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.
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:
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 …
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.]
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:
- Part 1: what we knew a month ago
- Part 2: why serial position matters
- Part 3: the evidence from ontogenetic series
- Part 4: is Suuwassea a juvenile of a known diplodocid?
- Part 5: is Haplocanthosaurus a juvenile of a known diplodocid?
- Part 6: more reasons why Haplocanthosaurus is not a juvenile of a known diplodocid
and the post that started it all:
- 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.