| Hyaenodon | |
|---|---|
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| 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: synonyms of species:
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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 and North America from the Middle Eocene to the Early Miocene, from 38 to 17 million years ago, existing for 21 million years. [22]
The genus currently consists of over a dozen species, although reanalysis of the genus is needed. [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. It was thought that their extinction in North America was the result of competition with carnivorans, however several studies have shown that the most likely cause of their extinction was their inability to adapt to open environments, as well as the decline of their preferred prey.
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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. [49]
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. [38] 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). [50] 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). [51]
H. horridus was one of the largest North American species. While m1 regressions suggested it could have weighed 91.8 kg (202 lb), [52] 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). [50] H. megaloides, the largest North American species, was three times heavier than H. horridus, weighing 30–120 kg (66–265 lb). [50] It has been suggested that the size decrease among North American Hyaenodon species may have been the result of competition with nimravids. [50] In Europe, the largest species was H. geravisi, weighing 50 kg (110 lb). [51] 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. [53] H. weilini was another very large species, described to be similar in size to H. gigas and H. mongoliensis. [38]
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. [54] Furthermore, a 2025 study found that based on elbow morphology found that H. crucians and H. horridus were a pounce-pursuit predators. [55] Much like H. horridus, H. eminus, H. gigas, and H. pervagus were recovered as a cursorial predators. [56] 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. [57] Dental microwear patterns suggest 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. [58] The tooth wearing on P4 of H. gigas suggests the primary function of the tooth was bone-cracking. [38] A 2024 study found that canine bite mechanic efficiency increased with tooth macrowear in Hyaenodon. [59]
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. [60]
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. [61] 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. [62]
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. [49]
H. horridus roamed North America from 36.5 to 31.4 Ma. [63] This species 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. [64] [65] 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). [66]
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 . [67]
Fossil evidence suggests that H. horridus could've predate on other large carnivores smaller than itself, such as Dinictis. [68] [69]
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. [51]
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 . [70]
During 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. [71] With H. brevirostrus being the last species in North America, which disappeared in the late Oligocene. [72] In Europe, they had already vanished earlier in the Oligocene, with the youngest species, H. weilini, persisting in the early Miocene of China. [38] The cause of their extinction has been debated by experts, with many suggesting that their extinction was due to competition with carnivorans. [73] [71] [74] 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. [75] Serio et al. (2024) 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. [74]
However, this has been contested by many experts, who argued instead their extinction correlated with abiotic changes in their environments. [66] [55] Christison et al. (2022) found that only small hyaenodonts and smaller carnivorans had a significant niche overlap compared to large hyaenodonts and 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. [66] Castellanos 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 addition to the low diversity of the clade by the start of the Oligocene. [55]
Additionally in Europe, Hyaenodon and amphicyonids preferred different environments, with the former hunting in more open environments suggesting niche partitioning between the two predators. [51]