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Dromomeron




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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