Hyaenodonta ("hyena teeth") is an extinct order of mostly carnivorous placental mammals of clade Pan-Carnivora from mirorder Ferae.[6][7] Hyaenodonts were important mammalian predators that are believed to have arose either in the Late Cretaceous[8][9] or Early Paleocene[10] within Europe, and persisted well into the Late Miocene.[11][12]Hyaenodonts were found across Africa, Eurasia, and North America throughout the early Cenozoic. They also occupied a variety of ecosystems, many of which inhabited forested environments, while some lineages, such as Hyaenodon and Megistotherium, may have been more adapted for more open environments.
Hyaenodonts were considerably more widespread and successful than the oxyaenids, the other clade of mammals originally classified along with the hyaenodonts as part of Creodonta.[13] In 2015 phylogenetic analysis of Paleogene mammals, by Halliday et al., monophyly of Creodonta was supported and was placed in the clade Ferae, closer to Pholidota than to Carnivora.[14] However, order Creodonta is now considered to be a polyphyleticwastebasket taxon containing two unrelated clades assumed to be closely related (or ancestral) to Carnivora. With Hyaenodonta and Oxyaenodonta now being considered their own distinct orders.[15][16][17][18][19][20][21][22][23][24][25]
They generally ranged in size from 30–140cm (1ft 0in– 4ft 7in) at the shoulder.[13] While Simbakubwa kutokaafrika may have weighed up to 1,500kg (3,300lb) (surpassing the modern polar bear in size[10]), this estimate is suspect due to being based on skull-body size ratios derived from felids, which have much smaller skulls for their body size. Other large hyaenodonts include two close and later-surviving relatives of Simbakubwa, Hyainailouros, and Megistotherium (the latter likely being the largest in the group), and the much earlier-living Hyaenodon gigas and Hyaenodon mongoliensis (The largest species of the genus, Hyaenodon), both species had a skull length of 60cm (2.0ft) and are estimated to be similar in size to Hyainailouros.[26] Most hyaenodonts, however, were in the 5–15kg (11–33lb) range, equivalent to a mid-sized dog.[27]
Compared to carnivorans, who have one pair of carnassial teeth, hyaenodonts possessed as many as three sequential pairs of carnassials or carnassial-like molar teeth in their jaws. Their skulls are known to high have sagittal crests which supported attachment for powerful temporalis muscles.[28] Hyaenodonts, like all "creodonts", lacked post-carnassial crushing molar teeth, such as those found in many carnivoran families, especially the Canidae and Ursidae.[28]
The anatomy of their skulls show that they had a particularly acute sense of smell, while their teeth were adapted for shearing, rather than crushing.[13] Hyaenodonts differed from Carnivora in that they replaced their deciduous dentition slower in development than carnivorans.[16][17] Studies on Hyaenodon show that juveniles took 3 to 4 years in the last stage of tooth eruption, implying a very 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.[17]
Postcranial anatomy
Hyaenodonts were ancestrally plantigrade,[29] but the later, larger forms were generally digitigrade or semidigitigrade.[10][29] At least one hyaenodont lineage, subfamily Apterodontinae, was specialized for aquatic, otter-like habits.[18]Hyainailouros had a humerus as robust as an average felid. Compared to the humerus of a tiger, the deltoid scar of Hyainailouros was more distally than tigers.[30]
Hyaenodonts soon dispersed into Africa and India, implying close biogeographical connections between these areas.[18][19] Afterwards, they dispersed into Asia from either Europe or India, and finally, North America from either Europe or Asia.[36][37]
They were important hypercarnivores in Eurasia, Africa, and North America during the Oligocene, but declined towards the end of the epoch, with nearly the entire order becoming extinct by the close of the Oligocene. Several representatives of this order, including hyainailourids Megistotherium, Simbakubwa, Hyainailouros, Sectisodon, Exiguodon, Sivapterodon, Metapterodon, and Isohyaenodon, the prionogalid Prionogale, the teratodontid Dissopsalis and the youngest species of genus Hyaenodon, H. weilini, survived into or evolved during the Miocene, of which, only Dissopsalis, Hyainailouros, and Metapterodon persisted into the Late Miocene.[11][12]
Extinction
The decline and extinction of the hyaenodonts has been debated by experts. Many experts have argued that their extinction was due to competition with the carnivorans.[10][38][39] Experts had hypothesized that competitive displacement from the invading carnivorans forced hyainailourids to vary in size and become more hypercarnivorous.[31][40] Borths and Stevens, in their 2019 paper, argued that cooperative carnivorans had larger and more complex brains, suggesting this enabled them to steal carcasses from large, solitary hyainailourines.[10] Lang and colleagues found that the evolutionary success of carnivorans compared to hyaenodonts may have been largely influenced by the retention of a basal morphotype throughout their evolutionary history. The authors also suggested that carnivorans likely contributed in some way to the extinction of hyaenodonts, with the difference in functional morphology and adaptive potential of their carnassials possibly being a factor.[35]
However, this hypothesis has been contested by experts.[41][42][43] Among hyainailourids, the discovery of Simbakubwa suggests the evolution of large hyainailourines was thought to have been changes in the herbivore fauna instead of competition with carnivorans.[10] Additionally, studies have found brain size have little to no correlation with sociality in carnivorans,[44][45][46] with the relative size of the anterior brain having a larger correlation to sociality than overall brain size.[47][48][49] Morales and colleagues argued the decline of hyainailourids was due to the increase in aridity, as they were more adapted for forested environments as opposed to savannas, steppes, or deserts.[42]
Christison and colleagues analyzed hyaenodonts and carnivorans from the Cypress Hills Formation and found that only the smaller hyaenodonts and carnivorans had significant niche overlap, while larger hyaenodonts and carnivorans (Such as Hyaenodon horridus and Hoplophoneus) had very distinct niches. They argue this would suggest that competitive replacement wasn’t the driving factor the decline and extinction of North American hyaenodonts. Instead, global climatic cooling of the earliest Oligocene resulted in drier, more open landscapes and resulted in the extinction of large browsing herbivores, including brontotheres, who were replaced by grazing mammals such as equids and rhinoceroses, which were better suited for the open environments. Because rhinoceroses didn’t reach their large sizes until the Miocene, hyaenodonts were at a disadvantage due to the lack of large prey available. In addition, because of their shorter legs, they were likely at a disadvantage in the increasingly open environments.[32] Despite recovering North American hyaenodonts as a relatively cursorial clade, Castellanos hypothesized due their short distal limbs, hyaenodonts had less cursorial adaptations than contemporary amphicyonids and couldn’t exploit open environments as well which likely led to their extinction.[50]
In Europe, Hyaenodon and amphicyonids were able to coexist by preferring different habitats, with Hyaenodon preferring more open environments.[51] Instead, the extinction of hyaenodonts is believed to have been caused by climatic changes rather than competition with carnivorans.[40][51]
↑ Van Valen, L. (1967). "New Paleocene insectivores and insectivore classification." Bulletin of the American Museum of Natural History 135(5):217-284
↑ Solé F. (2010) "Les premiers placentaires carnassiers européens (Oxyaenodonta, Hyaenodontida et Carnivora): origine, évolution, paléoécologie et paléobiogéographie; apport des faunes de l'Eocène inférieur du Bassin de Paris." Paris: Muséum National d'Histoire Naturelle. 703 p.
↑ Morlo, M., Gunnell, G. F. and Polly, P. D. (2009). "What, if not nothing, is a creodont? Phylogeny and classification of Hyaenodontida and other former creodonts." Journal of Vertebrate Paleontology Program and Abstracts, 2009:152A.
↑ Solé, F.; Lhuillier, J.; Adaci, M.; Bensalah, M.; Mahboubi, M.; Tabuce, R. (16 July 2013). "The hyaenodontidans from the Gour Lazib area (?Early Eocene, Algeria): implications concerning the systematics and the origin of the Hyainailourinae and Teratodontinae". Journal of Systematic Palaeontology. 12 (3): 303–322. doi:10.1080/14772019.2013.795196. S2CID84475034.
1 2 Barry, John C. (January 2025). "Siwalik Creodonts and Carnivorans". At the Foot of the Himalayas: Paleontology and Ecosystem Dynamics of the Siwalik Record. ISBN978-1421450278.
1 2 3 Lambert, David and the Diagram Group (1985): The Field Guide to Prehistoric Life. Facts on File Publications, New York. ISBN0-8160-1125-7
1 2 3 Bastl, Katharina Anna (2013). "First evidence of the tooth eruption sequence of the upper jaw in Hyaenodon (Hyaenodontidae, Mammalia) and new information on the ontogenetic development of its dentition". Paläontologische Zeitschrift. 88 (4): 481–494. doi:10.1007/s12542-013-0207-z. S2CID85304920.
↑ Polly, P. D. (1994). "What, if anything, is a creodont?". Journal of Vertebrate Paleontology. 14 (Supplement 3): 42A. doi:10.1080/02724634.1994.10011592.
↑ Morlo, M.; Gunnell, G.; Polly, P. D. (2009). "What, if not nothing, is a creodont? Phylogeny and classification of Hyaenodontida and other former creodonts". Journal of Vertebrate Paleontology. 29 (Supplement 3): 152A. doi:10.1080/02724634.2009.10411818.
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