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Hyaenodon

Hyaenodon is an extinct genus of carnivorous placental mammals belonging to the family Hyaenodontidae within the clade Hyaenodonta (formerly classified under the order Creodonta), known for their role as dominant predators during the Paleogene.^[1] These mammals, whose name translates to "hyena tooth," were not closely related to modern hyenas but occupied similar ecological niches as hypercarnivores with specialized dentition for shearing flesh.^[2] Species of Hyaenodon exhibited a wide range of body sizes, from small fox-like forms weighing around 10 kg to large lion-sized individuals approaching 100 kg, and displayed sexual dimorphism in some cases.^[1] The genus ranged temporally from the late Eocene to the early Miocene, spanning approximately 38 to 16 million years ago, likely originating in Asia, with fossils documenting its presence across North America, Europe, and Asia.^[1]^[2] In North America, Hyaenodon species were among the largest carnivores of the Oligocene, characterized by large heads, long slender legs suited for running, and powerful jaws adapted for bone-crushing and predation on medium to large ungulates.^[3] Eurasian records reveal diverse adaptations, including cursorial locomotion in larger forms and potentially semi-arboreal habits in smaller species, allowing them to exploit varied environments from forests to open plains.^[4] Around 30 species have been described, with notable examples including the small European H. exiguus and the massive North American H. horridus.^[1] Hyaenodon played a pivotal role in Paleogene mammalian communities, often serving as apex predators and scavengers before being largely replaced by the rising Carnivora order in the Miocene.^[5] Their morphology, including large maxillary sinuses and specialized paranasal structures, suggests efficient respiratory and sensory adaptations suited to terrestrial hunting, though with less acute olfaction than modern carnivorans.^[2] The evolutionary success of Hyaenodon highlights the adaptive radiation of hyaenodonts following the Cretaceous-Paleogene extinction, filling carnivorous guilds until environmental changes and competition led to their decline.^[4]

Taxonomy and phylogeny

Etymology and discovery

The genus name Hyaenodon derives from the Greek words hyaina (hyena) and odous (tooth), coined to highlight the superficial resemblance of its dentition to that of hyenas.^[1] The type species, H. leptorhynchus, was formally described in 1838 by French naturalists L. Laizer and J. de Parieu based on a lower jaw fragment representing an unknown carnivorous mammal.^[6] This holotype (FSL 70000) originated from late Oligocene deposits near Cournon-d'Auvergne in Puy-de-Dôme, France.^[7] The initial description of Hyaenodon occurred amid a surge of early 19th-century fossil discoveries of Paleogene mammals across Europe, including Eocene and Oligocene sites that revealed diverse carnivorous forms.^[4] These finds often led to initial misidentifications, with Hyaenodon's shearing carnassials and robust canines mistaken for those of extinct hyenas, reflecting limited understanding of mammalian evolution at the time.^[1] Subsequent explorations expanded the genus's known distribution, with American paleontologist Joseph Leidy reporting the first North American specimens in the 1850s, including H. cruentus from Oligocene strata of the White River Formation in Wyoming.^[8] These discoveries underscored Hyaenodon's transcontinental presence and prompted early comparisons between European and North American faunas.^[9]

Taxonomy

Hyaenodon is the type genus of the Hyaenodontidae, a family of extinct carnivorous mammals within the order Hyaenodonta, which is now recognized as a distinct clade sister to Carnivoraformes within the larger group Pan-Carnivora (mirorder Ferae).^[10] Previously classified under the polyphyletic order Creodonta, Hyaenodonta has been elevated to its own ordinal status based on phylogenetic analyses distinguishing it from other early carnivoramorphans.^[11] Within Hyaenodontidae, Hyaenodon belongs to the subfamily Hyaenodontinae and tribe Hyaenodontini, characterized by specialized carnassial dentition adapted for shearing flesh.^[12] The type species is Hyaenodon leptorhynchus (Laizer and de Parieu, 1838), based on a lower jaw fragment (holotype FSL 70000) from the late Oligocene of France, exhibiting the genus's characteristic elongated rostrum and robust carnassials.^[6] The genus encompasses approximately 20-30 valid species distributed across Eurasia, North America, and Africa from the Eocene to Miocene, reflecting high taxonomic diversity with body sizes ranging from fox-like to bear-sized forms.^[13] Notable examples include H. horridus Leidy, 1853, the largest North American species from the late Eocene to Oligocene White River Formation, estimated at 40-60 kg;^[14] H. gigas Dashzeveg, 1985, a giant Asian form from the Oligocene of Mongolia, one of the largest species in the genus;^[15] and H. brachyrhynchus from the Eocene of Europe, a smaller species known from dental remains indicating a more gracile build.^[15] A recent addition is H. lingbaoensis Li et al., 2025, a small-bodied species from the Middle Eocene Chuankou Formation in China's Lingbao Basin, represented by a partial maxilla preserving deciduous and permanent teeth that highlight early hyaenodontine dental evolution.^[16] Taxonomic revisions in the 2020s have resolved numerous junior synonyms through detailed morphometric and phylogenetic comparisons of cranial and dental material. For instance, several North American taxa previously distinguished by minor size variations have been consolidated, reducing the number of valid species in the White River Group.^[9] The outdated inclusion of Hyaenodontidae within Creodonta has been critiqued and abandoned in favor of clade-based classifications emphasizing shared derived traits like sectorial premolars.^[11] Recent research from 2023 to 2025 has confirmed the validity of several Asian species through new fossil discoveries, including additional remains of H. eminus and H. pervagus from the Oligocene of central Mongolia, which provide better resolution of their dental and postcranial morphology.^[15] These updates underscore ongoing refinements to the genus's systematics, integrating molecular clock estimates and comparative anatomy to clarify interspecies boundaries.^[17]

Phylogeny

Hyaenodon represents a derived genus within the subfamily Hyaenodontinae of the extinct order Hyaenodonta, positioned as part of a clade that includes closely related taxa such as Eurotherium and Proviverra. A 2019 phylogenetic analysis using cranial and dental characters recovered Hyaenodon as sister to a group encompassing Eurotherium species and certain proviverrine genera like Proviverra, emphasizing shared derived features in dentition and skull morphology that distinguish this North American-Eurasian subclade from more basal hyaenodonts.^[18] This positioning highlights Hyaenodon's evolutionary advancement within Hyaenodontinae, characterized by hypercarnivorous adaptations that emerged during the Eocene.^[19] In broader mammalian phylogeny, Hyaenodonta is classified as a group of stem pan-carnivoramorphs within the mirorder Ferae, distinct from the crown group Carnivora and more closely allied to pangolins (Pholidota) than to true carnivorans. The order diverged from its sister clade, the Oxyaenodonta, around the late Paleocene, with molecular and fossil clock estimates placing this split approximately 60-62 million years ago near the Paleocene-Eocene boundary.^[20]^[21] Hyaenodonta's placement as stem taxa is supported by parsimony-based cladistic analyses that recover the clade nested within Ferae, outside the Carnivoraformes, with consistency indices around 0.45-0.50 indicating moderate homoplasy in character evolution.^[22] Key phylogenetic insights derive from a 2020 cladistic study of European hyaenodonts, which recovered the Hyaenodontoidea as a monophyletic clade uniting proviverrines and hyaenodontines, including Hyaenodon, based on a matrix of 120 dental and cranial characters. Updates in 2024, incorporating new Tanzanian fossils, refined basal divergences within Hyaenodonta to the early Eocene, emphasizing Afro-Arabian contributions to the clade's radiation while integrating Asian material to resolve polytomies in earlier trees. In cladograms, Hyaenodon branches after the limnocyonine subfamily, reflecting a progressive increase in body size that paralleled ecological diversification into larger predatory niches during the late Eocene to Oligocene.^[23]^[24] Ongoing debates center on Hyaenodonta's origins, contrasting an African cradle hypothesis—supported by Paleocene fossils like Lahimia from Egypt—with a Laurasian dispersal model favoring Asian or European ancestry based on early Eocene dispersals across continents. Recent 2025 discoveries of basal Hyaenodon species from Eocene deposits in China's Lingbao Basin bolster the Asian cradle hypothesis, suggesting initial diversification in Asia before westward and transatlantic dispersals, with parsimony analyses yielding retention indices of 0.65-0.70 for these revised topologies.^[25]^[16]

Description

Overall morphology and size

Hyaenodon species exhibited a carnivorous body plan broadly resembling that of modern canids or ursids, with elongated skulls, robust limbs adapted for terrestrial locomotion, and a posture transitioning from plantigrade in ancestral or smaller forms to digitigrade or semi-digitigrade in larger, more derived species. These mammals, part of the extinct clade Hyaenodonta within Pan-Carnivora, were not closely related to hyenas (family Hyaenidae) despite superficial similarities in dentition and build, instead representing an early divergent lineage of placental carnivorans specialized for hypercarnivory.^[1]^[26]^[1] Body size varied widely across the genus, reflecting ecological diversity; smaller species such as H. exiguus measured approximately 1–2 m in length and weighed 10–20 kg, akin to a red fox in scale, while larger forms like H. horridus reached up to 3 m in length and 30–50 kg, comparable to a large wolf. Overall genus body masses ranged from about 10 kg to nearly 100 kg, with estimates derived from limb bone dimensions and dental metrics establishing this scale. Sexual dimorphism was minimal, though present, with males typically slightly larger than females based on cranial and postcranial variation.^[1]^[6] Proportions included a short neck, powerful shoulder girdle for forceful movements, and a long tail inferred to provide balance during agile pursuits or climbing in smaller taxa. Smaller European species displayed more generalized, semi-arboreal adaptations with forelimb features supporting scansorial habits, whereas larger North American species trended toward cursorial builds with elongated limbs for efficient terrestrial travel. North American Hyaenodon were generally larger on average than European counterparts, likely influenced by continental faunal dynamics and prey availability. Growth series from fossil assemblages reveal pronounced ontogenetic size increases, with juveniles scaling up substantially to adult dimensions.^[1]^[26]

Skull and dentition

The skull of Hyaenodon features an elongated rostrum housing large maxillary sinuses that occupy 25-30% of the transitional skull area at the level of M1 and extend anteriorly to near P2, contributing to a snout adapted for olfactory and respiratory functions distinct from those in Carnivora.^[2] A prominent sagittal crest rises along the cranium to anchor powerful temporalis muscles for jaw adduction, while the zygomatic arches are reduced in robustness, facilitating greater gape angles by minimizing masseteric stress during wide-mouth biting.^[2]^[27] The braincase is relatively small compared to the facial region, with a limited olfactory compartment and large frontal sinuses forming a prominent forehead and supraorbital processes, but not extending beneath the sagittal crest.^[2] Adult Hyaenodon specimens exhibit a complete dentition of 44 teeth, following the formula I3/3, C1/1, P4/4, M3/3, typical of hyaenodontids and indicative of a hypercarnivorous lifestyle.^[28] The upper carnassials, primarily P4 and M1, form sectorial shearing blades with reduced protocones and simple paracones for slicing flesh, while the lower molars possess trenchant heels on their paraconids and protoconids to complement this action without specialized bone-crushing morphology seen in hyaenids.^[29] Premolars are sectorial and blade-like, incisors are conical for gripping, and the overall dentition lacks robust carnassial talonids or metaconids, emphasizing flesh-tearing over grinding or pulverizing.^[29] Tooth enamel in Hyaenodon is notably thick relative to comparably sized carnivorans like foxes, providing durability against wear from abrasive foods such as bone fragments and tendons, as evidenced by microwear patterns of heavy striations and pits.^[30] Variations occur across species; for instance, larger forms like H. gigas display proportionally enlarged canines suited to subduing bigger prey, contrasting with the more modest canines in smaller species such as H. wyomingensis.^[26] Jaw mechanics in Hyaenodon support a powerful bite adapted for hypercarnivory, with lever arm models and morphofunctional analyses revealing high mechanical efficiency at the canines and carnassials, particularly as teeth wear to maintain shearing performance—unlike many extant carnivorans where efficiency declines.^[31] This configuration, driven by the elongated rostrum and robust crest, enables effective prey dispatch without the extreme durophagy of hyenas.^[32]^[27]

Postcranial skeleton

The axial skeleton of Hyaenodon consisted of a robust vertebral column with a flexible spine, characterized by flat zygapophyses that permitted a wide range of motion for agile terrestrial locomotion.^[33] This structure supported the genus's predatory lifestyle across diverse Paleogene environments, with the tail being well-developed for balance during movement.^[33] The forelimbs were robustly built, featuring a humerus with a prominent deltopectoral crest extending at least halfway down the shaft, which anchored strong shoulder muscles for powerful extension and flexion. The elbow joint showed features like a tightly curved semilunar notch and anteriorly oriented radial facet, enhancing stability and range of motion, while the manus retained some dexterity with semi-opposable digits suited for grappling or digging into prey.^[33] These adaptations reflect a generalized yet capable forelimb morphology, varying slightly with body size as noted in overall skeletal scaling. Hindlimbs in Hyaenodon displayed an elongated femur with a short neck and prominent greater trochanter, contributing to a cursorial build particularly in larger species, where the tibia and tarsals like the astragalus featured a deeply grooved trochlea for stable parasagittal motion.^[33] The stance was digitigrade, with reduced calcaneal asymmetry limiting mediolateral translation and promoting efficient forward propulsion on terrestrial substrates.^[33] Postcranial variations among Hyaenodon species reflected locomotor diversity, with smaller forms like H. exiguus showing scapular features such as a deep glenoid fossa and evidence of musculus levator claviculae development indicative of climbing capacity in semi-arboreal settings.^[34] In contrast, larger North American species exhibited more pronounced cursorial traits in limb proportions and thoracic structure, adapted for terrestrial pursuits.^[34]

Paleobiology

Predatory behavior

Hyaenodon species were primarily solitary or small-group ambush predators, relying on their cursorial limb adaptations for short pursuits in forested or woodland habitats during the Eocene and Oligocene.^[35]^[1] These mammals targeted medium-sized ungulates and smaller herbivores, such as early equids like Mesohippus and larger forms like Megacerops, with prey focus masses ranging from approximately 50 to 800 kg depending on species body size.^[36] Evidence from associated faunas in Eocene deposits indicates a preference for such prey in tropical to subtropical forest environments, where dense vegetation favored ambush tactics over prolonged chases.^[37] Feeding mechanics centered on powerful shearing bites delivered by specialized carnassial teeth, which sliced flesh and dismembered carcasses efficiently, as seen in the short, robust snouts and blade-like premolars and molars of specimens like Bastetodon syrtos.^[37] Microwear analysis reveals heavy gouging, pitting, and scratching on enamel surfaces, consistent with frequent consumption of tough, meaty tissues and occasional bone, supporting a hypercarnivorous diet exceeding 80% vertebrate protein based on tooth morphology and ecomorphological indices.^[36]^[38] Opportunistic scavenging of larger carcasses supplemented active hunting, akin to modern hyaenids, with jaw wear patterns indicating repeated high-force bites throughout adulthood and even enhanced canine piercing efficiency in older individuals.^[31]^[1] Prey selection involved size-based niche partitioning to minimize competition with contemporaneous nimravids, where larger Hyaenodon species like H. grandis (over 400 kg) exploited bigger herbivores unavailable to smaller felid-like nimravids, while smaller H. microdon overlapped more but still focused on mid-sized mammals.^[36]^[35] This strategy allowed Hyaenodon to dominate apex predatory roles in North American and Eurasian ecosystems until the late Oligocene.^[36]

Ontogeny and development

The ontogeny of Hyaenodon is primarily inferred from juvenile dental material, revealing a prolonged period of tooth replacement that extended over approximately 3–4 years. In European specimens, the lower jaw eruption sequence proceeds as m1 → dp1 → m2 → i → p2 → c/p3/m3 → p4, with the permanent carnassial p4 erupting last after m3, while the upper jaw follows M1 → P1 → P2 → P4 → M2 → P3 → C. This sequence contrasts with North American Hyaenodon, where P1 erupts before M1 and the canine is among the last teeth to emerge.^[39] The delayed eruption of carnassials, particularly the replacement of deciduous dp4 by p4, postponed the development of full shearing capability for bone-crushing predation until late in ontogeny.^[39] Growth during this phase involved a modest increase in tooth size, with permanent teeth measuring 12–16% larger than their deciduous predecessors, as evidenced by comparisons in juvenile fossils.^[39] Thin sections of deciduous canines from North American specimens exhibit four incremental rings, supporting an extended juvenile period of 3–4 years before completing dental eruption.^[39] Although direct skeletal growth data are limited, the timing of dental maturation suggests sexual maturity was reached around 3–4 years, aligning with the cessation of tooth replacement.^[40] Developmental variations appear among Hyaenodon species, with smaller forms potentially exhibiting accelerated maturation compared to larger ones, though specific timelines remain inferred from dental sequences across taxa. These patterns imply a prolonged period of dependency on parental provisioning, as the delayed onset of full predatory efficiency would have limited juvenile hunting success, suggesting some form of parental care in Hyaenodon social structure.^[39]

Brain anatomy

The brains of Hyaenodon species displayed encephalization quotients (EQs) that were generally lower than those of modern carnivorans, which typically range around 1.0, reflecting a more primitive level of encephalization overall. For instance, calculations for Hyaenodon horridus yield an EQ of 0.37 using Jerison's (1973) method or 0.36 using Eisenberg's (1981) method, while Hyaenodon crucians shows values of 0.85 (Jerison) or 0.98 (Eisenberg).^[18]^[41] Endocast reconstructions indicate brain volumes of approximately 125 cm³ for H. horridus and 47 cm³ for H. crucians, with the cerebrum significantly larger than the cerebellum in both cases.^[18]^[41] Structurally, the brain featured expanded, thick, tube-shaped olfactory bulbs that comprised 6.0–8.2% of the total volume, underscoring a reliance on olfaction, alongside a highly convoluted neocortex marked by three prominent furrows per cerebral hemisphere (lateral, suprasylvian, and ectosylvian).^[42]^[41] Sensory adaptations evident in Hyaenodon endocasts include prominent optic nerves (cranial nerve II), suggesting enhanced visual processing potentially suited for low-light conditions, and a well-developed vestibular system (cranial nerve VIII) indicative of adaptations for maintaining balance during agile pursuits.^[42] The relatively modest overall brain size and limited neocortical expansion compared to later mammals imply no evidence for advanced social cognition, consistent with the low EQ values.^[18] Evolutionary trends within Hyaenodontidae, including Hyaenodon, show an increase in relative brain size and cerebral complexity in later species, with the cerebrum expanding disproportionately to the cerebellum and additional sulci developing over time, possibly in response to increasingly complex paleoecvironments.^[18]^[41] Recent analyses using 2021 CT scans of related hyaenodont endocasts have revealed detailed vascular patterns, such as prominent dural sinuses and arterial impressions, further illuminating these trends in neural architecture.^[42]^[41] In comparisons to other groups, Hyaenodon brains appear more primitive than those of derived Carnivora, with less pronounced neocortical folding and lower EQs, but exhibit parallels to early miacids in the progressive expansion of the neocortex and overall encephalization through the Eocene and Oligocene.^[43]^[41] The enclosure of the brain within the robust skull, as seen in Hyaenodon, provided protection but limited further volumetric expansion relative to body size.^[18]

Distribution and paleoecology

Temporal and geographic range

Hyaenodon inhabited Laurasia during the middle Eocene to early Miocene, spanning approximately 42 to 16 million years ago, with the genus representing one of the most persistent predatory mammal lineages of the Paleogene. The earliest records of the genus date to the late Eocene in Europe and Asia, though a 2025 discovery of Hyaenodon lingbaoensis from the middle Eocene Chuankou Formation in China's Lingbao Basin suggests an Asian origin potentially extending back further, challenging previous timelines for hyaenodont diversification. In North America, the genus appeared by the late Eocene, likely via dispersal across the Bering land bridge from Asia. The latest occurrences are from the early Miocene in Asia, with North American records tapering off by the late Oligocene.^[44] Geographically, Hyaenodon was widespread across North America, Europe, and Asia, with over 20 species documented from diverse localities reflecting faunal exchanges via the Turgai Strait between Europe and Asia. Key fossil-bearing formations include the late Eocene Bridger Formation in Wyoming, USA, where early hyaenodont relatives provide contextual records; the Oligocene Quercy Phosphorites in France, yielding abundant Hyaenodon jaws such as H. leptorhynchus; and the upper Eocene to Oligocene Ardyn Obo (Ergilin Dzo) Formation in Mongolia, home to species like H. chunkhtensis. Rare records from northern Africa indicate limited southward dispersal, possibly via Tethyan connections.^[45]^[1]^[46] Fossils of Hyaenodon are relatively abundant compared to other hyaenodonts, with hundreds of specimens known worldwide, predominantly isolated dentaries, maxillae, and teeth that highlight the genus's hypercarnivorous adaptations. Recent excavations in central Mongolia have uncovered postcranial elements, enhancing understanding of locomotor anatomy beyond cranial material.^[47]^[1]

Habitat and faunal associations

Hyaenodon species inhabited a variety of paleoenvironments across the Northern Hemisphere during the late Eocene to early Miocene, adapting to warm, humid conditions in forested settings early in their history and persisting into more seasonal landscapes later on. In the late Eocene, fossils from the Ergilin Dzo Formation in southeastern Mongolia indicate fluvial deposits within subtropical forest ecosystems, where Hyaenodon coexisted with early perissodactyls and other mammals in a humid, tropical climate conducive to dense vegetation.28[559:HCATHF]2.0.CO;2) During the Oligocene, Hyaenodon thrived in North American woodlands and floodplains, as preserved in the White River Group of South Dakota and surrounding regions, which consists of volcaniclastic sediments deposited in alluvial and lacustrine settings amid a cooling climate with increasing seasonality. These environments supported mixed biotas including early equids such as Mesohippus, oreodonts like Merycoidodon, rodents, lagomorphs, diverse birds (e.g., bathornithids), and reptiles, reflecting woodland habitats with access to water sources. In Europe, Oligocene specimens from the Quercy Phosphorites in France suggest similar adaptability to forested and open mixed landscapes, with karstic fissures preserving bones that indicate semi-arboreal niches tolerant of moderate aridity.^[8]^[34] In Asia, Hyaenodon extended into the early Miocene within the Xianshuihe Formation of the Lanzhou Basin, China, where fluvial deposits point to woodland or floodplain habitats under a regime of seasonal aridity, allowing the genus to persist amid faunal turnover with northern Eurasian affinities. Taphonomic evidence from lagerstätten like the Uinta Formation in Utah, part of the broader Green River lacustrine system, highlights exceptional preservation of associated vertebrates in anoxic lake bottoms, suggesting Hyaenodon frequented nearby riparian zones in these subtropical to temperate settings during the late Eocene transition.^[9]

Ecological role and interactions

Hyaenodon species occupied a top carnivore trophic level in Paleogene ecosystems, functioning as apex or mesopredators in the absence of advanced Carnivora, with dental and cranial adaptations indicating a hypercarnivorous diet focused on vertebrate prey.^[34] Stable isotope analyses of co-occurring creodonts, including hyaenodonts, reveal elevated δ¹⁵N values consistent with a high trophic position, supporting their role as dominant predators rather than omnivores or scavengers.^[48] In European Eocene faunas, Hyaenodon filled a hyena-like niche, exerting predation pressure on medium to large herbivores and influencing community structure through selective foraging.^[34] Interactions among Hyaenodon and contemporary carnivores involved competition for shared prey resources, particularly with early carnivorans like miacids and nimravids such as Dinictis.^[49] Evidence of intraguild predation includes puncture marks on a Dinictis felina skull from the Oligocene Brule Formation, attributed to bites from Hyaenodon based on tooth morphology and spacing, suggesting aggressive encounters that reduced competitor populations.^[50] Such conflicts likely intensified in resource-limited habitats, where overlapping prey preferences drove antagonistic behaviors.^[51] Niche partitioning among Hyaenodon species was facilitated by body size variation, enabling coexistence within diverse guilds; smaller forms (4–11 kg, e.g., Hyaenodon leptorhynchus) targeted diminutive artiodactyls and possibly insects, while larger individuals (up to 50 kg, e.g., Hyaenodon gervaisi) pursued bigger ungulates as hypercarnivores.^[52] This size-based differentiation minimized direct competition, as evidenced by sympatric assemblages in Chattian sites like Séon Saint-André, where multiple Hyaenodon taxa co-occurred with amphicyonids without apparent niche overlap.^[53] Dispersal across continents further reduced local competition by exploiting varied faunal assemblages. As primary predators, Hyaenodon exerted significant community impacts by shaping herbivore evolution through sustained predation pressure, favoring traits like increased body size and defensive morphologies in Eocene ungulates.^[54] In North American and European faunas, hyaenodonts comprised a substantial portion of carnivorous diversity, representing up to 9% of total mammalian taxa on average across Cenozoic assemblages, with modeling of Eocene sites indicating predators accounted for 10–15% of estimated community biomass.^[55] This predatory dominance influenced trophic cascades, promoting balanced herbivore populations and ecosystem stability during the Paleogene.^[56]

Extinction

Patterns of decline

The genus Hyaenodon exhibited a gradual decline beginning in the late Oligocene, with regional extinctions occurring in a staggered pattern across its range. In North America, the last records date to the early Arikareean land mammal age (late Oligocene to earliest Miocene, approximately 25–23 Ma), after which the genus disappeared entirely by around 20 Ma.^[9] In Europe, Hyaenodon vanished earlier, with final occurrences in the Chattian stage of the late Oligocene (approximately 28–23 Ma).^[9] Asian populations persisted longer, with sparse fossils indicating survival into the early Miocene (approximately 20–17 Ma), marking the latest global records for the genus.^[9]^[57] Geographically, the decline showed early local extinctions in Africa during the Oligocene (around 25 Ma), as evidenced by fossils from the Nsungwe Formation in Tanzania.^[58] This was followed by progressive contractions across Laurasia, with North American and European populations extinguishing first, while Asian lineages represented holdouts into the Miocene, primarily in regions like the Lanzhou Basin in China.^[9] The pattern lacked synchronized global die-offs, instead reflecting regional attrition tied to the broader fossil record.^[57] Notable last occurrences include H. brevirostris in North American sites during the late Oligocene and H. leptorhynchus in European Chattian deposits.^[9] In Asia, H. weilini represents a terminal species from early Miocene sediments (approximately 19–20 Ma) in the Xianshuihe Formation.^[9]^[57] Sparse records from Mongolia, such as those attributed to H. gigas, are from the Oligocene but remain fragmentary.^[9] Fossil abundance for Hyaenodon decreased markedly after the Oligocene, with records shifting from relatively common isolated teeth and cranial elements in Paleogene deposits to rare, fragmentary remains in Miocene contexts, indicating a substantial reduction in population density.^[9] Diversity also contracted sharply, from over 20 recognized species across the Eocene and Oligocene to fewer than 10 persisting into the late stages, primarily as low-diversity relicts without evidence of mass mortality events.^[57]^[9]

Causes and hypotheses

The extinction of Hyaenodon is attributed to a combination of environmental changes and ecological pressures during the Oligocene to early Miocene, particularly in Asia where the genus persisted longest. Climatic cooling and increasing aridification drove habitat fragmentation and shifts from forests to more open environments across Eurasia. These changes reduced refugia essential for Hyaenodon's ambush predation strategies, leading to decreased prey availability and population viability.^[58]^[59] Biotic factors, particularly the radiation of true Carnivora into Eurasian ecosystems during the Oligocene to early Miocene, further compounded these pressures through niche overlap and competitive exclusion. Amphicyonids (bear-dogs) and early felids, with superior cursorial adaptations for sustained pursuit in open terrains, outcompeted hyaenodonts in exploiting ungulate prey, while shared dentition features—such as sectorial carnassials for shearing flesh—intensified direct rivalry for hypercarnivorous roles. Although some analyses suggest indirect rather than head-on competition, the temporal correlation between carnivoran diversification and Hyaenodon decline supports a role for inter-guild displacement, especially as Carnivora exhibited greater locomotor efficiency and broader dietary flexibility.^[5]^[58] Additional hypotheses emphasize the vulnerability of hyaenodonts' dietary specialization, characterized by extreme hypercarnivory with limited capacity for omnivory or herbivory. Dental topographic analyses reveal a significant loss of dietary disparity within Hyaenodonta following the Eocene-Oligocene boundary, with incomplete recovery by the early Miocene, indicating reduced adaptability to fluctuating prey bases. Stable isotope studies of associated faunas, including carbon and nitrogen ratios in tooth enamel, further demonstrate minimal isotopic shifts in hyaenodont diets over time, suggesting low behavioral plasticity in response to environmental stressors compared to more versatile carnivorans.^[59] Debates persist over the primacy of these factors, with no consensus on a single cause; instead, a multi-factorial model prevails, integrating climatic forcing with biotic interactions. The extinction of Hyaenodon aligns with the early Miocene radiation of Ferae (encompassing modern Carnivora), which filled vacated apex and mesopredator roles amid biome turnover in Laurasia. The broader extinction of Hyaenodonta occurred later, in the late Miocene in Africa.^[58]^[59]

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[12]
Hyaenodon - Mindat
Aug 10, 2025 · Description. Hyaenodon ("hyena-tooth") is the type genus of the Hyaenodontidae, a family of extinct carnivorous fossil mammals from Eurasia, ...
[13]
Hyaenodon cruentus Leidy 1853 (placental) - PBDB Taxon
... Hyaenodon mustelinus, Hyaenodon neimongoliensis, Hyaenodon pervagus, Hyaenodon raineyi, Hyaenodon venturae, Hyaenodon vetus, Hyaenodon weilini, Hyaenodon ...
[14]
Hyaenodon horridus - Mindat
Aug 11, 2025 · Hyaenodon ("hyena-tooth") is the type genus of the Hyaenodontidae, a family of extinct carnivorous fossil mammals from Eurasia, North ...
[15]
New remains of Hyaenodontidae (Creodonta, Mammalia) from the ...
New specimens of hyaenodontid creodonts are described which were unearthed in Mongolia by the Austrian–Mongolian Paleontological expeditions.
[16]
A new small hyaenodon (Hyaenodonta: Hyaenodontinae) from the ...
Apr 5, 2025 · The discovery of Hyaenodon lingbaoensis, a new species of small Hyaenodontinae, is reported from the Middle Eocene Chuankou Formation in the Lingbao Basin of ...
[17]
Evolution and Phylogeny of Hyaenodonts and Carnivorous Mammals
Recent studies have accelerated the pace at which we uncover the nuances of hyaenodont evolution. A notable study from South China (2023) described a new ...
[18]
The cranium of Proviverra typica (Mammalia, Hyaenodonta) and its ...
Jul 29, 2019 · The two Eurotherium species are sister-taxa (PP = 99%), as are the two species of Matthodon (PP = 90%) and of Oxyaenoides (PP = 66%).
[19]
Hyaenodontidae), a new species from the late Uintan of Utah - PeerJ
Nov 22, 2019 · In contrast to the analyses just discussed, results of the current phylogenetic analysis do not place Hyaenodontinae phylogenetically proximate ...
[20]
The Neurocranium of Ekweeconfractus amorui gen. et sp. nov ...
Sep 24, 2021 · Hyaenodonta is currently placed in the Superorder Ferae together with extant Carnivora and Pholidota (Flynn and Wesley-Hunt, 2005; Friscia and ...
[21]
Dispersals of placental carnivorous mammals (Carnivoramorpha ...
Oct 15, 2013 · Hyaenodonta was present in Africa since the Paleocene and became extinct in the latest middle Miocene or earliest late Miocene (Lewis and Morlo ...
[22]
[PDF] The hyaenodonts (Mammalia) from the French locality of ... - HAL Um
Results of the phylogenetic analysis of Hyaenodonta character-taxon matrix. Results are visualized as an “all compact” (majority rule plus ...
[23]
[PDF] 2020 42 13 - Publications scientifiques du Muséum
— Results of the phylogenetic analysis of Hyaenodonta character-taxon matrix. Results are visualized as an “all compact” (majority rule plus compatible.
[24]
Reliable biogeography requires fossils: insights from a new species ...
Aug 7, 2024 · (a) Phylogeny. We first revisited the taxonomy of all extant and extinct species in the study group (Carnivoramorpha, Hyaenodonta, and ...
[25]
A new small hyaenodon (Hyaenodonta: Hyaenodontinae) from the ...
The discovery of Hyaenodon lingbaoensis, a new species of small Hyaenodontinae, is reported from the Middle Eocene Chuankou Formation in the Lingbao Basin of ...
[26]
Craniodental and humeral morphology of a new species of ...
Previous studies show an ancient split between two major clades of hyaenodonts that converged in hypercarnivory: Hyainailourinae and Hyaenodontinae. These ...
[27]
Zygomatic Arch of Hyaenodon (Hyaenodontidae: Creodonta)
This condition has been correlated with atrophy of the masseteric musculature, itself reduce because of stresses imposed along with increased gape. Physical ...
[28]
Craniodental and humeral morphology of a new species of ...
Apr 19, 2017 · Hyaenodonts were morphologically diverse, ranging from the small, weasel-sized Proviverra typica [3] to the wolf-sized Hyaenodon horridus [4], ...
[29]
Morphological and ecological diversity within the hyaenodontidan ...
The teeth of all species referred to Hyaenodon are similar in morphology, which complicates distinction at the species level and results in an emphasis on size ...
[30]
First evidence of the tooth eruption sequence of the upper jaw in ...
The comparison in size of milk to permanent teeth indicates a growth rate of 12–16 % in Hyaenodon.
[31]
Relationship between tooth macrowear and jaw morphofunctional ...
Nov 11, 2024 · Analyses of the fossil carnivore Hyaenodon suggests an increase in canine biting efficiency with increased tooth wear, but this may reflect ...
[32]
https://pmc.ncbi.nlm.nih.gov/articles/PMC11562772
[33]
https://doi.org/10.1080/02724634.2015.965308
[34]
https://doi.org/10.1111/joa.12545
[35]
[PDF] body mass estimates in extinct mammals from limb bone dimensions ...
Hyaenodon was studied intensively by Mellett (1977). He recognised two subgenera, Neohyaenodon and. Protohyaenodon, among ten North American Hyaenodon species.<|separator|>
[36]
https://doi.org/10.1093/jmammal/gyab123
[37]
https://www.tandfonline.com/doi/full/10.1080/02724634.2024.2442472
[38]
Low‐magnification microwear in Carnivora and dietary diversity in ...
Aug 6, 2025 · The molars show large shearing blades and heavy wear—part of the evidence that Hyaenodon was a carnivorous predator with bone-cracking ...
[39]
https://doi.org/10.1007/s12542-013-0207-z
[40]
[PDF] Deciduous dentition and dental eruption of Hyainailouroidea ...
Deciduous dental morphology and the sequence of permanent dental eruption offer developmental and phylogenetic insights into mammalian evolution.
[41]
app007712020 - Acta Palaeontologica Polonica
Encephalization quotient.—The encephalization quotient (EQ) is brain size divided by expected body size for an average mammal of the same body size. As a ratio, ...
[42]
Full article: The neurocranium of Ekweeconfractus amorui gen. et sp ...
The oldest known hyaenodont endocasts, such as T. velox and T. agilis, indicate a degree of neocortical expansion similar to the late Paleocene oxyaenid ...
[43]
Brains of early carnivores | Paleobiology | Cambridge Core
Feb 8, 2016 · It is commonly believed that the brains of the ancestors of modern carnivores (miacids) were superior to (e.g., larger than) those of other ...
[44]
A new small hyaenodon (Hyaenodonta: Hyaenodontinae) from the ...
Apr 5, 2025 · This specimen likely represents the smallest hyaenodont so far discovered in China and provides important insights into its tooth replacement ...
[45]
[PDF] Mammalian Faunal Zones of the Bridger Middle Eocene
A faunal list of the Mammalia recognized in the Bridger is given with type localities and their horizons, so far as known, and the number of specimens in the ...Missing: Quercy Ardyn Obo
[46]
(PDF) Hyaenodon chunkhtensis and The Hyaenodontid Fauna From ...
Aug 10, 2025 · Hyaenodon ... A femur from the late Eocene of Mongolia likewise corresponds to that of ostriches in derived traits, whereas cervical vertebrae ...
[47]
(PDF) New remains of Hyaenodontidae (Creodonta, Mammalia ...
Aug 10, 2025 · These Hyaenodon remains suggest a separate evolutionary lineage, with different species in Europe and Asia, even if the genus is also known ...
[48]
Dietary niches of creodonts and carnivorans of the late Eocene ...
Dec 11, 2021 · We therefore cannot definitively state that there was sufficient competition for prey between the late Eocene creodonts and carnivorans in ...
[49]
[PDF] UC Berkeley - eScholarship
This paper presents a taxonomic description of a new genus of carnivoramorphan from the. Eocene Epoch, which constitutes a critical period of major ...
[50]
[PDF] Tooth puncture marks on a 30 million year old Dinictis skull
During the attack, the lower jaw of Hyaenodon crushed the left zygomatic arch and its lower premolars created scratches on the left maxilla and another bite ...
[51]
https://doi.org/10.1086/284531
[52]
https://doi.org/10.2110/palo.2018.022
[53]
(PDF) Niche partitioning of the European carnivorous mammals ...
Aug 5, 2025 · We here describe the first partial cranium of Hyaenodon leptorhynchus, the type species of the taxonomically diverse and widely distributed ...
[54]
Carnivorous mammals from the middle Eocene Washakie Formation ...
Aug 1, 2021 · A well-developed deltopectoral crest of the humerus, which is present in several early oxyaenodontans, also may have a phylogenetically ...
[55]
[PDF] evolution in carnivorous mammals - The Palaeontological Association
There are all gradations between plantigrade and digitigrade. On the whole ... digitigrade posture, which considerably lengthens the limb and so adds to the.<|separator|>
[56]
New data on the mammalian fauna from the late middle Eocene (MP ...
Apr 14, 2023 · The Hyaenodonta were the most diverse carnivorous mammals in the European Eocene and were classically divided into three subfamilies: ...
[57]
(PDF) Hyaenodonts and carnivorans from the early Oligocene to ...
Aug 6, 2025 · ... HYAENODON Laizer and Parieu, 1838. Hyaenodon weilini new species ... Hyaenodon. leptorhynchus. Comptes Rendus de l'Académie des. Sciences ...
[58]
The first hyaenodont from the late Oligocene Nsungwe Formation of ...
Oct 11, 2017 · Their study focused on species in the genus Hyaenodon, a genus phylogenetically distant from Afro-Arabian hyaenodonts [29, 57]. Despite the ...
[59]
Widespread loss of mammalian lineage and dietary diversity in the ...
Oct 7, 2021 · Because Hyaenodonta goes extinct in the late Miocene, the veracity of the middle Miocene diversity drop that is also seen in primates and ...