And we have arrived at part 3 (of how many? I don’t know yet…) of my Ornithoscelida themed mind dump exercise. Am I doing this for you? Am I doing it for me, as some kind of strange catharsis? It remains to be seen.
Anyway, Lets crack on.
If you haven’e been following me lately, this is what you have missed out on:
Essentially, In these previous posts I have been covering the basics of hip (pubic) retroversion and skeletal pneumaticity, two ‘classic’ saurischian features that Seeley and others have historically pointed to as evidence for ornithischians being excluded from a dinosaurian subgroup or clade containing only theropods and sauropodomorphs.
In these posts I mainly talked about saurischians (just what happened with the scientific literature on this subject from the ’80s- to the ’00s). Now, perhaps, given the key role that they play in this story, it is time for me talk about ornithischian dinosaurs.
This remarkable clade has, for basically the whole of dinosaur research history, been regarded as a distinct subset within Dinosauria.
Seeley was right to note that perhaps the feature that gives them their name, the bird hips, was, with the obvious exception of birds themselves, a feature unique to the ornithischian clade.
However, there are also many other features that help make a strong case for ornithischian monophyly. Because of these, most scientists in the field (and rightly so), believe that, whatever their origin, ornithischians, once they exist, do form a neat subset of dinosaurs that descend from a single common ancestor, to the exclusion of all other dinosaurs and dinosauriforms*.
I will now quickly go over some of these distinct features, just so as to give you all a descent grounding in ornithischian anatomy and how it differs from other dinosaurs. Then, when I start rambling on about their position in the tree, as well as their possible relationships with animals that most scientists (but not all) consider to be non-dinosaurian dinosauriforms*. you might have a better sense of what I am driving at.
*Some palaeontologists have suggested that Ornithischia shares a close relationship with Silesauridae, a group of (as most would say) non-dinosaurian dinosauriforms i.e. close dinosaur relatives.
First, we have the hips. As already discussed at length, ornithischians get their names from the fact that, like modern birds, they retrovert their pubic bone to point backward, rather than forward, and their is much debate as to why they do this.
We now know, because of discoveries made since 1887 when Seeley made his inital observations, that it isn’t juts the birds and the ornithischian dinosaurs that retrovert their pubes. Many clades within Theropoda try it at some stage, either in full, or partially, including: raptors, therizinosaurs and some ceratosaurs. Chilesaurus also does it, and whether this is convergent with, or homologous to, the same feature in Ornithischia, is currently the subject of debate.
So what else, apart from the hips?
Well, there has also long been recognised a suite of features unique among ornithischians that help us to diagnose them. Odd features of the skull, teeth, axial column, limbs… they all add up to a pretty convincing picture that the groups we think are all ornithischian actually do descend from a single common ancestor. The same features seem to crop up in stegosaurs, ceratopsians, hadrosaurs and lambeosaurs, little heterodontosaurs, and everything in between.
First, we have the predentary.
This feature is essentially a cropping tool; a lower beak in the lower half of the jaw that would have, in life, been covered in a keratinous sheath. Almost all known ornithischians have this feature, from the heterodontosaurs at the base of the tree, to the most derived Late Cretaceous ceratopsians.
In fact, it is so ubiquitous, another name that was once used for the Ornithischia was Predentata.
Then, there is the palpebral, a bony strut or spike that spans the orbit (eye socket). On the figure above, it is the little red bone. What this feature was for was to support an eyelid of some kind, and again, seems to be unique to ornithischians.
Moving into the axial column (spine), another feature that we see that is common in almost all ornithischians is the ossification of tendons that run along the top of the vertebrae. It is assumed that these ossified (turned to bone) tendons provided extra support to the vertebral column, possibly to assist in carrying the increased gut that is required for a herbivorous lifestyle (all ornithischians are, as far as we know, herbivores or omnivores).
Then we have things like the pendant fourth trochanter in the femur and the broad, blade like anterior trochanter.
What are these?
Well, on the femur of most reptiles, and other creatures, there are often flange like expansions, near the head and, sometimes, on the femoral shaft, and we call these trochanters. These strange juts of bone are for muscle attachment, providing grip during muscle contraction and expansion, driving locomotion. In ornithischians, the anterior trochanter, which sits next to the femoral head, is separated from the greater trochanter (also near the head) by a deep cleft, and the anterior tochanter takes the shape of a blade. This stands in contrast with some basal theropods, herrerasaurids and with sauropodomorphs, all of which have ridge like anterior trochanters that aren’t separated from the rest of the features on the proximal end of the femur by a deep groove or cleft.
Then, further down the femoral shaft, we have the fourth trochanter, situated at the rear, roughly halfway between the proximal and distal ends. This feature, which projects out as a tab or crest, is for the attachment of a long muscle (with an even longer name) that runs to the tail of dinosaurs and is responsible for pulling the leg backward with force, propelling the animal forward. In most dinosaurs, this crest is a exactly that, a crest, rising and sinking back into the bone. In ornithischians, the trochanter takes a different form. We call it pendant, because it projects down, as a rod-like structure, with a pronounced gap between its underside and the lower half of the femur. Again, this appears to be unique to ornithischians, and quite why they have developed such an unusually shaped area of muscle attachment is currently unknown.
But, in spite of all these unique features, which give good evidence of ornithischian monophyly, where the ornithischians fit into the dinosaurian tree isn’t any more clear. Given this odd set of features seem to appear almost all at once, and are all derived and shared among just the ornithischians (synapomorphies), they do little to affrim or reject hypotheses about what the ornithischians are most closely related to within the dinosauromorph lineage.
If, perhaps, some of these features were observed in other, non-ornithischian dinosaurs and dinosauriforms (or at least, the beginnings of, or versions of, these features) then maybe we could start placing Ornithischia with a little more confidence.
Some have claimed that certain features may be present in other, non-ornithischian taxa, and I think i may have identified some too. But whether such features in other taxa are homologous, or simply coincidence, or perhaps even just a misinterpretation of the anatomy of certain specimens, remains to be seen, and is the topic of my next post.
Dr Matthew Baron
1. Nothronychus_BW.jpg: ArthurWeasley – Nothronychus_BW.jpg, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=5954555
2. Carol Abraczinskas, Paul C. Sereno – Sereno PC (2012) Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs. ZooKeys 226: 1-225. doi:10.3897/zookeys.226.2840. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3491919/figure/F59/), CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48935748
4. By Daderot – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=37559701