The artiodactyls are arguably the most successful order of large land mammals (as long as we don’t mention a dominant lone species of large aggressive primates). Just perusing a list of artiodactyl names reveals how universal and important they are: goats, pigs, cows, giraffes, hippos, sheep, Protoceratidae… wait. What? Among the familiar families of artiodactyls there is an unfamiliar name—an entire vast lineage of hoofed animals completely gone forever. These were the Protoceratidae, hooved animals analogous to their cousins the deer, giraffes, and the camels. The Protoceratidae ranged across North America from the Eocene through the late Pliocene (46 million to 5 million years ago). For 41 million years great herds of these animals grazed the Rocky Mountains, the Great Plains, Appalachia, and Mexico (although the Laramide orogeny was still ongoing as they evolved, and the Great Plains were at first a great forest).
Protoceratidae had complex stomachs for breaking down grasses and other tough vegetation which they grazed upon. Initially the protoceratids were tiny like the smallest deer, but as time progressed some grew to the size of elk. Although the bones in their legs were somewhat different from deer and camels they would have looked similar at a distance…except for a stand-out feature. Male Protoceratidae had a rostral bone—a powerful y-shaped spar of bone jutting from their nose. It is believed that this was a sexual display, meant to impress female Protoceratidae and for sparring with other males for territory and mates (although anybody who has ever been jabbed in the face with a sharpened bone by an elk-size animal would probably testify that the rostral bone could be used defensively). The protoceratids also had a pair of more conventional horns on either side of their head like deer and cattle.
I wish I could show you more of this extinct family. They lived for a long time and took many shapes and appearances as they spread across the continent into many different niches. A species of particular note was Synthetoceras tricornatus, which was the largest of the protoceratids and which was endemic to most of the continent during the Miocene. Look at how lovely they are. I am ready to move to Texas and start a ranch for them—if they weren’t all dead.
So, what killed off the Protoceratids after 40 million years of success? It seems like they were outcompeted by other, more familiar forms of artiodactyls which developed as the Cenezoic wore on (and which were better suited to vast tracts of grassland—which came to dominate the landscape as the forests died back). The last protoceratid, the Kyptoceras, lived in semi-tropical forests of Florida during the Miocene. Perhaps it was like the Saola, an ever-dwindling wraith that lived deep in the rainforest and was seldom seen until one day it was gone completely. It is an appropriately melancholy picture of the last descendant of a once-great house.