DEPOSITIONAL-ENVIRONMENTS OF A MIDDLE JURASSIC TERRESTRIAL VERTEBRATEASSEMBLAGE, HUIZACHAL CANYON, MEXICO

A large assemblage of vertebrate fossils from the La Boca Formation in Huizachal Canyon, near Ciudad Victoria, Tamaulipas, Mexico, is preser ved in a unique setting: debris-flow sediments deposited over a landsc ape with at least modest topographic relief. Angular feldspar lathes i n the rock suggest that deposition was associated with penecontemporan eous volcanism. The similar to 8,000-specimen collection from Huizacha l Canyon includes the tritylodontid Bocatherium mexicanum, the burrowi ng diapsid Tamaulipasaurus morenoi, three types of mammals, two crocod yliforms, two types of dinosaurs, a pterosaur, and three sphenodontian s. The fossils generally occur as small (similar to 5 mm), isolated bo nes or bone fragments, but 35 articulated or associated specimens have been found. No turtles or fishes have been identified, implying a ter restrial source for this assemblage. Biostratigraphic and preliminary radiometric evidence suggest an early Middle Jurassic age, making this assemblage the only substantial terrestrial Middle Jurassic vertebrat e fauna known from North America.