Middle Jurassic ankylosaur tracks from Mexico

Abstract

A track-bearing locality in one of the shores of the ñumí River, near the City of Tlaxiaco, northwestern Oaxaca, southern Mexico, preserves the footprints of sauropod and theropod dinosaurs; it is located within the stratigraphic sequence of the Zorrillo-Taberna Indiferenciadas Formation, of Middle Jurassic (Bajocian) age. A well-preserved manus-pes set of ankylosaurian affinity (Tetrapodosaurus) is preserved on a rock block from this formation. These tracks are preserved as convex hyporeliefs. The pes impression is tetradactyl, with ventrally curved toe tips, suggesting that these were deeply impressed in the sediment. Manus impression is pentadactyl, with short and blunt digits. A curved structure preserved together with these footprints is herein interpreted to be a tail drag. The stratigraphic unit from whence this block came was identified and two additional ankylosaurian tracks, were found preserved in situ as natural sandstone casts. An isolated handprint, has the same features that fit the ankylosaurian hand morphology; however, some features such as a medio-lateral concavity are indicative of the presence of a well-developed palmar pad, and could suggest the functional mechanics of the ankylosaurian manus. The Zorrillo-Taberna Indiferenciadas Formation is a coal-bearing formation; after the global record, ankylosaur footprints are concentrated in coal-bearing and floodplain facies, thus suggesting, that at least the North American Ankylosauria were adapted to similar paleoecological conditions from Middle Jurassic to Early Cretaceous times. It represents the second report of ankylosaurian tracks from Mexico and the southernmost record of the ichnogenus Tetrapodosaurus in North America. In this way, it suggests a geographic continuum in the record of the Ankylosauria to southern North America during Middle Jurassic times. With their Middle Jurassic age, these footprints represent the oldest ankylosaurian ichnofossils known to date.