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Re: Removing segnosaurs from Theropoda



At 12:21 PM 8/30/95, Stan Friesen wrote:
> From: head@lust.isem.smu.edu (Jason Head)
> > As for "Thecodontians", you don't have to be a
> > cladist (and/or strictly adhere to monophyletic groupings), to recognize
> > that "thecodont" has NEVER described an actual group of related taxa.  It
> > was a garbage can ( a la megalosaur), for poorly understood archosaurs, and
> > has since been abandoned.
> >
>
>And, pray, what other name is available for stem-archosaurs?
>
>       Pray, there are multiple names to describe the varied lineages of
>basal archosaurs (which have already been posted).  But first, Colbert,
>1965, defines thecodonts as follows: "Another reptilian order that arose
>during the Triassic period were the thecodonts, the first of a group of
>related reptilian orders, collectively known as the archosaurs.  The
>thecodonts evolved as pseudosuchians, a varied group of reptiles, some
>large, some small, and as the phytosaurs, uniformly large,
>crocodilian-like animals.  All thecodonts became extinct at the end of the
>Triassic period."  Pseudosuchia is now crurotarsi (or vice versa,
>depending on which phylogeny you choose).   Phytosaurs are now considered
>nested w/ in crurotarsi, rendering Colbert's definition a bit odd.   In
>addition, if you follow the definition of thecodont as per Colbert, what
>is to become of Proterosuchus and our beloved Euparkeria?   In addition,
>how does one use the indentary plates of "thecodontia" to resolve
>dinosaurian systematics, when the plates appear and disappear throughout
>the various lineages so paraphyletically defined?  Which member of the
>thecodonts will you use for your outgroup(s): aetosaurs? riojasuchids?
>Rutiodon?  Using paraphyletic groups in fine for paleontology in
>situations where you want to discuss a lineage of critters, w/ out
>involving their DIRECT descendants.  That was the crux of my posting a few
>weeks back.   What, besides Triassic extinctions, can be discussed about
>thecodonts, that doesn't also imply dinos', pterosaurs, and crocs'?  If
>you want to snip birds off of the dino' tree to talk about the big, dead
>fellas, fine- but to snip dinos' pteros' AND crocs' off of archosauria (ne
>"thecodontia")?  What information will that give you?


        Colbert, E. H., 1965. THE AGE OF REPTILES. The World Naturalist, R.
Carrington (editor).

        Sereno, P. C., 1991. Basal Archosaurs: Phylogenetic Relationships
and Functional Implications.  Society of Vertebrate Paleontology Memoir 2,
vol .11







Jason J. Head
Dept. of Geological Sciences
Southern Methodist University
Dallas, Tx. 75275


        "I usually just make sh*t up" - discussion on oral presentations.