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Segnosaurs again [was Oviraptors]



In a message dated 97-04-30 12:44:50 EDT, th81@umail.umd.edu (Thomas R.
Holtz, Jr.) writes:

<< Greg Paul no longers considers them prosauropod descendants.  The abundant
 new material (including Alxasaurus, and the redescribed skull of
 Erlicosaurus) demonstrate the theropod, indeed the coelurosaurian nature of
 the material. >>

Segnosaur material is hardly "abundant." One good skull and a few partial to
fragmentary postcranial skeletons.

How swiftly we forget that a number of what are now considered prosauropods
were once classified as coelurians: _Ammosaurus_ and _Anchisaurus_, to name
two such genera, and that all prosauropods were once classified as theropods.
Why might that have been? Because there's considerable similarity between
>small< prosauropods and coelurians, including such basic features as lightly
built, pneumatic skeletons, which could well account for the so-called
"coelurosaurian nature" of segnosaurs. Larger prosauropods are less like
coelurians because they're more derived than the small forms, and I do not
consider them candidates for segnosaur ancestry.

The similarities of some features of _Alxasaurus_ and _Erlikosaurus_ material
to features found in some coelurian genera can easily be explained as
convergence, since it is very likely that ancestral segnosaurs were small,
lightly built, bipedal cursorial forms like their prosauropod predecessors.
(This also explains their absence from the fossil record.) The braincase and
palate of _Erlikosaurus_ as figured by Perle et al. bear little resemblance
to known coelurian material (which is quite scarce) but are quite similar to
those of the prosauropod _Plateosaurus_. I've read that paper several times,
and I have yet to find >any< feature of the skull of _Erlikosaurus_ that bars
a prosauropod ancestry for  segnosaurs, or any of the characters of
_Erlikosaurus_ that are listed as strictly theropod characters (e.g.,
interdental plates, finely serrated teeth) being absent from other dinosaur
groups. I've seen no published analysis of known segnosaur forelimb material
that establishes it as unequivocally maniraptoran. In _Alxasaurus_ the two
carpal elements that become the so-called "semilunate" carpal in
_Therizinosaurus_ are still unfused (see Russell & Dong's paper, fig. 8). On
the other hand, I consider the structure of the segnosaur foot as making it
virtually impossible for coelurians or any other theropods above the level of
_Herrerasaurus_ to be segnosaur ancestors. As soon as I find anything in the
literature convincing of a theropod relationship for segnosaurs, I'll let
everyone know.