From Greenhouse to Icehouse: The Marine Eocene-Oligocene Transition

By Donald R. Prothero; Linda C. Ivany et al. | Go to book overview

CHAPTER 2
Turnovers in Marine Gastropod Faunas
During the Eocene-Oligocene Transition,
West Coast of the United States

Richard L. Squires


ABSTRACT

A detailed inventory of the genera and subgenera of marine gastropods known from Paleogene rocks in Washington, Oregon, and California, in combination with chronostratigraphic control provided by recent paleomagnetic studies of key formations, revealed a loss of many warm-water taxa and the addition of new cooler water taxa associated with global cooling during the Eocene-Oligocene transition (about 40–30 Ma). Only full-stage ranges are known for most of the taxa, thereby giving the appearance that this overall change in the fauna took place as three turnover events. More detailed biostratigraphies are needed to prove if the three turnovers (especially the last two) are truly coincident with stage boundaries, or if there was just one gradual turnover (about 12 million years-long).

From Paleocene through middle Eocene time, nearly all the marine gastropods on the West Coast of the United States and Baja California, Mexico, were shallow-water, warm-water forms. Faunal diversity reached its highest (143 genera/subgenera) for the West Coast Paleogene during the early Eocene. At the beginning of the late Eocene, about 36.5 Ma, the first turnover took place. Forty-eight percent of the previous taxa disappeared, but many warm-water taxa survived. There was also a major influx of coolerwater taxa, and it was the largest such influx for the region during the entire Paleogene. The first pteropod gastropods also appeared on the West Coast at this time, and bathyal, chemosynthetic cold-seep gastropods became more common, especially in Washington.

About 33 Ma and 28.5 Ma, second and third turnovers took place. Like the first one, warm-waterholdover taxa survived, and new arrivals of coolerwater taxa took place. Unlike the first turnover, however, faunal diversity was much lower, and the percentage of new arrivals was much lower. Following the third turnover, there was a record low number (40) of gastropod genera/subgenera for the West Coast Paleogene rock record.

Certain gastropods (e.g., Calyptraea, Crepidula, Euspira, Glossaulax, Neverita, Sinum, Turritella) survived all three turnovers and the arrival of cooler waters. Several of these were solely represented by long-lived species that persisted through most of the Paleogene.


INTRODUCTION

During the Eocene-Oligocene transition (about 40–30 Ma), a global lowering of atmospheric and oceanic temperatures initiated the oceanic psychrosphere and the Antarctic glaciation (Berggren et al., 1998). This interval was the most significant episode of climatic change and extinction since the end of the Cretaceous, and warm-adapted species were the primary victims (Berggren and Prothero, 1992). Indepth paleoclimatic studies have been made on Gulf Coast Eocene-Oligocene mollusks (Dockery, 1986; Hansen, 1987, 1992), but similar detailed studies on Pacific Coast Eocene-Oligocene mollusks have never been attempted. Late Paleogene marine gastropods from California, Oregon, and Washington have much potential for studying the changes in paleoclimate during the Eocene-Oligocene transition because the faunas are common to abundant, taxonomically diverse, span about 16° of latitudinal difference, and

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