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Amynodontidae

(Redirected from Metamynodontini)

Amynodontidae is an extinct family of rhinocerotoids, known from the Eocene to Oligocene of North America, Europe, and Asia. They were among the dominant groups of herbivorous mammals in the Northern Hemisphere, and were most diverse in Asia. Amynodonts had a hippopotamus-like build, with massive bodies, short and robust limbs, and tusk-like canine teeth. They have been nicknamed swamp rhinoceroses[2][3][4] due to historically being interpreted as semiaquatic; modern research suggests that only some members of the group had semiaquatic lifestyles.

Amynodonts
Temporal range: Middle Eocene–Late Oligocene
Mounted skeleton of Amynodon
Mounted skeleton of Metamynodon
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Placentalia
Order: Perissodactyla
Superfamily: Rhinocerotoidea
Family: †Amynodontidae
Scott & Osborn, 1883[1]
Genera and subgroups

Derived amynodonts are generally grouped into the lightly built and terrestrial tapir-like cadurcodontines and the semiaquatic metamynodontines. Amynodonts increased in body size over the course of their evolutionary history, and ranged in weight from just over a hundred kilograms ("Amynodon" sinensis) to close to three tonnes (Zaisanamynodon).

Description

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Skull of Zaisanamynodon
Life restoration of Amynodontopsis

Amynodonts had several hippopotamus-like features, such as massive bodies and short, robust limbs.[5] They were medium-sized to large animals.[6] It was traditionally believed that the entire family was adapted to semiaquatic life, but different amynodonts are now known to have been adapted to different lifestyles.[6] The derived metamynodontines had clear adaptations towards a semiaquatic lifestyle but the other group of derived amynodonts, the cadurcodontines, were more lightly built and terrestrial, probably similar to modern tapirs.[6][7] Amynodon, not placed in either tribe, has been interpreted as a terrestrial browser that lived in open forest-savanna habitats,[6] though isotope analyses of its teeth may also indicate a semiaquatic lifestyle.[3]

The most significant synapomorphy (shared unique trait) in the amynodont skull were preorbital fossae (grooves in front of the eye orbits). The fossae differed in size and position depending on lineage; in primitive amynodonts (such as Amynodon), there are large fossae in front of the orbits.[6] In the more derived amynodont groups, the condition of the fossae becomes a distinguishing trait; metamynodontines have reduced fossae whereas cadurcodontines have fossae that extend medial to the orbit (towards the middle of the orbits).[6] Amynodont skulls had sagittal crests, a feature shared with some other early rhinocerotoids, such as Hyrachyus.[6] Another feature unique to amynodonts, which separates them from other rhinocerotoids, is the quadratic outline of third upper molar.[8]

Amynodonts had greatly enlarged and tusk-like canine teeth.[6][9] This further differentiates them from other rhinocerotoids, who either have small canines or lack them entirely. When other rhinocerotoid groups developed tusks, they were typically developed from the incisor teeth.[9] The fossil evidence suggests that canine size in amynodonts was a sexually dimorphic trait.[6]

Amynodonts had four toes on their manus (front feet) and three on their pes (hind feet),[6] a primitive condition among perissodactyls.[10]

History of discovery

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The first amynodont to be described was Cadurcotherium cayluxi in 1873, described by Paul Gervais from the Oligocene of France.[9] The first known North American amynodont fossils were found in the Uinta Formation of Utah and described by Othniel Charles Marsh. In 1875, Marsh named the new species Diceratherium advenum based on a fossil skull. Marsh soon realized that the fossil was distinct from Diceratherium, a previously described true rhinoceros (Rhinocerotidae), and moved it to the new genus Amynodon, as A. advenus, in 1877.[6]

In 1887, the new amynodont genus Metamynodon was described based on fossils from the White River Group.[6] Based on Metamynodon, William Berryman Scott and Henry Fairfield Osborn were able to establish several important family-level characteristics of the amynodonts, including the enlarged sagittal crest and the loss of P1 (the first premolar tooth).[6][9]

The first discoveries of amynodonts in Asia were made in Oligocene deposits in Pakistan and in Eocene deposits in Myanmar in the 1910s and 1920s. In the 1920s and 1930s, amynodont fossils were also discovered in Mongolia and China.[9]

Classification

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External relations

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Front teeth of various extinct rhinocerotoids: the amynodont Metamynodon (A), the hyracodont Hyracodon (B), the paracerathere Paraceratherium (C), and the rhinoceros Trigonias (D)

Marsh believed Amynodon to be a rhinoceros and did not separate it from other rhinoceroses above the genus level.[6] The separate family Amynodontidae was created in 1883 by Scott and Osborn in order to contain the two genera Amynodon and Orthocynodon, and distinguish them from both the rhinocerotids and the hyracodontids, another extinct rhinocerotoid group.[6] Orthocynodon would later be designated as a junior synonym of Amynodon by Osborn in 1890.[8] In 1889, Osborn published the first detailed comparison of Amynodontidae, Hyracodontidae, and Rhinocerotidae, and established the Amynodontidae as clearly distinct based mainly on their large canines and their shortened face (a trait later found to only apply to derived members of the family).[6]

The relationship between amynodonts and other rhinocerotoids is disputed.[6] Leonard B. Radinsky argued in 1969 that amynodonts were not rhinocerotoids at all, but that they had evolved independently from tapiroid ancestors.[6] In 1998, Donald Prothero placed the amynodonts in Rhinocerotoidea, as the sister taxon to a clade formed by Hyracodontidae and Rhinocerotidae.[6] Recent revisions do not reach a consensus on rhinocerotoid relations. Amynodonts have been recovered both as the sister group of paraceratheres,[11] or in a more basal position as the sister group of the clade containing eggyosodonts, paraceratheres, and rhinocerotids.[12]

Internal systematics

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Partial skull of Sellamynodon, from Late Eocene or Early Oligocene Romania

The first attempt to establish the relations between amynodont genera was done by Miklós Kretzoi in 1942. Kretzoi established four amynodont subfamilies: Amynodontinae, Cadurcotheriinae, Metamynodontinae, and Paramynodontinae.[6] In 1954, Vera Gromova rejected Kretzoi's classification scheme and did recognize any subgroups inside Amynodontidae.[9]

In the 1980s, William P. Wall attempted to refine the internal classification of the amynodonts. Wall was the first to perform a phylogenetic analysis of the group, and proposed two amynodont subfamilies: Rostriamynodontinae (containing only Rostriamynodon) and Amynodontinae (containing all other amynodonts). The genus Caenolophus was placed as Amynodontidae incertae sedis.[6] Wall further divided the Amynodontinae into three tribes, based on the preorbital fossae. The Amynodontini were characterized by a large fossae that does not extend medial to the orbit, the Cadurcodontini by large fossae that did extend medial to the orbit, and the Metamynodontini by reduced fossae.[6] Wall placed the genera Amynodontopsis, Cadurcodon, Sharamynodon, and Sianodon in Cadurcodontini, and Cadurcotherium, Megalamynodon, Metamynodon, Paramynodon, and Zaisanamynodon in Metamynodontini.[9]

The cladogram below presents the results of Wall's 1989 phylogenetic analysis of amynodont genera:[13]

Amynodontidae
Rostriamynodontinae

Rostriamynodon

Amynodontinae

In 1986, Donald Prothero, Earl Manning, and C. Bruce Hanson included Wall's data in a wider analysis of the Rhinocerotoidea. The recovered relationships were identical to Wall's analysis, but Prothero, Manning, and Hanson used different names for the subdivisions, proposing the subfamilies Cadurcodontinae (=Cadurcodontini) and Paramynodontinae (=Metamynodontini).[14]

In 2017, Alexander Averianov, Igor Danilov, Jin Jianhua , and Wang Yingyong published a new phylogenetic analysis of the amynodonts as part of their description of the species Cadurcodon maomingensis.[9] Their analysis recovered a sequential branching of basal amynodonts, and two supported clades of derived amynodonts, roughly corresponding to Wall's Cadurcodontini and Metamynodontini, and these names were thus retained.[9] The names "Rostriamynodontinae", "Amynodontinae", and "Amynodontini" were not retained, with more basal amynodonts merely referred to as early diverging amynodontids.[9] The results differed from Wall's in placing Cadurcotherium and Zaisanamynodon with the cadurcodontines rather than metamynodontines.[9] Amynodontopsis was recovered as relatively basal, outside the cadurcodontines.[9]

In 2018, Jérémy Tissier, Damien Becker, Vlad Codrea, Loïc Costeur, Cristina Fărcaş, Alexandru Solomon, Marton Venczel, and Olivier Maridet performed a new phylogenetic analysis as part of the description of several new amynodont fossils from Eastern Europe. The results were similar to those of Averianov and colleagues, but differed in placing Amynodontopsis at a more derived position, with the metamynodontines, as well as in the assessment of the species Procadurcodon orientalis and Zaisanamynodon protheroi.[15] Averianov et al. considered Procadurcodon to be valid, with Z. protheroi as a possible synonym,[9] whereas later authors have considered Procadurcodon as synonymous with Zaisanamynodon, and Z. protheroi valid.[11][15] In 2023, a more comprehensive phylogenetic analysis by Léa Veine-Tonizzo, Tissier, Maia Bukhisianidze, Davit Vasilyan, and Damien Becker produced a result similar to the 2018 analysis.[11]

List of genera

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  Disputed validity
  Disputed classification
Genus Temporal range Location Synonym(s) Notes Ref
Amynodon Middle Eocene United States Orthocynodon [6][8]
Amynodontopsis Middle–Late Eocene China
Hungary?
Mexico
Romania?
United States		 The assignment of European fossils to Amynodontopsis has been questioned.[16] [6][15][16]
Armania Late Eocene Mongolia Fragmentarily known but relatively specialized rhinocerotoid.[17] Variously classified as an amynodont[18][19] or a hyracodont.[20][21] [21]
Cadurcodon Late Eocene–Middle Oligocene Bulgaria
China
Kazakhstan
Mongolia		 Paracadurcodon, Sianodon [9][15]
Caenolophus Middle–Late Eocene China
North Korea		 Euryodon Very primitive amynodont, originally described as a hyracodont.[9][22] [9]
Cadurcotherium Oligocene Bosnia-Herzegovina
France
Germany
Kazakhstan
Pakistan
Spain
 Switzerland		 Cadurcamynodon [9][23]
Gigantamynodon Late Eocene China
Mongolia		 Type species is a nomen dubium,[9] may be a synonym of Zaisanamynodon.[24][25] Status of other species variable.[25] [9]
Hypsamynodon Late Eocene Mongolia Has been proposed to be a synonym of Cadurcotherium.[9] [9]
Huananodon Middle or Late Eocene China Based on poor fossil material[22] but the teeth are distinct[26] and characteristic of amynodonts[27] [16][22]
Megalamynodon Middle Eocene United States [6]
Metamynodon Middle Eocene–Middle Oligocene United States Cadurcopsis [6]
Paramynodon Middle–Late Eocene Myanmar [9]
Rostriamynodon Middle Eocene China [9]
Sellamynodon Late Eocene or Early Oligocene Romania [15]
Sharamynodon Middle–Late Eocene China
Kazakhstan
Kyrgyzstan		 Andarakodon, Lushiamynodon [9][15][28]
Teilhardia Middle Eocene China Very primitive amynodont, originally described as a hyracodont.[29] [29]
Toxotherium Late Eocene Canada
United States		 Uncertain placement wihin Perissodactyla. Possibly an "aberrant and diminutive" amynodont.[30] [30][31]
Zaisanamynodon Late Eocene China
Japan
Kazakhstan
Russia
United States		 Procadurcodon Largest known amynodont.[9] [11][32]

Species incertae sedis

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Several additional amynodont fossils have been reported and assigned to the genera listed above but their classification at the genus level has been questioned in recent taxonomic revisions.[9]

Taxon Temporal range Location Synonym(s) Notes Ref
"Amynodon" altidens Late Eocene China May belong to Sharamynodon [8][33]
"Amynodon" alxaensis Early Oligocene China May belong to Sharamynodon [8][34]
"Amynodon" hungaricus Late Eocene Hungary [15]
"Amynodon" lunanensis Late Eocene China May belong to Sharamynodon or Amynodontopsis [8][20][35]
"Amynodon" reedi Middle Eocene United States More similar to Zaisanamynodon than Amynodon, recovered outside Amynodon in phylogenetic analysis [15][36]
"Amynodon" sinensis Late Eocene China "Sianodon" chiyuanensis?, "S". honanensis?, "S." mienchiensis? Smallest known amynodont [9]
"Amynodon" watanabei Middle–Late Eocene Japan May belong to Sharamynodon [9][13]
"Lushiamynodon" obesus Late Eocene China Problematic, based on fragmentary remains [9][37]
"Lushiamynodon" sharamurenensis Late Eocene China Problematic, based on fragmentary remains [9][38]
"Lushiamynodon" wuchengensis Late Eocene China Problematic, based on fragmentary remains [9][39]
"Sianodon" gaowangouensis Early Oligocene China Does not belong to Cadurcodon since it had three pairs of upper incisors [9]
"Sianodon" ulausensis Late Eocene China Does not belong to Cadurcodon since it had three pairs of upper incisors [9]

Evolutionary history and paleobiogeography

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Map of amynodont fossil finds from the Middle–Late Eocene (top) and Oligocene (bottom)

Amynodonts probably originated in Asia[5][6][9] during the Early Eocene.[9] The Early–Middle Eocene deposits of the Irdin Manha Formation in China have yielded fossils of the oldest well-defined member of the group, Rostriamynodon.[6] Caenolophus, contemporary with Rostriamynodon, may be the most primitive amynodont; it is fragmentarily known but may have been anatomically intermediate between later amynodonts and more primitive rhinocerotoids.[9] Teilhardia, known from the same area, is also interpreted as a primitive amynodont, and has been suggested to be even more primitive than (and ancestral to) Caenolophus.[29]

Amynodonts spread to North America shortly after their initial appearance[5] and achieved a Holarctic distribution.[6] They became one of the dominant herbivore groups of their time.[9] In North America, the temporal range of the amynodonts extends from the Middle Eocene to the Early Oligocene.[6] The earliest known North American example of the group is Amynodon, from the late Bridgerian and Uintan land mammal ages.[6][9] The amynodonts were at their greatest taxonomic diversity in Asia, especially in Central Asia.[9] Amynodonts increased in body size over the course of their evolutionary history, the smallest known amynodont is the relatively basal "Amynodon" sinensis, estimated at 127 ± 15 kg (280 ± 33 lbs), whereas the largest are the derived Zaisanamynodon borisovi, at 2442 ± 257 kg (5384 ± 567 lbs), and Zaisanamynodon protheroi, at 2720 kg (5997 lbs).[9]

Migration across Beringia was highly important in amynodont evolution.[6] The ancestors of the North American Amynodon and Amynodontopsis bodei were probably amynodonts that migrated from Asia separately; Amynodontopsis bodei is more derived than Amynodon[9] but likely descended from the Asian species Amynodontopsis jiyuanensis.[16] Phylogenetic analyses suggest that there may have been as many as four or five dispersal events where new amynodonts spread from Asia into North America.[9] There were also several dispersal events into Europe. Cadurcotherium is the only genus recorded from Western Europe[9] but Cadurcodon and Sellamynodon are known from Eastern Europe.[15] Amynodonts do not appear to have reached Northern Europe.[15] The migration of amynodonts into Europe in the Oligocene may have been facilitated by the preceding Grande Coupure extinction event.[15]

The Middle to Late Eocene was a time of great environmental change, which may have facilitated the evolutionary radiation of the amynodonts. In North America, the environment changed during this time from lush semitropical forests to being dominated by open forests and grasslands.[6] The timeframe corresponding to the late Uintan and the Duchesnean land mammal ages in North America saw the emergence of the cadurcodontines and metamynodontines, illustrating diversification and pursuits of different lifestyles.[6] Similar to North American examples, Asian amynodonts also appear to diversify around the boundary between the Eocene and Oligocene.[6] The metamynodontines experienced evolutionary radiation in North America, represented by the genera Megalamynodon and Metamynodon.[9] Following the extinction of the brontotheres in the Late Eocene, Metamynodon was the largest mammal in Early Oligocene North America.[40] The record of carcudontines in North America is sparse and depends on the phylogenetic position of Amynodontopsis and the assessments of the species Zaisanamynodon protheroi and Amynodon reedi.[9][15]

Amynodonts were more successful in Asia than in North America, perhaps due to the increasing aridity in North America going into the Oligocene.[6] Metamynodontines survived in North America into the Oligocene; Metamynodon lived in river systems across most of the continent, but was a rare element of the Oligocene fauna and was extinct by the end of the Whitneyan land mammal age (late Early Oligocene).[6] Amynodonts in Europe disappeared in the latest Oligocene.[15] Amynodonts survived the longest in Asia,[6][9] where Cadurcotherium persisted in modern-day Pakistan until the latest Oligocene.[9] The last Asian amynodonts were previously believed to have reached into the early Miocene, but this dating has proven incorrect.[15]

References

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  1. ↑ Scott, W. B.; Osborn, Henry F. (1883). "On the Skull of the Eocene Rhinoceros, Orthocynodon, and the Relation of this Genus to other Members of the Group". Contributions from the E. M. Museum of Geology and Archæology of Princeton College. 3: 1–22. hdl:2027/mdp.39015057223565.
  2. ↑ Velichko, A. A.; Wright, Herbert Edgar (2005). Cenozoic Climatic and Environmental Changes in Russia. Geological Society of America. p. 145. ISBN 978-0-8137-2382-2.
  3. 1 2 Michalski, John Michael (2024). Tales in the Teeth: Using Isotopic Analysis to Understand the Paleoecology and Social Behavior of Brontotheres of the Intermountain Basins During the Middle Eocene (Master's thesis). Utah State University. p. 38.
  4. ↑ Akhmetiev, M. A.; Manchester, S. R. (2000). "A New Species of Palaeocarpinus (Betulaceae) from the Paleogene of the Eastern Sikhote-Alin". Paleontological Journal. 34 (4): 469–474.
  5. 1 2 3 Prothero, Donald R. (2016). The Princeton Field Guide to Prehistoric Mammals. Princeton University Press. p. 196. ISBN 978-0-691-15682-8.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Wall, William P. (1998). "Amynodontidae". Evolution of Tertiary Mammals of North America: Volume 1: Terrestrial Carnivores, Ungulates, and Ungulate Like Mammals. Cambridge University Press. pp. 583–588. ISBN 978-0-521-35519-3.
  7. ↑ Wall, W. P.; Heinbaugh, K. L. (1999). "Locomotor adaptations in Metamynodon planifrons compared to other amynodontids (Perissodactyla, Rhinocerotoidea)" (PDF). National Parks Paleontological Research. 4: 8–17. Archived from the original (PDF) on 19 February 2014. Retrieved 1 September 2018.
  8. 1 2 3 4 5 6 Wall, William P. (1982). "The Genus Amynodon and Its Relationship to Other Members of the Amynodontidae (Perissodactyla, Rhinocerotoidea)". Journal of Paleontology. 56 (2): 434–443. ISSN 0022-3360. JSTOR 1304471.
  9. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Averianov, Alexander; Danilov, Igor; Jin, Jianhua; Wang, Yingyong (2017). "A new amynodontid from the Eocene of South China and phylogeny of Amynodontidae (Perissodactyla: Rhinocerotoidea)". Journal of Systematic Palaeontology. 15 (11): 927–945. Bibcode:2017JSPal..15..927A. doi:10.1080/14772019.2016.1256914. ISSN 1477-2019.
  10. ↑ Rose, Kenneth D. (2006). "Brontotheriidae". The Beginning of the Age of Mammals. Johns Hopkins University Press. pp. 250–252. ISBN 978-0-8018-8472-6.
  11. 1 2 3 4 5 Veine-Tonizzo, Lea; Tissier, Jérémy; Bukhsianidze, Maia; Vasilyan, Davit; Becker, Damien (2023). "Cranial morphology and phylogenetic relationships of Amynodontidae Scott and Osborn, 1883 (Perissodactyla, Rhinocerotoidea)". Comptes Rendus. Palevol. 22 (8): 109–142. doi:10.5852/cr-palevol2023v22a8.
  12. ↑ Bai, Bin; Meng, Jin; Zhang, Chi; Gong, Yan-Xin; Wang, Yuan-Qing (2020). "The origin of Rhinocerotoidea and phylogeny of Ceratomorpha (Mammalia, Perissodactyla)". Communications Biology. 3 (1): 509. doi:10.1038/s42003-020-01205-8. ISSN 2399-3642. PMC 7490376. PMID 32929169.
  13. 1 2 Wall, William P. (1989). "The phylogenetic history and adaptive radiation of the Amynodontidae". The Evolution of Perissodactyls. Oxford University Press. pp. 341–354. ISBN 0-19-506039-3.
  14. ↑ Prothero, Donald R.; Manning, Earl; Hanson, C. Bruce (1986). "The phylogeny of the Rhinocerotoidea (Mammalia, Perissodactyla)". Zoological Journal of the Linnean Society. 87 (4): 341–366. doi:10.1111/j.1096-3642.1986.tb01340.x.
  15. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Tissier, Jérémy; Becker, Damien; Codrea, Vlad; Costeur, Loïc; Fărcaş, Cristina; Solomon, Alexandru; Venczel, Marton; Maridet, Olivier (2018). "New data on Amynodontidae (Mammalia, Perissodactyla) from Eastern Europe: Phylogenetic and palaeobiogeographic implications around the Eocene-Oligocene transition". PLOS ONE. 13 (4) e0193774. Bibcode:2018PLoSO..1393774T. doi:10.1371/journal.pone.0193774. ISSN 1932-6203. PMC 5905962. PMID 29668673.
  16. 1 2 3 4 Wang, Xiao-Yang; Wang, Yuang-Qing; Zhang, Rui; Zhang, Zhong-Hui; Liu, Xiao-Ling; Ren, Li-Ping (2020). "A new species of Amynodontopsis (Perissodactyla: Amynodontidae) from the Middle Eocene of Jiyuan, Henan, China". Vertebrata PalAsiatica. 58 (3). doi:10.19615/j.cnki.1000-3118.200313. ISSN 2096-9899. Archived from the original on 18 July 2025.
  17. ↑ Dashzeveg, Demberelyin (1991). "Hyracodontids and Rhinocerotids (Mammalia, Perissodactyla, Rhinocerotoidea) from the Paleogene of Mongolia" (PDF). Palaeovenebrata. 21 (1–2): 1–84.
  18. ↑ von Koenigswald, W.; Holbrook, L.T.; et al. (March 2011). "Diversity and Evolution of Hunter-Schreger Band Configuration in Tooth Enamel of Perissodactyl Mammals". Acta Palaeontologica Polonica. 56 (1): 11–32. Bibcode:2011AcPaP..56...11K. doi:10.4202/app.2010.0021. S2CID 33679289.
  19. ↑ Lopatin, A. V. (2020). "A Review of the Mesozoic and Cenozoic Mammals of Mongolia". Paleontological Journal. 54 (7): 779–808. Bibcode:2020PalJ...54..779L. doi:10.1134/S0031030120070084. ISSN 1555-6174.
  20. 1 2 Tsubamoto, Takehisa; Tsogtbaatar, Khishigjav; Tsogtbaatar, Chinzorig; Egi, Naoko (2022). "A brief review of the updated fossil vertebrate fauna of the upper Eocene Ergilin Dzo Formation, southeastern Mongolia". Memoirs of the Faculty of Science, Ehime University. 24: 64–83. doi:10.60217/0002002445. ISSN 0919-5203.
  21. 1 2 Bai, Bin; Meng, Jin; Janis, Christine M.; Zhang, Zhao‐Qun; Wang, Yuan‐Qing (2020). "Perissodactyl diversities and responses to climate changes as reflected by dental homogeneity during the Cenozoic in Asia". Ecology and Evolution. 10 (13): 6333–6355. doi:10.1002/ece3.6363. ISSN 2045-7758. PMC 7381588. PMID 32724516.
  22. 1 2 3 Wall, William P.; Manning, Earl (1986). "Rostriamynodon grangeri n. gen., n. sp. of amynodontid (Perissodactyla, Rhinocerotoidea) with comments on the phylogenetic history of Eocene Amynodontidae". Journal of Paleontology. 60 (4): 911–919. Bibcode:1986JPal...60..911W. doi:10.1017/S0022336000043079. ISSN 0022-3360.
  23. ↑ Ménouret, Bernard (2018). "Le genre Cadurcotherium (Rhinocerotoidea) en Europe; synthèse des connaissances et révision systématique". Revue de Paléobiologie, Genève. 37 (2): 495–517. doi:10.5281/zenodo.2545121.
  24. ↑ Lucas, Spencer G.; Emry, Robert J.; Bayshashov, Bolat U. (1996). "Zaisanaminodon, a Late Eocene Amynodontid (Mammalia, Perissodactyla) from Kazakhstan and China". Tertiary Research. 17 (1–2): 51–58.
  25. 1 2 Lucas, Spencer G.; Emry, Robert J. (1996). "Biochronological Significance of Amynodontidae (Mammalia, Perissodactyla) from the Paleogene of Kazakhstan". Journal of Paleontology. 70 (4): 691–696. Bibcode:1996JPal...70..691L. doi:10.1017/S0022336000023647. ISSN 0022-3360. JSTOR 1306530.
  26. ↑ Bai, Bin; Meng, Jin; Janis, Christine M.; Zhang, Zhao-Qun; Wang, Yuan-Qing (2020). "Perissodactyl diversities and responses to climate changes as reflected by dental homogeneity during the Cenozoic in Asia". Ecology and Evolution. 10 (13): 6333–6355. Bibcode:2020EcoEv..10.6333B. doi:10.1002/ece3.6363. ISSN 2045-7758. PMC 7381588. PMID 32724516.
  27. ↑ Bai, Bin; Meng, Jin; Mao, Fang-Yuan; Zhang, Zhao-Qun; Wang, Yuan-Qing (2019). "A new early Eocene deperetellid tapiroid illuminates the origin of Deperetellidae and the pattern of premolar molarization in Perissodactyla". PLOS ONE. 14 (11) e0225045. Bibcode:2019PLoSO..1425045B. doi:10.1371/journal.pone.0225045. ISSN 1932-6203. PMC 6839866. PMID 31703104.
  28. ↑ Lucas, Spencer G.; Emry, Robert J. (2001). "Sharamynodon (Mammalia: Perissodactyla) from the Eocene of the Ily basin, Kazakstan and the antiquity of Asian amynodonts" (PDF). Proceedings of the Biological Society of Washington. 114 (2): 517–525.
  29. 1 2 3 Radinsky, Leonard B. (1967). "A Review of the Rhinocerotoid Family Hyracodontidae (Perissodactyla)". Bulletin of the American Museum of Natural History. 136 (1).
  30. 1 2 Radinsky, Leonard B. (1969). "The Early Evolution of the Perissodactyla". Evolution. 23 (2): 308–328. doi:10.2307/2406794. ISSN 0014-3820.
  31. ↑ Koenigswald, Wighart Von; Holbrook, Luke T.; Rose, Kenneth D. (2011). "Diversity and Evolution of Hunter-Schreger Band Configuration in Tooth Enamel of Perissodactyl Mammals". Acta Palaeontologica Polonica. 56 (1): 11–32. doi:10.4202/app.2010.0021. ISSN 0567-7920.
  32. ↑ Lucas, Spencer G.; Emry, Robert J. (1996). "Biochronological Significance of Amynodontidae (Mammalia, Perissodactyla) from the Paleogene of Kazakhstan". Journal of Paleontology. 70 (4): 691–696. ISSN 0022-3360.
  33. ↑ Xu, Yu-Xuan; Chiu, Chan-siang (1962). "Early Tertiary Mammalian Fossils from Lunan, Yunnan" (PDF). Vertebrata PalAsiatica. 6: 313–332.
  34. ↑ Qi, Tao (1975). "An Early Oligocene Mammalian Fauna of Ningxia" (PDF). Vertebrata PalAsiatica. 13: 217–224.
  35. ↑ Chow, Minchen; Xu, Yu-Xuan; Zhen, Shuo-nan (1964). "Amynodon from the Eocene of Lunan, Yunnan". Vertebrata PalAsiatica. 8: 355–360.
  36. ↑ Stock, Chester (1939). "Eocene Amynodonts from Southern California". Proceedings of the National Academy of Sciences of the United States of America. 25 (6): 270–275. Bibcode:1939PNAS...25..270S. doi:10.1073/pnas.25.6.270. ISSN 0027-8424. JSTOR 87289. PMC 1077766. PMID 16577895.
  37. ↑ Chow, Minchen; Xu, Yu-Xuan (1965). "Amynodonts from the upper Eocene of Honan and Shansi". Vertebrata PalAsiatica. 9 (2): 190–204.
  38. ↑ Bai, Bin; Wang, Yuan-Qing; Li, Qian; Wang, Hai-Bing; Mao, Fang-Yuan; Gong, Yan-Xin; Meng, Jin (2018). "Biostratigraphy and Diversity of Paleogene Perissodactyls from the Erlian Basin of Inner Mongolia, China". American Museum Novitates (3914): 1–60. doi:10.1206/3914.1. ISSN 0003-0082.
  39. ↑ Wang, J. (1978). "Fossil Amynodontidae and Ischyromyidae of Tongbo, Henan". Vertebrata PalAsiatica. 16 (1): 22–29.
  40. ↑ Prothero, Donald R.; Schoch, Robert M. (2002). Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals. Johns Hopkins University Press. ISBN 978-0-8018-7135-1.
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