Ornithischia – the bird-hipped dinosaurs (but not the actual birds)… what are they?
Well, the most famous members of this clade of dinosaurs have to be Triceratops, Stegosaurus, Iguanodon and Pachycephalosaurus… maybe some people outside of dinosaur palaeontology will also be familiar with a few of their relatives.
Anyway, quite what they are, how they evolved and where they fit into the wider family of dinosaurs is what my PhD was on (available in no good book stores). In all honesty, I am, after five years of research on the topic, still not sure about the answers to any of those questions… and here is why.
Firstly, back in March last year Paul Barrett, Dave Norman and I did the whole Ornithoscelida thing and set a new record for a palaeontology paper’s meme:citation ratio… In sum, the main find of our research was a new tree topology for Dinosauria, in which Ornithischia and the meat eating Theropoda (T. rex and stuff) were united into a group for the first time since T. H. Huxley proposed a group called Ornithoscelida back in 1870…
Then, Paul Barrett and I followed up this study by reanalysing the bizarre Middle Jurassic ‘Frankenstein’s dinosaur’ Chilesaurus diegosaurezi using the new dataset. This taxon came out, rather surprisingly, as an early diverging member of Ornithischia.
Both papers have since had replies written to them, and have given us a chnace to write our own rebuttals. In this back-and-forthing with our colleagues, the position of Ornithischia and of Chilesaurus danced around a bit…
With Langer et al.’s reply alternative interpretations of the anatomical characters and their scores for certain taxa were presented in a revised dataset, and the traditional model of dinosaur relationships was recovered. However, this was shown to be not statistically significantly different to our Ornithoscelida hypothesis, or to the oft forgotten Phytodinosauria hypothesis. Here is our rebuttal, in case you want to read it.
The controversy surrounding the position of Chilesaurus is already the topic of a previous post, and so I wont repeat it here.
To cut a long story shorter, I then went on a bit of a tangent from the pure data coding and finer points of anatomy debates and explored the possibility that maybe, just maybe, we have had entirely the wrong impression of what Ornithischia actually is… so wrong, in fact, that it has completely hindered our attempts to complete a fair and truly objective broad study of their early evolution. My hunch (and that is all that it is at the moment) is that Ornithischia does not in fact trace its roots all the way back to the very first days of early dinosaur evolution and that actually, it may well be a subset of one of the original groups, for which we have a stronger Triassic fossil record, the theropods.
Here is my thinking…
Ever since Seeley in 1887-1888(ish) said Ornithischia and Saurischia were the two types of dinosaur, we have assumed, for one reason or another, that Ornithischia should have as long a fossil record as Saurischia. When dinosaur monophyly was reinstated in the 1970s, it was taken to be the case that these two fundamental groups must have diverged from one another directly following on from the most recent common ancestor of all dinosaurs. This would make Ornithischia and Saurischia sister-taxa – a view widely held for decades and supported by numerous quantitative studies. For this to be the case, Ornithischia would have to be present as far back as the Carnian (= the earliest subdivision of the Late Traissic Period). This is because Saurischia has a good record in the Carnian, according to most phlyogenetic hypotheses… My problem with this is that, as good as the anatomical observations of Seeley were, his view was based upon what he could glean from younger, Jurassic/Cretaceous taxa, and working within the mind-frame of a Victorian era creationist. Seeley wouldn’t have thought in the same way that modern evolutionary scientists do… the retroversion of the pubis in ornithischians, for example, was pointed to as a clear indication that Ornithischia was distinct from the two groups within Saurischia… we now know that the Saurischia hip set up is a primitive condition and that a single reversal in one group is essentially useless and uniformative when we try splitting and uniting the various groups on that feature.
So too, it turns out, is the pneumatic excavation of bone in derived sauropods and theropods… given that these features don’t go all the way back to the roots of these groups, we cannot say that they are a feature that proves their common descent. it is likely that such deep excavation in bone is convergent.
What is more, the fossil record of Ornithischia in the Triassic Period has always been terrible (and is getting much much worse). I think that, because people’s ideas about the ornithischian origin were so firmly fixed within the traditional ‘Seeleyan’ model, scientists working on the topic of early dinosaur evolution have always needed Triassic ornithischian specimens. For this reason, and for a lack of other good comparative materials for a lot of it, many Triassic specimens have been assigned to Ornithischia over the years. Anything small and reptilian with herbivore-like dentition had been chucked into Ornithischia, like Revueltosaurus and Pisanosaurus.
The trouble is, we now know, from other good fossil finds, that a tonne of these animals are actually not ornithischians, or even dinosaurs at all. That sexy looking Revueltosaurus is actually a weird type of herbivorous crocodile. In fact, by 2007, the entire Triassic fossil record for Ornithischia had collapsed down to just 3 specimens… 3! Namely Eocursor parvus, Pisanosaurus mertii and an unnamed thing from Argentina… Then, some more recent evidence had suggested that Eocursor and the Argentinian specimen might be younger than we thought, and not Triassic in age at all. Leaving just the broken pile of cruddy, weathered and deeply mysterious pile of bones that is Pisanosaurus as the lone specimen that may still allow people to make a valid case for the existence of Triassic (and more importantly, Carnian) Ornithischia.
Then, two studies, one being my own, and another by Agnolin & Rozadilla suggested that Pisanosaurus, like its other ‘Triassic ornithischian’ friends was indeed not an ornithischian, or even a dinosaur. We both suggested that Pisanosaurus might just represent another new member of the recently discovered clade of herbivorous dinosaur close cousins, the silesaurids. If this is the case, then good-bye unambiguous Triassic ornithischian record.
Why is this important… Well, it would mean that maybe, just maybe, my hunch that ornithischians are a slightly later appearing and derived subset within one of the two saurischian clades is correct… It might go some way to explaining why taxa like Chilesaurus could possess a suit of derived theropod features and the makings of a few primitive ornithischian features… it might also explain why the earliest good ornithischian fossils that we have from the Early Jurassic are so weird, so unlike basal dinosaurs and other dinosauriforms and, in many respects, so similar to many theropod taxa… Maybe wen haven’t spotted it yet because we have been trained to think of the earliest ornithischians as just weird outliers, the exception that proves the rule, kind of thing, and have thus ignored a lot of these quite important anatomical traits (or dismissed many as coincidences). What if Ornithischia is actually a very latest Triassic (Rhaetian) or even Earliest Jurassic (Hettangian) diverging clade… No modern studies that have assessed anatomical data with parsimony or Bayesian based methods have sampled far enough up the various branches of the tree to properly test it… Maybe Ornithischia is actually so far removed from the base of the dinosaur tree that no studies, including my own, have been able to properly place them… Its an intriguing thought and one that needs examining properly.
Dr Matthew Grant Baron
Reveultosaurus – By National Park Service – http://www.nps.gov/pefo/photosmultimedia/fossils-and-reconstructions-gallery.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=24784343
Chilesaurus – By Nobu Tamura email:firstname.lastname@example.org http://spinops.blogspot.com/ – http://spinops.blogspot.dk/2015/05/chilesaurus-diegosuarezi.html?q=Chilesaurus, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50352749
Others pulled from own previous blogs and talks etc.