UPPER CRETACEOUS (TURONIAN) CRUSTACEA FROM THE AUSTIN GROUP, NORTHEASTERN MEXICO: EVIDENCE FOR A PELAGIC ASSEMBLAGE

Four species represent the crustacean component of the fauna on the lithographic limestone that crop out at El Rosario quarry, Coahuila, northeastern Mexico. Planktic foraminifers, ammonoids, inoceramid bivalves, fishes, marine reptiles, a pterosaur and plant remains were preserved in a Turonian-Coniacian sequence, equivalent to the Austin Group. The paleoenvironment has been interpreted as an anoxic, low energy bottom at a water depth of at least 50 m. The only benthic elements of this assemblage are inoceramid bivalves, which cold tolerate such adverse anoxic conditions. The other elements include only pelagic forms and a pterosaur. In this type of environment, the relative abundance of decapod crustaceans and cirripedians seems to be unusual. Clusters of the cirripedian Stramentum are found attached to ammonite shells in discrete patches. Located distribution of these cirripedians on the flattened amonite shells suggest either that Stramentum lived attached while the amonte was still alive, or that they used the shell as as substrate once the ammonite died and was deposited in the sea bottom, which seems improbable due to the interpreted anoxic conditions. The most abundant crustacean is a calappid species, previously reported as a carcineretid from lower Turonian deposit of Colombia and Germany. Although carapace is not clear in any specimen, certain featues suggest a calappid affinity, but the most interesting detail is the flattened shape of the last two pairs of pereiopods. Chelae are preserved in most specimens; they are spinose and sharp, suggesting a possible carnivorous diet. A second species is represented by a new genus and species of homolid crab. Two pairs of pereiopods are extremely flattened and wide, while one pair has an unciform dactylus. No clear shape of chelae is observed in the scarce but nearly complete specimens. These two species may have lived associated to a kind of floating algae, with pereiopods adapted for swimming and hold to algal masses, such as portunid and majid crabs do nowdays on sargassum communities. Complete and articulated callianassid chelae are the fourth crustacean element. Other pereiopods and remains of cephalthorax were preserved in most specimens. Presence of these benthic crustaceans is explained based on tolerance to anoxic conditions.