Abstract

Cosmopolitan Jurassic ammonites and some pelecypods began differentiating in mid-Bajocian time into Boreal, Tethyan and Pacific realms, although there was some intermingling at the edges of the realms and considerable latitudinal shifting. During the Callovian and early Kimmeridgian, Boreal ammonites extended into California; in the late Oxfordian and Portlandian Tethyan forms extended north to southwestern Oregon. Similar shifts took place in Eurasia at the same times and in the same directions, probably due to regional changes within oceans and perhaps also to changes in solar radiation. The primary cause of differentiation probably was the general continental emergence beginning in the Bathonian, which established physical barriers about the middle of the Bajocian. These isolated the realms and resulted in cooling of the Arctic Ocean, and alterations in previous ocean current patterns.

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