Using positional homology in aetosaur (Archosauria: Pseudosuchia) osteoderms to evaluate the taxonomic status of Lucasuchus hunti

ABSTRACT

The Otis Chalk quarries in the Upper Triassic Dockum Group of West Texas have produced aetosaur material that most workers have suggested represents two distinct morphotypes. We use characters from aetosaur specimens with articulated or semi-articulated carapaces in which the anteroposterior placement of osteoderms can be established with certainty to compare homologous osteoderms in the Otis Chalk material. This study confirms that the genera Longosuchus and Lucasuchus are distinct morphotypes, which differ in that the former taxon has paramedian osteoderms with random pitted ornamentation and low pyramidal bosses that contact the posterior margin, and spines on the lateral osteoderms that are posteriorly emarginated, whereas the latter taxon has paramedians with a strongly radial ornamentation and large conical eminences, and spines on the lateral osteoderms that are not posteriorly emarginated. Both taxa also have paramedians that are overlapped anteriorly by the laterals, a character that may be a synapomorphy of desmatosuchine aetosaurs. The arguments that these morphotypes represent ontogenetic stages or sexual dimorphs of a single biological species cannot be corroborated using either comparisons with modern pseudosuchians, other aetosaur taxa, or stratigraphic ranges. Longosuchus is known only from the type area and has no utility as an index taxon of the Otischalkian land-vertebrate faunachron, although Lucasuchus suggests a tentative correlation between part of the Dockum Group of Texas and the Pekin Formation of North Carolina.

ACKNOWLEDGMENTS

A portion of this work is derived from an M.S. thesis in Geology by the senior author. We would like to thank Tim Rowe, Lyn Murray, and John Maisano (TMM), and Andrew Heckert (NMMNH) for access to specimens in their care. Sterling Nesbitt provided photographs of the AMNH material. Randall Irmis and Michelle Stocker provided photos of some of the TMM material. Reviews by three anonymous persons, Randall Irmis, and handling editor Christian Sidor improved the manuscript. This is Petrified Forest National Park paleontological contribution no. 18.