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Re: Avimimus (was Re: Dinosaur Web Pages' Re-Opening)



In a message dated 97-09-05 23:06:52 EDT, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes:

<< At 08:12 PM 9/4/97 -0400, Dinogeorge wrote:
 >I'm skeptical of the _Alvarezsaurus_-_Mononykus_ connection, since it is
 >documented only in a couple of cladistic studies that seem to do little
more
 >than balance a random assortment of characters against an equally random
 >assortment of characters without much regard for character suites.
         Ok, you got me, what does this mean? Should character sets, rather
 than being random, be specifically designed to prove a point? How do you get
 random character assortments? Or, for that matter, non-random ones? What is
 a character suite? >>

If you read Clark et al's paper on the skull of _Erlikosaurus_, you'll find a
few dromaeosaurid characters mentioned, a few ornithomimid characters
mentioned, as well as a few characters belonging to each of a number of other
theropod groups. Leaving aside the problem that some of these characters also
occur in non-theropod dinosaurs groups, what are we to make of this
assortment? Was there once a theropod in which _all_ of these characters were
expressed, so that _Erlikosaurus_ could acquire its share of them? Was this
same theropod also ancestral to dromaeosaurids, ornithomimids, and all those
other theropod groups--groups that then went on to >lose< each other's suites
of characters? So that dromaeosaurids are the theropods that retained those
dromaeosaurid characters also found in _Erlikosaurus_; ornithomimids are the
theropods that lost those dromaeosaurid characters but retained those
ornithomimid characters found in _Erlikosaurus_; and so forth? This is
parsimony??

People on this list at various times have noted (I'm paraphrasing here) that
segnosaurs have the lower jaw of a derived ornithomimid, the skull of a
derived troodontid, the pelvis of a derived dromaeosaurid, and a random
assortment of derived oviraptorid features. The hind feet are unlike those of
any theropod. But one can find a similar lower jaw among prosauropods, a
similar skull among prosauropods, and a similar pelvis among the prosauropods
(although there's nothing like the ilium of a segnosaur in any other dinosaur
group; it's a segnosaur autapomorphy)--as well as similar hind limbs and
feet. (Here I'm using the word "similar" to mean "sharing several characters
with," not just "looking like.") I can show you lots of cranial characters of
_Erlikosaurus_ that also occur in _Plateosaurus_, for example; the point
being that in segnosaurs we can find suites of prosauropod characters but not
suites of theropod characters. So why shouldn't I think that segnosaurs are
derived prosauropods, or at least close to that lineage?