EASTERN NORTH PACIFIC PALEOGENE VERTEBRATE DIVERSITY: AN OVERVIEW OF THE FOSSIL RECORD AND ITS BIASES

The eastern North Pacific is an active continental margin characterized by accreted terranes and significant movement along long-ranging strike-slip fault systems, which together have produced a geologically complex coastline. Marine fossils of Paleogene age occur along the length of the North American coastline. However, little has been published on vertebrate diversity in the Paleogene North Pacific, and there has been no review of these marine vertebrate faunas to date. Combining data from the literature and previously unpublished collections, Paleogene occurrences were compiled from Baja California to Alaska. These were then updated to current stratigraphic nomenclature and age assignments, and paleolatitudes were estimated based on models developed for specific terranes and faults.

Marine vertebrates are consistently rarer than invertebrates and patchily distributed. Vertebrates are known from fewer than 30% of the nearly 200 fossiliferous marine rock units in all time intervals. Paleocene and Eocene sites are predominantly small assemblages of shark and fish. By contrast, Oligocene faunas are dominated by marine mammals. Recovered taxonomic richness is low and independent of the number of localities or formations sampled. Rather than reflecting a lack of past diversity, low richness appears to result from a lack of collecting efforts in many areas in combination with limited outcrops. Historically, most finds are opportunistic and only a few areas have been systematically collected for vertebrates. Accessible exposure also strongly influenced the patchy distribution of faunas through time and is, in turn, a reflection of the active margin’s reorganization of accreted terranes. In particular, the emergence and accretion of Siletzia and transport of the Yakutat block along the Fairweather-Queen Charlotte fault acted as a significant control on the distribution and subsequent discoverability of Paleocene and Eocene sites.