Epicyon

Epicyon ("more than a dog") is a large, extinct, canid genus of the subfamily Borophaginae ("bone-crushing dogs"), native to North America. Epicyon existed for about 7 million years from the early Clarendonian age of the Late Miocene to the late Hemphillian age of the Early Pliocene.^[2][1] E. haydeni is the largest known canid of all time, with the type species reaching 2.4 m (7.9 ft) in length, 90 cm (35 in) in shoulder height and approximately 100–125 kg (220–276 lb) in body mass.^[3][4][5] The largest known humerus specimen belonged to an individual weighing up to 170 kg (370 lb).^[6]

Epicyon Temporal range: Late Miocene to Early Pliocene (Clarendonian to Hemphillian), 12–5 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted E. haydeni skeleton
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Carnivora
Suborder:	Caniformia
Family:	Canidae
Subfamily:	†Borophaginae
Tribe:	†Borophagini
Subtribe:	†Borophagina
Genus:	†Epicyon Leidy, 1858
Type species
†Epicyon haydeni Leidy, 1858
Other Species[1]
†E. aelurodontoides †E. saevus

Taxonomy

Epicyon was first named by Joseph Leidy in 1858 as a subgenus of Canis. It was also mentioned as belonging to the Aelurodontina by Matthew and Stirton in 1930. Later studies indicates that it was not a species of Canis, but a borophagine.

Epicyon haydeni, the type species, existed from 10-5 million years ago. It is synonymous with Aelurodon aphobus, Osteoborus ricardoensis, Osteoborus validus, and Tephrocyon mortifer, and was named by Joseph Leidy as a subgenus. It was recombined as Aelurodon haydeni by Scott and Osborn in 1890. Further study by Matthew in 1899, Matthew and Gidley in 1904, VanderHoof and Gregory in 1940, McGrew in 1944, Bennett in 1979, (1979) and Becker (1980). It again was recombined as Epicyon haydeni by Baskin in 1980, Voorhies in 1990, (1990), Baskin (1998), Wang et al. in 1999.

Mandible of Epicyon saevus

Epicyon saevus existed from 12 to 7 million years ago. It is synonymous with Aelurodon inflatus and was named by Joseph Leidy in 1858 or 1859. In the late 1880s-early 1900s, Scott, Matthew, Cope and Matthew, Troxell recombined the animal as Aelurodon saevus. It was recombined as Epicyon saevus by Baskin in 1980, Munthe in 1989, Voorhies in 1990, and Wang et al. 1999.

Epicyon aelurodontoides existed from 9 to 7 million years ago and was named by X. Wang and others in 1999. This species was found south of the Young Brothers Ranch, Kansas.^[1]

Description

Restoration

Epicyon had a massive head and powerful jaws that were well adapted for bone-crushing, with enlarged fourth premolars like some hyenas, giving its skull a lion-like shape rather than having a skull similar in shape to that of a wolf; the adaptation would have allowed Epicyon to scavenge as well as hunt, giving it access to the nutritious marrow other contemporary carnivores could not access.^[4]

E. haydeni was the largest known species of canid; it is estimated to have had a body length of 2.4 m (7.9 ft), a shoulder height of 90 cm (35 in) and a body mass of approximately 100–125 kg (220–276 lb),^[3][4][5] with the largest known specimen weighing up to 170 kg (370 lb).^[6]

E. saevus is estimated to have a shoulder height is up to 56 cm (22 in) and body mass up to 66.5 kg (147 lb).^[5]

Paleobiology

Predatory behavior

The molars of E. haydeni were grindstone-like teeth that allow for a canid diet that includes both meat and plant and insects. The proportional size of an animal's molars is a great measure of the nutritional diversity of its diet.^[7] Based on fossilized feces and its robust teeth and jaw muscles it is believed to have consumed large amounts of bone and share a similar digestive tract to modern day hyenas due to their ability to break down bones.^[7][8] The deadly bite of a E. haydeni was delivered by the canine teeth, which are placed near the front of the upper and lower jaws, the shortening of the jaws can be an effective method for getting the canines closer to the mandibular condyle, thereby increasing the biting force.^[7]

The small clavicle, flexible back, and digitigrade posture of E. haydeni are all postcranial features shared with other canids and are likely adaptations designed to increase the animal's stride length. Examinations of the limb proportions and toughness of the skeleton suggests that E. haydeni was less cursorial than hyenas or modern wolves but more cursorial than other borophagine species like Aelurodon.^[9][10] Unlike hyenas, E. haydeni must have used their rearmost lower premolar (p4) and upper carnassial (P4) to crack large bones (ibid.). Smaller bones and bone fragments were likely crushed with the carnassials and post-carnassial molars just as in extant canids.^[11] Due to its larger size and less gracile skeleton, E. haydeni was less cursorial and unable to run as long a distance as E. saevus, instead it relied on bursts of speed.^[9]

Predatory behavior for Epicyon heavily depends on the methods used. Schwab et al. (2019) found E. haydeni likely practiced pounce predation based on the anatomy of its bony labyrinth,^[12] while Figueirido et al. (2015) analysis on elbow morphology suggests E. haydeni was more of an ambush hunter.^[13] On the other hand, Martín-Serra et al. (2016) found E. haydeni practiced pursuit or pounce-pursuit predation, while E. saevus practiced pounce-pursuit predation based on forelimb analysis. Their analysis also suggests borophagines predatory behavior was not equivalent to any living species.^[14]

Social behavior

Whether or not Epicyon was gregarious or solitary is unclear. Tomida and colleagues believed E. haydeni was gregarious as it was very prevalent in the fossil record, and was one of the most common carnivores in North America at the time.^[15] Valkenburgh and colleagues also argued gregariousness because of their adaptation to hunting large prey and canids not being able to grapple large prey like felids can.^[11]

This interpretation was called into question by Ki Anderson, as craniodental and elbow joint morphology of borophaginae resembled that of pantherinae instead of recent canids, arguing that the latter are unsuitable modern analogues and not a reliable indicator. The author of the paper admitted that this is not enough to refute or support pack hunting among borophaginae as lions are capable of grappling prey, but they still hunt in social groups, showing the complexity of social behavior in carnivorans.^[5]

Paleoecology

Skull and mandible casts of E. saevus, Florida Museum of Natural History

Fossil specimens range from Florida to California and have been found in Nebraska, Montana, Kansas, Texas, New Mexico, Colorado, Oklahoma, Idaho, Oregon, Arizona within the United States, as well as in Alberta, Canada.^[15][16]

In North America, in places such as Coffee Ranch in Texas, Epicyon shared territory with the bear Agriotherium and the feliform Barbourofelis, machairodont feline Amphimachairodus coloradensis, and fellow canid Borophagus. All of these animals were potential competitors that would have occasionally conflicted with Epicyon for food and territory. Prey for Epicyon included herbivores such as the camel Aepycamelus, the pronghorn Cosoryx, horses such as Neohipparion and Nannippus, the peccary Prosthennops, and the rhinoceroses such as Teleoceras, all of which could provide a suitable meal through hunting or scavenging.^[17][18]

Epicyon was one of the last of the borophagines, and shared its North American habitat with several other canids, including:

• Borophagus 23.3 to 3.6 million years ago (Mya)
• Carpocyon from 20.4 to 3.9 Mya
• Paratomarctus from 16.3 to 5.3 Mya
• Aelurodon from 16.0 to 5.3 Mya
• Canis lepophagus from 10.3 to 3.6 Mya^[4]