The Cenozoic Era: Paleogene and Neogene Periods (65–2.6 Ma)

Most of western California was still underwater during the Paleogene, as evidenced by marine fossils, but continued to form and emerge throughout the Cenozoic. Fossil evidence indicates the first archaic mammals appeared in the Jurassic Period of the Mesozoic, approximately 225–210 Ma. Mammalia and Aves, evolutionarily constrained by predation and competition from non-avian Archosaurians, were already present in California and adaptively radiated throughout the Cenozoic across a diverse landscape. Migration among the northern continents was extensive during the early Paleogene. Generally, Paleogene California hosted a warm-humid climate and associated vegetation. The Miocene was a time of major change throughout the California landscape. Around 23 Ma, North America crashed into South America, forming the minor supercontinent America. Throughout the late Neogene, sea levels fluctuated with glaciations into the late Pleistocene. These fluctuations resulted in extinctions of estuary biota from the Miocene and Pliocene and the input of new biota from other areas. In general terms, throughout the Paleogene and Neogene, California was part of a mostly contiguous forest that was present across both sides of the Bering land bridge, which also served as a migratory corridor.