Hyaenodonta

skull of Hyaenodon horridus

Comparison of carnassial teeth of wolf and typical hyaenodontid and oxyaenid

Hyaenodonts are characterized by long skulls, slender jaws, slim bodies and a plantigrade stance. 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]) and Hyaenodon gigas (the largest species from genus Hyaenodon) was as much as 1.4 m high at the shoulder, 3.0 m long and weighed about 330 kg, most of hyaenodonts 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]

Because of their size range, it is probable that different species hunted in different ways, which allowed them to fill many different predatory niches. Smaller ones would hunt in packs during the night like wolves, and bigger, fiercer ones would hunt alone during the daylight, using their sheer size and their mighty jaws as their principal weapon. 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 3–4 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]

• Mammal classification
• Ferae
• Creodonta