RECONSTRUCTING EVOLUTIONARY RELATIONSHIPS IN CRASSATELLID BIVALVES ACROSS THE END-CRETACEOUS MASS EXTINCTION IN THE U.S. GULF AND ATLANTIC COASTAL PLAIN

Species of the bivalve family Crassatellidae are abundant and widespread throughout the U.S. Coastal Plain during the late Cretaceous. After the Cretaceous/Paleogene mass extinction, these bivalves experienced a moderate recovery and reradiation by the early Eocene. This research builds on earlier morphometric analyses of this clade to develop a phylogenetic framework for the genera Crassatella and Bathytormus in the U.S. Gulf and Atlantic Coastal Plain. The questions addressed include: (1) what major clades can be differentiated within this family, (2) are the two major genera monophyletic, and (3) what effects did the K/Pg extinction have on phylogenetic patterns?

The ingroup included all nine crassatellid bivalve species that occur in the U.S. Coastal Plain from Campanian to late Eocene. Analyses were performed with two different outgroup hypotheses: (1) Pachythaerus similoides Vokes 1946 (early Aptian, Lebanon) and Pachythaerus vindinnensis d’Orbigny, 1844 (Cenomanian, France) and (2) Crassatella hodgei (Stephenson, 1923)(early Campanian, U.S.). Specimens were obtained from eight museum collections, as well as field sites in Alabama, Mississippi, and Texas; ingroup sample sizes ranged from 12 to 188 per species.

The data matrix includes 64 characters, which describe shell shape, muscle attachments, hinge structure, and ornament; 27 characters are derived from landmark-based linear measurements. Phylogenetic analyses included both maximum parsimony and Bayesian phylogenetic approaches (tip-dating + fossilized birth-death (FBD)). We use stepping-stone analyses to assess a variety of character evolution models, with strongest support given to uncorrelated relaxed clock rates among branches and Gamma variation among discrete characters, and a strict clock Brownian motion model for the continuous characters.

Choice of outgroup affected parsimony-based results. Analyses using Pachythaerus as the outgroup yielded monophyly of both Crassatella and Bathytormus, while analyses using C. hodgei suggested that the genus Crassatella is paraphyletic with Bathytormus embedded into the main Crassatella clade. Moreover, Bayesian analyses suggest that Bathytormus is diphyletic, with a Cretaceous and Paleocene clade independently acquiring general “bathytormid” features from a “Crassatella” form.