Paleoclimate archives and evolutionary patterns preserved in neogene scallop shells: Implications for linking climate change, paleobiogeography, and morphologic evolution of Chesapecten along the Atlantic Coastal Plain, USA

Models of future climate predict a 2-4.5 degree Celsius increase in global temperature by the 21st century with uncertain consequences on evolution, ecology, and geographic distribution of organisms. This work combines geochronology, paleoclimatology, and paleoecology/evolution to assess variation in geographic patterns of global warming in the recent geologic past and its effects on the evolution, ecology, and biogeography of Chesapecten. Neogene sediments from the US Atlantic Coastal Plain yield deposits that mark the last two episodes of globally warm climates (i.e., Mid--Miocene Climate Optimum (MMCO) and the Middle Pliocene Warm Interval (MPWI)) and serve as analogues of future warming. The following studies employ shells of Chesapecten to reconstruct morphologic evolution and seasonality.;The genus Chesapecten is a morphologically complex group comprising seven nominal species in the Miocene and Pliocene of the U.S. Atlantic Coastal Plain. Species of Chesapecten were originally defined as a morpho--stratigraphic species (i.e., characterized by morphology and by gaps or hiatuses in deposition). The first chapter uses multivariate and descriptive statistics to: (1) determine the number and nature of true morphotaxa of Chesapecten (i.e., species defined solely by morphology), (2) nomenclatorially reconcile these morphotaxa with previously named nominal species, and (3) evaluate the biostratigraphic significance of the morphotaxa. This study demonstrates the existence of one novel morphogenus and two morphosubgenera of Chesapecten and emends the traditional systematics of Chesapecten. Morpho--stratigraphic species are distinguishable only when a species by stratum analysis is conducted. In the second chapter, seasonal temperatures are reconstructed along a meridional (∼27-37 degree N) and biogeographic gradient during the MMCO and MPWI to test whether global warming shifts biogeographic provenances northward. This study reports diminished seasonality and expansion of the tropical biogeographic province to mid latitudes during the MMCO and increased seasonality and expansion of the warm--temperature province in mid--latitudes during the MPWI. The third chapter evaluates whether mid--latitude MPWI shells form annual growth increments. MPWI shells from the warm--temperate province form winter growth lines contrary to modern bivalves. This dissertation provides a glimpse into Neogene seasonality during warm climates and is pivotal to understanding ecological and evolutionary responses to future warming.