So, yesterday I wrote part 2 of my series on Ornithoscelida. In it I outlined some old concepts and arguments supporting the traditional model of early dinosaur evolution that I had to wrestle with at the start of my PhD, some of which have since been cited by others in response to the publication of our article in Nature.
In the post, I talked about skeletal pneumaticity, a feature observed in a number of dinosaurs and in some close dinosaur relatives.
What I was trying to argue, perhaps in a less than clear way, was that Seeley, when forming his ideas about Ornithischia and Saurischia, was observing the heavy, well developed excavations into the axial columns of Jurassic sauropods and derived theropods. In both of these groups we see examples of very well developed skeletal pneumaticity, with deep fossae that form large blind pockets within, or, in some cases, completely penetrate, the vertebrae.
The point I would make here is that these features, while very similar in the later (read, younger) theropods and sauropodomorphs, when we start heading back down the lineages, towards the ‘basal’ members of the two groups, what we see is a dramatic shift away from such deep and well-developed fossae and foramina (hole in the bones), towards simple small fossae, faint lamination, even absence of anything vaguely pneumatic. What this says to me, and I think is fair to say generally, is that it appears to be quite clear that the heavy pneumatisation in derived theropods and derived sauropodomorphs is convergent, even if the propensity to develop such features, presaged by very shallow fossae and laminae on some vertebrae, is not.
As an example, I pointed to an animals called Pantydraco.
This cute little critter is a ‘basal’ sauropodomorph, meaning that it is one of the earlier branching members of this clade and falls outside of what we call ‘more derived’ subgroups within Sauropodomorpha.
Found in Wales, Pantydraco was once thought to be a species of the contemporaneous British taxon Thecodontosaurus, many specimens of which have been found in Triassic fissure fill deposits of South West England. We now think that it is a distinct genus, and my analyses, plus those of a few others, have placed it in a position slightly closer to the base of the sauropodomorph lineage than its English close cousin.
In my previous post, I said that I had observed no pneumatic features in Pantydraco myself, and that, much like other ‘basal’ sauropodomorphs, its anatomy suggested that the pneumatic features that are seen in the two saurischian groups are convergent.
I still stand by this argument. Although I may have been wrong about Pantydraco itself.
As has been very kindly pointed out by Matt Wedel, Pantydraco has, on some of its vertebrae (on its cervicals, if you want to know) small fossae that Matt says are “suggestive of pneumaticity”. There is a paper on them, which can be found here:
He also rightly pointed out that, when making arguments about ‘basal’ sauropodomorphs lacking pneumaticity, it is perhaps better that I highlight examples were there is clearly none, such as Saturnalia and Thecodontosaurus itself.
For one, I think my point remains valid, even if there are hints of some kind pneumatic feature in some of the vertebrae of Pantydraco.
When Seeley and others pointed to ‘pneumatic features’ as evidence for a monophyletic Saurischia, they were most certainly talking about the deep, well-developed fossae of taxa like Diplodocus and Allosaurus.
Taxa such as Saturnalia are generally considered even more ‘basal’ within Sauropodomorpha than Pantydraco, which I think just furthers my other point about how, if you trace these lineages back far enough, until you are into the Triassic representatives, the amount of pneumaticisation lessens and then vanishes quite possibly BEFORE the common ancestor of Sauropodomorpha and Theropoda.
Heinrich Mallison also very kindly pointed out that other ‘basal’ sauropodomorphs like Plateosaurus have clear fossae within some vertebrae. Again though, I would argue, while these features are quite well developed, they are still by no means the deep fossae or foramina that Seeley was looking at, and that, despite the fact that it often used as a exemplar ‘basal’ sauropodomorph in analyses and comparisons, Plateosaurus is actually a fair few steps up from the base of the sauropodomorph tree. In my analyses, as in many others, Plateosaurus serves as a kind of ‘bench-mark’ some way up the sauropodomorph lineage, on the way towards the true sauropods. Indeed, a group called Plateosauria exists that includes Plateosaurus and all ‘more derived sauropodomorphs’ to the exclusion of quite a sizeable sample of ‘more basal’ taxa from the Triassic and Jurassic, including Saturnalia Pantydraco and Thecodontosaurus, but also taxa such as Buriolestes, Chromogisaurus, and the very newly named Bugalosaurus. Also possibly in amongst these taxa are Eoraptor and Guaibasaurus (both of which flit in and out of various dinosaurian lineages; something deserving of a post in its own right), and other cool ‘proto-sauropodomorphs’ like Panphagia and Pampadromaeus.
Why highlight all of these taxa?
Well, I think the point that I am trying to make here is that, especially given the scores of discoveries in recent decades, our understanding of what ‘basal’ sauropodomorphs are, and what their anatomy looks like, shouldn’t be condensed to just include Plateosaurus, as has so often been the case. Yes we have known about it for a long time, and yes, it was forever that classic example of what sauropods looked like before they were sauropods, and yes, we have lots of specimens and yes, there are some good ones in the AMNH in New York, but honestly, I think if we do really want to know what ‘basal’ sauropodomorphs looks like, we do need to turn our attention to the specimens that I named above, and maybe be open to the idea that pneumatic features simply aren’t there at the very base of the tree.
The other issue I touched upon in my last post, and that was also picked up on in a comment by Heinrich Mallison, is the fact that these smaller, lets call them ‘proto-pneumatic’ features that appear on some but not all the vertebrae of some specimens, like Pantydraco and, lest we forget them, very early theropods like Coelophysis (which does have distinct but shallow fossae on its cervical [neck] vertebrae), may actually have their roots way down in the avemetatarsalian lineage.
As a quick overview for those that are perhaps not familiar with that last term, Avemetatarsalia is the lineage within Archosauria (= birds + crocodiles, and all their extinct relatives, including dinosaurs) that is more closely related to birds than to crocodiles. What this means in terms of the familiar is that it is essentially the birds, the other dinosaurs, and the pterosaurs (and a handful of other, less familiar taxa).
What has been quite interesting is that, in the last decade or so, some newly discovered or described specimens that fall within the avemetatarsalian lineage, but that are not dinosaurs, appear, in some ways, to have pneumatic precursory features in some of their vertebrae, similar to those observed in Pantydraco and other ‘basal’ saurischian forms; the possible ‘earliest dinosaur’ Nyasasaurus also has them, and this might, as I said in my previous post, suggest that very simple pneumatic features, or the precursors of them, are perhaps common to all dinosaur lineages ancestrally, and are perhaps lost in Ornithischia as a secondary adaptation.
This would negate any claims that pneumaticity can and should be used to argue for a monophyletic Saurischia, as secondary loss in one subgroup within three tells us diddly-squat about the possible interelationships of those subgroups.
If we have to call what is seen in the neck vertebrae of Pantydraco ‘pneumatic’ then I think we would have to extend this to also include what we see in non-dinosaurian avemetatarsalians. My point being that, either way you cut it, the fact that ornithischians seem to lack pneumatic features in general (bar some possible, but as yet unproven, examples) does not easily allow us to simply lump them all together and place them firmly outside the other members of Dinosauria, the theropods and the sauropodomorphs.
I hope that this clears things up for those of you who, like Matt and Heinrich, may have wondered about certain arguments in my last post. Either I have cleared things up, or made them a lot, lot worse. Either way, I think I am just about done writing about pneumatic features in dinosaurs, at least for now.
Dr Matthew Grant Baron
- Nobu Tamura (http://spinops.blogspot.com) – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=19459514