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RE: Dinosaur hips

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Jean-michel BENOIT
> Dear list members, I would like to know when the ancestral line
> of dinosaurs split into saurischian and ornithischian ( in terms
> of taxon ),

At least by the beginning of the Late Triassic (by which time we have
definite members of Saurischia and Ornithischia), and quite possibly by the
Middle Triassic (as there are sauropodomorphs (and hence saurischians) from
a Madagascan unit dated tentatively as Middle Triassic).  Could be even
earlier, but there is no direct record to support this.

> and the morphological or biological or comportamental
> need, if any, that led to this separation.

Damned if we know... Presumably the ancestral dinosaur will have all
features shared in common by the basal members of Theropoda,
Sauropodomorpha, and Ornithischia, but lack the specializations of any one
branch.  Hence the ancestral dinosaur most likely had serrated blade-like
teeth, a hand with reduced manual digits IV and V, a semi-opposable thumb,
have only two sacral vertebrae, a forward-pointing pubis, and be about a
meter long.  The very earliest members of each of the lineages would have
retained these features, but would (over time) have evolved the more typical
features of the clades as we know them.

As a very rough hypothesis, you could have ornithischians evolving as
fast-moving low browsers, sauropodomorphs as slower high-browsers, and
theropods as specialized small carnivores.  Speculations as to the ancestral
life habit for eusaurischians (sauropodomorphs + theropods) hinges on
whether herrerasaurids are theropods or are basal saurischians.

Hope this helps.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796