Mike in love

January 14, 2013

DSCN5581-mike-in-love

Matt took this photo in the basement of the Museum für Naturkunde Berlin, back in 2008 when we were there as part of the field-trip associated with the Bonn sauropod conference.

Hopefully all you long-time SV-POW! readers will recognise the specific vertebra that I’m in love with.

Friday evening I was in a pub with Mike, Darren, John Conway, and Emma Lawlor. We were killing time waiting for the Pink Giraffe Chinese restaurant down the street to open. I was chatting with John about “All Todays”, his speculative presentation with Cevdet Kosemen (a.k.a. Nemo Ramjet) on how future sentients might reconstruct Holocene animals if they were known only from fossils. Like his “All Yesterdays” presentation last year, John’s flights of scientific fancy had fired my imagination and gotten me thinking about how paleontology forms sort of a skin or membrane between the bubble of what we know and the surrounding ocean of what we don’t. I decided that we should pass a pad around and each sketch a speculative sauropod.

My own entry is based on the holotype of Mamenchisaurus hochuanensis, which was found almost complete except for the skull (naturally) and forelimbs. I have often joked that diplodocids were basically bipeds whose forelimbs happened to reach the ground. Mamenchisaurs were probably not that back-heavy, but their presacral vertebrae were extremely pneumatic and if our hypothetical future paleontologists had no other sauropod material to work with, I think it’s possible that they would reconstruct the M. hochuanensis holotype as a biped.

I’m not sure there’s much to say about Mike’s brachiosaur, beyond the Ebert-like observation that if a brachiosaur dressed up in a coat and top hat and went cruising for dames, this, I am forced to conclude, is more or less how it would look.

John Conway also drew a mamenchisaur, this time Mamenchisaurus youngi with its bizarrely bent-back sacrum. John’s explanation for the weird sacrum brings to mind ground sloths and–for those who saw “All Yesterdays” at SVPCA 2011–a certain black-feathered therizinosaur. I’d also like to note that he knocked this out in about 5 minutes, thus demonstrating the difference between a professional artist and a mere doodler like myself.

Darren’s hindlimb-less sauropod complements my bipedal Mamenchisaurus. Here the animal, evidently known from only the front half of the skeleton, has been restored as a giant bird. Dig the giant thumb claws and spreading metapodials. Surely, you say, future paleontologists of any species or machine culture would know a pectoral girdle when they saw one. But I’ll bet a sauropod scapulocoracoid could pass for an ilium, if said future paleontologists were still in the early stages of understanding the morphology and diversity of vertebrates. Remember that Seeley described the sauropod Ornithopsis as “a gigantic animal of the pterodactyle kind” based on its pneumatic vertebrae. There is also a long and honorable (?) tradition of mistaking sauropods for hadrosaurs (Sonorasaurus), theropods (Bruhathkayosaurus), and tree trunks (Sauroposeidon), so don’t be too quick to rule this out.

What I want to see next is a skeletal reconstruction of Darren’s sauro-bird, using only elements from the front half of a sauropod skeleton. Anyone want to give it a shot?

Our penultimate entry is Emma’s rendering of an evil bastard snake devouring an innocent baby sauropod. Tragically this one is not speculative–we have very good fossil evidence that the scene shown here really happened, probably a lot. She tried to make it up to us with a smiley face on the next page, but it was too late. We were so depressed after this that we could barely choke down four courses of excellent Chinese food.

One more for the road: a totally new depiction of the enigmatic sauropod Xenoposeidon by yours truly. I expect to see this incorporated into future talks and papers dealing with European sauropod diversity in the Early Cretaceous. Just credit me as you normally would.

That’s all, folks. I hope that speculative sauropod sketches get to be a Thing, and that we see lots more of them from future conferences.

YPM 5449, a posterior dorsal vertebra of Sauroposeidon, from D’Emic and Foreman (2012:fig. 6A and C).

Another recent paper (part 1 is here) with big implications for my line of work: D’Emic and Foreman (2012), “The beginning of the sauropod dinosaur hiatus in North America: insights from the Lower Cretaceous Cloverly Formation of Wyoming.” In it, the authors sink Paluxysaurus into Sauroposeidon and refer a bunch of Cloverly material to Sauroposeidon as well. So in one fell swoop Sauroposeidon goes from being one of the most poorly represented Early Cretaceous North American sauropods, based on just four vertebrae from a single individual, to one of the best-known, most complete, and most widespread, based on at least seven individuals from Texas, Oklahoma, and Wyoming.

The web of connections among the different sets of material is complex, and involves the Sauroposeidon holotype OMNH 53062 from the Antlers Formation of southeastern Oklahoma, the type and referred material of Paluxysaurus from the Twin Mountains Formation of northern Texas described by Rose (2007), sauropod material from the Cloverly Formation of north-central Wyoming described and illustrated by Ostrom (1970), and UM 20800, a scap and coracoid newly excavated from one of Ostrom’s old quarries.  D’Emic and Foreman argue that (1) the Cloverly material is referable to Sauroposeidon based on the shared derived characters of a juvenile cervical, YPM 5294, and the Sauroposeidon holotype, and (2) Paluxysaurus is not distinguishable from the Cloverly material and in fact shares several autapomorphies with the Cloverly sauropod. Which means that (3) Paluxysaurus is Sauroposeidon.

But that’s not all! All the new material suggests different phylogenetic affinities for Sauroposeidon. Instead of a brachiosaurid, it is now posited to be a basal somphospondyl. That’s not super-surprising; as we noted back in 2000 (Wedel et al. 2000), if Sauroposeidon was a brachiosaurid it had evolved some features in parallel with titanosaurs, most notably the fully camellate internal structure of the cervical vertebrae. And it also makes sense because other basal somphospondyls include Erketu and Qiaowanlong, the cervicals of which are similar to Sauroposeidon in some features. D’Emic and Foreman (2012) cite a forthcoming paper by Mike D’Emic in the Journal of Systematic Paleontology that contains the cladistic analysis backing all this up, but the case based on comparative anatomy is already pretty strong.

If anyone is unconvinced by all of these referrals, please bear in mind that we haven’t heard the whole story yet, quite probably for reasons that are outside of the authors’ control.  I am inclined to be patient because I have been in that situation myself: Wedel (2003a) was intended to stand on the foundation of evidence laid down by Wedel (2003b), but because of the vagaries of publication schedules at two different journals, the interpretive paper beat the descriptive one into press by a couple of months.

Mid-cervical originally described as Paluxysaurus, now referred to Sauroposeidon, from Rose (2007:fig. 10).

Anyway, if anyone wants my opinion as “Mr. Sauroposeidon“, I think the work of D’Emic and Foreman (2012) is solid and the hypothesis that Paluxysaurus is Sauroposeidon is reasonable. So, if I think it’s reasonable now, why didn’t I synonymize the two myself? Partly because I thought there was a pretty good chance the two were not the same, based mostly on FWMSH 93B-10-8 (which I referred to as FWMSH “A” in Wedel 2003b, since I had only seen in on display without a specimen number), which I thought looked a lot more like a titanosaur cervical than a brachiosaur cervical. But of course I thought Sauroposeidon was a brachiosaur until a couple of months ago, and if it ain’t, and if brachiosaurs and basal somphospondyls have similar cervicals, that objection is considerably diminished. And partly because I’ve had other things to be getting on with, and stopping everything else to spend what would realistically be a few months looking into a possible synonymy (that I didn’t strongly suspect) wasn’t feasible in terms of time or geography. So I’m glad that D’Emic and Foreman have done that work, and I’m excited about the new things they’ve uncovered.

And I’m honored to bring you a new life restoration of Sauroposeidon by uber-talented Bob Nicholls, which we think is the first to show Sauroposeidon in its new guise as a basal somphospondyl. Click through for the mega-awesome version.

Same critter, different views. If anyone wants to GDI this baby, you now have everything you need. Many thanks to Bob for permission to post these and the following making-of images. Please visit him at Paleocreations.com to see a ton of awesome stuff, and give him some love–or at least a few thousand “likes”–on Facebook.

This is Bob’s first foray into 3D modeling, but you’d never know from the quality of his virtual sculpt. And let me tell you, that dude works fast. He sent this initial version, showing Sauroposeidon as an attenuated brachiosaur (sorta like this) on August 23, to solicit comments from Mike and me.

I wrote back and let Bob know about the new work of D’Emic and Foreman, and suggested that he could probably be the first to restore Sauroposeidon as a somphospondyl. Mike and I also voiced our opposition to the starvation-thinned neck, and Mike suggested that the forelimb was too lightly muscled and that the ‘fingers’ were probably too prominent. The very next day, this was in our inboxes:

I wrote back:

Whatever Sauroposeidon was, its neck was fairly tall and skinny in cross-section. It looks like the neck on your model sort of tapers smoothly from the front of the body to the head. I think it would be much narrower, side-to-side, along most of its length, and would have a more pronounced shoulder-step where it met the body.
The bottom view is very useful. It shows the forefeet as being about the same size as the hindfeet. AFAIK all or nearly all known sauropod tracks have much bigger hindfeet than forefeet. Certainly that is the case with Brontopodus birdi, the big Early Cretaceous sauropod tracks from Texas that were probably made by Sauroposeidon. The forefeet should be about 75-80% the width of the hindfeet, and only about half a long front-to-back. Even if you don’t quite get to those numbers, shrinking the forefeet a bit and subtly up-sizing the hindfeet would make the model more accurate.
Mike’s commentary was much shorter–and funnier:
I like how freaky it looks. It looks WRONG, but in a good way.
Bob toiled over the weekend and came back with this subtly different, subtly better version:

I had one more change to recommend:

I’m sorry I didn’t suggest this sooner, but it only just now occurred to me. With the referral of Paluxysaurus and the Cloverly material to Sauroposeidon, we now have dorsal vertebrae, and they are loooong, much more similar in proportion to the dorsals of Brachiosaurus altithorax than those of Giraffatitan brancai. So, as much as I like the compact little body on your Sauroposeidon, I think it was probably fairly long in the torso. You probably already have Mike’s Brachiosaurus paper [Taylor 2009] with the skeletal recon showing the long torso–in the absence of an updated skeletal recon for Sauroposeidon, I’d use Mike’s Figure 7 as a guide for reconstructing the general body proportions.

Bob lengthened the torso to produce the final version, which is the first one I showed above. He sent that over on August 29–the delay in getting this post up rests entirely with me.

So. It is still very weird to think of “my” dinosaur as a somphospondyl rather than a brachiosaur. I had 15 years to get used to the latter idea. But suddenly having a lot more material–essentially the whole skeleton, minus some stinkin’ skull bits–is pretty darned exciting, and the badass new life restoration doesn’t hurt, either.

Now, would it be too much to wish for some more Brontomerus?

References

In the recent post on OMNH 1670, a dorsal vertebra of a giant Apatosaurus from the Oklahoma panhandle, I half-promised to post the only published figure of this vertebra, from Stovall (1938: fig. 3.3). So here it is:

And in the second comment on that post, I promised a sketch from one of my notebooks, showing how much of the vertebra is reconstructed. Here’s a scan of the relevant page from my notebook. Reconstructed areas of the vert are shaded (confusingly, using strokes going in opposite directions on the spine and centrum, and the dark shaded areas on the front of the transverse processes are pneumatic cavities), and measurements are given in mm.

Next item: is this really a fifth dorsal vertebra?

Apatosaurus louisae CM 3018 D4 and D5, in anterior (top), left lateral, and posterior views, from Gilmore (1936: plate 25).

Here are D4 and D5 of A. louisae CM 3018. They sort of bracket OMNH 1670 in terms of morphology. D4 has a broader spine, and D5 has a narrower one. The spine of D5 lacks the slight racquet-shaped expansion seen in OMNH 1670, but the overall proportions of the spine are more similar. On the other hand, the transverse processes of D4 taper a bit in anterior and posterior view, as in OMNH 1670, and unlike the transverse processes of D5 with their more parallel dorsal and ventral margins. But honestly, neither of these verts is a very good match (and the ones on either side, D3 and D6, are even worse).

Apatosaurus parvus UWGM 15556 (formerly A. excelsus CM 563) D4 (left) and D3 (right) in anterior (top), right lateral, and posterior views, from Gilmore (1936: plate 32).

Here are D3 and D4 of A. parvus UWGM 15556. D3 is clearly a poor match as well–it is really striking how much the vertebral morphology changes through the anterior dorsals in most sauropods, and Apatosaurus is no exception. D3 looks like a dorsal in lateral view, but in anterior or posterior view it could almost pass for a posterior cervical. If I was going to use the term “cervicodorsal”, indicating one of the vertebrae from the neck/trunk transition, I would apply it as far back as D3, but not to D4. That thing is all dorsal.

And it’s a very interesting dorsal from the perspective of identifying OMNH 1670. It has fairly short, tapering transverse processes. The neural spine is a bit shorter and broader, but it has a similar racquet-shaped distal expansion. I’m particularly intrigued by the pneumatic fossae inscribed into the anterior surface of the neural spine–in Gilmore’s plate they make a broken V shapen, like so \ / (or maybe devil eyes). Now, OMNH 1670 doesn’t have devil eyes on its spine, but it does have a couple of somewhat similar pneumatic fossae cut into the spine just below the distal racquet–perhaps a serially modified iteration of the same pair of fossae as in the A. parvus D4. It’s a right sod that D5 from this animal has its spine blown off–but it still has its transverse processes, and they are short and tapering as in OMNH 1670.

Apatosaurus sp. FMNH P25112, dorsal vertebrae 1-10 and sacrals 1 and 2, Riggs (1903: plate 46)

Here are all the dorsals and the first couple sacrals of FMNH P25112, which was originally described as A. excelsus but in the specimen-level analysis of Upchurch et al. 2005) comes out as the sister taxon to the A. ajax/A. parvus/A. excelsus clade. Note the striking similarity of the D5 here with D4 of the A. parvus specimen in Gilmore’s plate (until the careful phylogenetic work up Upchurch et al. 2005, that A. parvus specimen, once CM 563 and now UWGM 15556, was considered to represent A. excelsus as well). But  also notice the striking similarity of D6 to OMNH 1670. It’s not quite a dead ringer–the transverse processes are longer and have weird bent-down “wingtips” (XB-70 Valkyrie, anyone?)–but it’s pretty darned close, especially in the shape of the neural spine.

So what does this all mean? First, that trying to specify the exact serial position of an isolated vertebra is nigh on to impossible, unless it’s something that is one-of-a-kind like an axis. Second, after doing all these comparos I think it’s unlikely that OMNH 1670 is a D4–those are a bit too squat across the board–but it could plausibly be either a D5 or a D6. Third, I’m really happy that it doesn’t seem to match any particular specimen better than all the rest. What I don’t want to happen is for someone to see that this vertebra looks especially like specimen X and therefore decide that it must represent species Y. As I said in the comments of the previous post, what this Oklahoma Apatosaurus material needs is for someone to spend some quality time seeing, measuring, and photographing all of it and then doing a phylogenetic analysis. That sounds like an ambitious master’s thesis or the core of a dissertation, and I hope an OU grad student takes it on someday.

If you were intrigued by my suggestion that the big Oklahoma Apatosaurus rivalled Supersaurus in size, and wanted to see a technical comparison of the two, I am happy to report that Scott Hartman has done the work for you. Here’s one of his beautiful Apatosaurus skeletal reconstructions, scaled to the size of OMNH 1670, next to his Supersaurus silhouette. This is just a small teaser–go check out his post on the subject for a larger version and some interesting (and funny) thoughts on how the two animals compare.

References

  • Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
  • Riggs, E.S. 1903. Structure and relationships of opisthocoelian dinosaurs, part I: Apatosaurus Marsh. Field Columbian Museum Publications, Geological Series 2(4): 165–196.
  • Stovall, J.W. 1938. The Morrison of Oklahoma and its dinosaurs. Journal of Geology 46:583-600.

Left: the Queen of England, 163 cm.  Middle, the Oklahoma apatosaur dorsal, 135 cm.  Right, classic “big Apatosaurus” dorsal, 106 cm.  To scale.

Something I’ve always intended to do but never gotten around to is posting on some of the immense Apatosaurus elements from the Oklahoma panhandle. Here’s one of the most impressive, OMNH 1670, an isolated dorsal. Notice that the tip of the neural spine is ever-so-shallowly bifurcated, which in Apatosaurus indicates a D4, D5, or D6. The low parapophyses and fat transverse processes are similar to D4, but Apatosaurus D4s usually have somewhat broader spines, so I’m guessing this thing is a D5. These things vary and I could easily be off by a position in either direction.

Next to it is D5 of CM 3018, the holotype specimen of Apatosaurus louisae (from Gilmore 1936: plate 25), which has served as the basis for many of the published mass estimates of the genus Apatosaurus. OMNH 1670 is 135 cm tall, compared to 106 cm for D5 of CM 3018. If the rest of the animal scaled the same way, it would have been 1.27^3 = 2 times as massive. Mass estimates for CM 3018 are all over the map, from about 18 tons up to roughly twice that, so the big Oklahoma Apatosaurus was probably in Supersaurus territory, mass-wise, and may have rivaled some of the big titanosaurs (Update: see the two giant diplodocids square off in a cool follow-up post by Supersaurus wrangler Scott Hartman). Here’s a fun rainy-day activity: take any skeletal reconstruction of Apatosaurus, clone it in Photoshop or GIMP, scale it up by 27%, and park it next to the original. It looks a lot bigger. So I’m continually surprised that Apatosaurus is so rarely mentioned in the various roundups of giant sauropods, both in the technical literature and in popular articles online. This vertebra was figured by Stovall (1938)–if I get inspired, I’ll dig up that figure and post it another day (hey, look, I did!).

Fun fact: in Apatosaurus the tallest (most posterior) dorsals are 1.3-1.5 times as tall as D5 (Gilmore 1936: 201). So D10 from this individual was probably between 1.7 and 2 meters tall–not quite in Amphicoelias fragillimus territory but getting closer than I’ll bet most people suspected.

NB: if you try to use the scale bar lying on the centrum of OMNH 1670 to check my numbers, you will get a wonky answer. The problem is that the vertebra is so large that it is almost impossible to get far enough back from it (above it, in this case, since it is lying on a padded pallet) to get a shot free from distortion due to parallax. For this shot, the pallet with the vert was on the floor, and I was standing on top of the tallest ladder in the OMNH collections, leaning out over the vert to get centered over the prezygapophyses, and shooting straight down–in other words, I had done everything possible to minimize the visual distortion. But it still crept in. Anyway, trust the measurements, which I–and presumably Gilmore–made with a good old reliable tape measure.

References

  • Gilmore, C.W. 1936. Osteology of Apatosaurus with special reference to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum 11:175-300.
  • Stovall, J.W. 1938. The Morrison of Oklahoma and its dinosaurs. Journal of Geology 46:583-600.

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.

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.
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