Hyainailouridae

Hyainailouridae ("hyena-like cats") is a paraphyletic family of extinct predatory mammals within the polyphyletic superfamily Hyainailouroidea within extinct order Hyaenodonta. Fossil records show hyainailourids arose during the Middle Eocene, although Early Eocene origin is suspected, the family persisted into the Late Miocene. Fossils of this group have been found in Eurasia, Africa, and North America.

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 paraphyletic Hyainailourinae.

The largest known hyainailourid, as well as the largest hyaenodont, was Megistotherium. This taxa was estimated to have weighed 500–880 kg (1,100–1,940 lb), while 1,794–3,002 kg (3,955–6,618 lb) has been proposed, it exceeds the biomechanical limit of 1,100 kg (2,400 lb) for terrestrial mammalian carnivores, making the estimate unreliable. Hyainailouros was another large hyainailourid, with the largest species, Hyainailouros sulzeri, stood 100 cm (3 ft 3 in) at the shoulders and weighed 500 kg (1,100 lb). However, other members such as Pakakali, Mlanyama, and Isohyaenodon, were smaller within the 5–15 kg (11–33 lb) range, equivalent to a bobcat or fossa.

Early hyainailourids, such as Kerberos and Apterodontinae, had plantigrade locomotion, with feet flat on the ground, similar to bears. However, this trait was later lost as later hyainailourids, such as Simbakubwa, had semi-digitigrade locomotion which was more energy efficient in open environments. Unlike Hyaenodon, Hyainailouros wasn’t a cursorial predator but it was a capable jumper.

At least one hyainailourid lineage, apterodontines, was semi-aquatic and was fossorial.

Many hyainailourids, especially Miocene hyainailourids; were hypercarnivores. However, there were some hyainailourids that were mesocarnivorous such as Pakakali. Apterodon was believed to have fed on hard-shelled animals such as crustaceans. While Pakakli was found to eaten small vertebrates, occupying a niche similar to modern day gray foxes. Some hyainailourids, such as Kerberos, Megistotherium and Hyainailouros, showed adaptations towards osteophagy.

The larger hyainailourines were more specialized into taking down larger prey due to their large size and their relatively long face compared to carnivorans and hyaenodontids. The large size of Hemipsalodon enabled it to hunt brontotheres such as Megacerops. Kerberos was believed to have hunted large herbivores such as Choeropotamus, Lophiodon, and Palaeotherium. The largest hyainailourines, Hyainailouros, Megistotherium, and Simbakubwa were all believed to have been specialized on megaherbivores such as rhinoceroses and proboscideans.

The endocast of Megistotherium was found to have the largest relative neocortex of any of the hyaenodonts. Due to the earliest members having the smallest relative neocortexes, it is thought that throughout their evolution, the neocortex expanded in size. Despite the derived nature of its endocast, a 2019 study found that Megistotherium still had a relatively low EQ score of 0.24-0.29, which is lower than Apterodon, who had an EQ score of 0.34-0.35. However, a 2022 study estimated an even higher EQ score for Megistotherium with a range of 0.49-0.61. However despite this, both studies have found that hyainailourids had lower EQ score than hyaenodontids.