THE TURRITELLA NASUTA GROUP OF THE EOCENE GULF AND ATLANTIC COASTAL PLAIN

Turritellid gastropods are some of the most recognizable of all fossil gastropods, and being widespread both geographically and stratigraphically, they are extremely well-suited as index fossils in biostratigraphy. More than 55 turritellid species are known from the Paleogene deposits of the U.S. Gulf and Atlantic coastal plains, among them Turritella nasuta and its four named subspecies: T. n. brazita, T. n. felli, T. n. houstonia, and T. n. smithvillensis from the upper Middle Eocene Claibornian stage of Texas to South Carolina. Similarities in shell geometry and morphology between these forms, including a uniquely straight and cylindrical columella, suggest that they are closely related, but the interrelationships and relationships with other species have not seen much intense study for more than 80 years. Based on a landmark-based geometric morphometric analysis, we conclude that T. n. houstonia, T. n. smithvillensis, and T. n. felli are sufficiently distinct in morphology, in tandem with their temporal and spatial ranges, to be recognized as separate subspecies. T. n. houstonia warrants further study, as the morphological variance between its occurrences in the McBean Formation of South Carolina and lower Claiborne Group of east Texas may be different enough to identify them as separate subspecies. T. n. brazita is also noticeably different from other forms, but more specimens are needed before a definite conclusion can be reached. The distinctive columella of these forms suggests that T. nasuta was a separate turritellid lineage which immigrated to the Gulf Coast in the Middle Eocene from an unknown source, adding to the overall biogeographic complexity of the Paleogene fauna at this time.