A NEW PHYLOGENY OF EOCENE ATHLETA (GASTROPODA:VOLUTIDAE) FROM THE US GULF COASTAL PLAIN AND A NOTE ON GENUS LEVEL TRAITS

Gastropods of the genus Athleta Conrad, 1853 are common and well preserved in Eocene deposits of the Gulf Coastal Plain. The status of the genus Athleta is muddled by the lack of a holotype and the fact that later designated types were based solely upon the presence of an extra parietal callus, a trait which appears numerous times within the Athletinae and was suggested to be the result of an ambiguous “unfavorable environment” on gerontic forms. However, SEM photographs of protoconchs and early teleoconch characters of Athleta from the Gulf Coast and of Athleta and Volutospina from western Europe corroborate a separation of the two genera based on the development of the early whorl’s posterior-most spiral ridge; Volutospina maintain tabulate or concave shoulders that make it possible for the subsutural ridge to have spines as an adult.

In 1964, Fisher et al. documented directional evolution within a clade they referred to as the “Athleta petrosa stock”. Four species were described (A. petrosa, A. lisbonensis, A. tuomeyi, and A. dalli) and depicted in a phylogenetic tree. Fisher et al. named Volutocorbis limopsis as the presumed (but unlikely) ancestor since it was the only Athletinae present regionally when A. petrosa originated. Species discriminant analysis reveals the lineage A. petrosa to consist of two subspecies with morphological differences sufficient enough to warrant elevation to species status (A. petrosa and A. symmetrica). Their phylogeny is further re-evaluated by using new quantitative methods of tree construction, and expanded here with the addition of 7 additional co-occurring Athleta species of the Gulf Coast. Morphological data consists of a total evidence matrix of 35 discrete and 10 continuous characters. Unsurprisingly, the new tree is markedly different from the original of 1964 and surprisingly, the prevalence of stasis is striking.