Hyaenodon
Hyaenodon is an extinct genus of carnivorous placental mammals from the tribe Hyaenodontini, part of the subfamily Hyaenodontinae, that belonged to the now extinct order Hyaenodonta. The genus was found lived in Eurasia and North America from the Middle Eocene to the Early Miocene, from 38 to 17 million years ago, existing for. Hyaenodon first evolved in Asia, probably evolving from Propterodon.
The genus currently consists of at least 40 species, although due sexual dimorphism and intraspecific variation, they were likely fewer species within the genus. The species within the genus ranged in size from H. filholi, who weighed, to H. gigas and H. mongoliensis, who were estimated to be similar in size to Hyainailouros. The largest species were likely the apex predators of their time, with fossil records suggesting they could've occasionally hunted smaller predators. Several species within the genus were cursorial predators, either being ambushing or pounce-pursuit predators. The canines of the animal mediolaterally compressed much like canids, landing shallow bites on their prey. Unlike canids such as wolves, they were thought to have been solitary predators.
The genus saw a decline during the Late Eocene to Early Oligocene, with only one species, H. weilini, being present in the Miocene. Initially, experts hypothesize their decline and extinction was the result of competition with carnivorans. However, over the recent years, this hypothesis has been questioned. Instead, experts now hypothesize cause of their decline and eventual extinction was the inability to adapt to open environments.
Classification and phylogeny
Taxonomy
Description
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. Intraspecific dimorphism has also been reported in the genus, although its ecological significance is unclear. H. crucians, from the early Oligocene of North America, was estimated to have weighed around. H. microdon and H. mustelinus, from the late Eocene of North America, were even smaller and weighed probably about. 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, regressions based on limb morphology suggest the species was instead a bit smaller, with adults weighing on average, with the largest adults weighing no more than. H. megaloides, the largest North American species, was three times heavier than H. horridus, weighing. It has been suggested that the size decrease among North American Hyaenodon species may have been the result of competition with nimravids. In Europe, the largest species known species was H. geravisi, weighing. The largest species was H. gigas, followed by H. mongoliensis. Both species had a skull length of and were thought to have been larger than lions, potentially rivaling Hyainailouros in size. ''H. weilini was another very large species, described to be similar in size to H. gigas and H. mongoliensis''.
Postcranial remains
Postcranial remains have been recovered for Hyaenodon, although the fossil records within Eurasia are rather scarce compared to North America. While the neck of the animal was relatively short, the body and tail were long. Compared to Hyainailouros, the spine of Hyaenodon was longer and more robust, suggesting it didn't bore its head as low. Cervical vertebrae of Hyaenodon was more relatively short and resembled that of a felid than a canid.The neural spines were prominent and was posterior projecting Hyaenodon in comparison to carnivorans. Within Hyaenodon, the transverse processes and neural spine of the thoracic vertebrae were larger and more robust than what is seen with carnivorans. The more developed spines would've supported a larger head with hypothetical nuchal ligament.
The femur of Hyaenodon was found to have been nearly as long as the humerus, which was short and massive, supporting a large, rounded head. Compared to carnivorans, the greater tuberosity has a more irregular shape and faces anteromedially. It would've been an insertion site for the infraspinatous muscles, which isn't seen in carnivorans. Despite this, the humerus of Hyaenodon was similar to that of wolves and hyenas. Compared to hyenas, it shows an anconeal fossa, a well-developed brachial flange and a similarly elongated trochlea shape. Despite this, the trochela shape of Hyaenodon is more similar in to the red fox.
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
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. Hunter-Schreger bands observed in the tooth enamel of H. horridus are zigzag, suggesting that this species was osteophagous, whereas those of H. brevirostris and H. mustelinus transition from undulating at the base of the tooth to zigzag at the tip, indicating that these species were not as well adapted for feeding on bone. 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. Despite having adaptations towards bone consumption, compared to Hyainailouros, the dentition of Hyaenodon was geared more towards shearing meat and less towards bone crushing. 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. Due to the presence of relatively straight ungual phalanges, short phalanges, intermediate long metatarsals, cadually oriented olecranon, with a long olecranon, suggests Hyaenodon was a terrestrial animal. A 2003 study found that based on elbow morphology H. horridus was a cursorial predator and was the most cursorially adapted Oligocene carnivore sampled within the study. Furthermore, a 2025 study found that based on elbow morphology found that H. crucians and H. horridus were a pounce-pursuit predators. Much like H. horridus, H. eminus, H. gigas, and H. pervagus were recovered as a cursorial predators. On the other hand, analysis on the bony labyrinth of H. exiguus suggests this species was semi-arboreal.Despite being a cursorial predator, Hyaenodon probably still tended to hunt within short distance.
Brain anatomy and senses
While it has typically been assumed that Hyaenodon had a very massive skull, but a small brain, this has been called into question. Flink and colleagues found that Hyaenodon, had encephalization quotient of 0.36-0.37 and 0.42-0.46, for H. horridus and H. crucians respectively. This similar to basal and some modern carnivorans such as cougars, Hesperocyon gregarius, Hoplophoneus primaevus, and striped hyena and exceeding jaguars, Daphoenus, Hoplophoneus primaevus, and Eusmilus bidentatus. Their discovery found that hyaenodonts had relative brain sizes that overlapped with both extinct and extant carnivorans. 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.The olfactory bulbs were found to have been large in Hyaenodon, suggesting smell was the primary method of finding prey. However, compared to Neogene carnivorans of similar size, Hyaenodon sense of smell wasn't as acute, which would've been disadvantageous in prey detection.
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. Hyaenodon first appeared in North America during the Middle Eocene with the appearance of H. venturae, likely the result of immigration from Asia. File:Hyaenodon and Leptomeryx.jpg|thumb|right|Reconstruction of H. horridus and Leptomeryx evansi by W. B. Scott The most well known species, H. horridus, roamed North America from 36.5 to 31.4 Ma. This species was found in the Calf Creek locality of Cypress Hills Formation. The herpetofauna present within the locality suggests Calf Creek had a tropical or subtropical climate. 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.The predators present in Calf Creek likely practiced niche partitioning via different body sizes, with H. horridus focusing on prey that weighed and faced little competition from carnivorans. While Hoplophoneus could've induced some competition pressure via pack hunting, H. horridus could still hunt prey outside of the most probable range of the carnivoran. On the other hand, H. microdon was thought to have faced intense competition from five contemporary carnivorans.
File:Archaeotherium.jpg|left|thumb|Restoration of Archaeotherium eating roots. Several species of both taxa were contemporary with each other across North America
H. horridus was also found in Brule Formation of South Dakota. The paleoenvironment of Brule Formation was believed to have been a woodland grassland and gallery forest, populated in part by hackberry trees. 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. Despite being one of the top predators, it still probably lost its kills to an adult Archaeotherium.