Lissamphibians and Squamates from the Upper Cretaceous (upper Campanian) El Gallo formation of Baja California, Mexico

CHAVARRÍA-ARELLANO, María Luisa; Universidad Nacional Autónoma de México, Facultad de Ciencias, Departamento de Biología Evolutiva, Ciudad de México, CDMX, México. MONTELLANO-BALLESTEROS, Marisol; Universidad Nacional Autónoma de México, Instituto de Geología, Ciudad de México, CDMX, México. GARDNER, James D.; Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada.

Abstract 

The El Gallo formation in Baja California, México, contains a moderately diverse, but still not well understood, non-marine assemblage of plants, invertebrates, and vertebrates dating from the Late Cretaceous (late Campanian and possibly “Edmontonian” equivalent).  Historic collections made by the Los Angeles County Museum in the mid-1960s to mid-1970s and newer collections made by staff of the Universidad Nacional Autónoma de México and University of Washington from 2004 onwards, from multiple localities in the El Gallo formation, have yielded isolated, associated, and rare articulated lissamphibian and squamate bones.  The lissamphibian component consists of the typical Late Cretaceous North American trio of anurans, caudates, and albanerpetontids.  Bones currently available for the last two groups are uncommon and cannot be identified to lower taxonomic levels.  Anurans are represented by larger numbers of skull and postcranial bones, with maxillae indicating the presence of two and, possibly, three taxa.  So far the best documented squamate is the borioteiioid Dicothodon bajaensis, which is known by over a dozen jaws of different-sized individuals and at different ontogenetic stages, plus isolated teeth, skull fragments, vertebrae, and post-cranial material.  Other squamates are represented by rare vertebrae of the primitive snake Coniophis and by larger numbers of vertebrae, jaws, and other skull elements pertaining to at least six additional taxa of lizards: Anguidae, Contogenys, Exostinus, Odaxosaurus, Varanoidea, and probable Chamopsiidae.  The lissamphibian and squamate assemblage from the El Gallo formation helps fill a poorly sampled temporal interval for those groups in the North American record and its location in the southwestern portion of the continent makes it biographically interesting.  Patterns of diversities and occurrences evident within the assemblage promise to refine our understanding of the evolutionary histories of lissamphibians, snakes, and lizards in western North America during the latter part of the Cretaceous.