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Re: Removing segnosaurs from ...



In a message dated 95-08-31 00:02:04 EDT, swf@ElSegundoCA.ATTGIS.COM (Stan
Friesen) writes:

>I seem to remember that the herrerasaurs had a tetradactyl foot.
>Is this correct?  I suggest that a seperate descent from the
>herrerasaurs is another possiblity that needs to be considered
>here.

Herrerasaurians may be thought of as "theropods before the hallux
retroverted." Ditto lagosuchians: paraphyletic grades through which the
theropod-avian lineage passed following divergence from the common ancestor
of Dinosauria. The fossil record of these forms is pretty bad, so we cannot
yet establish whether there are monophyletic suprageneric subgroups within
these paraphyletic grades; each genus appears to be an autapomorphic stem
group.

When Dale Russell, working with _Alxasaurus_ material, first mentioned to me
that he thought segnosaurs might be theropods, I was very enthusiastic, and I
imagined that he had discovered a specimen that actually linked them to
herrerasaurians. I could easily have bought that: the retroverted pubis and
the pedal architecture can be derived pretty straightforwardly from those
features of _Herrerasaurus_. But there is really nothing else in the
_Alxasaurus_ material that is herrerasaurian. If someone can point out some
robust herrerasaurian features in segnosaurs, I'd be very interested.

G.O.