Synthetoceras is a protoceratid from the late Miocene of North America, around 10 million years ago. The protoceratids first appear in the fossil record around 46 million years ago. The early members of the group were small, hornless, forest-living animals, similar to today’s mouse deer. Later animals were more deer or antelope-like in size and shape, and were often characterized by bizarre combinations of horns on the nose and above the eyes. Unlike deer or antelopes, however, protoceratids had relatively short legs and unfused metapodial bones in their feet. The later forms seemed to have mostly lived on the subtropical coastal plains of Miocene North America, and may have been semi-aquatic browsers, like today’s moose.

There is some debate among paleontologists regarding the relationship of the protoceratids relative to other artiodactyls. Some researchers have argued that the pattern of blood circulation in the neck vertebrae suggests a close relationship with the camels, but more recent studies involving CT scanning of protoceratid skulls support a relationship with the ruminants, even though they lack the fused foot bones that characterize members of this group.