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Triassic 'dinosaurs' of North America - New paper




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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