Hopefully you have now said you ‘hellos’ so we can actually talk about the animal. Published today, Aardonyx was described by a team led by Adam Yates of Dracovenator fame. The beast in question is a new sauropodomorph dinosaur from the Early Jurassic of South Africa and has a few rather neat things to tell us about the evolution of sauropods. As ever I don’t want to simply rehash the paper as I’m sure a number of bloggers will cover most of the things going on and I’m not sure it serves a huge purpose for everyone to say the same things! There’ll be plenty of discussion I’m sure online, so I’ll try to be specific.
For those who have missed out, the sauropods are those giant saurischian dinosaurs that have long necks and tails and four massive legs (like Diplodocus and Brachiosaurus), but before them came the ‘prosauropods’ (or if you want to be a bit more technical: non-sauropodan sauropodomorphs) a paraphyletic group of animals which are superficially similar – giant herbivores with little heads and big necks and tails, but importantly were largely bipedal. Since the first dinosaurs and their ancestors were bipeds, it should be obvious that at some point some of the sauropodomorphs eventually changed from being bipeds to quadrupeds, and since *all* sauropods are quadrupeds then this happened sometime in the ‘prosauropod’ group.
Aardonyx is important as it can tell us a fair bit about this transition from biped to quadruped. There is a long and ongoing debate about which prosauropods (I’ll drop the ‘ s for now) were or were not bipeds. This largely centers around their ability to rotate the arm (or otherwise) so that they could place the palm of the hand flat on the ground (as with the often incorrectly reconstructed theropods). If not then it would seem unlikely that they were walking on all fours, despite their often large size (some exceed 10 m in length). However to make matters more interesting, there a very young prosauropods known that appear to be only able to walk on all fours suggesting that some may have switched from being a quadruped when first hatching and at some point becoming a bipedal adult.
So into this debate steps Aardonyx. What can it tell us about this shift from supposedly bipedal prosauropods to quadrupedal sauropods? The phylogenetic position of Aardonyx is very close to the sauropods and critically is the prosauropod closest to the sauropods which is still fully capable of bipedality, with the fully quadrupedal Melanosaurus being the only prosauropod more derived than Aardonyx and lying outside the Sauropoda. In other words, based on the phylogeny alone, one might expect that Aardonyx might show some adaptations towards being able to move as a quadruped like its more derived relatives, and not just as a biped as with many of the more basal ones.
And this is what we see – Aardonyx lacks any of the major specialisations for quadrupedality seen in the more derived taxa implying it was bipedal (as do some other characteristics). However, the forearm and the femur were modified such that they hold an intermediate position between more basal and more derived taxa implying that Aardonyx represents something of an intermediate between the taxa below and above it (phylogenetically speaking) and was perhaps on the way to quadrupedality with a slower gait and the occasional use of all four limbs for walking. This gives us a good idea of what may have caused the shift and how it first occured.
Finally I must say thank to Adam for sending me the paper early and all these pictures to use, and of course congratulations to him and his team on this interesting research. Adam already has his own post up about the animal over on his blog if you want more juicy detaily.
Incidentally in case you were wondering, the name does not relate to aardvarks or digging, just that the bones and especially the claws were embedded in exceptionally hard rocks that were hard to prepare.
Share this Post