Hyaenodon

Hyaenodon Temporal range: Middle Eocene to Early Miocene (Bartonian to Burdigalian) 38–17  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted H. sp. skeleton, Science Museum of Minnesota
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	† Hyaenodonta
Superfamily:	† Hyaenodontoidea
Family:	† Hyaenodontidae
Subfamily:	† Hyaenodontinae
Tribe:	† Hyaenodontini Leidy, 1869 [1]
Genus:	† Hyaenodon Laizer & Parieu, 1838
Type species
†Hyaenodon leptorhynchus Laizer and Parieu, 1838
Species
[see classification]
Synonyms [2]
synonyms of genus: Gobipterodon(Lavrov, 1999) [3] Macrohyaenodon(Lavrov & Emry, 1998) Macropterodon(Lavrov, 1999) Megalopterodon(Dashzeveg, 1964) Microhyaenodon(Lavrov, 1999) [3] Neohyaenodon(Thorpe, 1922) Prohyaenodon(Lavrov, 1999) Protohyaenodon(Stock, 1933) Pseudopterodon(Schlosser, 1887) Taxotherium(Blainville, 1841) synonyms of species: H. brachyrhynchus: Canis brachyrhynchus(Blainville, 1841) Hyaenodon brachyrhenchus(Lavrov, 1999) Hyaenodon cuvieri(Pictet, 1853) [4] Hyaenodon leptorhynchus(Dujardin, 1840) [5] Hyaenodon parisiensis(Laurillard, 1845) [6] Hyaenodon vulpinum(Filhol, 1877) Hyaenodon vulpinus(Gervais, 1873) Nasua parisiensis(Blainville, 1841) Pterodon brachyrhynchus(Pomel, 1846) [7] Pterodon cuvieri(Pomel, 1846) Taxotherium parisiense(Blainville, 1841) H. brevirostrus: Hyaenodon brevirostris(Joeckel, 1997) [8] Protohyaenodon brevirostrus(Mellett, 1977) H. chunkhtensis: Microhyaenodon chunkhtensis(Lavrov, 1999) H. crucians: Hyaenodon leptocephalus(Scott, 1888) [9] Hyaenodon minutus(Douglass, 1902) Hyaenodon paucidens(Osborn & Wortman, 1894) [10] Protohyaenodon crucians(Mellett, 1977) Prtohyaenodon crucians(Lavrov, 1999) Pseudopterodon minutus(Douglass, 1902) H. dubius: Hyaenodon aymardi(Filhol, 1882) [11] H. exiguus: Hyaenodon exigus(Lavrov, 1999) Pterodon exiguum(Gervais, 1873) H. filholi: Hyaenodon compressus(Filhol, 1876) Hyaenodon vulpinus(Filhol, 1876) Microhyaenodon filholi(Lavrov, 1999) Pseudopterodon ganodus(Schlosser, 1887) H. gervaisi: Hyaenodon ambiguous Hyaenodon ambiguus(Martin, 1906) H. gigas: Macropterodon zelenovi(Lavrov, 1999) Neohyaenodon gigas(Lavrov, 1999) H. heberti: Hyaenodon arnaudi(Depéret, 1917) [12] H. horridus: Hyaenodon cruentus(Leidy, 1853) Neohyaenodon horridus(Thorpe, 1922) Neohyaenodon semseyi(Kretzoi, 1941) [13] H. incertus: Gobipterodon exploratus(Lavrov, 1999) Hyaenodon exploratus(Polly, 1993) [14] Neohyaenodon incertus(Lavrov, 1999) Pterodon exploratus(Dashzeveg, 1985) H. leptorhynchus: Canis leptorhynchus(Blainville, 1841) Hyaenodon bavaricus(Dehm, 1935) [15] Hyaenodon cayluxi(Filhol, 1876) Hyaenodon martini(Depéret, 1917) Hyaenodon milloquensis(Martin, 1906) Pterodon leptorhynchus(Pomel, 1846) H. macrocephalus: Neohyaenodon macrocephalus(Lavrov, 1999) H. megaloides: Neohyaenodon megaloides(Mellett, 1977) H. microdon: Microhyaenodon microdon(Lavrov, 1999) Protohyaenodon microdon(Mellett, 1977) H. milvinus: Neohyaenodon milvinus(Lavrov, 1999) H. minor: Hyaenodon aimi(Cooper, 1926) [16] Hyaenodon hantonensis(Lydekker, 1884) [17] H. mongoliensis: Epipterodon mongoliensis(Lavrov, 1999) Megalopterodon mongoliensis(Dashzeveg, 1964) Neohyaenodon mongoliensis(Morlo & Nagel, 2006) [18] Pterodon mongoliensis(Van Valen, 1967) [19] H. montanus: Protohyaenodon montanus Neohyaenodon montanus(Mellett, 1977) H. mustelinus: Hyaenodon mustilinius Prohyaenodon mustelinus(Lavrov, 1999) Protohyaenodon mustelinus(Scott, 1894) H. pervagus: Hyaenodon neimongoliensis(Huang & Zhu, 2002) [20] H. pumilus: Microhyaenodon pumilus(Lavrov, 1999) H. raineyi: Microhyaenodon raineyi(Lavrov, 1999) Protohyaenodon raineyi(Gustafson, 1986) H. requieni: Hyaenodon heberti euzetensis(Depéret, 1917) Pterodon requieni(Gervais, 1846) H. venturae: Hyaenodon exiguus(Stock, 1933) Microhyaenodon venturae(Lavrov, 1999) Protohyaenodon exiguus(Stock, 1933) Protohyaenodon venturae(Mellett, 1977) H. vetus: Neohyaenodon vetus(Mellett, 1977) Pterodon californicus(Stock, 1933) H. yuanchuensis: Hyaenodon yuanchüensis(Young, 1937)

Hyaenodon ("hyena-tooth") is an extinct genus of carnivorous placental mammals from extinct tribe Hyaenodontini within extinct subfamily Hyaenodontinae (in extinct family Hyaenodontidae), that belonged to the now extinct order Hyaenodonta. ^[21] The genus was found lived in Eurasia and North America from the Middle Eocene to the Early Miocene, from 38 to 17 million years ago, existing for 21 million years. ^[22] Hyaenodon first evolved in Asia, ^[23] likely evolving from Propterodon . ^[24]

The genus currently consists of at least 40 species, ^[24] although due sexual dimorphism and intraspecific variation, they were likely fewer species within the genus. ^[23] The species within the genus ranged in size from H. filholi, who weighed 2 kg (4.4 lb), to H. gigas and H. mongoliensis, who were estimated to be similar in size to Hyainailouros . The genus saw a decline during the Late Eocene to Early Oligocene, with only one species, H. weilini, being present in the Miocene. Initially, experts hypothesize their decline and extinction was the result of competition with carnivorans. However, over the recent years, experts now suggest cause of their decline and eventual extinction was the inability to adapt to climatic changes in their environments.

Classification and phylogeny

Taxonomy

Tribe: †Hyaenodontini(Leidy, 1869) Genus: †Hyaenodon(Laizer & Parieu, 1838) †Hyaenodon brachyrhynchus(Blainville, 1841) [25] †Hyaenodon chunkhtensis(Dashzeveg, 1985) [26] †Hyaenodon dubius(Filhol, 1873) [27] †Hyaenodon eminus(Matthew & Granger, 1925) [28] †Hyaenodon exiguus(Gervais, 1873) [29] †Hyaenodon filholi(Schlosser, 1887) [30] †Hyaenodon gervaisi(Martin, 1906) [31] †Hyaenodon heberti(Filhol, 1876) [32] †Hyaenodon leptorhynchus(Laizer & Parieu, 1838) [33] †Hyaenodon lingbaoensis(Li, 2025) [34] †Hyaenodon minor(Lange-Badré, 1979) [35] †Hyaenodon pervagus(Matthew & Granger, 1924) [36] †Hyaenodon pumilus(Lavrov, 2019) [37] †Hyaenodon requieni(Gervais, 1846) [38] †Hyaenodon rossignoli(Lange-Badré, 1979) †Hyaenodon weilini(Wang, 2005) [39] †Hyaenodon yuanchuensis(Young, 1937) [40] Subgenus: †Neohyaenodon(paraphyletic subgenus)(Thorpe, 1922) [41] †Hyaenodon gigas(Dashzeveg, 1985) †Hyaenodon horridus(Leidy, 1853) [42] †Hyaenodon incertus(Dashzeveg, 1985) †Hyaenodon macrocephalus(Lavrov, 1999) [3] †Hyaenodon megaloides(Mellett, 1977) [43] †Hyaenodon milvinus(Lavrov, 1999) [3] †Hyaenodon mongoliensis(Dashzeveg, 1964) [44] †Hyaenodon montanus(Douglass, 1902) [45] †Hyaenodon vetus(Stock, 1933) [46] Subgenus: †Protohyaenodon(paraphyletic subgenus)(Stock, 1933) †Hyaenodon brevirostrus(Macdonald, 1970) [47] †Hyaenodon crucians(Leidy, 1853) †Hyaenodon microdon(Mellett, 1977) †Hyaenodon mustelinus(Scott, 1894) [48] †Hyaenodon raineyi(Gustafson, 1986) [49] †Hyaenodon venturae(Mellett, 1977)

Description

Skull of Hyaenodon horridus

Life reconstruction of H. horridus

The skull of Hyaenodon was long with a narrow snout—much larger in relation to the length of the skull than in canine carnivores, for instance. The neck was shorter than the skull, while the body was long and robust and terminated in a long tail. Compared to the larger (but not closely related) Hyainailouros , the dentition of Hyaenodon was geared more towards shearing meat and less towards bone crushing. ^[50]

The species within the genus vary in size, with most being small to medium sized predators, while some were among the largest terrestrial carnivorous mammals of their time. ^[39] Intraspecific dimorphism has also been reported in the genus, although its ecological significance is unclear. H. crucians, from the early Oligocene of North America, was estimated to have weighed around 10–25 kg (22–55 lb). H. microdon and H. mustelinus, from the late Eocene of North America, were even smaller and weighed probably about 5 kg (11 lb). ^[51] H. filholi was the smallest European species and within the genus, weighing 2 kg (4.4 lb). The type species, H. leptorhynchus, was estimated to have weighed 11 kg (24 lb). ^[52]

H. horridus was one of the largest North American species. While m1 regressions suggested it could have weighed 91.8 kg (202 lb), ^[53] regressions based on limb morphology suggest the species was instead a bit smaller, with adults weighing 41.42 kg (91.3 lb) on average and the largest adults wouldn’t have exceeded 60 kg (130 lb). ^[51] H. megaloides, the largest North American species, was three times heavier than H. horridus, weighing 30–120 kg (66–265 lb). ^[51] It has been suggested that the size decrease among North American Hyaenodon species may have been the result of competition with nimravids. ^[51] In Europe, the largest species was H. geravisi, weighing 50 kg (110 lb). ^[52] The largest species within the genus was H. gigas and H. mongoliensis, both species had a skull length of 60 cm (2.0 ft) and are similar in size to Hyainailouros. ^[54] H. weilini was another very large species, described to be similar in size to H. gigas and H. mongoliensis. ^[39]

Paleobiology

Reconstruction of Hyaenodon by Heinrich Harder (around 1920)

Predatory behavior

A 2003 study found that based on elbow morphology H. horridus was a cursorial predator and was the most cursorially adapted Oligocene carnivore within the study. ^[55] Furthermore, a 2025 study found that based on elbow morphology found that H. crucians and H. horridus were a pounce-pursuit predators. ^[56] Much like H. horridus, H. eminus, H. gigas, and H. pervagus were recovered as a cursorial predators. ^[57] On the other hand, analysis on the bony labyrinth of H. exiguus suggests this species was semi-arboreal and occupied a hyena-like niche. ^[22]

The Hunter-Schreger bands observed in the tooth enamel of H. horridus are zigzag, suggesting that this species was osteophagous, whereas those of H. brevirostris and H. mustelinus transition from undulating at the base of the tooth to zigzag at the tip, indicating that these species were not as well adapted for feeding on bone. ^[58] However, dental microwear patterns suggests that North American Hyaenodon had a diet more similar to lions, while European Hyaenodon microwear was more similar to spotted hyenas and bone crushing was likely a part of their diet. ^[59] The tooth wearing on P4 of H. gigas suggests the primary function of the tooth was bone-cracking. ^[39] A 2024 study found that canine bite mechanic efficiency increased with tooth macrowear in Hyaenodon. ^[60]

Tooth eruption

Studies on juvenile Hyaenodon specimens show that the animal had a very unusual system of tooth replacement. Juveniles took about 3–4 years to complete the final stage of eruption, implying a long adolescent phase. In North American forms, the first upper premolar erupts before the first upper molar, while European forms show an earlier eruption of the first upper molar. ^[61]

Brain anatomy and senses

While it has typically been assumed that Hyaenodon had a very massive skull, but a small brain, this has been called into question. ^[62] Flink and colleagues found that Hyaenodon, had encephalization quotient of 0.36-0.37 and 0.42-0.46, for H. horridus and H. crucians respectively. This similar to basal and some modern carnivorans such as cougars, Hesperocyon gregarius , Hoplophoneus primaevus , and striped hyena and exceeding jaguars, Daphoenus , Hoplophoneus primaevus , and Eusmilus bidentatus . Their discovery found that hyaenodonts relative brain sizes overlapped with both extinct and extant carnivorans. ^[63] The endocast of Hyaenodon stands out from other Hyaenodontoids as they had relatively high EQ, in addition to relatively gyrencephalic and neocorticalized brains, however the cause of the increase in EQ for the genus is still unknown. ^[62] It is thought compared to Neogene carnivorans of similar size, Hyaenodon sense of smell wasn’t as acute. ^[64]

Paleoecology

North America

Reconstruction of H. horridus and Leptomeryx evansi by W. B. Scott (1913)

Hyaenodon first appeared in North America during the Middle Eocene with the appearance of H. venturae. ^[23] The most well known species, H. horridus, roamed North America from 36.5 to 31.4 Ma. ^[65] This species was found in the Calf Creek locality of Cypress Hills Formation. ^[66] The herpetofauna present within the locality suggests Calf Creek had a tropical or subtropical climate. ^[67] In this locality, Hyaenodon coexisted with hyaenodonts such as H. microdon and the hyainailourid Hemipsalodon grandis. Carnivorans that were present in this formation were daphoeninae amphicyonids Brachyrhynchocyon dodgei and Daphoneus , nimravids Dinictis and Hoplophoneus , hesperocyonine canid Hesperocyon gregarius , and the subparictid Parictis. ^{[66] [68] [69]} In addition, Hyaenodon also coexisted with the entelodont Archaeotherium . Herbivores present in this locality include the equid Mesohippus , the hyracodontid Hyracodon priscidens , rhinoceroses such as Subhyracodon occidentalis , Trigonias osborni , and Penetrigonias sagittatus , tapirid Colodon occidentalis , brontothere Megacerops kuwagatarhinus , and the anthracothere Bothriodon advena. ^[69] The predators present in Calf Creek likely practiced niche partitioning via different body sizes, with H. horridus focusing on prey that weighed 166 kg (366 lb) and faced little competition from carnivorans. While Hoplophoneus could’ve induced some competition pressure via pack hunting, H. horridus could still hunt prey outside of the most probable range of the carnivoran. On the other hand, H. microdon was thought to have faced intense competition from five contemporary carnivorans. ^[70]

Restoration of Archaeotherium eating roots, several species of both Archaeotherium and Hyaenodon coexisted with each other across North America

H. horridus was also found in Brule Formation of South Dakota. ^[71] The paleoenvironment of Brule Formation was believed to have been a woodland grassland and gallery forest, populated in part by hackberry trees ( Celtis ). ^[72] Contemporary predators would've included fellow species H. crucians, the nimravid Hoplophoneus, the amphicyonid Daphoneus, and the entelodont Archaeotherium mortoni. Herbivores present include the early horse Mesohippus, hypertragulid Hypertragulus calcaratus , leptomerycid Leptomeryx evansi , tapirs such as Protapirus simplex and Colodon occidentalis, the camel Poebrotherium wilsoni , anthracotheres Aepinacodon americanus and Heptacodon occidentale , oreodonts Merycoidodon culbertsonii and Miniochoerus affinis . ^[71]

Fossil evidence suggests that H. horridus could've predate on other large carnivores smaller than itself, such as Dinictis. ^{[73] [74]}

Eurasia

In Europe, both H. geravisi and H. leptorhynchus were found in Séon Saint-André. Carnivorans present in this locality were amphicyonids Cynelos rugosidens and Pseudocyonopsisambiguus. All predators within this locality are believed to have practiced some niche partitioning. H. leptorhynchus is believed to have hunted small artiodactyls such as Bachitherium and Mosaicomeryx . However the larger predators would've hunted larger artiodactyls such as Anthracotherium cuvieri and Elomeryx borbonicus and perissodactyls such as Protaceratherium albigense and Ronzotherium romani . H. geravisi and Pseudocyonopsis were believed to competed for the same prey due to being similar in size to one another. Although, it is possible they preferred different environments as Hyaenodon was a cursorial predator and likely preferred open environments compared to amphicyonids, who preferred more closed environments such as forests. ^[52]

In East Asia, H. gigas lived during the Early Oligocene. ^[23] Within Khoer-Dzan, H. gigas coexisted with other predators like hyaenodonts H. eminus, H. incertus, H. mongoliensis, and H. pervagus, nimravids Eofelis and Nimravus intermedius , and entelodonts Brachyhyops trofimovi and Entelodon gobiensis . Contemporary herbivores include the anthracothere Bothriodon , the praetragulid Praetragulus electus , the brontothere Embolotherium andrewsi , chalicotheres Schizotherium avitum , and rhinocerotoids such as the paraceratheriid Urtinotherium parvum and rhinocerotid Ronzotherium orientale. ^[75]

Extinction

From the middle to late Eocene, hyaenodonts experienced a decline in diversity with only one genus, consisting of a few species by the end of the Eocene in North America. ^[76] With H. brevirostrus being the last species in North America, which disappeared in the late Oligocene. ^[77] In Europe, the last species to go extinct, H. exiguus and H. leptorhynchus, were last known in MP30 of the late Oligocene. ^{[52] [78]} The youngest species, H. weilini, persisted into the early Miocene in China, going extinct 17 Ma. ^[39]

The cause of their extinction has been debated by experts, with many suggesting that their extinction was due to competition with carnivorans, such as amphicyonids, hesperocyonines, hemicyonines, and hyenas. ^{[79] [76] [80]} Lang and colleagues theorize that the success of carnivorans compared to hyaenodonts was likely due to the retention of a basal morphotype throughout their evolutionary history. They also suggested that carnivorans possibly played a role in the extinction of hyaenodonts, probably due to the adaptive potential of their carnassials. ^[81] Serio and colleagues found that North American “creodonts” had a significant degree of morphological differentiation until the middle Eocene, with disparity among carnivorans increasing around the same time. The authors argue that carnivoran disparity negatively impacted the disparity of creodonts, suggesting carnivorans may have competitively replaced hyaenodonts. ^[80]

However, this has been contested by many experts, arguing their extinction correlated with abiotic changes in their environments. ^{[70] [56]} Christison and colleagues found that only small hyaenodont, H. microdon, experienced significant competition with the five contemporary carnivorans. The largest hyaenodonts such as H. horridus and Hemipsalodon , didn’t experience little to no competition from carnivorans. This suggests competition with carnivorans couldn't have been the driver of the extinction of North American hyaenodonts in the Late Eocene. Instead, they argued the highly specialized niche of hyaenodonts enhanced their extinction rates. The global cooling of the early Oligocene resulted in the extinction of large browsers such as brontotheres, as ecosystems became drier and more open. Brontotheres were replaced by equids and rhinoceroses, who were better adapted for open environments. But rhinoceroses wouldn’t reach large sizes until the Miocene epoch, leaving the gap of accessible large herbivores for the large, hypercarnivorous hyaenodonts. Furthermore, hyaenodonts tend to have relatively short legs, which may have been a disadvantage in open environments and likely played a role in their extinction. ^[70] Castellanos, in their 2025 paper, found that despite hyaenodonts showing adaptations towards cursoriality, because of their short distal limbs, hyaenodonts couldn’t exploit open environments as well as amphicyonids, which may have resulted in their extinction and in addition to the low diversity of the clade by the start of the Oligocene. ^[56]

In Europe, Hyaenodon and amphicyonids preferred different environments, with the former hunting in more open environments which would’ve limited competition between the two predators. ^[52] Due to this, the extinction of Hyaenodon in Europe was likely the result of the inability to survive in open environments due to the reduction of resources instead of competition with carnivorans. ^{[52] [82]}

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