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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?

The best paper I've seen recently about the Crurotarsi (the archosaurs
which share a more recent common ancestor with crocodillians than with
dinosaurs or pterosaurs) is:

Parrish, J.M.  1993.  Phylogeny of the Crocodylotarsi, with reference to
archosaurian and crurotarsan monophyly.  Journal of Vertebrate
Paleontology, 13:287-308.

His basic phylogenetic conclusions about the Crurotarsi are listed below:

Crurotarsi
        Ornithosuchidae (a group of mid-sized quadrupedal predators)
        Crocodylotarsi
                Parasuchia ("phytosaurs")
                Suchia
                        Prestosuchidae (a group of large quadrupedal
predators)
                        Rauisuchiformes
                                Aetosauria (armored, herbivorous forms)
                                Rauisuchia
                                        Rauisuchidae (more big predators)
                                        Paracrocodylomorpha
                                                Poposauridae (more big
preds)
                                                Crocodylomorpha

As you can see, there were multiple radiations of mid-to-large predatory
archosaurs within this group, mostly during the Triassic.  These animals
certainly did replace the Therapsida as the dominant large terrestrial
forms, and were replaced by dinosaurian archosaurs in turn.

However, as Mike Benton (Univ. of Bristol) has shown, the data from more
than 200 million years away are not finely resolved enough to demonstrate
adaptive superiority of crurotarsians versus therapsids.  Some suggestions
as to why the former became dominant over the latter at the dawn of the
Middle Triassic areimproved locomotory ability in crurotarsians and a
physiology better adapted to the high & dry, Pangaeic world.  Both are
possible, but hard to demonstrate (especially since some of the therapsids
had a fairly upright posture, while most of the crurotarsians did not).

In any case, just as the crurotarsians were part of a faunal turnover
replacing the therapsid-dominated communities, they were replaced
themselves by the dinosaur community (which also included the first true
mammals, turtles, pterosaurs, squamates (lizards & snakes), etc.).

Any other thoughts on these guys?
                                     
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