Hyaenodon

Hyaenodon ("hyena-tooth") is an extinct genus of carnivorous placental mammals from the tribe Hyaenodontini, part of the subfamily Hyaenodontinae (which is within the family Hyaenodontidae), that belonged to the now extinct order Hyaenodonta.Malcolm C. McKenna, Susan K. Bell (1997). "Classification of Mammals: Above the Species Level", Columbia University Press, New York, 631 pages. The genus was found lived in Eurasia and North America from the Middle Eocene to the Late Miocene, from 38 to 11.4 million years ago, existing for . Hyaenodon was thought to have evolved in Asia, probably evolving from Propterodon.

The genus currently consists of at least 40 species, although due to sexual dimorphism and intraspecific variation, there were likely fewer species within the genus. The species within the genus ranged in size from H. filholi, which weighed , to H. gigas and H. mongoliensis, which were estimated to be similar in size to Hyainailouros.

Smaller species, such as H. microdon, were mesopredators, having to compete with contemporary small carnivorans. On the other hand, the largest species were likely the apex predators of their time, hunting large herbivores of their environments. Fossil records also suggests they were also capable of hunting smaller predators. Several species within the genus were cursorial predators, either being ambushing or pounce-pursuit predators. The canines of the animal mediolaterally compressed much like canids, landing shallow bites on their prey. Unlike canids such as wolves, they are thought to have been solitary predators.

The genus saw a decline during the Late Eocene to Early Oligocene, with only a few species being present in the Miocene. Many experts hypothesize their decline and extinction was the result of competition with carnivorans. Although, this hypothesis has been challenged over the recent years. With many experts instead hypothesizing the cause of their decline and eventual extinction was the inability to adapt to open environments.

Classification and phylogeny

Classification Hyaenodon was a type genus of the family Hyaenodontidae,Leidy J. (1869.) "On the extinct Mammalia of Dakota and Nebraska: including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America." Journal of the Academy of Natural Sciences Philadelphia 7: 1–472. a group of hyaenodonts that arose during the Late Paleocene or Early Eocene. The family is considered to be a sister clade to proviverrines, a clade of hyaenodonts endemic to Europe, forming the forming the superfamily, Hyaenodontoidae. Hyaenodonts were once considered to be part of "Creodonta", (1985): The Field Guide to Prehistoric Life. Facts on File Publications, New York. although recent analysis recovered hyaenodonts as an entirely separate order rom oxyaenids.Solé, Floréal & Smith, Thierry (2013). "Dispersals of placental carnivorous mammals (Carnivoramorpha, Oxyaenodonta & Hyaenodontida) near the Paleocene-Eocene boundary: a climatic and almost worldwide story" Geologica Belgica 16/4: 254–261Prevosti, F. J., & Forasiepi, A. M. (2018). "Introduction. Evolution of South American Mammalian Predators During the Cenozoic: Paleobiogeographic and Paleoenvironmental Contingencies"

Taxonomy Tribe: †Hyaenodontini Genus: †Hyaenodon †Hyaenodon brachyrhynchus Henri Marie Ducrotay de Blainville (1841.) "Ostéographie ou description iconographique comparée du squelette et du système dentaire des mammifères récents et fossiles." Tome 2: Secondates et Subursus, 123 p.; Viverras, 100 p. et atlas, 117 pl. Baillėte édit. Paris. †Hyaenodon chunkhtensis Dashzeveg D. (1985.) "Nouveau Hyaenodontinae (Creodonta, Mammalia) du Paléogène de Mongolie." Annales de Paléontologie 71:223–256 †Hyaenodon dubius Filhol, H. (1873.) "Sur les Vertébrés fossiles trouvés dans les dépôts de phosphate de chaux du Quercy." Bull. Soc. Pholomath. Paris (6) 10, 85-89. †Hyaenodon eminus Matthew W. D. & Granger W. (1925.) "New creodonts and rodents from the Ardyn Obo Formation of Mongolia." American Museum Novitates 193:1–11. †Hyaenodon exiguus Gervais P. (1873.) "Mammifères dont les ossements accompagnent les dépôts de chaux phosphatée des départements du Tarn-et.Garonne et du Lot." Journal de Zoologie, Paris, 2:356-380 †Hyaenodon filholi M. Schlosser (1887.) "Die Affen, Lemuren, Chiropteren, Insectivoren, Marsupialier, Creodonten und Carnivoren des Europaischen Tertiars." Beitrage zur Paleontologie Osterreich-Ungarns un des Orients 6:1-224 †Hyaenodon gervaisi R. Martin (1906.) "Revision der obereocænen und unteroligocænen Creodonten Europas." Rev. Suisse Zool., 14, (3), pp. 405-500 †Hyaenodon heberti H. Filhol (1876.) "Recherches sur les phospohorites du Quercy. Étude des fossiles qi'on y rencontre et spécialement des Mammifères." Annales des Sciences Géologiques, Paris, 7(7):220 p., pl. 11-36; 1877:art. 1, 340 p., 28 pl. †Hyaenodon leptorhynchus Laizer, L. D. and Parieu, D. (1838.) "Description et détermination d'une mâchoire fossile appartenant à un mammifère jusqu'à présent inconnu, Hyaenodon leptorhynchus." Comptes Rendus de l'Académie des Sciences Paris, 7:442 †Hyaenodon lingbaoensis †Hyaenodon minor Lange-Badré, B. (1979.) "Les créodontes (Mammalia) d'Europe occidentale de l'Éocène supérieur à l'Oligocène supérieur." Mémoires du Muséum National d'Histoire Naturelle 42: 1–249 †Hyaenodon pervagus W. D. Matthew and W. Granger (1924.) "New Carnivora from the Tertiary of Mongolia." American Museum Novitates 104:1-9 †Hyaenodon pumilus †Hyaenodon requieni Gervais P. (1846.) "Mémoire sur quelques Mammifères fossiles du Vaucluse." Comptes rendus hebdomadaires des séances de l'Académie des sciences, Paris, T. 22, pp. 845-846. †Hyaenodon rossignoli †Hyaenodon weilini X. Wang, Z. Qiu and B. Wang (2005.) "Hyaenodonts and carnivorans from the early Oligocene to early Miocene of the Xianshuihe Formation, Lanzhou Basin, Gansu Province, China." Palaeontologia Electronica, Vol. 8, Issue 1; 6A: Pages 1-14 †Hyaenodon yuanchuensis C. Young (1937.) "An early Tertiary vertebrate fauna from Yuanchü." Bulletin of the geological society of China 17(3-4):413-438 Subgenus: †Neohyaenodon (paraphyletic subgenus) M. R. Thorpe (1922.) "A new genus of Oligocene Hyaenodontidae." American Journal of Science 3(16):277-287 †Hyaenodon gigas †Hyaenodon horridus Leidy, J. (1853.) "Remarks on a collection of fossil Mammalia from Nebraska." Proceedings of the Academy of Natural Sciences, Philadelphia, 6:392-394. †Hyaenodon incertus †Hyaenodon macrocephalus †Hyaenodon megaloides J. S. Mellett (1977.) "Paleobiology of North American Hyaenodon (Mammalia, Creodonta)." Contributions to Vertebrate Evolution 1:1-134 †Hyaenodon milvinus †Hyaenodon mongoliensis D. Dashzeveg (1964.) "On two Oligocene Hyaenodontidae from Erghilyin-Dzo (Mongolian People's Republic)." Acta Palaeontologica Polonica 9(2):263-274 †Hyaenodon montanus E. Douglass (1902.) "Fossil Mammalia of the White River beds of Montana." Transactions of the American Philosophical Society 20:237-279 †Hyaenodon vetus C. Stock (1933.) "Hyaenodontidae of the Upper Eocene of California." Proceedings of the National Academy of Sciences 19(4):434-440 Subgenus: †Protohyaenodon (paraphyletic subgenus) †Hyaenodon brevirostrus J. R. Macdonald (1970.) "Review of the Miocene Wounded Knee faunas of southwestern South Dakota." Bulletin of the Los Angeles County Museum of Natural History, Science 8:165-82 †Hyaenodon crucians †Hyaenodon microdon †Hyaenodon mustelinus William Berryman Scott (1894.) "The osteology of Hyaenodon" Journal of Academy of Natural Sciences, Philadelphia (2), 9, 291-323 †Hyaenodon raineyi E. P. Gustafson (1986.) "Carnivorous mammals of the Late Eocene and Early Oligocene of Trans-Pecos Texas." Texas Memorial Museum Bulletin 33:1-66 †Hyaenodon venturae

Description

Size 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. 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 . H. microdon and H. mustelinus, from the late Eocene of North America, were even smaller and weighed probably about . Another tiny North American species, H. venturae, was similar in size to H. microdon. H. filholi was the smallest European species and the smallest species within the genus, weighing . H. leptorhynchus, the type species, was estimated to have weighed .

H. horridus was one of the largest North American species. While m1 regressions suggested it could have weighed , regressions based on limb morphology suggest the species was instead a bit smaller, with adults weighing on average, with the largest adults weighing no more than . H. megaloides, the largest North American species, was three times heavier than H. horridus, weighing , possibly even , making it about the size of a lion. It has been suggested that the size decrease among North American Hyaenodon species may have been the result of competition with nimravids. In Europe, the largest species known species was H. geravisi, weighing . The largest species was H. gigas, followed by H. mongoliensis. Both species had a skull length of and were thought to have been larger than lions, potentially rivaling Hyainailouros. H. weilini was another very large species, described to be similar in size to H. gigas and H. mongoliensis.

Postcranial remains

Postcranial remains have been recovered for Hyaenodon, although the fossil records within Eurasia are rather scarce compared to North America. While the neck of the animal was relatively short, the body and tail were long. Compared to Hyainailouros, the spine of Hyaenodon was longer and more robust, suggesting it didn't bore its head as low. Cervical vertebrae of Hyaenodon was more relatively short and resembled that of a felid than a canid.

The neural spines were prominent and was posterior projecting Hyaenodon in comparison to carnivorans. Within Hyaenodon, the transverse processes and neural spine of the thoracic vertebrae were larger and more robust than what is seen with carnivorans. The more developed spines would've supported a larger head with hypothetical nuchal ligament.

The femur of Hyaenodon was found to have been nearly as long as the humerus, which was short and massive, supporting a large, rounded head. Compared to carnivorans, the greater tuberosity has a more irregular shape and faces anteromedially. It would've been an insertion site for the infraspinatous muscles, which isn't seen in carnivorans. Despite this, the humerus of Hyaenodon was similar to that of wolves and hyenas. Compared to hyenas, it shows an anconeal fossa, a well-developed brachial flange and a similarly elongated trochlea shape. Despite this, the trochela shape of Hyaenodon is more similar in to the red fox.

Compared to wolves, the radius and ulna of Hyaenodon are rather short. The anatomy of the radius suggests it lost any possible rotatory capabilities. The shaft of the radius was not as flattened as seen in carnivorans, although within the cross section it was quadratic, with the distal part being heavy. The ulna had a relatively long olecranon, along with a heavy shaft and a deep channel that runs into the radical notch to the styloid process.

Paleobiology

Predatory behavior