KILLER SPERM WHALES: EXPLORING THE LIFE HISTORY AND ECOLOGY OF NEOGENE PHYSETERIDS FROM THE ATLANTIC COASTAL PLAIN

The Family Physteridae (sperm whales) attained great diversity during the Miocene and Pliocene. Fossil specimens suggest the occurrence of at least 31 distinct species. Within the past decade, several studies have attempted to reconstruct the ecology and phylogeny of these early physeterid taxa, an endeavor made difficult by the scarcity and often poor preservational condition of remains. The Neogene phosphatic sands of the US Atlantic Coastal Plain (ACP), in particular at the Lee Creek Mine, North Carolina, lay claim to one of the largest early physeterid fossil assemblages in the world. Physeterid remains, especially teeth, are common in these strata along with a host of marine vertebrates ranging from turtles to crocodilians. Teeth are known for their archival properties, here we examine the teeth of two physeterid genera, Physeterula and Scalidcetus, to constrain the respective life histories and ecologies of these taxa.

Rhythmically accreted growth structures (‘growth layer groups’), analogous to tree rings, are visible in longitudinal cross sections of the teeth. As physeterid teeth grow continuously over ontogeny, these structures can be used to ascertain the age of the animal at death. Tallies of growth increments from multiple individuals reveal that life spans of both Physeterula and Scalidcetus were significantly shorter than the 65+ years noted in modern sperm whales. A relationship between tooth diameter and body size, as established in modern odontocetes, is used to approximate the body size of these early physeterids. Rather than attaining the large body sizes observed in the modern sperm whale, Physeter macrocephalus, these early physterids were comparable in size to modern killer whales, Orcinus orca. Functional upper and lower jaw dentition, not present in the modern taxon, is indicated by wear facets on teeth, suggesting an ecological analog with Orcinus as well. Despite similarities in ecology and body size, life spans of the Lee Creek whales are markedly shorter than those of Orcinus orca. To attain body sizes comparable to those of modern Orcinus in half the time requires rapid growth conceivably facilitated by an abundance of food. The highly productive waters of the ACP at this time likely supplied the resources necessary to sustain the high growth rates and metabolic demands of this top predator.