Hyaenodon

Hyaenodon Temporal range: Middle Eocene to Early Miocene (Bartonian to Burdigalian) 38.0–16.9  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 consists of many species that lived in Eurasia, Africa, and North America from the Middle Eocene to the Early Miocene, from 38 to 16.9 million years ago, existing for 21.1 million years. ^[22]

Classification and phylogeny

Taxonomy

Tribe: †Hyaenodontini(Leidy, 1869) Genus: †Hyaenodon(Laizer & Parieu, 1838) †Hyaenodon brachyrhynchus(Blainville, 1841) [23] †Hyaenodon chunkhtensis(Dashzeveg, 1985) [24] †Hyaenodon dubius(Filhol, 1873) [25] †Hyaenodon eminus(Matthew & Granger, 1925) [26] †Hyaenodon exiguus(Gervais, 1873) [27] †Hyaenodon filholi(Schlosser, 1887) [28] †Hyaenodon gervaisi(Martin, 1906) [29] †Hyaenodon heberti(Filhol, 1876) [30] †Hyaenodon leptorhynchus(Laizer & Parieu, 1838) [31] †Hyaenodon lingbaoensis(Li, 2025) [32] †Hyaenodon minor(Lange-Badré, 1979) [33] †Hyaenodon pervagus(Matthew & Granger, 1924) [34] †Hyaenodon pumilus(Lavrov, 2019) [35] †Hyaenodon requieni(Gervais, 1846) [36] †Hyaenodon rossignoli(Lange-Badré, 1979) †Hyaenodon weilini(Wang, 2005) [37] †Hyaenodon yuanchuensis(Young, 1937) [38] Subgenus: †Neohyaenodon(paraphyletic subgenus)(Thorpe, 1922) [39] †Hyaenodon gigas(Dashzeveg, 1985) †Hyaenodon horridus(Leidy, 1853) [40] †Hyaenodon incertus(Dashzeveg, 1985) †Hyaenodon macrocephalus(Lavrov, 1999) [3] †Hyaenodon megaloides(Mellett, 1977) [41] †Hyaenodon milvinus(Lavrov, 1999) [3] †Hyaenodon mongoliensis(Dashzeveg, 1964) [42] †Hyaenodon montanus(Douglass, 1902) [43] †Hyaenodon vetus(Stock, 1933) [44] Subgenus: †Protohyaenodon(paraphyletic subgenus)(Stock, 1933) †Hyaenodon brevirostrus(Macdonald, 1970) [45] †Hyaenodon crucians(Leidy, 1853) †Hyaenodon microdon(Mellett, 1977) †Hyaenodon mustelinus(Scott, 1894) [46] †Hyaenodon raineyi(Gustafson, 1986) [47] †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. ^[48]

The species within the genus varies in size, with most species were small to medium sized predators, while some species were among the largest terrestrial carnivorous mammals of their time. ^[37] 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). ^[49] 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). ^[50]

H. horridus was one of the largest North American species. While m1 regressions suggested it could’ve weighed 91.8 kg (202 lb), ^[51] regressions based on limb morphology suggests the species was instead a bit smaller, with adults weighing 41.42 kg (91.3 lb) on average and may not have exceeded 60 kg (130 lb). ^{[49] [48]} H. megaloides, the largest North American species, was three times heavier than H. horridus, weighing 30–120 kg (66–260 lb). ^[49] It has been suggested that the size decrease among North American Hyaenodon species, may have been the result of competition with nimravids. ^[49] In Europe, the largest species was H. geravisi, weighing 50 kg (110 lb). ^[50] The largest species within the genus was H. gigas, which could’ve weighed 378 kg (833 lb) and measured 3 m (10 ft) in length. ^[48] H. weilini was another very large species, described to be similar in size to H. gigas and H. mongoliensis. ^[37]

Paleobiology

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. ^[52] Furthermore, a 2025 study found that based on elbow morphology found that H. crucians and H. horridus were a pounce-pursuit predators. ^[53] Much like H. horridus, H. eminus, H. gigas, and H. pervagus were recovered as a cursorial predators. ^[54] 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]

Zigzag Hunter–Schreger bands in the enamel indicate that European and North America Hyaenodon had slightly different niches. Microwear patterns suggests that North American Hyaenodon diet was more similar to lions, on the other hand European Hyaenodon microwear was more similar to spotted hyenas and bone crushing was likely a part of their diet. ^[55] A 2024 study found that canine bite mechanic efficiency increased with tooth macrowear in Hyaenodon. ^[56]

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. ^[57]

Brain anatomy

While it has typically been assumed that Hyaenodon had a very massive skull, but a small brain, this has been called into question, as a recent study found that hyaenodonts, including Hyaenodon, had encephalization quotient similar to basal and some modern carnivorans such as cougars, Hesperocyon gregarius , Hoplophoneus primaevus , and striped hyena. ^[58] The endocast of Hyaenodon stands out from other Hyaenodontoids as they had relatively high EQ, in addition to relatively gyrencephalic and neocorticalized brains, although the increase in EQ for the genus is still unknown. ^[59]

Paleoecology

Reconstruction of Hyaenodon by Heinrich Harder (around 1920)

The various species of Hyaenodon competed with each other and with other hyaenodont genera (including Sinopa , Dissopsalis and Hyainailurus ), and played important roles as predators in ecological communities as late as the Early Miocene in Asia and preyed on a variety of prey species such as early horses like Mesohippus , brontotheres, early camels, oreodonts and even early rhinos. ^[48]

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

H. horridus roamed North America from 36.5 to 31.4 Ma. ^[53] This was found in the Calf Creek locality of Cypress Hills Formation. It would’ve coexisted with hyaenodonts such as H. microdon and 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. ^{[60] [61]} 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). ^[62]

H. horridus was also found in Brule Formation of South Dakota. Contemporary predators would’ve included the nimravid Hoplophoneus, amphicyonid Daphoneus, fellow species H. crucians, and entelodont Archaeotherium mortoni . Herbivores present include hypertragulid Hypertragulus calcaratus , leptomerycid Leptomeryx evansi , and oreodonts Merycoidodon culbertsonii and Miniochoerus affinis . ^[63]

Fossil evidence suggests that H. horridus could’ve predate on other large carnivores smaller than itself, such as Dinictis. ^{[64] [65]}

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 likely preferred more closed environments such as forests. ^[50]

Extinction

During the middle to late Eocene, hyaenodonts experienced a decline in diversity with only a few species by the end of the Eocene in North America. ^[66] With H. brevirostrus being the last species in North America, which disappeared in the late Oligocene. ^[67] In Europe, they had already vanished earlier in the Oligocene, with the youngest species, H. weilini, persisting in the early Miocene of China. ^[37] The cause of their extinction has been debated by experts, with many suggesting that their extinction was due to competition with carnivorans. ^{[68] [66] [69]} Lang et al. (2021) argues the success of carnivorans compared to hyaenodonts was likely due to the retention of a basal morphotype throughout their evolutionary history. They also suggest that carnivorans possibly played a role in the extinction of hyaenodonts, probably due to the adaptive potential of their carnassials. ^[70] Serio et al. (2024) found ‘creodonts’ (Including hyaenodonts) 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, which they argue supports competitive replacement. ^[69]

However, this has been contested by many experts, who argued instead their extinction correlated with abiotic changes in their environments. ^{[62] [53]} Christison et al. (2022) found that only small hyaenodonts had significant competition with carnivorans compared to large hyaenodonts. They found that competition with carnivorans couldn’t have been the driver of the decline of hyaenodonts in the late Eocene. Instead, they argued the highly specialized niche of hyaenodonts enhanced their extinction rates, with the global cooling of the early Oligocene made them unsuitable for open environments due to their short legs. ^[62]

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