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Re: thanks and a note about _Alxasaurus_
In a message dated 2/2/99 2:24:50 AM EST, email@example.com writes:
<< The pelvis hardly asserts a relationship with ornithschians, becuase
the the obturator prong in the segnosaurs occludes the pubis, which it
does not in any other group except birds. >>
Presence of obturator on ischium in segnosaurs is autapomorphy (note obturator
is absent in primitive ornithischians). So this is by no means necessarily an
avian(!) condition. Or are you arguing that segnosaurs are more closely
related to >birds< than to any other theropods?
<<The form of the ilium is nearly identical to
caenagnathids, *Caudipteryx*, and *Rinchenia* except for the lateral
deflection of the anterior blade in which case it resembles some
sauropods, pachyderms, and giant ground sloths. An apparent feeding
adaptation, if you ask me.>>
Form of segnosaur ilium is totally unique to dinosaurs and not "nearly
identical" by any stretch of imagination to ilium of caenagnathids,
oviraptorids, and so forth. Likewise sacrum. Study segnosaur ilium in dorsal
view, not just lateral view, which can be misleading. Segnosaur ilium has
hypertrophied anterior ramus, already well developed in advanced prosauropods.
Many other features also autapomorphic. Vague resemblances of a few scattered
theropod ilia representing several different groups to segnosaur ilia are
quite clearly convergent, since ilia of other theropods from same groups quite
unlike segnosaur ilia. Grouping of segnosaurs with ornithomimids in
Ornithomimosauria (Sereno 1998) hopelessly wrong (sorry, Paul)--cannot imagine
two dinosaur groups whose members resemble one another less.
Do not forget all known segnosaurs (even Chinese Lower Jurassic jaw) date from
>much< later than presumed divergence of segnosaur lineage within
Phytodinosauria, >plenty< of time for lots of autapomorphies to accumulate
(long branch convergence).
I'll believe segnosaurs are theropods and cheerfully admit grave error when
someone finally exhibits clear series of fossils showing intermediate stages
between [write your favorite theropod group here] and segnosaurs. Good start:
exhibit clear ornithomimosaur with opisthopubic pelvis and/or functionally
tetradactyl pes; or clear segnosaur with arctomet foot.