Even-toed ungulates (pigs, cows, deer, and their relatives) are the most common large land mammals on Earth today. Their fossil evidence first appears in the Eocene (about 55 million years ago). One of their distinctive features is a special anklebone (astragalus) that allows them to run with greater power and stability. Double grooves in the astralagus articulate with the tibia (lower leg bone), making a strong ankle joint that can move forward and backward without giving way side to side. They also are “fore-gut fermenters” with a digestive system that allows them to efficiently process vegetation. The first even-toed ungulates were rabbit-sized, unlike most artiodactyls today. By the end of the Eocene, they had begun to diversify into many shapes and sizes. Most of these early lineages eventually went extinct, but several experienced a remarkable global diversification. Ancestors of modern antelope, camels, and peccaries exploited grassland habitats that formed during the Miocene in North America. Ancestors of modern deer and bison diversified in Eurasia and later migrated across the Bering Land Bridge to North America.

It might seem odd to group Cetaceans (whales) with Artiodactyls (hooved animals such as pigs, deer, camels, llamas, and hippos). But that is exactly what the combination of their scientific names into Cetartiodactyla is about. For some time, scientists had proposed that whales descended from land mammals, with the focus on the hooved fossil Mesonychia. In the 1990s, DNA sequences from many different genes revealed a closer relationship between whales and hippos than between hippos and any other hooved mammals. While it is not yet known what common ancestor whales and hippos share, the newly discovered relationship is getting attention. It appears that whales and hippos may have branched off from Artiodactyls as long as 60 million years ago. Scientists have proposed a new group called Whippomorpha (wh[ale] + hippo[potamus]; morphe = form) to include whales and hippopotamus and exclude other hooved animals.