PALEOCENE CRUSTACEA FROM COAHUILA, NORTHEASTERN MEXICO; STRATIGRAPHIC AND PALEOBIOGEOGRAPHIC IMPLICATIONS

Paleocene decapod crustaceans are scarce and important to understanding evolution and distribution of recent and extinct taxa. This is the first report of Paleocene decapods from Mexico. Specimens of the genera Enoploclytia, Linuparus, Symethis, Paraverrucoides, Tehuacana, and Viapinnixa have recently been collected from the Paleocene Rancho Nuevo Formation, the youngest unit of the Parras Basin in Coahuila, northeastern Mexico. Additionally, a palm of a callianassiid and a pair of small chelae belonging to Paguristes sp. has also been collected. A pair of large chelae (40 cm in length) and two incomplete carapaces representing Enoploclytia sp. cf. E. tumimanus Rathbun if confirmed will represent one of the few cases of a crustacean species that survived the K/T extinctions, as that species was previously reported from the uppermost Maastrichtian beds of the Escondido Formation, Coahuila. Occurrence of Enoploclytia within Paleocene strata extend the stratigraphic range from Upper Cretaceous to Lower Tertiary. One small carapace of Linuparus sp. cf. L. wilcoxensis Rathbun is significant, as this same genus is found within upper Maastrichtian beds of the subjacent Las Encinas Formation, Parras Basin. The raninid Symethis sp. cf. S. johnsoni Rathbun is represented by a single carapace, considerably bigger than specimens reported from the Paleocene of Alabama. Paraverrucoides alabamensis (Rathbun) and Tehuacana tehuacana Stenzel are also present. These two species were previously reported from the Paleocene of Alabama and Texas, respectively. A well-preserved ventral portion of P. alabamensis will add unknown morphologic details to this species and extend their paleobiogeographic distribution. Juvenile and adult specimens of Viapinnixa sp. expand the paleobiogeographic range of this genus, as the other known Paleocene occurrence is from the middle Paleocene of Greenland. This occurrence confirms southward migration of the genus, as the only other species is found within Lower Eocene deposits of Chiapas, southern Mexico. Presence of the nautiloid Cimomia haltomi Aldrich within outcrops of the Rancho Nuevo Formation, suggests a middle Paleocene age for this crustacean assemblage. Bivalves, nautiloids and serpulid annelids have been previously reported for this unit.