Arctocyonidae (from Greek arktos and kyôn, "bear/dog-like") is an extinct, possibly Polyphyly family of Placentalia which lived from the late Cretaceous to the early Eocene. They were initially regarded as Creodonta, though have since been reassigned to an order of their own, the Arctocyonia. Some have suggested that arctocyonids are ancestral to modern-day Artiodactyl, or that they form a sister group. However, more recent Phylogenetics suggest that arctocyonids may represent an artificial grouping of extinct Ungulate, or that they might be an assemblage of unrelated placentals related to Pangolin, Pantodonta, and Periptychidae.
Members of Arctocyonidae are characterised by long skulls, with large Sagittal crest and very large canines. In the case of Arctocyon proper, the lower canines especially were large enough to require a diastema on the upper jaw to accommodate them. Arctocyonids varied considerably in size and morphology. Smaller genera, like Chriacus, were about the size of a coati, while larger ones, such as Arctocyon, weighed up to and stood at the shoulder. Many arctocyonids have climbing adaptations, suggesting that they were either descended from arboreal taxa, or were arboreal themselves. The North American Anacodon was more robust than other genera, and had adaptations for Fossorial as well as climbing. Most genera appear to have been Omnivore, though Anacodon showed signs of an increase in Herbivore.
Taxonomy
The family Arctocyonidae was named by Christoph Gottfried Andreas Giebel in 1855, as a carnivoran subfamily that included
Arctocyon, the amphicyonid
Amphicyon, and the ursid
Agriotherium.
It was elevated to family level by Scottish zoologist Andrew Murray.
At some point thereafter, arctocyonids became a family within
Creodonta.
In 1975,
Malcolm McKenna erected a new order, Arctocyonia, to accommodate them, placing them within Ungulata.
Since then, Arctocyonidae has largely been treated as a family of its own, though how that family is divided has been another matter. William Diller Matthew, in 1937, divided it into four subfamilies (Arctocyoninae, Chriacinae, Oxyclaeninae, and
Triisodontidae);
the latter is now regarded as a family of its own. That same year, George Gaylord Simpson suggested that arctocyonids could instead be divided into Arctocyoninae, Oxyclaeninae, and Triisodontinae.
In 1978, Leigh Van Valen erected a new subfamily, Loxolophinae.
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 Artiodactyl
Description
Arctocyonidae, if monophyletic, was a morphologically disparate lineage. Some genera, such as
Chriacus, were fairly small, and bore adaptations for an arboreal or scansorial lifestyle.
Others, like
Anacodon, were very large and robust, having adaptations for both arboreal and fossorial lifestyles.
, particularly
A. mumak, appears to have been the most terrestrial,
though likely descended from arboreal ancestors.
Most arctocyonids are fragmentary, making it difficult to determine body size.
Chriacus likely weighed , and was slightly larger than a modern
coati.
may have weighed up to ,
and had an estimated shoulder height of . Remains of
Mentoclaenodon suggest a very large body size, though exactly how large it grew to is unclear,
and it may have been outsized by
Arctocyon mumak.
Skulls and dentition
The skulls of arctocyonids were fairly long, with a small
Neurocranium and very large
Sagittal crest and occipital crests,
combined traits of
Herbivore and
Carnivore mammals.
The
zygomatic arch of
Arctocyon specifically was very large, with a posterior angle anterior to the
mandibular fossa, almost forming a right angle.
In most genera, the
Incisor were small and unspecialised, though others, like
Thryptacodon, had modified them into
Toothcomb convergent with those of
Lemur.
Though the canines likely had a role in feeding in certain genera,
in others, like
Anacodon, they did not.
In the case of
Arctocyon, the lower canines were longest, to the extent that they were accommodated by a gap (or
diastema) between the upper canines and
Premolar.
The cheek teeth were tricuspid (three-cusped) and were often bunodont. The premolars were simple, if fairly sharp,
while the molars were blunt and resembled those of bears.
Overall, the dental morphology of arctocyonids suggests that they were omnivorous to varying degrees.
Postcranial elements
In
Arctocyon, the mammillary processes of the dorsal vertebrae were robust (suggesting powerful musculature), and the
caudal vertebrae appear to have been tightly interlocked, suggesting that the tail was fairly rigid.
In
Chriacus, however, there are signs that the tail may have been prehensile.
Arctocyonid limbs were fairly typical in length, with stout
Ulna and
Fibula shafts.
In
Anacodon, particularly, the limbs were very robust.
In
Chriacus, the ankle joints were flexible and allowed the hind feet to rotate, enabling them to climb downward.
's ankles had very little lateral movement.
Five digits were present on all limbs. The
Phalanx bone were narrow and long, though the innermost and outermost digits were slightly reduced.
In the case of
Chriacus, the innermost digit (the hallux) was divergent.
On all digits, the
Ungual were laterally compressed and quite clawlike.
In some genera, like
Anacodon, they may have been used for digging.
Biology
Diet and feeding
Arctocyonid cheek teeth were bunodont, and the
Carnassial seen in other predatory mammal clades were essentially absent.
Overall, the dental morphology of arctocyonids suggests that they were omnivorous, to varying degrees.
may have been among the least carnivorous, having flat, crenulated cheek teeth
while
Arctocyon corrugatus was among the most carnivorous.
See also
