Hyainailouridae

Relations 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. Taxonomy ==Characteristics ==

The largest known hyainailourid, as well as the largest hyaenodont, was Megistotherium. This taxa was estimated to have weighed , while has been proposed, it exceeds the biomechanical limit of for terrestrial mammalian carnivores, making the estimate unreliable. Hyainailouros was another large hyainailourid, with the largest species, Hyainailouros sulzeri, stood at the shoulders and weighed . However, other members such as Pakakali, Mlanyama, and Isohyaenodon, were smaller within the range, equivalent to a bobcat or fossa. Locomotion Early hyainailourids, such as Kerberos and Apterodontinae, had plantigrade locomotion, with feet flat on the ground, similar to bears. Diet Many hyainailourids, especially Miocene hyainailourids; were hypercarnivores. Kerberos was believed to have hunted large herbivores such as Choeropotamus, Lophiodon, and Palaeotherium. 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. ==Evolution==

Hyainailouridae were one of the two families of the superfamily, Hyainailouroidae, which appeared during the early Paleocene. It is hypothesized that hyainailourides evolved during the early Eocene in Afro-Arabia. Fossil evidence suggests the family migrated out of Africa at least four times. The Lutetian/Bartonian Dispersal Phase was the first migration of hyainailourids to Laurasia. The appearance of Kerberos and Paroxyaena was related to a faunal modification of mammals. They may have diapered into Europe via the Iberian Peninsula. == Extinction ==

During the late Eocene to early Oligocene, hyainailourids went extinct in North America and Europe. with the last genus Metapterodon going extinct around 10.1 Ma.'' Lang and colleagues argued that carnivorans may have played a role in the extinction of hyaenodonts with the adaptive potential of their carnassials and the difference in functional morphology. Borths and Stevens, in their 2019 paper, argued that the giant hyainailourines were outcompeted by social carnivorans due to their larger, more complex brains. The discovery of Simbakuwba suggests the evolution of large hyainailourines was due to changes in the herbivore fauna instead of competition with carnivorans. with the relative size of the anterior brain playing a bigger role in correlation to sociality. Christison and colleagues analyzed hyaenodonts and carnivorans from Calf Creek locality of Cypress Hills Formation. Their analysis found that the largest hyaenodont and carnivore, Hemipsalodon grandis and Hoplophoenus, had very distinct niches, with the former focusing on large herbivores such as brontotheres. They argue this would suggest that competitive replacement wasn’t the driving factor the decline and extinction of North American hyaenodonts. Instead, global climatic cooling during the earliest Oligocene resulted in drier, more open landscapes and which saw the extinction of large browsing herbivores, including brontotheres. Following the extinction of bronotheres, they were replaced by grazing herbivores such as equids and rhinoceroses, which were better suited for the open environments. Because rhinoceroses didn’t reach their large sizes until the Miocene. Hyaenodonts such as Hemipsalodon were at a disadvantage due to the lack of large prey available. In addition, because of their shorter legs, they were likely at a disadvantage in the increasingly open environments. ==References==