Latitudinal gradients in tertiary molluscan faunas of the Pacific coast

Tertiary molluscan faunas of the middle latitudes of the marginal eastern North Pacific are characterized by warm-water taxa whose descendants now live in more southerly latitudes. A series of profiles in which cumulative percentages of warm-water faunal elements are plotted against latitude show progressive northward decreases in the percentage of these elements in the faunas of Pacific coast Tertiary stages. Systematic changes in the relative position of these latitudinal gradients during the Middle and Late Tertiary are related to climatic change in the Pacific Basin.

Widespread tropical marine climate in the middle latitudes of the eastern North Pacific during the Eocene is indicated by widespread faunal units characterized by high levels of taxonomic diversity. Succeeding Early Oligocene faunas are less diverse, suggesting cooler climatic conditions. Unusually low representations of warm-water genera characterize the molluscan faunas of the Acila shumardi Zone in central California (latitude 34°–37°N). The anomalously cool-water aspect of these faunas may record the occurrence of upwelling along a bold linear segment of the Pacific coast. During the Late Oligocene or the Early Miocene, they are replaced by faunas of unusually warm-water aspect resulting in positive anomalies in Miocene latitudinal faunal gradients in central California. The Miocene anomalies seem to result from the development of an irregular Neogene coastline with extensive, newly established shallow-water embayments.