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Re: Phytosaurs, crocodilians and thecodonts



>I'm trying to sort out the distinction between phytosaurs, crocodilians and
>crocodile-like thecodonts in the early and middle Mesozoic. Does anyone have
>a handle on these creatures and their impact on the therapsidae and other
>reptiles of the period?

As to your first question, the best way to distinguish parasuchians (aka
phytosaurs) vs. crocodilians is the position of the nostrils.  In
crocodilians, most of the long part of the jaw is the bone called the
maxilla, which is behind the nostril.  Thus, in crocs, the nostrils are at
the end of the snout.  In parasuchians most of the long part of the jaw is
the premaxilla, the bone in front of the nostril.  Thus, in parasuchians,
the nostrils are close to the eyes, and the snout sticks out in front of
them.

As explained in the previous posting, parasuchians and crocodilians occupy
differnt positions within a large, diverse group of archosaurs which share
common features of the ankles, hips, skulls, and other parts of the body. 
While many of the crurotarsians of the Triassic looked fairly similar
(i.e., large skulled quadrupedal predatory archosaurs), there were some
very differnt groups within them.  These include the
superficially-croc-like parasuchians, the pigsnouted, armored aetosaurians,
small cursorial crocodylomorphs, and toothless Lotosaurus.
                                     
Thomas R. Holtz, Jr.
Vertebrate Paleontologist in Exile
U.S. Geological Survey
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092

email:  tholtz@geochange.er.usgs.gov 
Phone:  703-648-5280
FAX:            703-648-5420