PALEOCLIMATIC IMPLICATIONS OF THE CHRONOSTRATIGRAPHIC DISTRIBUTION OF CRETACEOUS TO RECENT NERITID AND CYPRAEOIDEAN MARINE GASTROPODS OF THE PACIFIC SLOPE OF NORTH AMERICA

Neritid and cypraeoidean (cypraeid and some ovulid) gastropods are highly diagnostic of warm-water, nearshore marine conditions. A total of 37 species of neritids and 48 species of cypraeoideans are known from the Early Cretaceous to Recent (a time span of 130 m.y.) of the Pacific slope of North America (northern and southern Baja California, Mexico; California; Oregon; and Washington). The number of species per stage is closely similar between both taxonomic groups, and their records are also similarly discontinuous.

Both groups of gastropods show peaks in diversity during the warm-water times of the early Late Cretaceous (Turonian), early to middle Eocene ("Capay" and "Domengine" "stages"), and early middle to late middle Miocene ("Temblor Stage"). During the Turonian (globally, the warmest time of the Late Cretaceous) and the "Capay"-"Domengine" stages (globally, the warmest time of the Cenozoic), these gastropods ranged as far north as 45° and 48°, respectively. During the "Temblor Stage" and Pliocene, their ranges were only as far north as 34° and 33°, respectively. Pleistocene through Recent occurrences of both groups are no farther north than 31° and are associated with warm waters of the Gulf of California and southward. One exception is the Pliocene to Recent cypraeid Zonaria spadicea, which is confined today to warm-temperate waters of southern California and northern Baja California, Mexico.

On the Pacific slope of North America, both the neritids and cypraeoideans were absent during the Early Cretaeous Barremian and Aptian stages, the Late Cretaceous Coniacian and Santonian stages, the early Paleocene Danian Stage, and the Oligocene Matlockian and Juanian stages. Their absences in the Barremian, Aptian, and Danian stages are probably caused by preservational and collecting problems. Their absences in the Coniacian, Santonian, Matlockian, and Juanian stages coincided with global climatic cooling.