NEOGENE SPERM WHALES ON THE ATLANTIC COASTAL PLAIN: CONSTRAINING LIFE HISTORY AND ECOLOGY FROM TEETH

The Family Physeteridae (sperm whales) attained its greatest diversity during the Miocene and early Pliocene. Fossils from Neogene strata worldwide suggest the presence of at least 16 distinct species. Little is known of the life history and ecology of these early physeterids, and studies relying on anatomical clues to constrain these parameters, are often limited by poor specimen preservation . We believe that teeth can be a vital tool in efforts to this end. We examine a set of Neogene sperm whale teeth recovered from the Lee Creek Mine, Aurora County, North Carolina. Here and at other locations along the Atlantic Coastal Plain, a succession of late Paleogene and Neogene phosphatic sands contain a rich marine vertebrate fauna, including a diversity of whales. Modern sperm whale size and dietary preferences preclude life on a continental shelf, so the occurrence of fossil relatives in a coastal setting is unexpected and begs further analysis.

Rhythmically accreted growth structures (growth layer groups, GLG’s) are visible in longitudinal cross sections of physeterid teeth. As sperm whale teeth grow continuously over ontogeny, these structures, analogous to tree rings, can be used to ascertain the age of the whale at death. We tallied GLG’s in a suite of teeth from the Lee Creek Mine and find that the average life span of these ancient whales was 13.2 years, significantly lower than the 60+ year life spans noted in modern sperm whales. Because the largest of these teeth attained sizes similar to those of living whales, it is unlikely that our fossils simply came from juveniles. Shorter life span and faster growth therefore seem to characterize this ancient population relative to modern whales. Provided that the teeth have not been altered diagenetically, tooth chemistry and its variability over ontogeny can offer additional insight into their life histories. In modern whales, δ¹³C values tend to decrease with age, related to migration to high latitude feeding grounds. We find that δ¹³C trajectories in these fossil whales are flat over ontogeny, indicating lifetime subsistence on the coastal plain. These data suggest that the coincident high productivity conditions, indicated by prevalence of phosphates, allowed for the evolution of sperm whale populations fundamentally distinct from the exclusively oceanic taxon of today.