Hyaenodon

Size The species within the genus vary in size, with most being small to medium-sized predators, while some were among the largest terrestrial carnivorous mammals of their time. Another tiny North American species, H. venturae, was similar in size to H. microdon. H. filholi was the smallest European species and the smallest species within the genus, weighing . H. leptorhynchus, the type species, was estimated to have weighed . H. horridus was one of the largest North American species. While m1 regressions suggested it could have weighed , Cervical vertebrae of Hyaenodon was more relatively short and resembled that of a felid than a canid. Compared to wolves, the radius and ulna of Hyaenodon are rather short. The anatomy of the radius suggests it lost any possible rotatory capabilities. The shaft of the radius was not as flattened as seen in carnivorans, although within the cross section it was quadratic, with the distal part being heavy. The ulna had a relatively long olecranon, along with a heavy shaft and a deep channel that runs into the radical notch to the styloid process. == Paleobiology ==

(around 1920) Predatory behavior The canines of Hyaenodon were mediolaterally compressed, similar to canids, this was ideal for slashing bites. This suggests Hyaenodon landed shallow bites on potential prey and likely didn't perform the killing bite seen in felids. However, dental microwear patterns suggests that North American Hyaenodon had a diet more similar to lions, suggesting it ate mostly meat with various intakes of bone. On the other hand, European Hyaenodon microwear were more similar to that of spotted hyenas, suggesting bone cracking was likely a major part of their diet. The tooth wearing on P4 of H. gigas suggests the primary function of the tooth was for bone-cracking. A 2024 study found that canine bite mechanic efficiency increased with tooth macrowear in Hyaenodon. Ontogeny Studies on juvenile Hyaenodon specimens show that the animal had a very unusual system of tooth replacement. Juveniles took about 3–4 years to complete the final stage of eruption, implying a 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. Locomotion Initially, Hyaenodon was thought to have been a semi-plantigrade walker, however other experts considered it to have been a digitigrade. Furthermore, a 2025 study found that based on elbow morphology found that H. crucians and H. horridus were pounce-pursuit predators. Much like H. horridus, H. eminus, H. gigas, and H. pervagus were recovered as cursorial predators. On the other hand, analysis on the bony labyrinth of H. exiguus suggests this species was semi-arboreal. The endocast of Hyaenodon stands out from other hyaenodontoids as they had relatively high EQ, in addition to relatively gyrencephalic and neocorticalized brains, however the cause of the increase in EQ for the genus is still unknown. Despite having neocorticalized brains compared to other hyaenodonts, the neocortex of Hyaenodon was only moderately folded. Social behavior Due the small size of the neocortex, some experts proposed that Hyaenodon probably didn't hunt in packs. Further evidence to suggest it was a solitary predator was fossil evidence of defecation, as defecating on food was an indication of a solitary predator. ==Paleoecology==

North America During the early Paleogene, North America consisted of subtropical swampy, densely forested habitats which supported predators such as oxyaenids. However, during the Middle Eocene, these habitats were replaced by more temperate, open forests. Because of these environmental changes, hyaenodonts would replace oxyaenids, as well as mesonychians and miacoids. This species was found in the Calf Creek locality of Cypress Hills Formation. In this locality, Hyaenodon coexisted with hyaenodonts such as H. microdon and the hyainailourid Hemipsalodon grandis. Carnivorans that were present in this formation were daphoeninae amphicyonids Brachyrhynchocyon dodgei and Daphoneus, nimravids Dinictis and Hoplophoneus, hesperocyonine canid Hesperocyon gregarius, and the subparictid Parictis. In addition, Hyaenodon also coexisted with the entelodont Archaeotherium. Herbivores present in this locality include the equid Mesohippus, the hyracodontid Hyracodon priscidens, rhinoceroses Subhyracodon occidentalis, Trigonias osborni, and Penetrigonias sagittatus, tapirid Colodon occidentalis, the brontothere Megacerops kuwagatarhinus, and the anthracothere Bothriodon advena. Contemporary predators would've included fellow species H. crucians, the nimravid Hoplophoneus, the amphicyonid Daphoneus, and the entelodont Archaeotherium mortoni. Herbivores present include the early horse Mesohippus, hypertragulid Hypertragulus calcaratus, leptomerycid Leptomeryx evansi, tapirs such as Protapirus simplex and Colodon occidentalis, the camel Poebrotherium wilsoni, anthracotheres Aepinacodon americanus and Heptacodon occidentale, oreodonts Merycoidodon culbertsonii and Miniochoerus affinis. Fossil evidence suggests that H. horridus could've predate on other predators such as Dinictis and juvenile Archaeotherium. Hyaenodon first appears in the fossil records around MP17a of the Bartonian stage of the Eocene marked by the appearance of H. brachyrhynchus, H. minor, and H. requieni. Mammalian predators present in this locality other than H. requieni included the hyainailourine Pterodon dasyuroides and the amphicyonine Cynodictis lacustris. Contemporary herbivores include perissodactyls (palaeotheres), endemic (amphimerycids, Anoplotheriidae, choeropotamids, and xiphodontids) and non-endemic artiodactyls (dichobunides, tapirulids, and anthracotheres). Other mammalian fauna includes primates (adapids and omomyids), rodents (ischyromyids, theridomyids, and glirids), soricomorphs (nyctitheriids), and tribosphenidans (Herpetotheriidae). The massive drop in temperature stemmed from the first major expansion of the Antarctic ice sheets that caused drastic pCO2 decreases and an estimated drop of ~ in sea level. During the Grand Coupure, the extinction rate of western European mammalian lineages increased to 60%. The turnover saw the extinction of frugivorous/folivorous families of endemic artiodactyls such as xiphodontids and choeropotamids, as well as the extinction of several hyaenodonts including H. requini and the hyainailourines. The Grande Coupure event also marked a large faunal turnover would mark the arrivals of later anthracotheres, entelodonts, ruminants (Gelocidae, Lophiomerycidae), rhinocerotoids (Rhinocerotidae, Amynodontidae, Eggysodontidae), carnivorans (later Amphicyonidae, Amphicynodontidae, Nimravidae, and Ursidae), eastern Eurasian rodents (Eomyidae, Cricetidae, and Castoridae), and eulipotyphlans (Erinaceidae). However, some experts found closed forests and temperate to subtropical climates still prevailed in Europe, although the humidity wasn't as high as the Priabonian. The species was found within the Khoer-Dzan locality of the Ergilin Dzo Formation. Sedimentary analysis of Khoer-Dzan found the presence of root traces which suggests red soils, which is associated with floodplain environments, with no evidence of lacustrine environments. Within this locality, H. gigas coexisted with other predators like hyaenodonts H. eminus, H. incertus, H. mongoliensis, and H. pervagus, nimravids Eofelis and Nimravus intermedius, and entelodonts Brachyhyops trofimovi and Entelodon gobiensis. Contemporary herbivores include the anthracothere Bothriodon, the praetragulid Praetragulus electus, the brontothere Embolotherium andrewsi, the chalicothere Schizotherium avitum, and rhinocerotoids such as the paraceratheriid Urtinotherium parvum and rhinocerotid Ronzotherium orientale. == Extinction ==

From the middle to late Eocene, hyaenodonts experienced a decline in diversity with only one genus, consisting of a few species by the end of the Eocene in North America. In Europe, the last species to go extinct, H. exiguus and H. leptorhynchus, were last known in MP30 of the late Oligocene. One of the youngest species, H. weilini, lived during the Early Miocene of China, going extinct 17 Ma. Lang and colleagues theorize that the success of carnivorans compared to hyaenodonts was likely due to the retention of a basal morphotype throughout their evolutionary history. They also suggested that carnivorans possibly played a role in the extinction of hyaenodonts, probably due to the adaptive potential of their carnassials. Friscia and Valkenburgh argued that while "creodonts" didn't limit the evolution of carnivorans in North America, carnivorans may have limited the ecological evolution of creodonts, with the large hypercarnivorous ecomorphs being the "last stand" of the order. MP30 and MN1 saw an increase in aridity, and was associated with a cooler climate. It saw the extinction of other groups of animals such as Theridomorpha. Despite being recovered as a cursorial predator, European Hyaenodon went extinct because of the expansion of open environments as it reduced the access to resources. ==References==