Artiodactyls are an order of even-toed ungulates. These ungulates walk with their weight born equally on their 3rd and 4th toes, which are usually covered with a thick nail referred to as a hoof. This group first appeared in the Eocene Period about 54 MYA. Artiodactyls fall into two basic groups, omnivores with a simple stomach including the pig and hippo, and ruminating (cud chewing) animals including the camel, goat and antelope. Artiodactyls outnumbered the larger more diverse perissodactyls. In the Miocene Period, as grasses became the major vegetation across the plains, these even toed ungulates with their complex stomachs quickly became the dominant herbivores.