UNDERSTANDING ENDEMISM IN THE NORTHEAST PACIFIC: INFERENCES FROM PALEOBIOGEOGRAPHY AND THE EARLY ONTOGENY OF CENOZOIC GASTROPODS

The late Paleogene and early Neogene fossil marine gastropods of the northeast Pacific, from Alaska to southern California, are abundant, widespread, and so distinct that they define West Coast marine biostratigraphic stages. Research and analyses of these faunas reveal patterns of their origination, extinction, and endemism throughout the Cenozoic, particularly across the Eocene/Oligocene boundary. This period marked a profound climatic change from greenhouse to icehouse conditions that initiated at least two well-documented faunal turnovers of tropical to cool-water gastropod taxa.

This study explores the relationship between buccinid gastropod species in these turnover faunas and their mode of development (i.e. either “direct developing” or free-swimming veligers). Protoconch morphology is used to infer developmental mode and is characterized using the criteria of Shuto (1974) and Hansen (1980), Scanning Electron Microscope (SEM) micrographs, and the image analyzing software Image J. Buccinid genera including Bruclarkia, Molopophorus, Eosiphonalia, and Lirabuccinum from the Olcese Sand, Pittsburg Bluff, Sooke, Keasey, and Lincoln Creek Formations are included in the analyses to address: (1) the modes of development in these gastropod clades before and after the Eocene/Oligocene climatic transition; (2) their paleogeographic distribution along the Northeast Pacific coastline during the Eocene, Oligocene, and Miocene; and (3) the evolution of buccinid larval types within lineages through time. For the first time, ontogenetic strategy will be inferred from these fossil shells and used to understand species' dispersal potential, sensitivity to extinction, and evolutionary history from a developmental perspective.