Re: Erlic/kosaurus

[longrich@Princeton.EDU (Nick Longrich)]

At 7:24 PM 5/28/96, Dinogeorge@aol.com wrote:

>Which postcranial characters are these? Pneumatization of the axial
>skeleton, which runs rampant through Theropoda and even Prosauropoda
>and Sauropoda?  Semilunate carpal, which I have yet to see in any
>segnosaurian specimen, but which everybody says is there (like the
>emperor's new clothes)? Don't forget that Huene once thought
>prosauropods were closely related to coelurosaurians.  The
>coelurosaurian features you may see in segnosaurs could be their
>coelurosaur-like prosauropod characters in various derived states.

        There are a couple very general similarities- for one, the
distal expansion of the pubis, which at least superficially resembles
the pubic "boot" of tetanurans. Expansion of the distal end of the
pubis is pretty minimal in the prosauropods, although some sauropods
(Apatosaurus) seem to have had some.
        The other is that the hand is, as far as we know,
three-fingered.  This could be a convergency, but only a couple of
groups besides the theropods got down to four- psittacosaurs,
according to the Dinosauria, and the duckbills. And among the
theropods, only tetanurans got down to three.  On the other hand, this
is a pretty functional character, and functional characters tend to
evolve quickly (and so can be less reliable for cladistic
analysis). As far as I know, digits IV and V in dinosaurs were
unclawed, so there would be less incentive to keep them around
(probably why tetanures lost them).
        If the Therizinosaurs/Segnosaurs were prosauropods or their
relatives, that would leave a gap in their fossil record leading back
to the early Jurassic, which leaves them plenty of time to evolve all
those weird characters on their own (more time than they'd get if they
came from coelurosaur stock). One has to wonder if the fossil record
actually is THAT bad, but considering how we have stuff like Minmi and
the Abelisaurs that almost come out of nowhere, the answer would
probably be yes.