Hyaenodonta

Hyaenodonta Temporal range: 63.8–8.8 Ma PreꞒ Ꞓ O S D C P T J K Pg N early Paleocene to late Miocene (Suspected Late Cretaceous origin, but unconfirmed by fossils yet)[1][2]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Mirorder:	Ferae
Clade:	Pan-Carnivora
Order:	†Hyaenodonta Van Valen, 1967[3]
Subgroups
[see classification]
Synonyms
Hyaenodontida (Solé, 2010)[4] Hyaenodontidae (Leidy, 1869) Proviverroidea (Morlo, 2009)[5]

Hyaenodonta ("hyena teeth") is an extinct order of hypercarnivorous placental mammals of clade Pan-Carnivora from mirorder Ferae.^[6][7] Hyaenodonts were important mammalian predators that arose during the early Paleocene in Europe^[8] and persisted well into the late Miocene.^[9]

Hyaenodonts are characterized by long, often disproportionately large skulls, slender jaws, and slim bodies. They generally ranged in size from 30 to 140 cm at the shoulder.^[10] While Simbakubwa kutokaafrika may have been up to 1,500 kg (3,300 lb) (surpassing the modern polar bear in size^[11]), 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 (the largest species from genus Hyaenodon), which may have been as large as 1.4 m high at the shoulder, 3.0 m long and weighed about 330 kg. Most hyaenodonts, however, were in the 5–15 kg range, equivalent to a mid-sized dog.^[12] 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.^[10]

Hyaenodonts were ancestrally plantigrade, but the later, larger forms were generally digitigrade or semidigitigrade. Because of their size range, it is probable that different species hunted in different ways, which allowed them to fill many different predatory niches, with small or medium-sized forms filling roles similar to mustelids or smaller felids of today while the larger forms functioned as apex predators focusing on larger prey, wielding their mighty jaws as their principal weapon as they lacked grasping forelimbs. The carnassials in a hyaenodonts are generally the second upper and third lower molars. However, some hyaenodonts possessed as many as three sequential pairs of carnassials or carnassial-like molar teeth in their jaws.^[13] Hyaenodonts, like all creodonts, lacked post-carnassial crushing molar teeth, such as those found in many carnivoran families, especially the Canidae and Ursidae, and thus lacked dental versatility for processing any foods other than meat.^[13]

Hyaenodonts differed from Carnivora in that they replaced their deciduous dentition slower in development than carnivorans.^[14] 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.^[15]

At least one hyaenodont lineage, subfamily Apterodontinae, was specialised for aquatic, otter-like habits.^[16]

Having evolved in Europe during the Paleocene,^[8] hyaenodonts soon after spread into Africa and India, implying close biogeographical connections between these areas.^[16][17] Afterwards, they dispersed into Asia from either Europe or India, and finally, North America from either Europe or Asia.^[18][19]

They were important hypercarnivores in Eurasia, Africa, and North America during the Oligocene, but declined towards the end of the epoch, with almost 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 survived long enough to go extinct at the close of the Miocene.^[9] Traditionally, this has been attributed to competition with carnivorans, but no formal examination of the correlation between the decline of hyaenodonts and the expansion of carnivorans has been recorded, and the latter may simply have moved into vacant niches after the extinction of hyaenodont species.^[20]

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.^[10] 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.^[21] However, order Creodonta is now considered to be a polyphyletic wastebasket taxon containing two unrelated clades assumed to be closely related (or ancestral) to Carnivora.^{[8][14][15][16][17][22][23][24][25][26][27]}

• Mammal classification
• Ferae
• Creodonta