Mountains create more isolation through climate, geological and disturbance diversity than lowlands and so often have more species, especially at middle elevations. The Klamath-Siskiyous also may have shielded life from the second worst disaster ever 65 million years ago. Even the timing was bad coming after and during great outpourings of carbon dioxide from Siberian volcanoes. North America was the worst hit, being covered by a lateral blast from a seven to ten mile wide asteroid traveling over 20 times faster than a bullet. First to spread across North America was a cloud of hot vaporized rock. Next was a tidal wave likely hundreds of times higher than the one in the Indian Ocean. Seawater probably washed over our continent except perhaps the top of the Klamath-Siskiyous-Belt Mountains.
When ejected rock fell red-hot back to earth, even wet wood burned for there was 10% more oxygen back then. Forests became ash except perhaps for serpentine and shale areas in the high Klamath-Siskiyous and Appalachians. A “nuclear winter” was followed by super-warming by carbon dioxide from vaporized rock. And we complain about our weather! Only one primate apparently survived, appropriately named Purgatorius.