Laurasiatheria ├─Ferae └─┬─Cetartiodactyla │ └─Perissodactyla ├─Hippomorpha └─Tapiromorpha
The perissodactyls are the "odd-toed" ungulates. An ancient and venerable lineage, they were at one time an astonishingly diverse group, including among their number the largest land mammals ever to live (the indricotheres) as well as horses no bigger than a fox terrior (the eohippus) . Now only a few highly derived (specialised) representatives of three of the five main lineages remain, with each group very different in superficial appearnce to the other two. It's hard to imagine that animals as diverse as horses and rhinos are actually fairly closely related, but this is indeed the case. Certainly, rhinos and the representatives of the third perissodactyl lineage, the tapirs, do share a prehistoric look, and both indeed have hardly changed since the Miocene. Horses in contrast continued to evolve and flourish upto the Holocene, when their survival was ensured (unlike the magnificent rhino, hunted and poached almost to extinction) by becoming among the most beloved of the animals domesticated by man MAK120319
As a whole, the ungulates are the big, fast herbivores of the Cenozoic. This is the same role which was played by the ornithopods before them, and by various therapsid groups at times in the Late Paleozoic and Early Mesozoic. The perissodactyls got off to a late start, perhaps in the Late Paleocene, but arguably continued that trend to become bigger and faster -- but not both at the same time. The key innovations of the perissodactyls do seem to be related to further increases in mobility. The clavicle is completely absent, finally disposing of that awkward lock between the front limbs which was inherited from the first tetrapods. The astragalus (one of the main ankle bones) is shortened, sacrificing flexibility for the sake of strengthening the leg as a whole. The weight is concentrated on the middle of the foot, rather than being balanced between the two sides. These sorts of adaptations allow the evolution of extremely fast animals, like horses; or to massive animals with remarkable turning ability, like rhinos (and also, via convergence, elephants). The hind limbs can take a huge amount of pounding. The front limbs move more independently and allow changes in direction without loss of balance. On the other hand, these adaptations do not favor jumping ability or rapid postural changes. That sort of behavior requires a different sort of springy-ness, close limb coordination, and superior balance. Try a standing jump or rapid squat with your arms held out at different angles, or just standing on the edges of your feet, and you'll see the problem. A perissodactyl is a bit like a human on skiis. With practice, speed and agility are very good at any mass; the limiting factors are balance and posture.
The gradual replacement of perissodactly groups with terrestrial artiodactyls (pigs, goats, cattle, deer), may relate to long-term changes in climate and vegetation. Some of this relates probably to the unique digestive apparatus of the artiodactyls. But what exactly are the environmental factors responsible for the relative failure of the perissodactyls? -- ATW 020309, originally "altungulata", redefined by MAK120319
checked ATW050727, revised and updated MAK120319