Mesozoic that would move into their current positions during the next era. The climate of the Mesozoic was varied, alternating between warming and cooling periods. Overall, however, the Earth was hotter than it is today. Dinosaurs appeared in the Late Triassic and became the dominant terrestrial vertebrates early in the Jurassic, occupying this position for about 135 million years until their demise at the end of the Cretaceous. Birds first appeared in the Jurassic, having evolved from a branch of theropod dinosaurs. The first mammals also appeared during the Mesozoic, but would remain small—less than 15 kg (33 lb)—until the Cenozoic.
Geology of Arizona The subduction of the Farallon Plate beneath western North America remains a poorly understood event, although new seismic and computer tomography modeling is shedding light on the event. As the oceanic plate subsided under North America, it generated partial melting conditions that formed volcanoes on the surface in California and Arizona. Volcanic activity commenced in western Arizona around 205 million years ago, kicking off the Nevadan orogeny, which lasted until 145 million years ago, spanning the Triassic into the Jurassic. Granite intrusions 190 million years ago enriched many veins with minerals and generated copper porphyry gold deposits. Bisbee, Arizona, 92