Dromomeron

[Tim Williams <twilliams_alpha@hotmail.com>]

In all the excitement, I'm not sure the actual paper on the new 
dinosauromorph (_Dromomeron romeri_) was actually mentioned here.  Here's 
the reference...

Randall B. Irmis, Sterling J. Nesbitt, Kevin Padian, Nathan D. Smith, Alan 
H. Turner, Daniel Woody, and Alex Downs (2007).  A Late Triassic 
dinosauromorph assemblage from New Mexico and the rise of dinosaurs.  
Science 317: 358-361.

Abstract: "It has generally been thought that the first dinosaurs quickly 
replaced more archaic Late Triassic faunas, either by outcompeting them or 
when the more archaic faunas suddenly became extinct.  Fossils from the 
Hayden Quarry, in the Upper Triassic Chinle Formation of New Mexico, and an 
analysis of other regional Upper Triassic assemblages instead imply that the 
transition was gradual.  Some dinosaur relatives preserved in this Chinle 
assemblage belong to groups previously known only from the Middle and 
lowermost Upper Triassic outside North America.  Thus, the transition may 
have extended for 15 to 20 million years and was probably diachronous at 
different paleolatitudes."

So far, only hindlimb elements have been assigned to _Dromomeron_, and these 
closely resemble those of _Lagerpeton_.  In addition to _Dromemeron_, the 
Hayden Quarry also yielded a "_Silesaurus_-like dinosauriform" that may be 
the same as _Eucoelophysis_ (but so far there's no overlapping diagnostic 
material, which is required to establish that they are the same); a 
coelophysoid; _Chindesaurus bryansmalli_ (basal saurischian); and a host of 
non-dinosauromorph archosaurormorphs.

The accompanying phylogeny shows a clade comprising _Lagerpeton_ and 
_Dromomeron_  as the most basal dinosauromorph clade.  Irmis &c do not 
mention the name Lagerpetonidae, which is available for this new clade 
(Arcucci, 1986).  Higher up (as sister taxon to Dinosauria) is a clade 
comprising _Silesaurus_, _Eucoelophysis_ and the new Hayden Quarry 
"silesaur" (if not the same as _Eucoelophysis_).  Elsewhere, Nesbitt et al. 
(2007) suggested that _Pseudolagoschus_ (=?_Lewisuchus_) may belong to this 
"silesaur" clade.  Pterosaurs (represented by _Eudimorphodon_ and 
_Dimorphodon_) are recovered by Irmis &c as the sister taxon to 
Dinosauromorpha.

The tree in Fig. 3 includes a long ghost lineage for the 
Dinosauria+"silesaur" line after it diverged from basal dinosauriform line 
(_Marasuchus_).  I did wonder if certain poorly known ornithodirans may 
belong on this dinosaur line, such as _Avipes_ or _Saltopus_, but they may 
be too fragmentary or poorly preserved to be sure.

The only quibble I have with the _Dromomeron_ paper is that they call the 
crocodilian stem group "Pseudosuchia".  I much prefer Crocodylotarsi or 
Crurotarsi, rather than having a clade called "Pseudosuchia" that includes 
both Suchia and Crocodilia!

References

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

Arcucci, A. (1986). Nuevos materiales y reinterpretacibn de _Lagerpeton 
chanarensis_ Romer (Thecodontia, Lagerpetonidae nov.) del Triasico medio de 
La Rioja, Argentina.  Ameghiniana 23: 223-242.

Cheers

Tim

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