Anurans were common in western North America by the Paleocene, and Caudates were likely present by the Eocene. Both lineages are sensitive to changes in temperature and moisture, and because dispersal events are rare, due to habitat availability, they leave strong genetic signatures. Plethodon, Ensatina, Taricha, and Rana share similar Miocene divergence times in the Klamath-Siskiyou region and undoubtedly resided there as refugial populations during later glaciations in surrounding regions. A number of taxa were present along the coast prior to the formation of the Coast Ranges and the subsequent orogeny and associated embayments of the Coast Ranges had a strong influence on clade divergence. The Transverse Ranges breaks are important in many amphibian species that are also present in insects and reptiles and small mammals. The Santa Clara River Valley Embayment in the Pliocene is estimated to have existed about 3.3–1.6 Ma and likely served as important biogeographic barrier in this region. As elsewhere in the world, the amphibians of California are under extreme threat of extinction due to habitat loss, invasive species, disease, and climate change. The extreme loss of mesic and wetland habitats throughout California, but particularly in the Central Valley, has isolated populations and removed opportunities for adaptive radiation and gene flow.
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