THE EFFECTS OF THE END-CRETACEOUS MASS EXTINCTION ON PHYLOGENETIC RELATIONSHIPS IN CRASSATELLIDAE (MOLLUSCA, BIVALVIA) FROM THE U.S. GULF COASTAL PLAIN

Many studies have documented the effects of the Cretaceous/Paleogene (K/Pg) extinction on shallow marine invertebrate assemblages, but few have assessed the evolutionary consequences within a high-resolution stratigraphic and phylogenetic framework. During the Cretaceous, crassatellid bivalves from the U.S. Gulf Coastal Plain radiated rapidly. However, out of the seven genera recorded prior to the K/Pg boundary, only three survived the mass extinction, making the Crassatellidae a useful lineage in which to assess the effects of the K/Pg mass extinction on evolutionary relationships. The primary objectives of this study were to reconstruct a phylogeny of K/Pg Crassatellidae from the U.S. Gulf Coastal Plain, identify major clades within the family and examine the effects of extinction on phylogenetic patterns.

Ten ingroup species within Crassatellidae and two outgroup species within Astartidae were selected for phylogenetic analysis. An average of seven specimens per species, accessed from seven museum collections, were examined to sample morphological variation within species. Within each species, large specimens were selected to focus primarily on adult characters. Fifty-four morphological characters, including aspects of the hinge, external ornament, and outlines of the posterior, anterior and ventral margins, were used to describe morphological differences among the species. A maximum parsimony approach, with a branch and bound search and 1000 bootstrap replicates was used.

Preliminary results indicate that two ingroup species, Scambula perplana and Bathytormus pteropsis, cluster closely with the outgroup species, Lirodiscus tellinoides and Astarte sulcata. Within the ingroup, Crassatella tumidula may be more closely related to three Bathytormus species, B. alaeformis, B. clarkensis, and B. protextus than Crassatella s.s. However, further research must be done to verify this result.