The Rattlesnake Formation represents the Miocene epoch, and dates from 7 to 5 million years ago. The populations had adjusted to a significantly dryer, cooler climate. Grasses were more abundant than shrubs and trees, and grazers were more abundant than browsers. Pictured here are; pliohippus (in the modern lineage of the horse), dog, short face bear, peccaries, and tetralophodon (in the elephant family).