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.
Eleven ingroup species within Crassatellidae and three 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 Crassatellidae is paraphyletic. Bathytormus pteropsis, a species within Bathytormus, clusters closely with Scambula perplana, a species under Crassatella, and is found within other species of Crassatella. Three out of the four Bathytormus species, Bathytormus alaeformis, Bathytormus protextus, and Bathytormus clarkensis, group closely together, indicating common ancestry. 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.
Guardado, Rosemary, "The effects of the end-Cretaceous mass extinction on phylogenetic relationships in Crassatellidae (Mollusca, Bivalvia) from the U.S. Gulf Coastal Plain" (2021). Geology Senior Theses. Paper 7.