Originally posted by Deesquared I did not know that! How interesting.
FYI:
Horses, rhinos and tapirs are in the PERISSODACTYLA, commonly called odd-toed ungulates. There were other types in the past that have gone extinct.
Hippos, gazelle, antelope, cattle, giraffe, pigs, peccaries, camels, goats, sheep are in the ARTIODACTYLA, commonly called even-toed ungulates, or cloven-hoofed ungulates.
A critical feature of this group is an astragalus (= ankle bone) that has a joint at both ends, a feature unique to artiodactyls. It effectively gives their hind legs an extra segment. Other mammal hind legs have three segments = femur to the knee, tibia + fibula to the ankle, foot to the end of the toes. Artiodactyls have femur - tibia/fibula - astragalus - rest of the foot. The segment formed by the astragalus is very short, It's tricky to describe how it works, easier with a diagram, but basically the astragalus provides a "low gear" to "high gear" shift when the animal is running. It probably enables the springing-leaping ability of smaller artiodactyls, and also enables a faster acceleration from a standstill to full speed.
In the "rise of mammals" following extinction of the dinosaurs, perissodactyls expanded and diversified fairly early, then declined. Artiodactyls appear a bit later and increased dramatically almost to modern time, but as with many mammals, there's a decline associated with events during and following the ice ages. For example, camels were diversified in North America up to the Ice Ages at which time they spread into South America (llama, alpaca, vicunia) and also into Asia and beyond to Africa (dromedary and Bactrian camels). All the North American camels go extinct.
You might be interested to know that
1) there were once in NA small camels, average height about 24 inches (0.6m) that ran about in herds possibly containing thousands of them.
2) There were camels larger than a modern camel -
Camelops,
Alticamelus, Titanotylopus for three of them