RANGES OF VARIABILITY IN MARINE MOLLUSKS FROM “GREENHOUSE” AND “ICEHOUSE” CLIMATIC REGIMES: PLIO-PLEISTOCENE OF FLORIDA VS. LATE CRETACEOUS OF THE U.S. WESTERN INTERIOR SEAWAY AND GULF COASTAL PLAIN

Understanding connections between physical environment and evolutionary pattern involves documenting the relationships between evolutionary tempo, phenotypic variability, and environmental variability. Different theories, however, make different predictions about these relationships, and various studies have produced a range of results. We compare evolutionary tempo and mode in marine mollusks from the Late Cretaceous representative of a “greenhouse” climate regime, which was climatically relatively stable (i.e., less frequently/rapidly changing) and the Late Neogene representative of a climatically less stable “icehouse” climate regime. We examined species of the bivalve family Cardiidae and the gastropod family Turritellidae from the Late Cretaceous of the North American Western Interior Seaway (WIS) and Gulf Coastal Plain, and the Plio-Pleistocene of southern Florida. We studied two species of turritellids from the Cretaceous (Turritella kansasensis, T. codellana) and two from the Neogene (T. perattenuata,T. wagneriana), and four cardiids from the Cretaceous (Cardium speciosum, Pleuriocardium pauperculum, Trachycardium eufaulensis, Granocardium kummeli) and two from the Neogene (Tr. egmontianum, Tr. emmonsi)). Bivalve morphology was quantified by linear measurements and eigenshape analysis; gastropods by landmarks of spiral sculpture converted to linear measurements.

Our results are mixed: completed shape and size data for bivalves shows that the Florida Neogene species are significantly more variable than any of the four Cretaceous species, whereas preliminary data on the turritellids indicate that species from the two different climate regimes show roughly similar ranges of morphological variation, with the Neogene species showing slightly narrower ranges. These results suggest that the relationship between frequency and/or rate of climate change and evolutionary pattern is complex and deserving of further analysis.