Odd-toed ungulate

Odd-toed ungulates
Temporal range: 56–0 Ma

Ypresian-Holocene See below for possible Paleocene members.

Clockwise from left: plains zebra (Equus quagga), Indian rhinoceros (Rhinoceros unicornis) and South American tapir (Tapirus terrestris)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Theria
Infraclass: Eutheria
Magnorder: Boreoeutheria
Superorder: Laurasiatheria
(unranked): Scrotifera
Order: Perissodactyla
Owen, 1848
The white rhinoceros is the largest living perissodactyl

Perissodactyls, otherwise known as odd-toed ungulates, compose an order of mammals characterized by an odd number of toes and by hindgut fermentation with somewhat simple stomachs. As large grazers they digest plant cellulose in their intestines rather than in one or more stomach chambers, unlike the even-toed ungulates. The order includes three extant families: Equidae, including horses, donkeys, and zebras, Rhinocerotidae, rhinos and Tapiridae, tapirs, with a total of about 17 species. In spite of their very different appearance they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the term.

Anatomy

As an adaptation to different habitats and lifestyles, the odd-toed ungulates have developed distinct differences in their build. Common features are in the construction of the limbs and teeth. Rhinos are the largest members of the group, and the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time. At the other extreme, an early member of the order, the prehistoric horse Hyracotherium, had a shoulder height of only 20 cm (7.9 in). Apart from dwarf varieties of the domestic horse and the donkey, perissodactyls reach a body length of 180–420 cm (71–165 in) and a weight of 150 to 3,500 kg (330 to 7,720 lb). While rhinos are only sparsely hairy and exhibit a thick epidermis, tapirs and horses have a dense, short coat. Most species are grey or brown, although zebras and young tapirs are striped.

Limbs

The main axes of both the front and the rear feet pass through the third toe, which is always the largest. The remaining toes have been reduced to varying degrees. Tapirs, which are adapted to walking on soft ground, have four toes on their forefeet and three on their hindfeet. Living rhinos have three toes on both the front and hind feet. Modern equines possess only a single toe. Their feet are equipped with hooves, however, which cover the toe almost completely; rhinos and tapirs, by contrast, have hooves covering only the leading edge of the toes, with the bottom being soft.

The ulna and fibula are reduced in horses. A common feature that clearly distinguishes this group from other mammals is the saddle-shaped ankle between the astragalus and the scaphoid, which greatly restricts the mobility of the foot. The thigh is relatively short, and the clavicle is absent.

Skull and teeth

South American tapirs are among the few species of extant perissodactyl with a trunk

Odd-toed ungulates have a long upper jaw with an extended diastema between the front and cheek teeth, giving them an elongated head. The various forms of snout between families are due to differences in the form of the premaxilla. The lacrimal bone has projecting cusps in the eye sockets and a wide contact with the nasal bone. The temporomandibular joint is high and the mandible is enlarged.

Rhinos have one or two horns made of agglutinated keratin, unlike the horns of even-toed ungulates which have a bony core.

The number and form of the teeth vary according to diet. The incisors and canines can be very small or completely absent, as in the two African species of rhinoceros. In the horses, usually only the males possess canines. The surface shape and height of the molars is heavily dependent on whether soft leaves or hard grass makes up the main component of their diet. Three or four cheek teeth are present per half of the jaw, so that the dental formula of odd-toed ungulates is: 0-3 . 0-1 . 2-4 . 31-3 . 1 . 2-4 . 3 × 2 = 30-44

Internal anatomy

All perissodactyls are hindgut fermenters. Hindgut fermenters, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where the food is digested by bacteria. No gallbladder is present. The stomach of perissodactyls is simply built, while the cecum accommodates up to 90 litres (24 US gal) in horses. The intestine is very long, reaching up to 26 metres (85 ft) in horses. Extraction of nutrients from food is relatively inefficient, which probably explains why there are no small odd-toed ungulates; for large animals nutritional requirements per kilogram of body weight are lower and the surface-to-volume ratio is smaller.

Distribution

The distribution of most species, such as the Indian rhinoceros, has declined in recent decades.

The distribution of most perissodactyl species has declined in recent decades. Today's distribution area of odd-toed ungulates consists only of a small part of a once larger range. Members of this group are now found in Central and South America, in eastern and southern Africa and in central, southern, and southeastern Asia. During the peak of odd-toed ungulate variation, from the Eocene to the Oligocene, perissodactyls were distributed over much of the globe except for Australia and Antarctica. Horses and tapirs arrived in South America after the formation of the Isthmus of Panama in the Pliocene, around 3 million years ago. In North America, they died out around 10,000 years ago, while in Europe the tarpans disappeared in the 19th century. Hunting and restriction of habitat have led to the present-day species being reduced to fragmented relict populations. In contrast, domestic horses and donkeys, as livestock, have gained a worldwide distribution, and feral animals of both species are now also found in regions outside of their original range, such as in Australia.

Lifestyle and diet

Perissodactyls inhabit a number of different habitats. Tapirs are solitary and inhabit mainly tropical rainforests. Rhinos tend to live alone in rather dry savannas and, in Asia, wet marsh or forest areas. Horses inhabit open areas such as grasslands, steppes, or semi-deserts and live together in groups. Odd-toed ungulates are exclusively herbivores that feed, to varying degrees, on grasses, leaves and other plant parts.

Reproduction and development

A young South American tapir

Odd-toed ungulates are characterized by a long gestation period and a small litter size, usually delivering a single young. The gestation period is 330–500 days, being longest in the rhinos. Newborn perissodactyls are precocial; young horses and rhinos can follow the mother after a few hours. Infants are nursed for a relatively long period of time, often into their second year, reaching sexual maturity at around eight or ten. Perissodactyls are long-lived, with several species reaching an age of almost 50 years in captivity.

Taxonomy

Outer taxonomy

Traditionally, the odd-toed ungulates were classified with other mammals such as artiodactyls, hyraxes, mammals with a proboscis, and other "ungulates". A close family relationship was particularly suspected with hyraxes, proven by similarities in the construction of the ear, and the course of the carotid artery.

Richard Owen, who coined the expression odd-toed ungulate

Due to molecular genetic studies, however, serious doubts about the relationship of the ungulates were significantly raised in recent times, probably making this a polyphyletic group, which means that the similarities are the result of convergent evolution, and not the result of common ancestry. Elephant and hyraxes are now mostly in the superiority of the Afrotheria, are therefore are not closely related with the perissodactyls. These, in turn, are in the Laurasiatheria, a superorder that had its origin in the extinct continent Laurasia. The molecular genetic findings suggests that the sister taxon of the Perissodactyla, Cetartiodactyla, formed, in which the cloven (Artiodactyla) and the whales (Cetacea) are included; both groups together form the Euungulata.[1] Next outside are the bats (Chiroptera) and ferae (a common taxon of predators (Carnivora) and pangolins (Pholidota)).[2] Pegasoferae is in an alternative scenario, which states a greater unity between the perissodactyls and the predators.[3]

 Innere Systematik der Euungulata nach Welker et al. 2015[4]
 Euungulata 
 Cetartiodactyla 

 Artiodactyla (Paarhufer)

 Cetacea (Wale)



 Panperissodactyla 

 Perissodactyla (Unpaarhufer)

 „Meridiungulata“ (Südamerikanische Huftiere †;
  speziell Notoungulata und Litopterna)




According to studies that were published in March 2015, odd-toed ungulates are in a close family relationship with at least some of the so-called Meridiungulata, one from the Paleocene to Pleistocene in South America, occurring from a very diverse group of mammals whose systematic unity was largely unexplained. Some of these were the basis of their paleogeographic distribution. Afrotheria associated what some anatomical features such as the construction of the spine or the talus. However, it was by means of protein sequencing and the comparison with fossil collagen that they gained remnants of some phylogenetically young members of "Meridiungulata" (specifically Macrauchenia from the group of litopterna and Toxodon from the group of notoungulata), a close relationship can be worked out to perissodactyls. Both kinship groups, the odd-toed ungulates and litopterna-notoungulata are now in the higher-level taxon the Panperissodactyla. This kinship group stands within the Euungulata and the even-toed ungulates and whales (Cetartiodactyla). The separation of litopterna-notoungulata group of the perissodactyls took place probably before the Cretaceous-Paleogene extinction event. As a starting point for the development of the two groups probably can "condylarths" be taken into consideration, which represent a heterogeneous group of primitive ungulates that, in the Paleogene mainly, inhabited the northern hemisphere.[4][5]

Modern members

Odd-toed ungulates (Perissodactyla) consists of three living families with around 17 species - in the horse the exact count is still controversial. Rhinos and tapirs are more closely related to each other and are offset by the horses. The separation of horses from the rest perissodactyls carried out according molecular genetic analysis in the Paleocene before about 56 million years ago, while the rhinos and tapirs in the lower middle Eocene split before about 47 million years.

Internal classification of extant Perissodactyla[6][7][8]
 Perissodactyla 

 Equidae

 Equus ferus

 Equus asinus


 Equus kiang

 Equus hemionus

 Equus zebra

 Equus quagga

 Equus grevyi
 Tapiridae

 Tapirus indicus

 Tapirus bairdii

 Tapirus kabomani

 Tapirus pinchaque

 Tapirus terrestris






 Rhinocerotidae



 Rhinoceros unicornis

 Rhinoceros sondaicus

 Dicerorhinus sumatrensis

 Ceratotherium simum

 Diceros bicornis







Prehistoric members

Live reconstruction of Chalicotherium Anisodon grande (formerly Chalicotherium grande)

Fossils of perissodactyls occurred in a high speed, and multi-variant forms; the major lines of development include the following groups:

Higher classification of perissodactyls

 Internal classification of Perissodactyla[14]
 Perissodactyla 
 Tapiromorpha 

 Isectolophidae (†)
 Ancylopoda 

 Lophiodontidae (†)

 Chalicotheriidae (†)



 Ceratomorpha 
 Tapiroidea 

 Helaletidae (†)

 Tapiridae



 Rhinocerotoidea 

 Amynodontidae (†)

 Hyracodontidae (†)

 Rhinocerotidae







 Hippomorpha 
 Equoidea 

 Palaeotheriidae (†)

 Equidae



 Brontotherioidea 

 Lambdotheriidae (†)

 Brontotheriidae (†)





The relations of the large group of odd-toed ungulates among themselves are not fully understood. Initially, after the establishment of the concept Perissodactyla by Richard Owen in 1848, the present-day representatives were considered equal. In the first half of the 20th century began with the involvement of a stronger systematic differentiation of odd-toed ungulates, thereby placing them in two major suborders: Hippomorpha and Ceratomorpha. The Hippomorpha comprises today's horses and their extinct members (Equoidea), and Ceratomorpha consist of tapirs and rhinos plus their extinct members (Tapiroidea and Rhinocerotoidea).[15] The names Hippomorpha and Ceratomorpha were introduced in 1937 by Horace Elmer Wood, whereby it so responded to the criticism that was previously proposed by his three-year name Solidungula which had come for the grouping of horses and Tridactyla and for the rhinoceros/tapir complex.[16][17] The extinct brontotheriidae were also classified under Hippomorpha and therefore possess a close relationship to horses. Some researchers see this assignment because of similar dental features, but there is also the view of a very basal position within the odd-toed ungulates, which they then belong to the group of Titanotheriomorpha.[14][18]

Originally, the Chalicotheriidae were seen as members of Hippomorpha, presented in 1941. William Berryman Scott thought that as a claw-bearing perissodactyls, they were opposite and he pointed it into the new suborder Ancylopoda (where Ceratomorpha and Hippomorpha as odd-toed ungulates in the group of Chelopoda were combined).[19] The term Ancylopoda, in 1889, coined by Edward Drinker Cope, had been established for chalicotheres. However, further morphological studies from the 1960s showed a middle position of Ancylopoda between Hippomorpha and Ceratomorpha. Leonard Burton Radinsky saw all three major groups of odd-toed ungulates as peers, which he, inter alia, with the extremely long and independent phylogenetic reasoned development of the three lines.[20] In the 1980s, Jeremy J. Hooker in Zahnbau saw a general similarity of Ancylopoda to Ceratomorpha, especially in the earliest members, leading to the unification of the two submissions in the interim order, in 1984, Tapiromorpha (at the same time he expanded the Ancylopoda the Lophiodontidae ). The name Tapiromorpha goes back to Ernst Haeckel, who coined it in 1873; but he has long been considered synonymous to Ceratomorpha because Wood had not noticed him in 1937 with the establishment of Ceratomorpha due to its highly different use in the past.[21] Also in 1984, Robert M. Schoch used the conceptually similar term Moropomorpha which today applies to Tapiromorpha as synonymous.[22] Within the Tapiromorpha are the now extinct Isectolophidae which is a sister group of the Ancylopoda-Ceratomorpha group and are thus the most primitive members to look at this relationship complex.[18][23]

Evolutionary history

Origins

The development history of the Perissodactyla is comparatively well through fossils handed down; numerous findings leave this earlier form much richer and more widespread recognized group. As one of the oldest relatives of odd-toed ungulates in part is Radinskya from the late Paleocene of East Asia is considered.[24] The only, slightly more than 8 centimetres (3.1 in), measured skull belongs to a very small and original animal that on the easy π-shaped design of the enamel the rear molars similarities to perissodactyls and their relatives, especially the rhinos.[25] The origins of odd-toed ungulates are unclear, often suspects are found in formations in Asia. Finds from the western India could confirm this and the creation of the center in the South Asia Filter. From the Cambay shale formation, which the Lower Eocene antedates 54.5 million years come remains of Cambaytherium and Kalitherium which the family of Cambaytheriidae form.[26][27] Their teeth show comparable to Radinskya similarities to the early perissodactyls and Tethytherien.[28][29] Among other things, the saddle-shaped collection of Naviculargelenks, the bottom of the talus, and the mesaxonische design of the front and hind feet - the main axis of the foot went through the center beam (beam III) - for a close relationship with the perissodactyls. However, since the feet were deviating from the earliest perissodactyls that Cambaytherien today is considered its sister group. Perhaps the ancestors arrived on an island bridge from the Afro-Arab landmass on the Indian subcontinent drifting, which was an island and then north towards Asia.[30]

Phylogeny

Hyracotherium, an early relative of the horse, is one of the oldest known perissodactyls

The Perissodactyla appear relatively abruptly at the beginning of the Lower Eocene before about 56 million years ago, both in North America and Asia.[31] The oldest finds originate among others from Sifrhippus, an ancestor of the horses from the Willswood lineup in northwestern Wyoming.[32][33] The distant ancestors of tapirs appeared not too long after that in the Ghazij lineup in Balochistan, such as Ganderalophus, next also Litolophus which in the line of development of Chalicotheriidae stands, or Eotitanops from the group brontotheriidae.[34][35] Initially, the members of the different lineages still looked quite similar with an arched back and generally four toes on the front and three on the hind feet. Hyracotherium, which is considered member of the horse family, resembled outwardly Hyrachyus, the first member of rhinos and tapirs.[36] All were small when compared to later forms and lived as fruit and foliage eaters in cramped forests. The first if the mega-fauna emerged with the Brontotherien already in Middle and Upper Eocene, the known Megacerops from North America reached 2.5 metres (8.2 ft) shoulder level and could've weighed just over 3 metric tons (3.3 short tons). The decline of Brontotherien at the end of the Eocene stands in connection with the advent of competition from other herbivores.[10][37]

More successful lines of odd-toed ungulates emerged at the end of the Eocene when the cramped jungles gave way to steppe, such as Chalicotheriidae and the rhinos and their immediate relatives; their development also started with very small forms. Paraceratherium, the biggest mammal ever to walk the earth, evolved during this era.[38][39] They weighed up to 20 metric tons (22 short tons) and lived throughout the Oligocene in Eurasia. With the onset of the Miocene, the perissodactyls reached, before about 20 million years ago, the first time the connection of Africa with Eurasia. However, passed through the now following faunal groups of animals in the ancient settlement areas of odd-toed ungulates, such as the mammoths, whose competition also led to the extinction of some odd-toed ungulate lines. Even the rise of ruminants that have similar ecological niches occupied, and had a much more efficient digestive system, is associated with the decline in diversity of odd-toed ungulates. But a significant share of the decline of perissodactyls was due to climate changes during the Miocene towards a cooler and dryer climate, which was accompanied by the spread of open landscapes. However, some lines flourished as with those of horses and rhinos, by adapting numerous members by anatomical modifications to the tougher grass food. This resulted in open land forms that populated the newly created types of landscape. With the emergence of the Isthmus of Panama in the Pliocene, perissodactyls, as well as other mega-fauna, were given access to the one habitable remote continent: South America.[40][41] However, many perissodactyls became extinct at the end of the ice ages, as with American horses and the Elasmotherium, which was common among most mega-fauna. Whether over-hunting by humans (overkill hypothesis) or climatic changes, or a combination of both factors were responsible for the extinction of ice age mega-fauna, is controversial.[10]

Research history

Linnaeus (1707–1778) presented in 1758 in his seminal work Systema Naturae the horse (Equus) to the side of hippos (Hippopotamus). These contained, at that time, also the tapirs (Tapirus), precisely the tapir, which in Europe was the only known Tapir art at that time; Linnaeus considered Hippopotamus as terrestrial. Both genera referred to the group of Linnaeus Belluae. He united against the rhinos being paired with the glires, a group now consisting of the lagomorphs and rodents. Only Mathurin Jacques Brisson (1723–1806) severed, in 1762, the introduction of the concept of the tapir and the hippos, and also divided the rhinos from the rodents, but didn't united the three families as the odd-toed ungulates. In the transition to the 19th century, the individual perissodactyl genera with various other groups, associated with the proboscidean and even-toed ungulates, saw the establishment of the term "pachyderm" (Pachydermata); Étienne Geoffroy Saint-Hilaire (1772- 1844) and Georges Cuvier (1769–1832), in 1795, introduced the rhinos and elephants, the hippos, pigs, peccaries, tapirs and hyrax as pachyderms.[15][18][42][43] The horses were but largely as a of the other mammals separated group and were often classified under the name Solidungula.[44][45]

Henri Marie Ducrotay de Blainville (1777–1850), in 1861, classified ungulates by the structure of the feet, and so, animals differed with an even number of toes from those with an odd number of toes. He pushed the horses as solidungulate near the tapirs and rhinos as multungulate animals and referred all together as onguligrades. Richard Owen (1804–1892) invoked in his study fossil mammals of the Isle of Wight on Blainville and introduced the name Perissodactyla.[15][18]

Othniel Charles Marsh (1831–1899), in 1884, came up with the concept Mesaxonia. This comprises the present members of the odd-toed ungulates, including their extinct relatives, of which the hyrax was explicitly closed. Mesaxonia is now considered synonymous to Perissodactyla. He still used subordination terms (rhinos, horses, tapirs), while Perissodactyla stood as a designation for the entire order, including the hyrax. The assumption that the hyrax were part of Perissodactyla remained well into the 20th century.[46] Only with the advent of molecular genetic research methods had it been recognized that the hyrax is not closely related to the perissodactyls but more with the elephants and manatees.[1][47]

Interactions with humans

The quagga became extinct by the end of the 19th century.

The domestic horse and the donkey are especially used for transportation, working and pack animals and play an important role in human history. The domestication of both species began several millennia ago. Due to the motorisation of agriculture and the spread of the automobile traffic, such use has declined sharply in Western industrial countries; riding is usually operated more as a hobby or sport. In the less developed regions of the world, the use of these animals, however, is still widespread. To a lesser extent, horses and donkeys are also kept for their meat and their milk.

In contrast, the stocks of almost all other species of odd-toed ungulates have declined dramatically by hunting and habitat destruction. The quagga is extinct, and the Przewalski's Horse is considered extinct in the wild.

Present threat levels, according to IUCN (2012):[48]

References

  1. 1 2 Dan Graur; Manolo Gouy; Laurent Duret (1997). "Evolutionary Affinities of the Order Perissodactyla and the Phylogenetic Status of the Superordinal taxa Ungulata and Altungulata Molecular Phylogenetics and Evolution". Molecular Phylogenetics and Evolution 7: 195–200. doi:10.1006/mpev.1996.0391.
  2. ↑ Jingyang Hu; Zhang Yaping; Li Yu (2012). "Summary of Laurasiatheria (Mammalia) Phylogeny". Zoological Research 33: 65–74. doi:10.3724/sp.j.1141.2012.e05-06e65.
  3. ↑ Hidenori Nishihara; Masami Hasegawa; Norihiro Okada. "Pegasoferae, an unexpected mammalian clade revealed by tracking ancient retroposon insertions". Proceedings of the National Academy of Sciences USA 103: 9929–9934. doi:10.1073/pnas.0603797103. PMC 1479866. PMID 16785431.
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  5. ↑ Ross MacPhee; Frido Welker; Jessica Thomas; Selina Brace; Enrico Cappellini; Samuel Turvey; Ian Barnes; Marcelo Reguero; Javier Gelfo; Alejandro Kramarz (2014). "Ancient protein sequencing Resolves litoptern and notoungulate superordinal affinities". The History of Life: A view from the Southern Hemisphere: 186.
  6. ↑ Christelle Tougard; Thomas Delefosse; Catherine Hänni; Claudine Montgelard (2001). "Phylogenetic Relationships of the Five Extant Rhinoceros species (Rhinocerotidae, Perissodactyla) Based on Mitochondrial Cytochrome b and 12S rRNA gene". Molecular Phylogenetics and Evolution 19: 34–44. doi:10.1006/mpev.2000.0903. PMID 11286489.
  7. ↑ Cynthia C. Steiner; Oliver A. Ryder (2011). "Molecular phylogeny and evolution of the Perissodactyla". Zoological Journal of the Linnean Society 163: 1289–1303. doi:10.1111/j.1096-3642.2011.00752.x.
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  9. ↑ Cooper L. N., Seiffert E. R., Clementz M., Madar S. I., Bajpai S., Hussain S. T., Thewissen J. G. M. (2014). "Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls". PLoS ONE 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMID 25295875.
  10. 1 2 3 Donald R. (2009). "Evolutionary Transitions in the Fossil Record of Terrestrial Hoofed Mammals". Evolution: Education and Outreach 2: 289–302. doi:10.1007/s12052-009-0136-1.
  11. ↑ unknown (2004). "PERISSODACTYLA – odd-toed ungulates, i.e. horses, tapirs, rhinos and relatives". Mikko's Phylogeny Archive.
  12. ↑ Ravel Anthony, Orliac Maeva (2014). "The inner ear morphology of the 'condylarthran' Hyopsodus lepidus". Historical Biology 27: 8. doi:10.1080/08912963.2014.915823.
  13. 1 2 Cooper, L. N.; Seiffert, E. R.; Clementz, M.; Madar, S. I.; Bajpai, S.; Hussain, S. T.; Thewissen, J. G. M. (2014-10-08). "Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls". PLoS ONE 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMID 25295875.
  14. 1 2 Luke T. Holbrook; Joshua Lapergola (2011). "A new genus of perissodactyl (Mammalia) from the Bridgerian of Wyoming, with comments on basal perissodactyl phylogeny". Journal of Vertebrate Paleontology 31: 895–901. doi:10.1080/02724634.2011.579669.
  15. 1 2 3 George Gaylord (1945). "The Principles of Classification and a Classification of Mammals". Bulletin of the American Museum of Natural History: 252–258.
  16. ↑ Horace Elmer Wood (1934). "Revision of the Hyrachyidae". Bulletin of the American Museum of Natural History: 181–295.
  17. ↑ Horace Elmer Wood (1937). "Perissodactyl suborders". Journal of Mammalogy 18: 106. doi:10.1093/jmammal/18.1.106.
  18. 1 2 3 4 Robert M. Schoch (1989). "A brief historical review of perissodactyl classification". The Evolution of Perissodactyls. Oxford University Press. pp. 13–23.
  19. ↑ William Berryman Scott (1941). "Part V: Perissodactyla". The Mammalian fauna of the White River Oligocene Transactions of the American Philosophical Society New Series 28: 747–964. doi:10.2307/1005518.
  20. ↑ Leonard B. Radinsky (1964). "Paleomoropus, a new early Eocene chalicothere (Mammalia, Perissodactyla), and a revision of Eocene chalicotheres". American Museum Novitates: 1–28.
  21. ↑ JJ Hooker (1984). "A primitive ceratomorph (Perissodactyla, Mammalia) from the Early Tertiary of Europe". Zoological Journal of the Linnean Society 82: 229–244. doi:10.1111/j.1096-3642.1984.tb00545.x.
  22. ↑ Robert Milton Schoch (1984). "Two unusual specimens of the Yale Peabody Museum Helaletes in collections, and some comments on the ancestry of the Tapiridae (Perissodactyla, Mammalia)". Peabody Museum, Yale University: 1–20.
  23. ↑ Luke T. Holbrook (2001). "Comparative osteology of early Tertiary tapiromorphs (Mammalia, Perissodactyla)". Zoological Journal of the Linnean Society 132: 1–54. doi:10.1111/j.1096-3642.2001.tb02270.x.
  24. ↑ Malcolm C. McKenna; Chow Minna; Suyin Ting; Luo Zhexi (1989). "Radinskya yupingae, a perissodactyl-like mammal from the Late Palaeocene of China". The Evolution of Perissodactyls. Oxford University Press. pp. 24–36.
  25. ↑ Kenneth D. Rose (2006). The Beginning of the Age of Mammals. Johns Hopkins University Press. pp. 242–267.
  26. ↑ Sunil Bajpai; Vivesh Kapur; Debasis P. Das; BN Tiwari; N. Saravanan; Ritu Sharma (2005). "Early Eocene Land Mammals from Vastan Lignite Mine, District Surat (Gujarat), western India". Journal of the Palaeontological Society of India: 101–113.
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