PHYLOGENETIC RELATIONSHIPS IN CRASSATELLID BIVALVES ACROSS THE CRETACEOUS-PALEOGENE MASS EXTINCTION (U.S. COASTAL PLAIN)

Although several studies have assessed the effects of the K/Pg mass extinction on shallow marine ecosystems, few datasets explicitly focus on evolutionary impacts using a highly resolved phylogenetic and stratigraphic framework. The bivalve family Crassatellidae is abundant and taxonomically diverse in U.S. Coastal Plain assemblages immediately below the K/Pg boundary. Approximately 40% of crassatellid bivalve genera survived the mass extinction, making it possible to explicitly compare macroevolutionary trends in victims and survivors. The goals of this research were to: (1) reconstruct a phylogeny of Cretaceous and Paleogene crassatellids from the U.S Atlantic and Gulf Coastal Plain, (2) differentiate major clades within the family, and (3) examine the effects of extinction on phylogenetic patterns.

Twelve species, including ten crassatellids (ingroup) and two astartids (outgroup), were selected for phylogenetic analysis. Fifty-four characters and 14 digitized landmarks, describing shell outline, hinge structure, muscle attachments, and ornament, were used to characterize shell morphology. Specimens were sourced from seven different museum collections and sample sizes for each species ranged from three to thirteen. Data on chronostratigraphic occurrences were compiled from the Paleobiology Database, as well as taxonomically revised museum collections, to provide initial estimates of extinction, origination, sampling, and preservation. Computational phylogenetic analyses included both maximum parsimony and Bayesian phylogenetic approaches (tip-dating + fossilized birth-death (FBD)).

The parsimony-based tree indicates that the genus Crassatella is paraphyletic due to the inclusion of multiple genera, Bathytormus and Scambula into the main Crassatella clade. Species such as Crassatella vadosa, Crassatella hodgei, and Bathytormus pteropsis, terminate at the K/Pg boundary and most likely represent the ancestors of the local Paleocene species. Preliminary tip-dating+FBD analyses suggest elevated extinction in the Maastrichtian and a possible rebound pulse in the early Cenozoic. These analyses allowed for heterogeneous rates of change both among characters and across branches and also allowed for variable diversification rates over time.