Silesaurid (Archosauria: Dinosauriformes) remains from the base of the Dockum Group (Late Triassic: Otischalkian) of Texas provide new insights to the North American record of dinosauriforms.

Abstract

Silesaurids (Archosauria: Dinosauriformes) are found in Middle to Upper Triassic deposits across Pangea, but few stratigraphic sections record the evolution of the group in one geographic area over millions of years. Here, we describe silesaurid remains from the oldest of the Upper Triassic stratigraphic sequence from the base of the Dockum Group, from the type locality of the Otischalkian faunachronozone. Isolated limb bones diagnostic of silesaurids include humeri, femora, and tibiae of a seemingly unique Silesaurus-like taxon from the same locality (Otis Chalk Quarry 3). The femora consist of four specimens of different lengths that sample the variation of character states associated with ontogeny, also sampled previously in both silesaurids (e.g., Asilisaurus kongwe and Silesaurus opolensis) and within neotheropods within Dinosauria (e.g., Coelophysis bauri). Our observations of the variation in the silesaurid sample further reinforce the interpretation of high variation of morphological features common in dinosauriforms. Furthermore, we show that overpreparation of bone surfaces has hidden some of this variation in previous interpretations. The tibia growth series shows that the fibular crest of the tibia develops during ontogeny, yet another phylogenetically informative character for dinosaurs and their kin that is at least ontogenetically variable in silesaurids. The presence of silesaurids at the base of the Dockum Group (late Carnian or early Norian) conclusively shows that the group was present near the onset of deposition of Upper Triassic rocks and survived for millions of years in the same geographic area at low latitudes throughout the Late Triassic.