Arctocyon

''Arctocyon'' (from Greek ''arktos'' and ''kyôn'', "bear/dog-like") is an extinct genus of large placental mammals, part of the possibly polyphyletic family Arctocyonidae. The type species is ''A. primaevus'', though up to five other species may be known. Fossils of ''Arctocyon'' have been found in Europe and North America. ''Arctocyon'' was originally named as a subgenus of the bear-dog ''Amphicyon'', though was subsequently found to belong to a genus and family of its own. The relationship between arctocyonids and other placentals is unclear, with early classification efforts placing them as carnivores or creodonts, and later ones classifying them under Condylarthra, a wastebasket taxon for various early ungulates. More recent phylogenetic analyses suggest that arctocyonids are an artificial assemblage of several distantly-related placental lineages, and ''Arctocyon'' may be closely related to '' Loxolophus'' (another arctocyonid) and to pantodonts.

''Arctocyon'' was among the largest arctocyonids, if not the largest. ''A. primaevus'' measured at the shoulder, and weighed up to , while ''A. mumak'' was larger still. Like many other arctocyonids, ''Arctocyon'' had very large canine teeth. In the case of the lower canines, they were large enough that they had to be accommodated by a gap between the upper canines and the premolars. The occipital and sagittal crests were very large. The cheek teeth were fairly unspecialised, and resembled those of bears. ''Arctocyon'''s torso was rigid and well-muscled, and the tail was quite inflexible. The forelimbs were powerfully muscled, the claws were curved and laterally compressed, and the digits had a strong grasping ability, suggesting that at least some species, like ''A. primaevus'', were capable of climbing. Other species, such as ''A. mumak'', were more terrestrial, and may have occasionally burrowed.

Taxonomy

Early history

The earliest fossils of ''Arctocyon'' were found in the area around La Fère, France, by M. Fremanger. It was first described by Henri Marie Ducrotay de Blainville in 1841, on the basis of well-preserved fossil remains from the upper Paleocene sediments of France. Initially, it was assigned to ''Amphicyon'' as a subgenus, alternatively called ''Palaeocyon''. In 1855, Christoph Gottfried Andreas Giebel elevated the latter name to genus level, and erected the subfamily Arctocyoninae to incorporate it, including ''Arctocyon'', along with Agriotherium and ''Amphicyon''. In 1866, Arctocyonidae was elevated to family level by Scottish zoologist Andrew Murray, who further included '' Tylodon''. In 1892, William Berryman Scott named another arctocyonid, ''Claenodon'', which was subsequently synonymised with ''Arctocyon'' proper.

Taxonomy

Though initially classified within Carnivora, Arctocyonidae subsequently fell under the wastebasket taxon Creodonta. Subsequently, it fell under Condylarthra, a polyphyletic assemblage of basal ungulates. Arctocyonidae saw multiple taxonomic revisions over the coming decades, with anywhere from two to four subfamilies being recognised at a given time.The relationship between arctocyonids and other clades has long been uncertain. Since becoming the sole representatives of their own order, they have been suggested to be either ancestral to artiodactyls or close to the clade's stem.

The family's monophyly has also been called into question. In 2012, a phylogenetic analysis of '' Prolatidens waudrae'', a traditional arctocyonid, recovered it as a more basal ungulate; ''Arctocyon'', '' Landenodon'' and '' Thryptacodon'' were recovered as part of a clade sister to triisodonts and mesonychids; and the remainder of tested arctocyonids formed a polytomy basal to that clade and ''Diacodexis''. In 2015, Peter E. Kondrashov and Spencer G. Lucas recovered the family as an artificial assemblage of basal ungulates. That same year, a larger analysis by Thomas J. D. Halliday, Paul Upchurch and Anjali Goswami recovered arctocyonids as several entirely unrelated placental lineages. By their unconstrained strict consensus tree, ''Arctocyon'' is related to ''Periptychus'' and '' Protolitopterna'', Loxolophinae is related to pantolestids and pangolins. By their constrained strict consensus tree, ''Arctocyon'' and ''Loxolophus'' form a clade related to pantodonts and periptychids, and the rest of Arctocyonidae is recovered close to pangolins.

Internal systematics

The type species of ''Arctocyon'' is ''A. primaevus'', initially named by de Blainville in 1839 for remains recovered from France. Two additional species of ''Arctocyon'', ''A. corrugatus'' and ''A. ferox'', were named by Edward Drinker Cope in 1883, both from the Eocene of North America. Both were initially assigned to ''Mioclaenodon'',E. D. Cope. 1883. First addition to the fauna of the Puerco Epoch. Paleontological Bulletin 36:545-563 and subsequently ''Oxyclaenus''. A fourth species, ''A. mumak'', was named by Leigh Van Valen in 1978 and initially assigned to '' Arctocyonoides''. Its name derives from ''mûmakil'', a Haradrim name for oliphaunts (large, elephant-like creatures) in the works of J. R. R. Tolkien. Initially known from only a lower jaw, recovered from Palaeocene strata of North America, ''A. mumak'' is now known from a partial skeleton, which demonstrates that it was the largest ''Arctocyon'' species, and possibly the largest arctocyonid outright. ''A. mumak'' has occasionally been regarded as a junior synonym of ''A. acrogenius'', named in 1956 by Charles Lewis Gazin. ''Anacodon nexus'', also named by Gazin, has been tentatively assigned to ''Arctocyon''. Another possible species, ''A. matthesi'', has been described from Germany.

Description

''Arctocyon'', particularly ''A. mumak'', appears to have been among the most terrestrial arctocyonids, though likely descended from arboreal ancestors. ''A. primaevus'' may have weighed up to , and had an estimated shoulder height of . The biggest species, and probably the biggest arctocyonid overall, was ''Arctocyon mumak''.

Skull and dentition

''Arctocyon'''s skull was fairly long, with a small braincase and very large sagittal and occipital crests. The zygomatic arch was very wide, with a posterior angle anterior to the mandibular fossa, almost forming a right angle. Like in most arctocyonid genera, the incisors were small and unspecialised. Though both sets of canines were very large, the lower set was longest, to the extent that they had to be accommodated by a gap (or diastema) between the upper set and the premolars. Like in other amphicyonids, the cheek teeth were tricuspid (three-cusped) and were often bunodont. The carnassial tooth morphology seen in other predatory mammal lineages was absent. The premolars were simple, if fairly sharp, while the molars were blunt and resembled those of bears.

Postcranial skeleton

In ''Arctocyon'', the mammillary processes of the dorsal vertebrae were robust, suggesting powerful musculature. The posterior thoracic vertebrae were characterised by revolute zygapophyses, suggesting a strong degree of rigidity. The caudal vertebrae appear to have been tightly interlocked, suggesting that the tail, too, was fairly rigid. Like in other arctocyonids, the limbs were fairly typical in length, with stout ulnar and fibular shafts. Five digits were present on all limbs. The phalanges were narrow and long, though the innermost and outermost digits were slightly reduced. The joints of the digits were highly mobile, and the flexor muscles were developed, suggesting a strong grasping ability. On all digits, the unguals were laterally compressed and quite clawlike. ''Arctocyon'' was plantigrade, meaning that it walked with its feet flat on the ground like a modern bear. ''A. mumak'' had a well-developed furrow under the sustentaculum tali (a horizontal shelf on the calcaneus) and a great plantar tubercle on the navicular, suggesting a higher degree of terrestriality than in other species.

Paleoecology

''Arctocyon'' probably had an omnivorous diet: the molariform teeth indicate that ''Arctocyon's'' teeth could grind plant material, but the incisors, and in particular, the large canines, indicate the ability to feed on meat. The postcranial skeleton also suggests a mixed diet, even if the morphology is not found in any modern mammal. ''A. corrugatus'' appears to have been the most carnivorous species. The overall morphology of ''Arctocyon'' has a combination of arboreal and fossorial traits, suggesting that at least some species, i.e. ''A. primaevus'', were at least somewhat arboreal in habits. Others, like the larger ''A. mumak''., were more terrestrial, and may have been burrowers or even fossorial.

One study indicated that ''A. primaevus'', morphologically, was more similar to some extinct South American metatherians, such as sparassodonts, than to any other mammal. The general size and proportions are a mix between ''Borhyaena

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