, 11 are isolated vertebrae, so only three represent reasonably complete probably-diplodocoid series (MOR 592, AMNH 7535, and CM 555). From CM 555 they discuss only one vertebra, the C6; and AMNH 7535 is not mentioned at all outside of Table 1 and a passing mention the Supplementary Information, so the subadult data actually used in the paper consist of isolated vertebrae and one articulated series, MOR 592. (For the sake of comparison, in the first post on this topic I looked up 10 articulated series, only two of which–Diplodocus carnegii CM 84/94 and Camarasaurus lentus CM 11338–are even mentioned in Woodruff and Fowler [2012].)

In light of the previous post, on serial variation, the dangers of using isolated vertebrae should by now be apparent. Recall that even adult diplodocids are expected to have completely unsplit spines as far back as C5 (Apatosaurus) or C8 (Barosaurus) and as far forward as D7 (Apatosaurus) or D6 (Barosaurus), and only partially split spines in the adjacent positions. Furthermore, size is a notoriously unreliable criterion of age; MOR 790 8-10-96-204 from Figure 2 in Woodruff and Fowler (2012) also appears in their Figure 3 as the second-smallest vertebra in this ‘ontogenetic’ series, despite most likely coming from a well-fused adult approximately the same size as the D. carnegii individual that represents the end of the series. So without any evidence other than sheer size (if that size overlaps with the adult size range) and degree of neural spine bifurcation (which cannot help but overlap with the adult range, since the adult range encompasses all possible states), simply picking small vertebrae with unsplit spines and calling them juvenile is at best circular and at worst completely wrong–as in the case of MOR 790 8-10-96-204 examined in the last post.

Unfortunately it is not possible to tell what criteria Woodruff and Fowler (2012) used to infer age in their specimens, because they don’t say. Neural arch fusion is discussed in general terms in the Supplementary Information, but in the text and in the figures everything is discussed simply in terms of size. For example:

In the next largest specimen (MOR 790 7-26-96-89, vertebral arch 9.9 cm high), the neural spine is relatively longer still and widens at the apex…

The Supplementary Information provides more evidence that Woodruff and Fowler (2012) did not consider the confounding effects of size, serial position, and ontogenetic stage. In the section on the Mother’s Day Quarry in the Supplementary Information, they wrote:

Because of this size distribution it is not surprising that there are also different ontogenetic stages present which result in cervical centrum lengths varying between 12 and 30 cm.

Now, there may be different ontogenetic stages present in the quarry, and the cervicals in the quarry may vary in length by a factor of 2.5, but the latter does not demonstrate the former. In D. carnegii CM 84/94 the longest postaxial cervical (C14, 642 mm) is 2.6 times the length of the shortest (C3, 243 mm; data from Hatcher 1901). The size range reported as evidence of multiple ontogenetic stages by Woodruff and Fowler (2012) turns out to be slightly less than that expected in a single individual.

With that in mind, let’s look at each of the putative ontogenetic sequences in Woodruff and Fowler (2012):

### Anterior cervical vertebrae

Woodruff and Fowler (2012:fig. 3)

The proposed ontogenetic series used by Woodruff and Fowler (2012) for anterior cervical vertebrae consists of:

• CMC VP7944, an isolated ?Diplodocus vertebra from the Mother’s Day site, which is described in the text but not pictured;
• MOR 790 7-30-96-132, an isolated vertebra from the same site;
• MOR 790 8-10-96-204, another isolated vertebra from the same site;
• MOR 592, from a partial cervical series of a subadult Diplodocus but with the serial position unspecified;
• ANS 21122, C6 of Suuwassea (included in Fig. 3, but not discussed as evidence in the accompanying text)
• CM 555, C6 of a nearly complete (C2-C14) cervical series of a subadult Apatosaurus;
• CM 84/94, C7 of Diplodocus carnegii

CMC VP7944 is not pictured, but from the description in the text it’s perfectly possible that it represents a C3, C4, or C5, all of which have undivided spines even in adult diplodocids. It therefore contributes no information: the hypothesis that the spine is undivided because of ontogeny is not yet demonstrated, and the hypothesis that the spine is undivided because of serial position is not yet falsified.

MOR 790 7-30-96-132 is shown only from the front, so the centrum proportions and the shape of the neural spine cannot be assessed. The neural arch appears to be fused, but the cervical ribs are not. Again, we cannot rule out the possibility that it comes from an very anterior cervical and therefore its undivided spine could be an artifact of its serial position. It therefore contributes no information on possible ontogenetic changes in neural spine bifurcation.

As shown in the previous post, MOR 790 8-10-96-204 is probably a C4 or C5 of an adult or near-adult Diplodocus about the same size as or only slightly smaller than D. carnegii CM 84/94. It is small and has an undivded spine because it is an anterior cervical, not because it is from a juvenile. It therefore contributes no support to the ontogenetic bifurcation hypothesis.

The pictured vertebra of MOR 592 has a shallow notch in the tip of the spine, which is expected in C6 in Apatosaurus and Diplodocus and in C9 and C10 in Barosaurus. The serial position of the vertebra is not stated in the paper, but about half of the anterior cervicals even in an adult diplodocid are expected to have unsplit or shallowly split spines based on serial position alone. Based on the evidence presented, we cannot rule out the possibility that the shallow cleft in the pictured vertebra is an artifact of serial position rather than ontogeny. It therefore contributes no support to the ontogenetic bifurcation hypothesis.

ANS 21122 has an incompletely divided neural spine, which is in fact expected for the sixth cervical in adult diplodocids as shown by A. parvus CM 563/UWGM (in which C6 is missing but C5 has an unsplit spine and C7 a deeply bifid spine) and D. carnegii CM 84/94 (in which C6 is also shallowly bifid). A. ajax NMST-PV 20375 has a wider split in the spine of C6, but the exact point of splitting appears to vary by a position or two among diplodocids. The hypothesis that the spine of ANS 21122 C6 is already as split as it would ever have gotten cannot be falsified on the basis of the available evidence.

CM 555 C6: see the previous paragraph. Note that in ANS 21122 the neural arch and cervical ribs are fused in C6, and in C6 of CM 555 they are not.

CM 84/94 C7 has a deeply split spine, but this expected at that position. C6 of the same series has a much more shallow cleft, and C5 would be predicted to have no cleft at all (recall from the first post that the neural spines of C3-C5 of this specimen are sculptures). So any trend toward increasing bifurcation is highly dependent on serial position; if serial position cannot be specified then it is not possible to say anything useful about the degree of bifurcation in a given vertebra.

Summary. CMC VP7944 and MOR 790 7-30-96-132 could be very anterior vertebrae, C3-C5, in which bifurcation is not expected even in adults. Since they are isolated elements, that hypothesis is very difficult to falsify. MOR 790 8-10-96-204 is almost certainly a C4 or C5 of an adult or near-adult Diplodocus. ANS 21122 and CM 555 C6 are incompletely divided, as expected for vertebrae in that position even in adults. CM 84/94 has a shallowly divided spine in C6 and more deeply bifid spines from C7 onward, just like CM 555.

Verdict: no ontogenetic change has been demonstrated.

### Posterior cervical vertebrae

Woodruff and Fowler (2012:Fig. 4A)

The proposed ontogenetic series includes:

• OMNH 1267 and 1270
• MOR 790 7-26-96-89
• MOR 592
• CM 84/94

OMNH 1267 and 1270 are isolated neural arches of baby sauropods from the Black Mesa quarries. OMNH 1267 does not appear to be bifurcated, but it has a very low neural spine and it was probably sanded during preparation, so who knows what might have been lost. OMNH 1270 actually shows a bifurcation–Woodruff and Fowler (2012:3) describe it as having “a small excavated area”–but again it is not clear that the spines are as intact now as they were in life. More seriously,  since these are isolated elements (you can all join in with the refrain) their serial position cannot be determined with any accuracy, and therefore they are not much use in determining ontogenetic change. Although they are anteroposteriorly short, that does not necessarily make them posterior cervicals. The cervical vertebrae of all sauropods start out proportionally shorter and broader than they end up (Wedel et al. 2000:368-369), and the possibility that these are actually from anterior cervicals–not all of which are expected to have bifurcations–is difficult to rule out.

The other three vertebrae in the series have deeply bifurcated spines. In the text, Woodruff and Fowler (2012:3) make the case that the bifurcation in MOR 592 is deeper than in the preceding vertebra, MOR 790 7-26-96-89. However, the proportions of the two vertebrae are very different, suggesting that they are from different serial positions, and the centrum of MOR 790 7-26-96-89 is actually larger in diameter than that of the representative vertebra from MOR 592. So unless centrum size decreased through ontogeny, these vertebrae are not comparable. As usual, we don’t know where in the neck the isolated MOR 790 vertebra belongs, and we only see it in anterior view. Nothing presented in the paper rules out possibility that is actually an anterior cervical, and in fact the very low neural spines suggest that that is the case.

Allowing for lateral crushing, the vertebra from MOR 592 (again, we are not told which one it is) looks very similar to the D. carnegii CM 84/94 vertebra (C15–again, I had to look it up in Hatcher), and is probably from a similar position in the neck. In comparing the two, Woodruff and Fowler (2012:4) say that in CM 84/94, “the bifurcated area has broadened considerably”, but this clearly an illusion caused by the lateral compression of the MOR 592 vertebra — its centrum is also only half as wide proportionally as in the CM 84/94 vertebra.

Summary. The OMNH vertebrae are of unknown serial position and probably lost at least some  surface bone during preparation, so their original degree of bifurcation is hard to determine. The other three vertebrae in the series all have deeply bifid spines, but they are out of order by centrum size, MOR 790 7-26-96-89 might be an anterior cervical based on its low neural spines, and the “broadening” of the trough between MOR 792 and CM 84/94 is an artifact of crushing.

Verdict: no ontogenetic change has been demonstrated.

### Anterior dorsal vertebrae

Woodruff and Fowler (2012:Fig. 5A)

The ontogenetic series here consists of:

• MOR 790 7-17-96-45
• MOR 592
• CM 84/94

As usual, the serial positions of the MOR 592 and CM 84/94 vertebrae are presumably known but not stated in the paper. The D. carnegii CM 84/94 vertebra is D4. Comparisons to the MOR 592 vertebra are not helped by the fact that it is shown in oblique posterior view. Nevertheless, the two vertebrae are very similar and, based on the plates in Hatcher (1901), the MOR 592 vertebra is most likely a D4 or D5 of Diplodocus. The spines in the larger two vertebrae are equally bifurcated, so the inference of ontogenetic increase in bifurcation rests on the smallest of the three vertebrae, MOR 790 7-17-96-45.

MOR 790 7-17-96-45 is an isolated unfused neural arch, clearly from a juvenile. Its serial position is hard to determine, but it is probably not from as far back as D4 or D5 because it appears to lack a hypantrum and shows no sign of the parapophyses, which migrate up onto the neural arch through the cervico-dorsal transition. The element is only figured in anterior view, so it is hard to tell how long it is proportionally. Still, based on the single photo in the paper (which is helpfully shown at larger scale in Fig. 5B), it seems to be reasonably long, with the prezygapophyses, transverse processes, neural spines, and postzygapophyses well separated from anterior to posterior. In fact, I see no strong evidence that it is a dorsal neural arch at all–the arch of a posterior cervical would look the same in anterior view.

Given that MOR 7-17-96-45 lacks a hypantrum and parapophyses, it is not directly comparable to the two larger vertebrae. Although we cannot determine its position in the presacral series, its spine is shallowly bifurcated, to about half the distince from the metapophyses to the postzygapophyses. In Apatosaurus louisae CM 3018, the notch in D3 is about equally deep, and in C15 it is only slightly deeper, still ending above the level of postzygapophyses. So there is some variation in the depth of the bifurcation in the posterior cervicals and anterior dorsals in the North American diplodocids. Without knowing the precise serial position of MOR 7-17-96-45, it is difficult to derive inferences about the ontogeny of neural spine bifurcation.

Diplodocid anterior dorsal vertebrae. Left and right, dorsal vertebrae 3 and 4 of adult Apatosaurus louisae holotype CM 3018, from Gilmore (1936: plate XXV). Center, juvenile neural arch MOR 7-17-96-45, modified from Woodruff and Fowler (2012: fig. 5B), corrected for shearing and scaled up.

What this element does conclusively demonstrate is that the neural arches of posterior cervicals or anterior dorsals in even small, unfused juvenile diplodocids were in fact bifurcated to to a degree intermediate between  D3 and D4 in the large adult Apatosaurus louisae CM3018 — in fact, so far as neural cleft depth is concerned, it makes rather a nice intermediate between them.  (It differs in other respects, most notable that it is proportionally broad, lacks a hypantrum and parapophyses, etc.)

Summary. The two larger specimens in the ‘ontogenetic series’ are from similar serial positions and show the same degree of bifurcation. MOR 7-17-96-45 is from a more anterior position, based on its lack of hypantrum and parapophyses.  Although it is a juvenile, its degree of bifurcation is similar to that of anterior dorsal vertebrae in adult Apatosaurus (and that of C15 in A. louisae CM 3018, if MOR 7-17-96-45 is, in fact, a cervical).

Verdict: no ontogenetic change has been demonstrated.

### Posterior dorsal vertebrae

Woodruff and Fowler (2012:Fig. 6A)

The ontogenetic series consists of:

• OMNH 1261
• MOR 592
• CM 84/94

The D. carnegii CM 84/94 vertebra is D6, and based on its almost identical morphology the MOR 592 vertebra is probably from the same serial position. They show equivalent degrees of bifurcation.

OMNH 1261 is another isolated juvenile neural arch. The portion of the spine that remains is unbifurcated. However, the spine is very short and it is possible that some material is missing from the tip. More importantly, the last 3-4 dorsals in Apatosaurus, Diplodocus, and Barosaurus typically have extremely shallow notches in the neural spines or no notches at all. If OMNH 1261 is a very posterior dorsal, it would not be expected to show a notch even when fully mature.

Verdict: no ontogenetic change has been demonstrated.

Woodruff and Fowler (2012:Fig. 7)

### Caudal vertebrae

The ontogenetic series here consists of:

• MOR 592
• CM 84/94

The first thing to note is that the ‘bifurcation’ in MOR 592 is at right angles to that in the proximal caudals of D. carnegiiCM 84/94, so the one can hardly be antecedent to the other.

More importantly, antero-posterior ‘bifurcations’ like that in MOR 592 are occasionally seen in the caudal vertebrae of adult sauropods. Below are two examples, caudals 7 and 8 of A. parvus CM 563/UWGM 15556. In other words, in this character MOR 592 already displays adult morphology.

Verdict: no ontogenetic change has been demonstrated.

A. parvus CM 563/UWGM 15556 caudals 8 and 7 in right lateral view, from Gilmore (1936:pl.. 33)

Camarasaurus

The ontogenetic series here consists of:

• OMNH 1417
• AMNH 5761

OMNH 1417 is an isolated cervical neural spine, and the pictured vertebra of Camarasaurus supremus AMNH 5761 is a posterior cervical. In C. grandis and C. lewisi, all of the cervical vertebrae eventually develop at least a shallow notch in the tip of the neural spine, but as shown in the previous post there seems to be some variation between Camarasaurus species, and, likely, between individuals. In the absence of information about its serial position and the species to which it belonged, the lack of bifurcation in OMNH 1417 is uninformative; it could belong to an anterior cervical of C. supremus that would not be expected to develop a bifurcation.

Verdict: no ontogenetic change has been demonstrated. There is evidence that neural spine bifurcation developed ontogenetically in Camarasaurus, but it comes from the juvenile C. lentus CM 11338, described by Gilmore (1925), and the geriatric C. lewisi, described by McIntosh, Miller et al. (1996)–see the first post in this series for discussion.

## Conclusions

The ‘ontogenetic’ series of Woodruff and Fowler (2012) are not really ontogenetic series. In all of the diplodocid presacral vertebrae and in Camarasaurus, the smallest elements in the series are isolated vertebrae or neural arches for which the serial position is almost impossible to determine (and for the reader, completely impossible given the limited information in the paper) and even the taxonomic identifications are suspect (e.g., the OMNH material–how one reliably distinguishes the Apatosaurus and Camarasaurus neural arches is beyond me). The larger vertebrae in the presacral series are all compromised in various ways: one includes an adult masquerading as a juvenile (MOR 790 8-10-96-204 in the anterior cervicals), one is out of order by centrum size (MOR 790 7-26-96-89 and MOR 592 in the posterior cervicals), and two show no change in degree of bifurcation from the middle of the series to the upper end (MOR 592 and CM 84/94 in the anterior and posterior dorsals). The shallow longitudinal bifurcation in the MOR 592 caudal vertebra is similar to those found in caudal vertebrae of adult diplodocids, and is not antecedent to the transverse bifurcations discussed in the rest of the paper.

Crucially, when information on size and serial position is taken into account, none of the ‘ontogenetic series’ in the paper show any convincing evidence that neural spine bifurcation increases over ontogeny. The best evidence that bifurcation does increase over ontogeny comes from Camarasaurus, specifically the juvenile C. lentus CM 11338 described by Gilmore (1925) and geriatric C. lewisi BYU 9047 described by McIntosh et al. (1996), it was already recognized prior to Woodruff and Fowler (2012), and it has not caused any taxonomic confusion.

There is an asymmetry of interference here. To call into question the conclusions of Woodruff and Fowler (2012), all one has to do is show that the evidence could be explained by serial, intraspecific, or interspecific variation, taphonomy, damage during preparation, and so on. But to demonstrate that bifurcation develops over ontogeny, one has to falsify all of the competing hypotheses. I know of only one way to do that: find a presacral vertebral column that is (1) articulated, (2) from an individual that is clearly juvenile based on criteria other than size and degree of bifurcation, which (3) can be confidently referred to one of the known genera, and then show that it has unbifurcated spines in the same serial positions where adult vertebrae have bifurcated spines. Isolated vertebrae are not enough, bones from non-juveniles are not enough, and juvenile bones that might pertain to new taxa are not enough. It may be that this is not yet possible because the necessary fossils just haven’t been found yet. I am not suggesting that we stop doing science, or that the ontogenetic hypothesis of neural spine bifurcation is unreasonable. It’s perfectly possible that it’s true (though MOR 7-17-96-45 ironically suggests otherwise). But it’s not yet been demonstrated, at least for diplodocids, and to the extent that the taxonomic hypotheses of Woodruff and Fowler (2012) rely on an ontogenetic increase in bifurcation in diplodocids, they are suspect. That will be the subject of the next post.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## References

• Gilmore, C.W. 1925. A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument. Memoirs of the Carnegie Museum 10:347-384.
• Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
• Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
• McIntosh, J.S., Miller, W.E., Stadtman, K.L., and Gillette, D.D. 1996. The osteology of Camarasaurus lewisi (Jensen, 1988). BYU Geology Studies 41:73-115.
• Wedel, M.J., Cifelli, R.L., and Sanders, R.K. 2000. Osteology, paleobiology, and relationships of the sauropod dinosaur Sauroposeidon. Acta Palaeontologica Polonica 45(4):343-388.
• Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.

## Neural spine bifurcation in sauropods, Part 2: why serial position matters

### April 9, 2012

In the previous post in this series I looked at the some of the easily available raw data on neural spine bifurcation in Morrison sauropods. In this post I’ll explain how serial variation–that is, variation along the vertebral column in one individual–is relevant to the inferences made in the new paper by Woodruff and Fowler (2012). But first, a digression, the relevance of which will quickly become clear.

How do you recognize an adult sauropod?

There are only a handful of criteria that have been used to infer adulthood in sauropods. In rough order from least to most accurate–so far as I can tell!–they are:

1. sheer size
2. fusion of the neural arches to the centra
3. fusion of the sacral vertebrae to each other, and fusion of the sacral ribs to form the sacricostal yoke
4. fusion of the cervical ribs to the centra and neural arches
5. fusion of the scapula to the coracoid
6. presence of an external fundamental system in the cortices of the long bones

I’ll discuss each one in turn. (Please let me know in the comments if I’ve missed any.)

These vertebrae are rather dissimilar in size and form. Click through to find out why.

1. Size alone is pretty useless. The mounted Giraffatitan is a pretty damn big animal by anyone’s standards, but it’s demonstrably smaller than another individual from Tendaguru, and the scap-coracoid joint is unfused. On the other hand, there are things like dicraeosaurids that apparently matured at relatively small sizes (for sauropods). There is definitely some individual or low-level taxonomic variation. Marsh’s “Brontosaurusexcelsus holotype YPM 1980 is an adult but about the same size as the subadult Apatosaurus ajax holotype YPM 1860 that it ended up being generically synonymised with (see the sacra of the two taxa compared below). The giant Oklahoma Apatosaurus is about 1.4 times the size of A. louisae CM 3018 in most linear measures, but some of the neural arches and cervical ribs are unfused (the vertebra in the linked post is only a quarter bigger than the corresponding element in CM 3018, but there are other elements of the Oklahoma Apatosaurus that are proportionally even larger). On the flip side, I have seen some comparatively tiny Diplodocus material at BYU in which all of the neural arches are fused to the centra, despite the vertebrae being about half the size of those in the mounted D. carnegii CM 84/94. So I am very leery of size as a reliable indicator of age in sauropods. It is a bad criterion in general, and especially bad for cervical vertebrae, which can change so much along the column. C15 of D. carnegii CM 84/94 has a cotyle diameter almost four times that of C3 in the same animal.

Sacra of Apatosaurus excelsus YPM 1980 and A. ajax YPM 1860 at the same scale, from Ostrom and McIntosh (1966:plates 27 and 29)

2. People often cite closure of the neurocentral synostoses* as an indicator of adulthood, but again I am skeptical. There’s no doubt that the neurocentral synostoses do eventually close; my skepticism runs the other way, in that there are sauropods with closed neurocentral synostoses that do not appear to have reached full size. The HM SI** individual of Giraffatitan is one example–it’s about 75% of the size of the mounted (SII) individual, and only 66% the size of the giant HM XV2 (by cross-scaling through HM SII; SI and XV2 share no overlapping elements), and yet the neurocentral synostoses are all closed. Same deal with Apatosaurus CM 555, which has open joints as far back as C8 but is between one-half and two-thirds the size of A. louisae CM 3018. If you found a posterior cervical or anterior dorsal of CM 555 by itself, without the open joints on the more anterior vertebrae to guide you, you’d think it was full grown based on arch fusion. So it seems safest to say that neurocentral synostosis closure is a necessary but not sufficient condition for inferring adulthood.

* Hat tip to Jerry Harris, who alerted me that the term ‘sutures’ is reserved for skulls only, and that the joints between neural arches and centra are properly called synostoses. Thanks also to physical anthropologist Vicki Wedel, who confirmed this.

** Yes, I’m using the old Humboldt Museum numbers here, out of convenience, and because HM SII probably means more to more readers than the correct M.B. R. number that only six people have memorized.

3. Coalescence of the sacrum and formation of the sacricostal yoke have intuitive appeal. The sacricostal yokes are banana-shaped bars of bone formed by the union of the sacral ribs that articulate with the ilia–you can see them on either side of the apatosaur sacra in the image above, and in this post on the sacrum of Camarasaurus lewisi. Since the sacricostal yokes are the bony interfaces between the axial skeleton and the hindlimb girdles, we might expect them to be biomechanically important and for their formation to be closely related to the attainment of adult size. But I’m putting them fairly low on the list for reasons both practical and theoretical. On the practical side, fusion of the sacral vertebrae and ribs is hard to assess unless the sacrum has fallen apart. An intact sacrum might be intact because the bones were actually fused together, or because the unfused bits just happened to hang together through the process of fossilization (if that sounds unlikely, just remember that it’s true of almost every articulated fossil skull you’ve ever seen). On the theoretical side, the timing of sacral fusion seems to be variable. A. ajax YPM 1860 has fused neural arches and cervical ribs but a very incompletely fused sacrum, whereas D. carnegii CM 84/94 has the five sacral centra coossified and a sacricostal yoke uniting the ribs of S2-S5*, but some of the cervical ribs are unfused. Yes, I realize that discounting this criterion because it conflicts with other mutually conflicting criteria is a bit wonky, but (1) that’s the essential challenge of doing non-histological skeletochronology on sauropods–none of the signs seem to tell us what we want–and (2) I’m happy to fall back on the practical reason if you find the theoretical one unconvincing. Last item: I have seen both ‘sacricostal’ and ‘sacrocostal’ used in the literature–can anyone make a case for one being more correct than the other? ‘Sacrum’ is from the Latin sacer, ‘sacred’, apparently because the sacra of animals used to be sacrificed to the gods (not sacroficed–maybe there’s my answer?).

*Hatcher (1901) described an 11th dorsal and four sacral vertebrae, but he noted that the 11th dorsal “functions as a sacral” and “is coossified by the centrum with the true sacrals”. The D. carnegii holotype was one of the first nearly complete sauropod skeletons to be monographically described, and it was not yet clear that the typical number of sacrals for the North American diplodocids–and indeed for most other sauropods–is five (some primitve taxa have four, many titanosaurs have six).

4. Cervical rib fusion might be better. Giraffatitan HM SI and Diplodocus CM 84/94 both have their cervical neurocentral synostoses closed, but both have unfused cervical ribs as far back as C5. This suggests that cervical rib fusion proceeded from back to front (in at least those taxa) and that it followed neurocentral fusion. The sole exception that I have seen is a subadult Apatosaurus cervical from Cactus Park in the BYU collections, which has fused ribs but open neurocentral joints.

5. It’s hard to tell if fusion of the scapula to the coracoid is better or worse than cervical rib fusion, because the timing varies among taxa (hence the caveat that these criteria are in rough order). Giraffatitan HM SII has fused neural arches and fused cervical ribs but open scap-coracoid synostoses (yes, again, synostoses rather than sutures) ; Diplodocus CM 84/94 has a fused scap-coracoid but some unfused cervical ribs. This is probably another necessary but not sufficient condition.

6. The gold standard for determining cessation of growth is the formation of an external fundamental system (EFS) in the outer cortex of a bone. Unfortunately that requires destructive sampling (even if only drilling), is time-consuming, and has been done for few individual sauropods.

The upshot of all of the above is that the readily available ways of determining adulthood in sauropods are all inexact and frequently conflict with each other. Neural arch fusion does not indicate full growth–some sauropods appear to have fused their neurocentral joints when they were only two-thirds grown (in linear terms; 30% grown in terms of mass).

For the purposes of this post and the next, I am going to refer to the big mounted skeletons–Apatosaurus louisae CM 3018, Diplodocus carnegii CM 84/94, etc.–and individuals of like size as ‘adults’ to indicate that they had attained adult morphology, without implying that they were done growing or had EFSs, and also not implying that smaller individuals were necessarily subadult. ‘Adult’ here is used a term of convenience, not a biological fact.

Implications of serial changes in bifurcation for isolated elements

From here, this post picks up right where the last one in this series left off, so feel free to refer back to the previous post for any points that are unclear.

In the diplodocids, adults are expected to have unsplit spines as far back as C5, C6 may be only incompletely bifid (e.g., D. carnegii CM 84/94), and the spines in the posterior dorsals are expected to be either very shallowly notched at the tip or completely unsplit. Therefore it is impossible to say that an isolated vertebra belongs to a juvenile individual on the basis of neural spine bifurcation alone. Depending on how one defines “anterior cervical”, one half to one third of anterior cervicals are expected to have unsplit spines even in adults.

Serially comparable dorsal vertebrae in different Camarasaurus species or ontogenetic stages. Left: dorsal vertebra 7 (top) and dorso-sacral (= D11) (bottom) of Camarasaurus supremus AMNH 5760 and 5761 “Dorsal Series II”, both in posterior view, with unsplit neural spines. Modified from Osborn and Mook (1921: plate LXXI). Right: dorsal vertebrae 7-11 of Camarasaurus lewisi holotype BYU 9047 in posterodorsal view, with split spines. From McIntosh, Miller, et al. (1996: plate 5). Scaled so that height of D11 roughly matches that of C. supremus.

In Camarasaurus the picture is less clear. The immense C. supremus AMNH 5761 has unsplit spines in C3-C4 and in the last three or four dorsals, but some of those very posterior dorsals have extremely shallow depressions in the tips of the spines, with little consistency among the four individuals that somewhat confusingly make up that specimen. In the geriatric C. lewisi all of the post-axial presacral neural spines are at least incompletely bifid. Even in the very posterior dorsals there is still a distinct notch in the neural spine, not just a very slightly bilobed tip as in the posterior dorsals of C. supremus. Either this is an interspecific difference or some amount of ontogenetic bifurcation happened well into adulthood; current evidence is insufficient to falsify either hypothesis.  (That’s the trouble with n=1.)

A final thing to note: as I briefly mentioned in the earlier post, it is easier to detect deep bifurcations than shallow ones if the material is broken or incomplete. The neural spine tips are usually narrow, fragile, and easily broken or lost. If a vertebra is missing the top half of its spine but the bottom half is not split, it is usually impossible to say whether it would have been bifid or not. But if the spine is deeply bifurcated, even a small piece of bone from the base of the trough or one of the metapophyses is enough to confirm that it was bifid.

“Primitive” morphology can be an effect of serial position

Even in ‘adult’ sauropods like the big mounted Apatosaurus and Diplodocus skeletons, the anterior cervicals are less complex than the posterior ones. Compared to posterior cervicals, anterior cervicals tend to have simpler pneumatic fossae and foramina, fewer laminae, and unsplit rather than bifid spines. In all of these things the anterior cervicals are similar to those of juveniles of the same taxa, and to those of adults of more basal taxa. This is also true in prosauropods–in Plateosaurus, the full complement of vertebral laminae is not present until about halfway down the neck (see this subsequent post for details).

An important implication of this is that an isolated cervical might look primitive (1) because it comes from a basal taxon, or (2) because it is from a juvenile, or (3) because it is from near the front of the neck.

Woodruff and Fowler (2012:Fig. 2)

In their Figure 2, Woodruff and Fowler (2012) compare an adult Mamenchisaurus cervical, an isolated cervical of a putative juvenile Diplodocus (MOR 790 8-10-96-204), and a cervical of D. carnegii CM 84/94. The point of the figure is to show that the isolated ‘juvenile’ vertebra is more similar in gross form  to the Mamenchisaurus cervical than to the adult D. carnegii cervical.

Unfortunately the figure confuses ontogenetic and serial variation. Based on the proportions of the centrum and the shape of the neural spine, the isolated MOR cervical is probably from a very anterior position in the series. No measurements are given in the paper or supplementary information (grrr), but using the scale bar in the figure I calculate a centrum length of about 28 cm, a cotyle height of 7 cm, and an elongation index (EI, centrum length divided by cotyle diameter) of 4. That EI, combined with the overall shape of the neural spine and the very long overhang of the prezygapophyses, make the vertebra most similar to C4 and C5 of D. carnegii CM 84/94. But the D. carnegii cervical included in the figure is C12. It differs from the isolated cervical in having a forward-leaning, bifurcated neural spine, a much more complicated system of laminae with many accessory laminae, and more complex pneumatic sculpturing. All of these differences are more likely to be caused by serial variation than by ontogeny–the same characters separate C12 from C4 and C5 in the same individual.

Diplodocus carnegii CM 84/94 cervicals 2-15 in right lateral view, from Hatcher (1901:pl. 3)

So here’s how that figure would have looked, had the comparable C5 of CM 84/94 been used instead of C12:

Woodruff and Fowler (2012:Fig. 2), with Diplodocus carnegii CM 84/94 C12 replaced by C5.

It’s now immediately apparent B more closely resembles C than A, in the possession of overhanging prezygapophyses, non-overhanging postzygapophyses, elongation index, anterodorsal inclination of the cotyle margin, lack of anterior deflection of diapophysis, etc. The biggest differences between B and C are the shape of the neural spine and, for want of a better word, the ‘sinuosity’ of the ventral centrum margin in lateral view. Both characters are highly variably serially within an individual, among individuals in a species, and among species in Apatosaurus and Diplodocus, so it is hard to attach much weight to them.

What is MOR 790 8-10-96-204?

It gets more complicated. The isolated MOR vertebra is presented as an example of juvenile morphology. But does it actually belong to a juvenile?

Here’s what we know for certain about the vertebra:

• it has an EI of 4 (this is a proportion, so it’s still accurate even if the scale bar is off)
• the cervical ribs are fused to the neural arch and centrum

In addition, the figure appears to show that:

• it has a centrum length of 28 cm, although this could be off if the scale bar is incorrectly sized (which is why I prefer measurements to scale bars)
• the neural arch appears to be fused to the centrum. Admittedly, the image in the figure is small and I haven’t seen the specimen in person. But we know this much: the centrum and neural arch stayed together through the process of preservation and preparation, which does not usually happen unless they have at least started coossifying; the photo does not show an obvious line of fusion between the centrum and neural arch; and the cervical ribs are fused, which in almost all sauropod vertebrae happens after closure of the neurocentral synostoses.

Now, as we’ve just seen above, the morphology of MOR 790 8-10-96-204 is indistinguishable from the morphology of an anterior cervical vertebra in an adult, and it compares especially well to C4 and C5 of D. carnegii CM 84/94. The apparent centrum length (measured from the scale bar in the figure) of MOR 790 8-10-96-204 is 28 cm, compared to 29 cm and 37 cm for C4 and C5 of D. carnegii CM 84/94, respectively. So MOR 790 8-10-96-204 is roughly the same size as the adult C4 and about 80% of the size of the adult C5. Furthermore, its neural arch appears to be fused and its cervical ribs are fused to the neural arch and centrum, whereas the cervical ribs of the ‘adult’ D. carnegii CM 84/94 are not yet fused in C2-C5.

In sum, the isolated MOR vertebra shown in Woodruff and Fowler (2012:Fig. 2) is most likely a C4 or C5 of an adult Diplodocus similar in size to D. carnegii CM 84/94, and based on cervical rib fusion it may be from an individual that is actually more mature than CM 84/94. All of the differences between that vertebra and the D. carnegii C12 shown in the same figure are more easily explained as consequences of serial, rather than ontogenetic, variation.

MOR 790 8-10-96-204 and the Mother’s Day Quarry

MOR 790 8-10-96-204 is from the Mother’s Day Quarry (Woodruff and Fowler 2012:Table 1), which is supposed to only contain juvenile and subadult sauropods (Myers and Storrs 2007, Myers and Fiorillo 2009). Myers and Fiorillo (2009:99) wrote:

The quarry has a strikingly low taxonomic diversity, with one sauropod taxon and one theropod taxon present. However, the relative abundance of elements from these taxa is so uneven – diplodocoid sauropod material comprises 99% of the recovered bones – that the quarry is effectively monospecific (Myers and Storrs, 2007). The theropod material consists of isolated teeth only and is probably related to scavenging of the sauropod carcasses. All identifiable sauropod elements belong to either juvenile or subadult individuals (Fig. 2); none is attributable to a fully-adult individual (Myers and Storrs, 2007).

The Figure 2 cited in that excerpt shows two sauropod centra, a dorsal and a caudal, both with unfused neural arches. And yet here is MOR 790 8-10-96-204, similar in size and morphology to D. carnegii CM 84/94, and with at least partially closed neurocentral synostoses and fused cervical ribs. By all appearances, it belongs to an adult or nearly adult animal. It is hard to avoid the conclusion that the Mother’s Day Quarry includes at least one adult or near-adult Diplodocus. The only alternative is that MOR 790 8-10-96-204 is a juvenile in which the neural arch and cervical ribs fused very early.* But if that were the case, what basis would we have for thinking that it belonged to a juvenile, other than that it came from a quarry that only produced juveniles up until now? I trust that the circularity of that logic is clear. It is much more parsimonious to infer that MOR 790 8-10-96-204 is just what it appears to be–an anterior cervical of an adult or near-adult Diplodocus–and that the Mother’s Day Quarry is not exclusively filled with juvenile sauropods.

* Another wrench in the gears: if MOR 790 8-10-96-204 is a juvenile that had freakishly early fusion of its various bits, then clearly its ontogeny has departed from that of Diplodocus, all bets are off about developmental timing, and we shouldn’t be using it to make inferences about the normal ontogeny of diplodocids anyway. It’s damned if you do (it’s an adult), damned if you don’t (it’s a freak).

I’m not criticizing the work of Myers and Storrs (2007) on the taphonomy of the Mother’s Day Quarry or Myers and Fiorillo (2009) on age segregation in sauropod herds, by the way. It’s possible that they never saw MOR 790 8-10-96-204, or that if they did see the specimen they mistook it for a juvenile vertebra based on its size. All it takes is one bone to show that an animal is present in a quarry, and no number of other bones can prove that said animal is absent; if they only saw juveniles, the inference that the quarry only contained juveniles was sound (the operative word is was). If MOR 790 8-10-96-204 is a C5, it’s still only 80% the size of the same vertebra in D. carnegii CM 84/94, so maybe it was the oldest one in the group, or maybe it was an adult slumming with the juveniles, or maybe groups of juvenile sauropods often had one or more adults present to keep an eye on things. Or maybe it happened along earlier or later and just got buried in the same hole. There are a host of possibilities, most of which do not contradict the general conclusions of Myers and Storrs (2007) and Myers and Fiorillo (2009).

Conclusions

Size matters. Size alone is a horrible, horrible criterion for inferring age, especially in a clade (Diplodocoidea) in which adult size is known to vary, and especially with vertebrae. We should expect cervical vertebrae in a single individual to differ in diameter by a factor of 4.

Serial position matters. Not all vertebrae turn out the same. Even in adults, anterior cervicals look very different from posterior cervicals, and have different character states. Anterior cervicals and cervicals of juvenile individuals often look similar. The best way to tell them apart is to rely on articulated series–which is why I went to the trouble of writing the first post in this series.

Skeletochronology matters. The fact that MOR 790 8-10-96-204 has an apparently fused arch and fused cervical ribs should have been huge red flag that maybe it wasn’t actually a juvenile.

I went through that example at length because it shows how serial changes in size and morphology can mimic or suggest ontogenetic changes. In the next post I will examine the rest of the data Woodruff and Fowler (2012) used to support the hypothesis of ontogenetic control of neural spine bifurcation.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## References

• Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
• Myers, T.S., and Fiorillo, A.R. 2009. Evidence for gregarious behavior and age segregation in sauropod dinosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology 274:96-204.
• Myers, T.S., and Storrs, G.W. 2007. Taphonomy of the Mother’s Day Quarry, Upper Jurassic Morrison Formation, south-central Montana, U.S.A. PALAIOS 22:651–666.
• McIntosh, J.S., Miller, W.E., Stadtman, K.L., and Gillette, D.D. 1996. The osteology of Camarasaurus lewisi (Jensen, 1988). BYU Geology Studies 41:73-115.
• Osborn, H.F. and Mook, C.C. 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History 3:247-287.
• Ostrom, John H., and John S. McIntosh.  1966.  Marsh’s Dinosaurs.  Yale University Press, New Haven and London.  388 pages including 65 absurdly beautiful plates.
• Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.

## Neural spine bifurcation in sauropods, Part 1: what we knew a month ago

### April 5, 2012

The discussion over the new paper by Woodruff and Fowler (2012)–see this post and the unusually energetic comment thread that follows–made me want to go back to the literature and see what was known or could be inferred about neural spine bifurcation in the Morrison sauropods before the recent paper was published.

In this post I’m mostly going to stick with the “classic” specimens–those that are reasonably complete and have been monographed.  So this is the stuff that anyone could easily have figured out for themselves–I am just gathering it here so that it’s conveniently in one place.

## Cervical vertebrae

Apatosaurus louisae, CM 3018, and A. parvus, CM 563/UWGM 15556

Apatosaurus parvus CM 563/UWGM 15556 cervicals 7, 5, 4 and 3 in anterior and right lateral views, from Gilmore (1936:pl. 31)

CM 3018 is the mounted Apatosaurus at the Carnegie Museum in Pittsburgh, which was exhaustively described by Gilmore in his 1936 monograph. In that paper Gilmore also described CM 563, which is now mounted at the University of Wyoming Geological Museum and cataloged as UWGM 15556. Gilmore referred CM 563 to A. excelsus, but in a specimen-level phylogenetic analysis Upchurch et al. (2005) found CM 563/UWGM 15556 to belong to A. parvus–hat tip to alert reader LeeB. for catching the revised specific referral. On the subject of neural spine bifurcation, Gilmore wrote (1936:195):

Unfortunately the type of A. louisae lacks most of the spine tops, only  those of cervicals eight, ten and twelve being complete; thus the point of change from single to bifid spines cannot be determined in this specimen. In specimen No. 563, C.M., identified by Hatcher as pertaining to Brontosaurus, an identification with which I concur [Apatosaurus louisae and Brontosaurus excelsus were then considered by some to be separate genera], there are nine cervical vertebrae preserved, three of which I regard as cervicals, three, four, and five. These show the spines to be single as far posteriorly as the fifth vertebra. Since C. 7 shows a well defined notch between the metapophyses, it seems to be a fair conclusion that C. 6 in Apatosaurus is the first vertebra to show a notch on the summit of the spine.

In Camarasaurus supremus Osborn and Mook make the observation that, “In C. 5, the spine has a very slight median notch.” In C. lentus [Gilmore 1925:369], however, the first notched spine is that of C. 7. From C. 6 to C. 9 inclusive, the spinal notch increases steadily in depth. From C. 9 to C. 15 inclusive the spine is completely divided into two metapophyses.  [An odd statement, since Gilmore (1925:351) says that there are twelve cervicals in C. lentus. By "C. 15" he must mean the 15th vertebra, i.e. D3.]

In CM 3018 the spine of C7 appears to be split, based on Gilmore’s Plate 24 (note that for a deeply notched spine, it is not necessary to have the spine tips preserved; the base of the trough will do), and the spine of C6 might be. Gilmore doesn’t mention it in the text, but the plate seems to show C6 with broken metapophyses (at least in posterior view, there are break lines across the metapophyses at mid-height) but with a preserved trough in the middle, which if accurate is enough to confirm that the spine of C6 was also at least partially bifid.

In CM 563/UWGM 15556, there is little doubt: C3, C4, and C5 all have single spines, and the spine of C7 is deeply split. I am happy with Gilmore’s estimates of serial position, by the way. I don’t think the putative C3 could be further back, given how small and short it is, and it’s the wrong shape for an axis. C4 and C5 fall into line in both size and morphology. There is clearly at least one missing vertebra between C5 and C7. The C7 might just possibly be a C8 but I really don’t think it could be any farther back than that, and I agree with Gilmore that it makes more sense as a C7.

The mounted apatosaurs at the Yale Peabody Museum, the AMNH, and the Field Museum are not much help.* The cervical series of all three are heavily reconstructed, and at least for the YPM and AMNH specimens it is quite difficult to tell where the bone ends and the plaster begins.

* Before some outraged curator, collections manager, docent, or museum-goer gets up in my grill about this, I’m not saying they’re bad mounts, or that the necks are entirely fictional! It’s just a sad fact that the fragile neural spine tips are easily broken off and therefore rarely preserved intact. The museums are not defrauding anyone by sculpting in replacements–but that reconstructive work does make it hard to use those specimens as data if neural spine bifurcation is the character of interest. That’s all I’m saying.

Apatosaurus ajax, NSMT-PV 20375

Apatosaurus ajax NSMT-PV 20375, cervical vertebrae 3, 6 and 7 in anterior and posterior views. Modified from Upchurch et al. (2005: plate 2)

This is the new(ish) A. ajax described by Upchurch et al. (2005). They wrote (pp. 27-28):

The neural spine is unbifurcated in C3, but strongly bifurcated from C6 onwards; this seems to be the typical location for the onset of spine bifurcation in Apatosaurus, since it also occurs in this manner in CM 3018 and UWGM 15556.

Note that the intervening vertebrae are missing from that specimen, so we can’t tell exactly where the bifurcation began, but it is perfectly consistent with CM 563/UWGM 15556.

Diplodocus carnegii, CM 84/94

Diplodocus carnegii cervicals 2-15 in posterior view, from Hatcher (1901:pl. 6). Note that the bifid spines in C3-C5 are sculptures; there is no evidence that these spines were bifurcated when the vertebrae were intact.

Cervicals Three, Four, and Five.–All of these vertebrae are more or less injured. The neural spines and transverse processes especially are not well preserved. [...] Commencing with C. 3 the neural spines of these vertebrae have been restored as bifid both anteriorly and posteriorly, each spine consisting of a broad thin plate of bone formed by the union of the pre- and postzygapophyseal laminae of their respective sides. These are made to appear free anteriorly and posteriorly, but united, except at their apices, throughout the inner sides; conditions which prevail in the succeeding cervicals.

Cervicals Six, Seven, Eight, Nine and Ten.–These vertebrae differ so little in their more important characters that they may be very conveniently described together. They are all fairly well preserved and show certain characters which are gradually more emphasized in the succeeding vertebrae of the series. Commencing with C. 6 they  regularly increase in length posteriorly. The neural spines become more completely bifid, resulting in a pair of transversely placed perfectly free spines on the tenth cervical consisting of  triangular plates of bone diverging superiorly and terminating at the summit in a rather blunt, rounded process.

Unfortunately Hatcher’s plates do not show which areas have been reconstructed and which have not–a very common failing in these classic monographs, and one which Upchurch et al. (2005) happily did not duplicate. Hatcher says that certain characters are more emphasized in more posterior vertebrae, and that by C10 the metapophyses are “perfectly free”, which suggests that they might have been less than perfectly free before. The spines of C3-C5 have been reconstructed as bifid, but that was to make them consistent with the succeeding vertebrae. So CM 84/94 is like its neighbor in the Carnegie dinosaur hall, CM 3018, in that in both specimens we know that the spines are bifid by some point but we don’t know what was going on in C3-C5.

Barosaurus lentus, AMNH 6341

A. Barosaurus lentus AMNH 6431 cervical vertebra 8 in anterior, left side, and posterior views. B. Diplodocus carnegii CM 84 cervical 8 in anterior, left side and posterior views (from Hatcher 1901). (McIntosh 2005:fig. 2.2)

This is the mostly-complete skeleton famously mounted in a rearing pose in the AMNH rotunda. McIntosh (2005:47-48):

The neural spine of cervical 8 is flat across the top, and that of 9 shows the first trace of a divided spine (Fig. 2.2A). This division increases gradually in sequential vertebrae, being moderately developed in cervicals 12 and 13, and as a deep V-shape in cervicals 15 and 16. This development is in sharp contrast to Diplodocus, where cervical 3 already shows the first trace of division, and where the division is already quite deep in cervical 7 (Fig. 2.2B). By cervical 11 it is as well developed as cervical 16 of Barosaurus (AMNH 6341; Fig. 2.3A). In the spines, a further difference is that those of the last two cervicals (14 and 15) of Diplodocus project anterodorsally, whereas those of Barosaurus are all directed dorsally (Fig. 2.3B).

(McIntosh’s reference in the text to cervical 7 of Diplodocus carnegii is partly in error; the neural spine cleft is fairly deep in C7 but the vertebra shown in the figure is C8, as correctly noted in the figure caption.)

Suuwassea emilieae, ANS 21122

Suuwassea emilieae C6 in anterior, left lateral, and posterior views, from Harris (2006:Text-fig. 7A-C)

Harris (2006) on C2 (p. 1096, Text-fig. 4):

The spine gradually widens mediolaterally toward the distal end, which is rendered heart-shaped by a 12-mm-deep, sagittal, parabolic notch.

C3 (Text-fig. 5) is missing the end of its spine, enough remains to show that if it was bifid the cleft could not have been very deep.

C4 is missing.

C5 (p. 1099, Text-fig. 6):

The spinous process expands mediolaterally toward its apex, attaining maximal width just proximal to its terminus. A long, narrow crack at the distal end gives the appearance of bifurcation, but the collinear dorsal margin indicates that no true split was present.

C6 (p. 1101, Text-fig. 7, above):

The distal end of the spine is cleft by a parabolic, 11.8-mm-deep intraspinous sulcus, marking the initial stage of bifurcation.

Camarasaurus lewisi, BYU 9047, C. supremus AMNH 5761, C. lentus CM 11338 and YPM 1910

Camarasaurus supremus AMNH 5761, C2-C7 in anterior, left lateral, and poster views, from Osborn and Mook (1921:pl. 67)

McIntosh, Miller, et al. (1996:76) on Camarasaurus (= “Cathetosaurus“) lewisi:

The spines are placed well back on the arches and rise higher than in C. grandis, C. lentus, or C. supremus. The cleft in the bifid spine is much deeper and narrower than in any of those species. The cleft in cervical 3 of C. grandis (YPM 1905) is barely perceptible, very modest in numbers 4 and 5, and distinct in 6.

The metapophyses are transversely broad. If the arrangement portrayed by Osborn and Mook (1921) for the large adult C. supremus (AMNH 5761) is correct, the notch between them first appears in cervical 7, the same position reported by Gilmore (1925) for the juvenile C. lentus (CM 11338). However, a small depression is present in cervical 5 of the holotype (YPM 1910) of the latter species. In C. lewisi (BYU 9047) a narrow, deep, sharp cleft is already present in cervical 3 (Jensen, 1988). The depth continues to increase greatly to cervical 8, the last cervical in which reliable measurements of this feature can be taken, where it appears as a very steep, V-shaped notch. This feature appears to be unique to C. lewisi.

McIntosh’s comments here perfectly match the descriptions provided by Osborn and Mook (1921) and Gilmore (1925) so I have not bothered pasting in the relevant sections of those papers (though I did reread them to make sure everything matched).

Camarasaurus grandis, GMNH-PV 101 (WPL 1995) and YPM 1905

McIntosh, Miles, et al. (1996:11-12):

As in C. grandis YPM 1905, the bifurcation of the spine begins as an incipient notch on cervical 3, but it is not until cervical 5 that one observes the fully developed U-shaped trough, and even then the cleft is not nearly as prominent as in the cervicals of the diplodocids. In this respect GMNH-PV 101 Camarasaurus, and most of the other specimens of the genus, differ from the deep, narrow bifurcation seen in C. lewisi BYU 9047 (see Table C).

However, this description does not match the illustrations. While fig. 25 does show a very subtle notch in C3, fig. 26 shows no bifurcation whatsoever in C4 which has a distinctly convex neurapophysis. Subtle bifurcation returns in C5, and deepens slightly in C6, C7 and C8.

## Inferences on bifurcation in cervicals

Remember that here I am not trying to either support or challenge the work of Woodruff and Fowler (2012), I’m just looking at what had been published previously and seeing what inferences could be drawn from that evidence only.

1. There is no evidence in any of the North American diplodocoids of a bifid spine farther forward than C6. The bifid spines in the mounted skeleton of D. carnegii are sculptures; Hatcher was doing his best with imperfect fossils and limited information. The appearance of a split spine in C5 of Suuwassea is caused by a vertical crack and a small amount of missing bone. In the very large AMNH 6341 Barosaurus, the first partially split spine is on C9.

2. Adult sauropods can show unbifurcated spines, partially bifurcated spines, and fully bifurcated spines serially in the same individual. This is true even in very large individuals (e.g., Apatosaurus parvus CM 563/UWGM 15556, Barosaurus lentus AMNH 6341, Camarasaurus supremus AMNH 5761), so it is unlikely to be an artifact of ontogeny. Therefore single spines do not always indicate juveniles, bifid spines do not always indicate adults, and incompletely bifid spines did not always become fully bifid–in all of the specimens listed above, the most anterior bifid spines are only shallowly divided. We should probably describe vertebrae with shallow splits as ‘incompletely’ bifid rather than ‘incipiently’ bifid; the latter term implies that the bifurcation was going to deepen with time, which did not always happen depending on serial position.

3. The evidence from Camarasaurus is consistent with an ontogenetic increase in bifurcation. The juvenile C. lentus described by Gilmore (1925) has the first incipiently bifurcated spine at C7, whereas the larger, presumably adult individual of the same species represented by YPM 1910 has the first split at C5, as do the individuals that make up C. supremus AMNH 5761. In C. lewisi BYU 9047 and C. grandis YPM 1905, and arguably in C. grandis GMNH-PV 101 the first spine to be partially split is C3. It is tempting to interpret the difference between adult C. lentus and C. supremus on one hand (first split at C5) and C. lewisi and C. grandis on the other (first split at C3) as interspecific variation, but it might be individual variation considering that we are dealing with usually just one individual from each species (for neural spine bifurcation in adults; I am aware that there are other individuals not mentioned here).

## Dorsal vertebrae

I’m going to report the results here in a more compact form than I did for the cervicals. My convention of convenience will be: spines that are split over more than half the distance from the tips to either the postzygapophyses or transverse processes (whichever are higher) are described as deeply bifid, and those split over less than half that distance, including very shallow dorsal indentations, are described as shallowly bifid. The expected dorsal counts are 10 in Apatosaurus and Diplodocus, 9 in Barosaurus, and 12 in Camarasaurus.

Apatosaurus louisae CM 3018 (Gilmore 1936): Deeply bifid  in D1-D3, shallowly bifid in D4-D6, unsplit in D7-D9, D10 spine missing.

Apatosaurus parvus CM 563/UWGM 15556 (Gilmore 1936): Deeply bifid in D1-D3, shallowly bifid in D4, D5-D10 spines missing.

Apatosaurus ajax NMST-PV 20375 (Upchurch et al. 2005): Deeply bifid in D1-D4, shallowly bifid in D5-D6, unsplit in D7-D10.

Diplodocus carnegii CM 84/94 (Hatcher 1901): Deeply bifid in D1-D5, shallowly bifid in D6-D9, unsplit in D10.

Diplodocus longus USNM 10865 (Gilmore 1932): Deeply bifid in D1-D5, shallowly bifid in D6-D8, unsplit in D9-D10.

Barosaurus lentus YPM 429 (Lull 1919): Deeply bifid in D1, D4, and D5, unsplit in D6-D9 (NB: Lull interpreted the latter as D7-D10 on the expectation of 10 dorsals, based on Diplodocus).

Barosaurus lentus AMNH 6341 (McIntosh 2005): Deeply bifid in D1-D3, shallowly bifid in D4-D8, unsplit in D9.

Camarasaurus lentus CM 11338 (Gilmore 1925): Deeply bifid in D1-D3, shallowly bifid in D4-D6, unsplit in D7-D12.

Camarasaurus supremus AMNH 5761 (Osborn and Mook 1921): Deeply bifid in D1-D6, shallowly bifid in D7-D8, unsplit in D9-D12 (NB: a bit of guesswork here, since Osborn and Mook were working with disarticulated material and interpreted Camarasaurus has having 10 or 11 dorsals).

Camarasaurus lewisi BYU 9047 (McIntosh, Miller, et al. 1996): Deeply bifid in D1-D8, shallowly bifid in D9-D12.

## Inferences on bifurcations in dorsals

1. As with the cervicals, most adult sauropods have deeply bifid, shallowly bifid, and unsplit spines in serially adjacent vertebrae. In the diplodocids, the spines of D6-D10 (or D9 in Barosaurus) are always either unsplit or very shallowly indented at the tips.

2. The diplodocid genera show some interesting differences. In Apatosaurus the last four dorsals are always unsplit. In Diplodocus the spines are at least shallowly indented as far back as D8 or D9. Barosaurus goes both ways, with YPM 429 having unsplit spines in the four most posterior dorsals, and AMNH 6341 having an entirely unsplit spine only in the last dorsal.

3. In the diplodocids, deep splits are always confined to the first half of the dorsal series (D1-D5), and these are usually followed by a long run of vertebrae with very shallowly notched spine tips. The exception is Barosaurus YPM 429, which–if the vertebrae are truly consecutive (the series is missing at least two)–has a deep split in D5 and unsplit spines in D6-D9.

4. As with the cervicals, the evidence from Camarasaurus does not rule out an ontogenetic increase in bifurcation. In the juvenile C. lentus CM 11338, the spines are  only bifid as far back as D6; in the adult C. supremus AMNH 5761 to D7; and in the old C. lewisi BYU 9047 to D12. If these differences represent ontogenetic changes rather than interspecific differences (which also cannot be ruled out at this point), it is interesting that there is a bigger difference between the adult C. supremus and the old C. lewisi than between the juvenile C. lentus and the adult C. supremus: in other words, the greatest changes took place after adulthood was attained.

## The rest of the series

Links to all of the posts in this series:

and the post that started it all:

## References

• Gilmore, C.W. 1925. A nearly complete articulated skeleton of Camarasaurus, a saurischian dinosaur from the Dinosaur National Monument. Memoirs of the Carnegie Museum 10:347-384.
• Gilmore, C. W. 1932. On a newly mounted skeleton of Diplodocus in the United States National Museum. Proceedings of the United States National Museum 81:1-21.
• Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
• Harris, J.D. 2006. The axial skeleton of the dinosaur Suuwassea emilieae (Sauropoda: Flagellicaudata) from the Upper Jurassic Morrison Formation of Montana, USA. Palaeontology 49:1091-1121.
• Hatcher, J.B. 1901. Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum 1:1-63.
• Lull, R.S. 1919. The sauropod dinosaur Barosaurus Marsh. Memoirs of the Connecticut Academy of Arts and Sciences 6:1-42.
• McIntosh, J.S. 2005. The genus Barosaurus Marsh (Sauropoda, Diplodocidae); pp. 38-77 in Virginia Tidwell and Ken Carpenter (eds.), Thunder Lizards: the Sauropodomorph Dinosaurs. Indiana University Press, Bloomington, Indiana, 495 pp.
• McIntosh, J.S., Miles, C.A., Cloward, K.C., and Parker, J.R. 1996. A new nearly complete skeleton of Camarasaurus. Bulletin of the Gunma Museum of Natural History 1:1-87.
• McIntosh, J.S., Miller, W.E., Stadtman, K.L., and Gillette, D.D. 1996. The osteology of Camarasaurus lewisi (Jensen, 1988). BYU Geology Studies 41:73-115.
• Osborn, H.F. and Mook, C.C. 1921. Camarasaurus, Amphicoelias, and other sauropods of Cope. Memoirs of the American Museum of Natural History 3:247-287.
• Upchurch, P., Tomida, Y., and Barrett, P.M. 2005. A new specimen of Apatosaurus ajax (Sauropoda: Diplodocidae) from the Morrison Formation (Upper Jurassic) of Wyoming, USA. National Science Museum Monographs No. 26. Tokyo. ISSN 1342-9574.
• Woodruff, D.C, and Fowler, D.W. 2012. Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character. Journal of Morphology, online ahead of print.

## Changes through growth in sauropods and ornithopods

### March 29, 2012

By one of those happy coincidences that you sometimes get, today saw the publication of not one but two dinosaur ontogeny papers: this morning I was sent a copy of Woodruff and Fowler (2012) on ontogenetic changes in the bifid spines of diplodocoids, and tonight I was alerted to Werning (2012) on Tenontosaurus growth trajectories based on osteohistology.

It’s interesting to compare them.  The obvious conclusion is that, while sauropod vertebrae are intrinsically better than ornithopod long-bones, the latter make much, much better subjects for ontogeny studies.

# Sauropod Vertebrae

Woodruff and Fowler (2012:fig. 3). Ontogenetic development of diplodocid anterior cervical vertebrae documenting bifurcation from absent, through incipient, to fully developed. (A) MOR 790 7-30-96-132, Diplodocus sp. (B) MOR 790 8-10-96-204, Diplodocus sp. (C) MOR 592, Diplodocidae sp. (D) ANS 21122, Suuwassea, (image provided by J. Harris). (E) CM 555, Apatosaurus excelsus, (image provided by M. Wedel), (F) CM 84 (from Hatcher, 1901). Scale bar = 5 cm.

The problem that Woodruff and Fowler have is that they’re working from a small selection of mostly isolated elements, nearly all of them damaged by breakage and/or crushing, and uncontrolled for serial position beyond some basic binning.  As a result, the taxonomic identifications are really rather arbitrary and unsupported — as they say (p. 2), “for the purposes of this study, original taxonomic designations were not reexamined” — and indeed re-examination would not necessarily help much.

As a result, they are left with rather a circular argument, as follows:

• Small specimen X is Suuwassea.
• Small specimen Y is assigned to Apatosaurus, because someone once said so.
• But Y has a less bifid neural spine than adult Apatosaurus.
• So spine bifurcation increases through ontogeny in Apatosaurus.
• So small specimen X belongs to Apatosaurus, too.
• => Suuwassea is Apatosaurus [Note: I stupidly wrote "Diplodocus" here in the first posted version.  Now corrected to Apatosaurus.]

I don’t find this at all convincing.  (Neither does Matt: we discussed this briefly today, and at more length at the Bonn workshop where Cary presented this work.)  Leaving aside the observation that the conclusion fits in neatly with the Horner Lab’s ongoing everything-is-just-a-Triceratops-growth-stage project, there isn’t really much that Cary could have done differently here: the necessary specimens (i.e. multiple near-complete associated individuals of unambiguous taxonomic identity) just don’t exist.

(Mind you, figure 7A is about the least convincing evidence of bifurcation I’ve ever seen.)

# Ornithopod long bones

Werning (2012:fig. 2). A. Cross-section of OMNH 10144. This bone was invaded by bacteria before fossilization and thus much of the primary tissue is obscured. It is presented here in cross-section to illustrate vascular density and arrangement. B. Detail of A, showing general vascular patterning. The cortex is dominated by longitudinal canals arranged circumferentially. C. Cross-section of OMNH 8137. D. Detail of C, showing primary cortical tissues. The bone is woven, and most canals are longitudinal primary osteons (some anastomose circumferentially). Two LAGs (arrows) are shown. E. Cross-section of FMNH PR2261. F. Detail of E, showing mostly primary tissues of the midcortex at a transition to slower growth. Deeper in the cortex (upper left), bone is woven and osteocytes are dense and disorganized. Some secondary osteons are visible, but they do not overlap or obscure all of the primary tissues. Past the LAG (arrow), canals remain dense but decrease in diameter, bone tissue is weakly woven, and osteocytes decrease in number and become more organized. Scale bars: A = 2 mm; B, F = 1 mm; C = 4 mm; D = 0.5 mm; E = 10 mm.

By contrast, Sarah Werning has a much better story to tell, largely just because she’s working with much better material.  Tenontosaurus is known from many complete and near-complete individuals, there is no real uncertainty about the material all belonging to the same taxon, and the elements (long bones) are much less subject to crushing and breakage than vertebrae.

Of course it also helps that Sarah has done astonishingly careful, detailed work.  I don’t think I’m giving much away when I say that this paper has been many years in the making — as the Acknowledgements say “This work was completed in partial fulfillment of the requirements for the Master of Science degree, Department of Zoology, University of Oklahoma.”  And it’s been a looong time since Sarah was at Oklahoma.  But I get the sense that this reflects taking the time and putting in the effort to Get It Right, rather than standard-issue procrastination.

Perhaps the most impressive part of this paper, though, is methodological:

High-resolution histological images of the cross-sections are digitally reposited online for scholarly use at MorphoBank (http://MorphoBank.org), project p494; see Table 2 for a list of slides and accession numbers. Digital images larger than 25,000 pixels in either dimension were digitally scaled (reduced to 17,000–20,000 pixels in the larger dimension) to allow processing on MorphoBank and because most image editing software does not support editing of gigapixel jpg files. These edits were made after scale bars had been added. Images in full resolution can be obtained from the author.

As you can see, these are ridiculously high-resolution images.  By making them freely available, Sarah is doing everything she can to enable others to replicate or correct her work — or to use the data for other purposes that she’s not yet thought of.  This is a big win for the progress of science, and I want to publicly congratulate Sarah for doing it.  I would love to see this become standard behaviour.

(It’s a bit strange that the paper links onto the the MorphoBank home page rather than directly to the Tenontosaurus project.  Here you go.)

# PLoS ONE vs. Journal of Morphology

A final thought, before I finally go to bed.  When I saw Cary’s paper in the Journal of Morphology I thought to myself, “good for him, he’s got that into a really good journal” — which is true.  But then when Sarah’s came in, I found myself comparing PLoS ONE.  Here’s what I came up with:

Access.  A no-brainer: PLoS ONE wins hands down, being open-access while J. Morph. is paywalled.

Charges.  The flip side of the first category: J. Morph. wins because (as far as I can tell from the Author Guidelines) there is no publication charge.  PLoS ONE charges \$1350.

Length.  Another PLoS ONE win, because it imposes no limits whatsoever on length, figure count, etc.  Cary’s paper is no lightweight at 11 pages, but PLoS’s liberality with page-space means that Sarah’s is well over twice as long at 25 pages.  Now of course not all papers need to be long; but for those that do, cutting to a journal’s length limit is a painful and stupid process.

Image Colour.  Again, PLoS ONE wins — it’s very rare these days to see a greyscale specimen image in a PLoS journal.  But to be fair to J. Morph., the author guidelines do say: “All color figures will be reproduced in full color in the online edition of the journal at no cost to authors. For the printed version free color figures are at the editors discretion.”  So it looks like Cary and Denver dropped the ball on this.

Image Resolution.  No question.  PLoS ONE wins by a mile.  Click through the two representative images above: the PLoS one has 3.5 times as many pixels, and that’s after I added a 10% wide margin around the J. Morph. image.  And if you want to use these illustrations as basis for new images, remember PLoS also lets you download the original full-resolution submitted image in a lossless format.

DOI Resolution.  When I added the references below, I noticed that the DOI for the J. Morph. article doesn’t resolve yet, which is careless.  PLoS ONE wins in this category, resolving just fine.

Impact Factor.  Yes, it’s a stupid number and we all hate it.  But some people still seem to take it seriously, so we may as well look.  And PLoS ONE wins with 4.411 to J. Morph.’s 2.087 — more than double.

Putting it all together, based on the seven categories that I evaluated here (and no doubt I missed some), it looks like the only reason to go with J. Morph. ahead of PLoS ONE is to avoid the publication fee.  By this point, PLoS ONE has made itself an absolutely mainstream journal for palaeo, and the obvious first choice for most projects.

# References

• Werning, Sarah.  2012.  The ontogenetic osteohistology of Tenontosaurus tilletti. PLoS ONE 7(3):e33539. doi:10.1371/journal.pone.0033539
• Woodruff, D. Cary, and Denver W. Fowler.  2012.  Ontogenetic influence on neural spine bifurcation in Diplodocoidea (Dinosauria: Sauropoda): a critical phylogenetic character.  Journal of Morphology, online ahead of print.  doi:10.1002/jmor.20021 [Direct Link, since the DOI doesn't seem to work.]