Late Triassic tanystropheids (Reptilia, Archosauromorpha) from northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae

Abstract

We report on tanystropheids from the Late Triassic (middle Norian) Hayden Quarry of northern New Mexico (Chinle Formation, Hayden Quarry). These elements, consisting of isolated vertebrae and appendicular bones, represent the first unambiguously identified tanystropheid from western North America and likely the latest occurrence of the group, postdating Tanytrachelos in the eastern United States. A new phylogenetic analysis of early saurians identifies synapomorphies of tanystropheid subclades, which are recognized in the recovered vertebrae and a calcaneum. The femora are consistent with referral to Tanystropheidae. There is no clear association of the remains, however, so we refrain from erecting a new taxon. The analysis also indicates that the Hayden Quarry tanystropheid fossils belong to a newly recognized clade including the Late Triassic taxa Langobardisaurus and Tanytrachelos. Because most tanystropheid specimens are two-dimensionally crushed skeletons, the Hayden Quarry tanystropheid fossils provide valuable insights into the three-dimensional osteology of derived tanystropheids. The most striking feature of the Hayden vertebrae is a rugose, flattened expansion of the neural spines in the dorsal, sacral, and caudal regions, probably linked to a ligamentous bracing system. These fossils and others from Late Triassic sites in the American West suggest that tanystropheids underwent a previously unrecognized radiation in North America just prior to their extinction.