Triassic 'dinosaurs' of North America - New paper

[Tim Williams <twilliams_alpha@hotmail.com>]

This is what I call a GREAT paper...

Sterling J. Nesbitt, Randall B. Irmis, and William G. Parker (2007).  A 
critical re-evaluation of the Late Triassic dinosaur taxa of North America.  
Journal of Systematic Palaeontology 5 (2): 209?243.

SYNOPSIS: "The North American Triassic dinosaur record has been repeatedly 
cited as one of the most complete early dinosaur assemblages.  The discovery 
of _Silesaurus_ from Poland and the recognition that _Herrerasaurus_ and 
_Eoraptor_ may not be theropods have forced a re-evaluation of saurischian 
and theropod synapomorphies.  Here, we re-evaluate each purported Triassic 
dinosaur from North America on a specimen by specimen basis using an 
apomorphy-based approach.  We attempt to assign specimens to the most 
exclusive taxon possible.  Our revision of purported Late Triassic dinosaur 
material from North America indicates that dinosaurs were rarer and less 
diverse in these strata than previously thought.  This analysis concludes 
that non-dinosaurian dinosauriforms were present in North America in the 
Late Triassic.  Most of the proposed theropod specimens are fragmentary 
and/or indistinguishable from corresponding elements in the only well-known 
Triassic theropod of North America, _Coelophysis bauri_.  No Triassic 
material from North America can be assigned to Sauropodomorpha, because none 
of the purported ?prosauropod? material is diagnostic.  Recent discovery of 
the skull and skeleton of _Revueltosaurus callenderi_ from Arizona shows 
that it is a pseudosuchian archosaur, not an ornithischian dinosaur.  As a 
result, other purported North American ornithischian teeth cannot be 
assigned to the Ornithischia and therefore, there are no confirmed North 
American Triassic ornithischians.  Non-tetanuran theropods and possible 
basal saurischians are the only identifiable dinosaurs recognised in North 
America until the beginning of the Jurassic Period."

A few tidbits regarding putative Late Triassic theropods...

_Eucoelophysis baldwini_ - Based on the holotype (associated), this is a 
valid taxon, but not a theropod, or even a dinosaur.  Instead, 
_Eucoelophysis_ appears to be a basal ornithodiran or dinosauriform that may 
form a clade with _Silesaurus_ and _Pseudolagosuchus_.  Referred material 
may come from more than one taxon, but it is in any case non-diagnostic.

_Gojirasaurus quayi_ - Holotype is possibly a chimaera that includes bona 
fide coelophysoid material.  This is a 'metataxon', with no automorphies or 
unique combination of characters.

_Camposaurus arizonensis_ - Coelophysoidea indet., as posited by several 
previous studies.

_Protoavis texensis_ - Discussed briefly, though the authors do not endorse 
avian affinities.  The holotype is from Bull Canyon Formation, but the 
_Protoavis_ hypodigm incorporates material from both the Bull Canyon 
Formation and Tecovas Formation.  Parts of _Protoavis_ (especially the 
cervicals) may come from a drepanosaurid; other drepanosaurid material is 
known from the _Protoavis_ locality.  The tibia (TTUP 9201) and femur (TTUP 
9200) of the _Protoavis_ holotype appear to belong to a non-tetanuran 
theropod, possibly a coelophysoid.

_Spinosuchus caseanus_ - A valid taxon, courtesy of the distinctive neural 
spines; but no strong evidence that _Spinosuchus_ is a theropod, or even a 
dinosaur, as has been mooted.  No evidence that it's a trilophosaur either.  
The authors regard it as Archosauriformes incertae sedis.

_Chindesaurus bryansmalli_ - Certainly a valid taxon.  Maybe a basal 
saurischian; _Chindesaurus_ shows similarities to both _Herrerasaurus_ and 
_Saturnalia_.

_Caseosaurus crosbyensis_ - Perhaps the same as _Chindesaurus_, though 
otherwise it can only be demonstrated to be a dinosauriform.

_Arctosaurus osborni_ - Archosauriformes indet.

Cheers

Tim

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