Perissodactyla is an order of ungulates. The order includes about 17 living species divided into three families: Equidae, Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They typically have reduced the weight-bearing toes to three or one of the five original toes, though tapirs retain four toes on their front feet. The nonweight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, artiodactyls bear most of their weight equally on four or two of the five toes: their third and fourth toes. Another difference between the two is that odd-toed ungulates digest plant cellulose in their intestines, rather than in one or more stomach chambers as even-toed ungulates, with the exception of Suina, do.
Ungulates are members of the diverse clade Euungulata which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.
Artiodactyls are mammals belonging to the order Artiodactyla. Typically, they are ungulates which bear weight equally on two of their five toes: the third and fourth, often in the form of a hoof. The other three toes are either present, absent, vestigial, or pointing posteriorly. By contrast, most perissodactyls bear weight on an odd number of the five toes. Another difference between the two is that many artiodactyls digest plant cellulose in one or more stomach chambers rather than in their intestine as perissodactyls do. The advent of molecular biology, along with new fossil discoveries, found that cetaceans fall within this taxonomic branch, being most closely related to hippopotamuses. Some modern taxonomists thus apply the name Cetartiodactyla to this group, while others opt to include cetaceans within the existing name of Artiodactyla. Some researchers use "even-toed ungulates" to exclude cetaceans and only include terrestrial artiodactyls, making the term paraphyletic in nature.
Creodonta is a former order of extinct carnivorous placental mammals that lived from the early Paleocene to the late Miocene epochs in North America, Europe, Asia and Africa. Originally thought to be a single group of animals ancestral to the modern Carnivora, this order is now usually considered a polyphyletic assemblage of two different groups, the Oxyaenids and the Hyenodonts, not a natural group. Oxyaenids are first known from the Palaeocene of North America, while hyaenodonts hail from the Palaeocene of Africa.
Entelodontidae is an extinct family of pig-like artiodactyls which inhabited the Northern Hemisphere from the late Eocene to the Middle Miocene epochs, about 38-19 million years ago. Their large heads, low snouts, narrow gait, and proposed omnivorous diet inspires comparisons to suids and tayassuids (peccaries), and historically they have been considered closely related to these families purely on a morphological basis. However, studies which combine morphological and molecular (genetic) data on artiodactyls instead suggest that entelodonts are cetancodontamorphs, more closely related to hippos and cetaceans through their resemblance to Pakicetus, than to basal pigs like Kubanochoerus and other ungulates.
Mesonychia is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychians first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes, became extinct in the early Oligocene. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyles before the group became extinct.
Palaeotherium is an extinct genus of perissodactyl ungulate known from the Mid Eocene to earliest Oligocene of Europe. First described by French naturalist Georges Cuvier in 1804, Palaeotherium was among the first Paleogene mammals to be described.
Merycoidodontoidea, previously known as "oreodonts" or "ruminating hogs," are an extinct superfamily of prehistoric cud-chewing artiodactyls with short faces and fang-like canine teeth. As their name implies, some of the better known forms were generally hog-like, and the group has traditionally been placed within the Suina, though some recent work suggests they may have been more closely related to camels. "Oreodont" means "mountain teeth," referring to the appearance of the molars. Most oreodonts were sheep-sized, though some genera grew to the size of cattle. They were heavy-bodied, with short four-toed hooves and comparatively long tails.
Mesonychidae is an extinct family of small to large-sized omnivorous-carnivorous mammals. They were endemic to North America and Eurasia during the Early Paleocene to the Early Oligocene, and were the earliest group of large carnivorous mammals in Asia. Once considered a sister-taxon to artiodactyls, recent evidence now suggests no close connection to any living mammal. Mesonychid taxonomy has long been disputed and they have captured popular imagination as "wolves on hooves," animals that combine features of both ungulates and carnivores. Skulls and teeth have similar features to early whales, and the family was long thought to be the ancestors of cetaceans. Recent fossil discoveries have overturned this idea; the consensus is that whales are highly derived artiodactyls. Some researchers now consider the family a sister group either to whales or to artiodactyls, close relatives rather than direct ancestors. Other studies define Mesonychia as basal to all ungulates, occupying a position between Perissodactyla and Ferae. In this case, the resemblances to early whales would be due to convergent evolution among ungulate-like herbivores that developed adaptations related to hunting or eating meat.
Oromerycidae is a small, extinct family of artiodactyls closely related to living camels, known from the early to late Eocene of western North America.
Tillodontia is an extinct suborder of eutherian mammals known from the Early Paleocene to Late Eocene of China, the Late Paleocene to Middle Eocene of North America where they display their maximum species diversity, the Middle Eocene of Pakistan, and the Early Eocene of Europe. Leaving no descendants, they are most closely related to the pantodonts, another extinct group. The tillodonts were medium- to large-sized animals that probably feed on roots and tubers in temperate to subtropical habitats.
Phenacodontidae is an extinct family of large herbivorous mammals traditionally placed in the “wastebasket taxon” Condylarthra, which may instead represent early-stage perissodactyls. They lived in the Paleocene and Eocene epochs and their fossil remains have been found in North America and Europe.
Duerotherium is an extinct genus of Paleogene artiodactyls known only from the Iberian Peninsula during the Middle Eocene, which contains one species D. sudrei. It, like other members of the Anoplotheriidae, was endemic to western Europe. The anoplotheriine was described from a left fragment of a maxilla from the Mazaterón Formation of the Duero Basin in 2009. Its dentition is mostly typical of the Anoplotheriinae but differs by an elongated plus triangular 3rd upper premolar and very specific traits of the molars. It is thought to have been part of an endemic faunal assemblage that evolved within the Iberian Peninsula by the Middle Eocene, where climates were subtropical.
Cainotheriidae is an extinct family of artiodactyls known from the Late Eocene to Middle Miocene of Europe. They are mostly found preserved in karstic deposits.
Propyrotherium is an extinct genus of mammals, belonging to the order Pyrotheria. It lived during the Late Eocene, in what is now South America.
Pseudhyrax is an extinct genus of archaeohyracid notoungulate. It lived from the Late Eocene to the Early Oligocene, of what is now South America.
Rhyphodon is an extinct genus of notoungulate, who lived from the Middle to the Late Eocene in what is today South America.
Santiagorothia is an extinct genus of interatheriid notoungulate. It lived during the Early Oligocene, and its fossils were discovered in Argentina and Chile.
Periptychus is an extinct genus of mammal belonging to the family Periptychidae. It lived from the Early to Late Paleocene and its fossil remains have been found in North America.
Catodontherium is an extinct genus of Paleogene artiodactyls belonging to the family Anoplotheriidae. It was endemic to western Europe and had a temporal range exclusive to the middle Eocene, although its earliest appearance depends on whether C. argentonicum is truly a species of Catodontherium. It was first named Catodus by the French palaeontologist Charles Depéret in 1906, who created two species for the genus and later change the genus name to Catodontherium in 1908. The Swiss palaeontologist Hans Georg Stehlin renamed one species and classified two other newly erected species to Catodontherium in 1910. Today, there are four known species, although two remain questionable in genus placement.
