Hyainailouros

Hyainailouros ("hyena-cat") is an extinct polyphyletic genus of hyaenodont belonging to the family Hyainailouridae that lived during the Early to Late Miocene from 20.0 to 11.4 million years ago, making it one of the last known hyaenodonts. This genus comprises at least three species spread across Eurasia and Africa.^[6][7] Currently, there’s a debate whether or not Megistotherium was synonymous to Hyainailouros or a separate genus entirely.^{[8][9][10][11]} The extinction of large hyainailourines such as Hyainailouros, may have been due to the decline of large herbivores and competition with social carnivorans, as their larger, more complex brains, would’ve enabled them to steal kills from large, solitary hyainailourines like Hyainailouros.

Hyainailouros Temporal range: Early to Late Miocene (Burdigalian to Tortonian) 20.0–11.4 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Lower jaw of Hyainailouros sulzeri
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Hyaenodonta
Superfamily:	†Hyainailouroidea
Family:	†Hyainailouridae
Subfamily:	†Hyainailourinae
Tribe:	†Hyainailourini
Genus:	†Hyainailouros Biedermann, 1863
Type species
†Hyainailouros sulzeri Biedermann, 1863[1]
Other Species
†H. bugtiensis (Pilgrim, 1912)[2] †H. napakensis (Ginsburg, 1980)[3] †H. osteothlastes? (Savage, 1973)
Synonyms
synonyms of genus: Hainailouros (Lavrov, 1999)[4] Hyaenaelurus (Stehlin, 1907) Hyaenailurus (Rütimeyer, 1867) synonyms of species: H. napakensis: Hyainailouros nyanzae (Ginsburg, 1980) Pterodon nyanzae (Savage, 1965)[5] H. sulzeri: Hyainailouros maximus (Meyer, 1837) Hyaenailurus sulzeri (Biedermann, 1863)

Taxonomy

Hyainailouros is the type genus of the superfamily Hyainailouroidae, one of the two known superfamilies of the order Hyaenodonta, with Hyaenodontoidea representing the other superfamily. The genus is part of the subfamily known as Hyainailourine, within the family known as Hyainailouridae.^[12] This family of hyaenodonts was diverse, while hyainailourines consisted of terrestrial predators, the subfamily Apterodontinae consisted of hyainailourids with otter-like adaptations for a semi-aquatic lifestyle.^[13][14][15] The classification between Hyainailouros and Megistotherium has been debated by experts.^[8] Some experts consider H. bugtiensis to be synonymous with H. sulzeri.^[16]

Hyainailourine based on results by recovered by Matthew R. Borths and Nancy J. Stevens (2019):^[12]

Hyainailourinae	Falcatodon schlosseri Simbakubwa kutokaafrika Hyainailouros sulzeri Arrisdrift hyainailourine Hyainailouros napakensis Isohyaenodon andrewsi Sivapterodon Hyainailouros bugtiensis Megistotherium osteothlastes Leakitherium hiwegi Mlanyama sugu Megapterodon kaiseri Isohyaenodon zadoki Exiguodon pilgrimi

Evolution

Hyainailouroids were thought to have evolved during the Early Eocene,^[12] although a Middle Paleocene origin was also suspected for the superfamily.^[17] Despite its large size, Hyainailouros may have evolved from small-bodied hyainailourines weighing less than 15 kg (33 lb).^[14] Due to the appearance of the Gomphothere Land Bridge, the Hyainailouros lineage was able to disperse into Eurasia around 19.6 Ma, and would later disperse into Europe around 16.9 Ma.^[12]

Description

Size comparison of H. sulzeri, Amphicyon giganteus and Crocuta crocuta

The type species, H. sulzeri, was one of the largest members of the order Hyaenodonta. This species stood 100 cm (3 ft 3 in) at the shoulders,^[18] with a 2019 study estimating this species weighing between 266–1,276 kg (586–2,813 lb). H. bugtiensis was around the same size of H. sulzeri, if not slightly larger, weighing 267–1,744 kg (589–3,845 lb). Both species were similar in size to the closely related Simbakubwa. H. napakensis, on the other hand, was the smallest species of the genus, weighing just around 202–271 kg (445–597 lb).^[12] But this regression is problematic as hyaenodonts possessed very large heads in proportion to their body size. Many experts argue smaller sizes. Based on postcranial remains, some experts argued Hyainailouros was about the size of a tiger,^[19] although other experts still suggest higher estimates for H. sulzeri, suggesting it weighed 500 kg (1,100 lb).^[20] H. bugtiensis (referred as H. sulzeri) according to a 2025 study, was estimated to have weighed 430 kg (950 lb).^[16]

Paleobiology

Molar

Much like Simbakubwa, Hyainailouros walked with a semi-digitigrade stance and was probably capable of large, leaping bounds, although it likely wasn’t a fast runner.^[12][14] Compared to Hyaenodon, Hyainailouros was less adapted for meat shearing instead showed more adaptations towards bone crushing.^[18] It’s believed that the evolution of large hyainailourines was due to the evolution of large herbivores, which resulted in adaptations to hunt and scavenge proboscideans and rhinoceroses, which were typically ignored by carnivorans due to their large size. This specialized niche that remains largely unoccupied in modern ecosystems populated by more socially complex carnivorans and may have allowed them survive into the Neogene.^[12] Additionally, fossil evidence shows that Hyainailouros appeared in Europe around the same time as proboscideans, suggesting Hyainailouros followed their prey as they migrated out of Africa.^[14][21]

Regressions found that humerus of Hyainailouros is found to be as robust as an average felid, but less robust than Smilodon and American lion. Compared to the humerus of tigers, the deltoid scar was located more distally in the humerus of the hyainailourine than the humerus of tigers.^[19]

Paleoecology

Restoration of H. sulzeri (far left), Cynelos eurydon, Afrosmilus africanus and H. napakensis (far right)

H. sulzeri has been found in Europe and some parts of Africa,^[22][20] from 18 to 15 ma.^{[23] Including supplementary materials} Within Grand Morier of Miocene France, H. sulzeri coexisted with mammalian predators such as the hyena Protictitherium gaillardi, felids such as Miopanthera lorteti and Styriofelis, amphicyonids such as Agnotherium grivense, Amphicyon giganteus, Pseudarctos bavaricus, and Pseudocyon sansaniensis, the mustelid Trocharion albanense, and early ursid Hemicyon sansaniensis. Herbivores present in this locality include proboscideans such as the gomphothere Gomphotherium angustidens, deinothere Prodeinotherium bavaricum, and mammutid Zygolophodon turicensis, an ochotonid, extinct beavers Anchitheriomys and Steneofiber depereti, the early horse Anchitherium aurelianense, the chalicothere Anisodon grande, and rhinoceroses such as Lartetotherium sansaniense, as well aceratheriinaes Brachypotherium brachypus, Plesiaceratherium lumiarense, and Prosantorhinus.^[24] Due to its rarity, H. sulzeri probably wasn’t serious competitor towards Amphicyon in Europe.^[20]

Within Arrisdrift, Namibia, H. sulzeri coexisted with carnivorans such as the felid Diamantofelis, amphicyonids such as Amphicyon giganteus and Namibiocyon ginsburgi, and viverrids such as Orangictis gariepensis and Africanictis. Contemporary herbivores include proboscideans such as the deinothere Prodeinotherium hobleyi, amebelodont Afromastodon, and the gomphothere Gomphotherium, the hyracoid Prohyrax hendeyi, the bovid Homoiodorcas, the tragulid Dorcatherium pigotti, and the rhino Diceros. Other faunas found within Arrisdrift include reptiles such as the crocodilian Crocodylus gariepensis, tortoises such as Namibchersus namaquensis and Mesochersus orangeus, the boid Central African rock python, and monitor lizards. Birds present in this locality include accipitrids and struthionid.^[25]

H. bugtiensis was found in the Siwalik Hills and lived from 19 to 11.4 ma.^[16] Within the Bugti Hills, Hyainailouros coexisted with proboscideans such as the mammutid Zygolophodon metachinjiensis, the deinothere Prodeinotherium pentapotamiae, the choerolophodontid Choerolophodon corrugatus, gomphotheres such as Gomphotherium browni and Protanancus chinjiensis, the giraffid Progiraffa, and rhinoceroses such as Brachypotherium and Aprotodon fatehjangense.^[26]

Extinction

The extinction of large hyainailourines is hypothesized to have been the result of changing ecosystems and competition with carnivorans.^[12][27] While hyaenodonts in Namibia declined due to the inability to adapt to environments such as steppes, savannas, and deserts, a few lineages such as Megistotherium continued to persist despite the increase in aridity.^[28] Some experts argued that the presence of carnivorans may have resulted in some hyaenodonts to become larger and more hypercarnivorous.^[29][27][30] However, the discovery of Simbakubwa suggests the key factor of the large sizes of hyainailourines was changes in the herbivore fauna due to changes in the Afro-Arabia landscape instead of competition with carnivorans as they diversified later in the Miocene. Experts believed that large hyainailourines may have declined due to the decline of large herbivores, who tend to have slow generation times and may be particularly sensitive to environmental changes.^[12]

Since they were specialized on these herbivores, even if their population declined briefly, the large hyainailourines would’ve been affected by the changing resources, more so than the smaller carnivorans. This is seen in modern ecosystems where large hypercarnivores are affected more dramatically with environmental shifts than smaller mescarnivores.^[12] Although they may have been effective scavengers,^[20][12] experts argued social carnivorans may have been adept at stealing large carcasses from large, solitary hyainailourines, due their larger, more complex brains. This would’ve resulted in the extinction of large hyainailourines as they wouldn’t be able to obtain large amounts of food needed for survival.^[12] However, studies have shown that larger brains have little to no roles in sociality among carnivorans,^[31][32][33] instead within carnivoran families, sociality tends to correlate the anterior cerebrum volume relative to overall brain size.^[34][35][36]