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Re: Broader than Dinosauria [Additional Info]


WHOA, Hold on a minute,
Jaime, you're making some progress convincing me there might be some significant synapomorphies. Let's not spoil it by adding the twist-thumb too quickly.
Although the twist-thumb is apparently present in most theropods and prosauropods (any sauropods??), it is unfortunately known in only one ornithischian (Heterodontosaurus tucki). It may be more parsimonious to view the twist-thumb as having arisen separately in ornithischians and saurischians, rather than multiple losses in ornithischians (or are ornithischian thumbs rarely preserved?). I think there might be good reasons various workers have failed to follow Bakker in using the twist-thumb as a dinosaur synapomorphy.
----Cheers, Ken
P.S. By the way, I believe it was one of Jaime's posts that mentions the lack of a twist-thumb in segnosaurs. If they are the only theropods to lack the twist-thumb, it would be interesting to know why they were the only theropods to have lost it.
******************************************** 
From: "Jaime A. Headden" <qilongia@yahoo.com>
Reply-To: qilongia@yahoo.com
To: dinosaur@usc.edu
CC: Adam.Yates@bristol.ac.uk
Subject: Re: Broader than Dinosauria [Additional Info]
Date: Mon, 10 Sep 2001 13:29:31 -0700 (PDT)

Thanks to Adam Yates for providing information on *Saturnalia*'s pelvis, and allowing me to
confirm an hypothesis. A partially open medial acetabular wall, in the form of the triangular
apse, does appear to be a dinosaurian synapomorphy, even if it seems to have been convergently
expanded within Dinosauria. This does not contradict the use of the character.

I'm only hoping that more complete information on *Eoraptor* is in the offering as well, and new
finds continue to supplement our understanding....

As Ken is confronted with the large deltopectoral crest, _partially_ open acetabulum, and
ascending process of the astragalus as likely synapomorphies of the clade represented by the
formula {*Marasuchus* + Dinosauria}, it seems the major innovation of {Dinosauria <- *Marasuchus*}
is the features of the femur and tarsus indicating increased terrestrial bipedal cursorial
locomotion and the loss of the postparietal fenestra (questionable in *Lewisuchus*, since
parietals are absent from the specimen), that there are in fact secure synapomorphies for
Dinosauria. As Pete Buchholz points out, the "twist" thumb also appears to be a {Dinosauria <-
*Marasuchus*} synapomorphy.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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