The odd-toed ungulates or Perissodactyla are large to very large browsing and grazing mammals with relatively simple stomachs and a large middle toe. The members of the order fall into two groups: horses, which have only one toe and tend to be fast runners with long legs, and the suborder Ceratomorpha, which contains two families of slower-moving, thick-set animals with several functional toes: the tapirs and the rhinoceroses.
The odd-toed ungulates arose in what is now North America in the late Paleocene, less than 10 million years after the dinosaurs died out. By the start of the Eocene (55 million years ago) they had diversified and spread out to occupy several continents. The horses and tapirs both evolved in North America; the rhinoceroses appear to have developed in Asia from tapir-like animals and then reinvaded the Americas during the middle Eocene (about 45 million years ago). There were 12 families, of which only three survive. These families were very diverse in form and size; they included the enormous brontotheres and the bizarre chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached 12 tonnes—more than twice the weight of an elephant!
Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 million years ago) saw a major change: the even-toed ungulates with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many odd-toed species survived and prospered until the late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.
The three surviving families of odd-toed ungulate are classified as follows.
ORDER PERISSODACTYLA
Family Equidae: horses and allies, about 9 species in one genus
The Perissodactyla have been brigaded with the Artiodactyla (q.v.) to form the typical group of the ungulates, under the name of Diplarthra, or Ungulata Vera, and the features distinguishing the combined group from the less specialized members of the order Ungulata will be found under the heading of that order.
The Perissodactyla may be divided into the four followinf sections, namely the extinct Titanotheroidea, the Hippoidea represented by the horse tribe and their ancestors, the Tapiroidea typified by the tapirs, and the Rhinocerotoidea, which includes the modern rhinoceroses and their forerunners.
Rhinoceros Group.The last section of the Perissodactyla is that of the Rhinocerotoidea, represented by the modern rhinoceroses and their extinct allies.
Hooker, J. A primitive ceratomorph (Perissodactyla, Mammalia) from the early Tertiary of Europe.
EocenePerissodactyla from the Shinzhaly river, eastern Kazakhstan.
The complete mitochondrial DNA sequence of the greater Indian rhinoceros, Rhinoceros unicornis, and the phylogenetic relationship among Carnivora, Perissodactyla, and Artiodactyla (plus Cetacea).
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