[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Beipiaosaurus and sauropod ancestry

In a message dated 6/20/99 12:37:34 PM EST, HPBredow@aol.com writes:

<< "The specimen shows certain anatomical characteristics closer to those of 
 oviraptorids than advanced therizinosaurs. This suggests that the five-toed 
 feet of therizinosaurs were not inherited from prosauropod ancestors, but 
 were evolved independently from four-toed theropod ancestors. >>

This statement contains an error: the feet of segnosaurians are four-toed 
like those of prosauropods, not five-toed. The difference between 
segnosaurian feet and theropodan feet is that in theropods the first toe is 
strongly reduced (and, I argue, from a primitive retroverted condition), 
whereas in segnosaurians the first toe is enlarged and more or less aligned 
with the other three toes. It is not impossible for this condition to evolve 
from the typical theropodan foot, since the first toe is not entirely gone, 
just pretty unlikely. Paul Sereno tells me that there are errors in the 
published illustrations of segnosaurian feet that make it seem as if the 
first toe had rearticulated with the ankle. It did not; it sticks out from 
the side as in typical theropods.

On the other hand, I've now seen the Beipiaosaurus paper, and the arguments 
presented there about segnosaurian-theropod relationships are not 
particularly convincing. The only unambiguously segnosaurian feature of the 
skeleton is the lower jaw with teeth, which the authors even assert seems 
oversized for the rest of the skeleton but nevertheless still claim is part 
of the same individual. This, I feel, remains to be proved. There is 
>nothing< in the skeleton itself outside the jaw that identifies it as 
segnosaurian; it could very well be an ordinary oviraptorosaurian skeleton 
that a small segnosaurian jaw washed into. We all know that segnosaurians and 
oviraptorosaurians share a number of apomorphies; finding some of these 
features in this skeleton would be expected if it were either an 
oviraptorosaurian or a segnosaurian.