Hyainailouridae

Hyainailouridae used to be considered a subfamily of Hyaenodontidae, but cladistic study by Sole et al., (2013, 2015) treats it as a distinct family. Two subfamilies are recognized, Apterodontinae and Hyainailourinae.[1][2]

Hyainailourids 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. While some measured as much as 1.4 m high at the shoulder with head-body length up to 3.2 m and weighed up to 1,500 kg, most were in the 5–15 kg range, equivalent to a mid-sized dog. 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.

At least one hyainailourid lineage, Apterodontinae, was specialised for aquatic, otter-like habits.[3]

They were important hypercarnivores in Eurasia and Africa during the Oligocene, but gradually declined, with almost the entire family becoming extinct by the close of the Oligocene. Only Megistotherium and several of its sister genera, including Hyainailouros and Sivapterodon, survived into the Miocene.[4] Traditionally this has been attributed to competition with carnivorans, but no formal examination of the correlation between the decline of hyaenodontids and the expansion of carnivorans has been recorded, and the latter may simply have moved into vacant niches after the extinction of hyaenodontid species.