THE XOCHIXTLAPILCO ICHNOFAUNA, A MIDDLE JURASSIC DINOSAUR TRACK ASSEMBLAGE FROM THE MIXTECA TERRANE, SOUTHEASTERN MEXICO: ITS GEOLOGIC AND PALEONTOLOGIC SIGNIFICANCE

Red phyllarenite strata of the Bajocian-Bathonian Tecocoyunca Group partim located 6.3 km south of Huajuapan de León, Oaxaca have yielded the southernmost North American dinosaur record. Reevaluation of this ichnofauna discloses that it consists of footprints whose track makers are referred to small (ornithomimosaur) and moderately large (allosaurid) theropods, as well as to two small herbivorous forms, an undescribed sauropod taxon (at least of family rank), and a hypsolophontid ornithopod.

The track makers of this Middle Jurassic assemblage show both peculiar features and overall affinities with taxa of comparable age recorded in South America, Africa, Australia and Western Europe; it sampled a tropical community, perhaps set in a somewhat restricted or isolated setting, where limited space and resources might have induced selective pressures toward small size of the faunal components (particularly of the primary consumers); such setting also shielded the fauna from competition and exchange with continental faunas, thus promoting its identity, and accounting (in part at least) for the small size of the herbivores.

Recent models on the Mesozoic geologic and tectonic evolution of the Southeastern Mexico-Middle America region, portray the Mixteca Terrane during the Jurassic, as one of the several small continental-crust blocks set in the widening space between North America, Africa and South America, as Pangea became disassembled; further, there are not sufficient data to constraint the sea/land boundary of any particular block during the Jurassic and most of the Cretaceous. The paleogeographic island scenarios derived from such models are consistent with the proposed setting of the dinosaur assemblage, thus lending support to this hypothesis.