HIGH DIVERSITY OF FRESHWATER GASTROPODS FROM THE MAASTRICHTIAN CERRO DEL PUEBLO FORMATION, COAHUILA; ENVIRONMENTAL AND STRATIGRAPHIC IMPLICATIONS

Freshwater gastropods are relatively rare in the fossil record. The high diversity of freshwater gastropods found at the basal portion of the Cerro del Pueblo Formation, brings the opportunity to describe new forms and to compare them with other occurrences around the world. Preliminar examination of our material has yielded ten species, and some bivalves under study. Similitude has been found with previously described assemblages of freshwater molluscs from the Upper Cretaceous of Colorado, Nuevo Mexico, and Sonora. Some specimens found in Coahuila are the biggest ever reported for the genera to which they belong. Preservation as hematized remains and their occurrence in fine mudstone allowed observation of fine sculpture details of the shell. The gastropods were found in discrete layers of green and red mudstone, with the highest diversity found in the green mudstone. Red beds contain a single species of a big viviparid gastropod. Species preserved as hematized remains are also diverse. Bivalves are rarely found here, but seem to be common at the green mudstone beds. Fruits, bone fragments, coprolites and tortoise remains are found associated with the gastropods. Interbedded thin sandstone units contain the Maastrichtian ammonite Sphenodiscus pleurisepta. Paleoenvironment is interpreted to be interdistributary marshes, swamps and ponds, developed in a delta plain during a sea level fall episode. The reason why these gastropods are rare in the red beds remain unclear, but future studies of their distribution, systematics and biology of recent relatives would help to understand further ecologic significance.