Introducing Dromomeron gregorii a New Lagerpetid Dinosauromorph from the Lower Chinle Formation of Arizona and Dockum Group of Texas

The new issue (June 2009) of the Journal of Vertebrate Paleontology features an article by Sterling Nesbitt, Randy Irmis, and colleagues featuring new data on the Late Triassic North American lagerpetid dinosauromorphs first mentioned by Irmis et al. (2007). This new data includes:

1) A complete decription of the hindlimb material of Dromomeron romeri Irmis et al. 2007.

2) A description of a second species of Dromomeron, D. gregorii.

3) A phylogenetic analysis of basal dinosauromorphs.

4) A detailed discussion of ontogenetic changes in the femur of Dromomeron, which has implications for the phylogeny of early dinosaurs. Specifically that a well-developed anterior trochanter and trochanteric shelf is developed from a low rugosity as femoral size increases. Thus the lack of a trochanteric shelf in some dinosauromorph specimens (e.g., D. romeri and Lagerpeton) may simply indicate immaturity of the specimen rather than a taxonomic or phylogenetic distinction.

Note that Lagerpetidae is formally diagnosed in this paper as a converted clade name to include both species of Dromomeron and Lagerpeton chanarensis.

Also note that lagerpetids now are know from the basal Chinle Formation of Arizona, the upper Chinle of west-central New Mexico, and the lower and middle Dockum Group of Texas. These animals were much more diverse and widespread than previously believed. However, I'm still waiting to recover one in the Petrified Forest. They should be present along with our "silesaurid", Chindesaurus bryansmalli, and coelophysoid theropods.

Here is the abstract from the new paper:

Nesbitt, S.J., Irmis, R.B., Parker, W.G., Smith, N.D., Turner, A.H., and T. Rowe. 2009. Hindlimb osteology and description of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29:498-516.

ABSTRACT—The recent discovery of early dinosauromorphs from North America demonstrates that they were contemporaries with dinosaurs and other basal archosaurs during a substantial portion of the Late Triassic Period. Hindlimb material (femora, tibiae, a fibula, astragalocalcanea, and phalanges) of Dromomeron romeri, a non-dinosauriform dinosauromorph from the Petrified Forest Member of the Chinle Formation from north-central New Mexico, is described. A new species of Dromomeron from the lower portion of the Chinle Formation (eastern Arizona) and Dockum Group (northern Texas) is also described, based on several disarticulated femora and tibiae. D. romeri, Lagerpeton, and the new taxon form the sister group to all other dinosauromorphs and demonstrate that this clade, Lagerpetidae, persisted well into the Norian. Lagerpetidae is supported by several synapomorphies: femoral head hook-shaped in medial and lateral views; ventral emargination on the anterolateral side of the femoral head; an enlarged posteromedial tuber of the proximal end of the femur; femoral crista tibiofibularis larger than the medial condyle; anteromedial corner of the distal end of the femur forms 90 degree or acute (>90 degree) angle; and a posterior ascending process of the astragalus. An ontogenetic series of the femur of Dromomeron indicates that some character states previously used in phylogenetic analyses of early dinosaurs may be ontogenetically variable.

REFERENCES

Irmis, R. B., Nesbitt, S.J., Padian, K., Smith, N.D., Turner, A.H., Woody, D., and A. Downs. 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science
317:358–361.