Order PERISSODACTYLA – Equids, Rhinoceroses, Tapirs
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Order PERISSODACTYLA Order PERISSODACTYLA – Equids, Rhinoceroses, Tapirs Perissodactyla Owen, 1848. Quarterly Journal of the Geological Society of London 4: 103–141. Upper toothrows in altungulate Radinskya (late Paleocene) and Hyracotherium (Eocene). Tentative phylogenetic tree of Perissodactyla after Beninda-Emonds, 2007. Equidae (1 genus, 4 species) Asses, Zebras p. xx Rhinocerotidae (2 genera, 2 Rhinoceroses p. xx for true horses. North America became the centre of evolution of species) true horses, which occasionally migrated to other continents. The The perissodactyls are the order of herbivorous ‘odd-toed’ hoofed descendants of Protorohippus (once called Hyracotherium; Froehlich mammals that includes the living horses, zebras, asses, tapirs, 2002) evolved into many different lineages living side by side. The rhinoceroses and their extinct relatives. They were originally named collie-sized three-toed horses Mesohippus and Miohippus (from beds by Richard Owen (1848) as a group including horses, rhinos, tapirs dated about 30–37 mya) were once believed to be sequential segments and hyraxes, although no recent authors have accepted the inclusion on the unbranched trunk of the horse evolutionary tree. However, of hyraxes in Perissodactyla. Perissodactyls are recognized by a number they coexisted for millions of years, with five different species of two of unique specializations (Hooker 2005), but their single most diagnostic genera living at the same time and place. From Miohippus-like ancestors, feature is the structure of their feet. Most perissodactyls have either horses diversified into many different ecological niches. One major one or three toes on each foot, and the axis of symmetry of the foot lineage, the anchitherines, retained low-crowned teeth, presumably runs through the middle digit. for browsing soft leaves in the forests. Some anchitherines, such as The perissodactyls are divided into three subordinal groups (Prothero Megahippus, were almost as large as the living horse. Anchitherium & Schoch 1989): the Hippomorpha (horses and their extinct relatives); migrated from North America to Europe about 20 mya and was the the Titanotheriomorpha (the extinct brontotheres, found in Asia and first true horse to reach Europe. North America during the Eocene); and the Moropomorpha (tapirs, About 15 mya there were at least 12 different lineages of three- rhinoceroses, and their extinct relatives). toed horses in North America, each with slightly different ecological Perissodactyls were once thought to have evolved in Central America specializations, a situation analogous to the diversity of modern antelopes from the phenacodonts, an extinct group of archaic hoofed mammals in East Africa. The ancestors of this great radiation of horses are a group placed in the invalid taxon ‘Condylarthra’. However, in 1989, a of three-toed, pony-sized beasts that have long been lumped into the specimen recovered from deposits in China dated to about 57 mya was ‘wastebasket’ genus ‘Merychippus’. However, recent analyses have shown described and named Radinskya. This specimen shows that perissodactyls that the species of ‘Merychippus’ are ancestral to many different lineages originated in Asia at around this time and were unrelated to North of horses. True Merychippus was a member of the hipparion lineage, American phenacodonts. By 55 mya the major groups of perissodactyls a group of three-toed horses that developed highly specialized teeth, had differentiated, and migrated from Asia to Europe and North and had a distinctive concavity in the bone on the front of the face. America. Before 34 mya the brontotheres and the archaic tapirs were Hipparions were a highly diverse and successful group of horses, with the largest and most abundant hoofed mammals in Eurasia and North seven or eight different genera spread not only across North America, America. After these groups became extinct, horses and rhinoceroses but also migrating to Eurasia. Merychippines were also ancestral to became the most common perissodactyls, with a great diversity of lineages such as Calippus (a tiny dwarf horse), Protohippus and Astrohippus. species and body forms. Both groups were decimated during another On two different occasions (Pliohippus and Dinohippus), three-toed mass extinction about 5 mya, and today only five species of rhinoceros, horses evolved into lineages with a single toe on each foot. About 5 mya four species of tapir, and a few species of horses, zebras, and asses most of these three-toed and one-toed horse lineages became extinct, cling to survival in the wild. The niches of large hoofed herbivores leaving only Dinohippus to evolve into the modern horse Equus. The main have, to a large extent, been taken over by the ruminant artiodactyls, lineage of horses that survived the extinctions 5 mya were known as such as cattle, antelopes, deer, and their relatives. the equines. The living genus Equus first appeared about 3 mya and was From their Asian origin, the hippomorphs spread all over the widespread throughout the northern hemisphere. When the Isthmus of northern continents. In Europe, the horse-like palaeotheres substituted Panama rose about 2.5 mya, horses also spread to South America. There 408 Order PERISSODACTYLA and braincase. One of the easiest to spot is the crests on the crowns of the upper molars, which look like the Greek letter ‘pi’. The earliest rhinos, known as Hyrachyus, were widespread over Eurasia and North America about 53 mya. They apparently crossed back and forth between Europe and North America using a land bridge. From Hyrachyus, three different families of rhino diverged. Long before hippos evolved, one family of rhinocerotidae, the amynodonts, evolved into hippo-like amphibious forms, with stumpy legs and a barrel chest for living in ponds and rivers. The last of the amynodonts, which had a short trunk like an elephant, died out in Asia about 15 mya. The second family was known as the hyracodonts, or ‘running rhinos,’ because they had unusually long slender legs compared with other rhinos. They were particularly common in Asia and North America between 42 and 34 mya. The biggest of all was Paraceratherium Hyracotherium skeleton. (once called Baluchitherium or Indricotherium), which was six metres tall at the shoulder and weighed 20 000 kg. It must have browsed they evolved into distinctive horses with a short proboscis known as leaves from the tops of trees, as giraffes do today. Indricotheres were the hippidions. At the end of the last Ice Age (about 10,000 years ago), also the last of the hyracodonts, vanishing from Asia about 15 mya. horses became extinct in the New World. They were reintroduced to The third family is the true rhinoceroses, or family Rhinocerotidae. their ancestral homeland by Columbus in 1493. Feral horses that have They first appeared in Asia and North America about 40 mya, and lived escaped from domesticated stock are known as mustangs. side by side with the hyracodonts and amynodonts on both continents. The earliest moropomorphs, such as Homogalax, occur in strata Rhinocerotids can be distinguished from hyracodonts and amynodonts about 55 mya. They are virtually indistinguishable from the earliest by their distinctive molars, and by the development of a tusk-like horses, such as Protorohippus. From this unspecialized ancestry, a lower incisor that occluded against a chisel-like upper incisor. In the variety of archaic tapir-like animals diverged. Most retained the simple late Eocene and early Oligocene (about 37–28 mya), there were several leaf-cutting teeth characteristic of tapirs, and like brontotheres, they different genera of rhinocerotids, When the brontotheres died out died out about 34 mya when their forest habitats shrank. Only the about 34 mya, rhinocerotids were the largest land mammals in North modern tapirs, with their distinctive long proboscis, still survive in America, and remained so until mastodonts arrived about 16 mya. the jungles of Central and South America (three species), and South- Up until this point, all the rhinoceroses we have mentioned were East Asia (one species). All are stocky, pig-like beasts with short stout hornless. Rhinos with horns first appeared about 28 mya; two different legs and oval hooves, and a short tail. They have no natural defences lineages independently evolved paired horns on the tip of the nose. against large predators, so they are expert at fleeing through dense Both of these groups went extinct about 18 mya, when two new brush and swimming to make their escape. subfamilies immigrated to North America from Asia: the browsing The horse-like clawed chalicotheres are closely related to some of (leaf-eating) aceratherines, and hippo-like grazing teleoceratines. these archaic tapirs. When chalicotheres were first discovered, there Between 18 and 5 mya, browser–grazer pairs of rhinos were found was general disbelief that the claws belonged to a hoofed mammal all over the grasslands of Eurasia, Africa and North America. The related to horses and rhinos. However, many specimens have clearly teleoceratine Teleoceras was remarkably similar to hippos in its short shown that chalicotheres are an example of a hoofed mammal that limbs, massive barrel-shaped body and high-crowned teeth for eating has secondarily developed claws. To begin with, claws were considered gritty grasses. We know these animals were aquatic because they are useful for digging up roots and tubers, but the fossilized claws show no usually found in ancient lake or river sediments, and some extraordinary sign of the characteristic scratches due to digging. Instead, chalicotheres specimens buried in volcanic ash preserve the grass seeds of their last are now thought to have used their claws to haul down limbs and meal. branches to eat leaves (much as ground sloths did). Chalicotherium had A mass extinction event that occurred about 5 mya wiped out North such long forelimbs and short hindlimbs that it apparently knuckle- American rhinos, and decimated most of the archaic rhino lineages walked with its claws curled inward. Chalicotheres were always rare (especially the teleoceratines and aceratherines) in the Old World.