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Earth Claw is here!

November 11, 2009 33 comments

aard-new-reconToday saw the publication of a paper that I’ve working for years to produce. It is an extremely proud moment for me since it doesn’t just represent a new dinosaur I’ve named but a new dinosaur that was found and excavated on my own expeditions. So in a sense it is more ‘my’ dinosaur than any of the others I’ve named, all of which had been found, excavated, prepared and curated long before I ever set eyes on them. I’ll have a lot more to say about the discovery of this new dinosaur, which I and my co-authors have called Aardonyx celestae, in posts over the next few days.
For now I just want to introduce Aardonyx the dinosaur, and say a little about why I think its special beyond its sentimental significance to me.

What is it?
Aardonyx is a sauropodomorph dinosaur. That is a long-necked, small-skulled, plant-munching branch of the lizard-hipped dinosaur group. Sauropodomorpha includes (as the name would suggest) the well-known giant quadrupedal sauropods as well as a range of more primitive forms that don’t appear to have survived beyond the Early Jurassic. Aardonyx is a relatively primitive member of the group and is excluded from Sauropoda (at least under some definitions of Sauropoda), although it appears to be more closely related to them than most other non-sauropod (=basal) sauropodomorphs.

 

Where was it found?
The bones were excavated from a single small but dense bone-bed that we nicknamed Marc’s Quarry (after my student Marc Blackbeard who studied the taphonomy of the site as part of his honours degree). Marc’s Quarry is one of several sites on the farm Spion Kop, part of a game farm complex in the Senekal District of the Free State South Africa.

How much of it was found?
We were actually quite lucky. We found the bones of two individuals thoroughly comingled. Between the two of them we found a remarkable amount of the skeleton, including the frequently missing skull. The major missing elements were the nasal and quadrate bones of the skull, the humerus of the forelimb and the ilium of the pelvis.

 

12-aardonyx-skeletonA skeletal reconstruction of Aardonyx scaled to the size of the smaller individual (but using parts from both)

How big was it?
My skeletal reconstruction (scaled to the size of the smaller individual) is just over 7 metres in length when the neck is held straight out in front of the body and stands 1.5 metres high at the hips. Assuming no major proportional changes the larger individual would have been close to 9 metres long and nearly 2 metres high at the hips. The histological work of Anusuya Chinsamy suggests that neither individual was mature at the time of death, indicating that Aardonyx got even bigger. Thus it is one of the largest known non-sauropod sauropodomorphs.

When did it live?
The geology of the site is fairly clear. There are no complex folds or faults, just simple, horizontal layer-cake stratigraphy. Thus there can be little doubt that Marc’s Quarry lies in the upper Elliot Formation, between the lower Elliot Formation and the Clarens Formation. That puts it in the Early Jurassic, somewhere between 183 and 200 million years old. Exactly where in this time range is far less certain but I would guess closer to 183ma than to 200ma.

What makes it a new species?
Aardonyx is blessed with many characters that enable it to be distinguished from all other known basal sauropodomorphs. These unique characteristics include features of the skull, neck, forearm and foot. Within the skull the apparently giant bony aperture of the nostril is especially noticeable. It is true that is a feature of the reconstructed skull and could therefore be prone to error. However I am confident that this is not the case. Firstly The premaxilla and maxilla used to reconstruct definitely come from the same taxon (they share the same set of unusual features such as pointed, virtually unserrated and longitudinally ridged teeth and a wide band of dense ‘micropits’ along the alveolar margins of both bones. Secondly the scar on the maxilla for articulation with the posterior process of the premaxilla is a perfect match. Other unusual skull features include the relatively tiny size of the major blood vessel and nerve openings on the maxilla bone and a very deep, sharply defined groove on the inside of the maxilla- you simply don’t see that elsewhere.
The neck vertebrae are distinctive in their simplicity. Generally sauropodomorph vertebrae become more complex as you progress towards the trunk, with the neck rib attachment scars projecting on well marked processes that are supported by thin buttresses of bone. None of the neck vertebrae of Aardonyx, from front to back, have a trace of a bony lamina or a projecting rib attachment.
As we look at the rest of the anatomy we continue to see odd features, for example the radius carries an extremely large highly rugose biceps scar (present on both individuals) while the fourth metatarsus of the foot meets the ankle in a broad racket-like expansion. All this indicates that although Aardonyx is intermediate between more typical ‘prosauropods’ and basal true sauropods it had been doing plenty of its own evolution since it split from the sauropod line and had accrued a bunch of weird characters that make it highly recognizable.

What makes it special?
Aardonyx seems to lie at the very heart of the transition from basal sauropodomorphs, that we could label ‘typical’ or ‘core’ prosauropods to true sauropods. When I plugged Aardonyx into two different cladistic matrices Aardonyx came out as the sister taxon of Melanorosaurus (or Melanorosauridae) + Sauropoda in both cases. Moreover this result was one of the most robust nodes in the analyses. This is interesting because as we say in the paper we think that committed quadrupedality is diagnostic for the latter clade. However Aardonyx is probably not a quadruped that makes it the closest known outgroup to Sauropoda before their move on to all fours.
Aardonyx also reveals that the sequence of evolution of derived characteristics on the line leading to sauropods involved more convergences and/or reversals than we previously suspected. But more about that later.

PrintAardonyx in its place in a simplified family tree of dinosaurs. Image courtesy of Matt Bonnan

Yates, A. M., Bonnan, M. F., Neveling, J., Chinsamy, A. and Blackbeard, M. G. 2009. A new transitional sauropodomorph dinosaur from the Early Jurassic of South Africa and the evolution of sauropod feeding and quadrupedalism. Proc. R. Soc. B doi:10.1098/rspb.2009.1440