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Alternative explanations?

Posted by AndyFarke on 05 Mar 2009 at 05:44 GMT

As noted above, the functional morphology of the theropod forelimb [58], [81] makes assignment of Atreipus to theropods unlikely.

But. . .we also don't know a lot about functional morphology of the forelimb and wrist this close to the base of Theropoda. I think it might be a little dangerous to exclude a theropod origin just based on orientation of the manual digits alone. More follow-up work on the functional morphology of the manus and forearm are needed in order to more explicitly define when the shift from "palm down" to "palm in" happened. Ungual size, or other morphological aspects of the tracks, are more convincing evidence in my mind (and avoid problems of circularity).

RE: Alternative explanations?

JDHarris replied to AndyFarke on 05 Mar 2009 at 15:01 GMT

Well, we're arguing that the track maker here was likely something very basal -- a coelophysoid or slightly more derived theropod. Not herrerasaur-grade...but without postulating the existence of relatively derived tetanurans in the earliest Jurassic here when coelophysoids and _Dilophosaurus_ (itself debatably coelophysoid or slightly more derived)-grade animals are what are known, it's most parsimonious that these tracks were made by something quite basal. More to the point, in the other tracks that have been declared as theropod and have handprints (like _Atreipus_), the hand print morphology doesn't match what we know of really basal theropods like _Herrerasaurus_ (assuming this is actually a theropod and not a non-theropodan basal saurischian) -- I could see that they might be coelophysoid, but that's stretching it. They really do look more ornithischian (or non-dinosaurian dinosauriform). But still, it's a hypothesis that, yes, further discoveries and further functional morphological testing will help unravel, and allow us to better pinpoint when the inability to pronate evolved. I'd predict it is coupled with the loss of obligate quadrupedality, but then again, ornithischians seem to either have retained it into bipedality or quickly re-evolved it as various lineages returned to quadrupedality, so it's a good question!

The other problem here is that, early on in the dinosaurian diversification process, one would expect the ornithischians and saurischians to converge on each other morphologically -- this is why tracks like _Atreipus_ have been so difficult to pinpoint to taxon: basal ornithischians have very theropod-like feet, for example (or vice-versa, I suppose), and without hand prints would almost certainly be labeled as theropods. So it's the hand prints that are telling, and in those tracks, the claw impressions don't nicely match known theropods from the time period...but since we're lacking great comparative ornithischian manus material from the same time (with the single exception of a good _Heterodontosaurus_ skeleton), it's hard to assess without looking to other evidence. Even in later, bipedal ornithischians, like "hypsilophodontids," the feet aren't much different from many theropods, even in claw morphology, so while I definitely concur that things like ungual size are good criteria, they alone aren't sufficient (as you suggest), particularly nearer the base of the dinosaurian tree.

RE: RE: Alternative explanations?

tholtz replied to JDHarris on 20 Dec 2010 at 14:29 GMT

I might suggest that the anatomy (tiny manus, large tridactyl pes) and stratigraphy (Late Triassic only) of _Atreipus_ may indicate these are tracks of silesaurids rather than dinosaurs.

No competing interests declared.