Amphicyon

Extinct genus of carnivores

Amphicyon is an extinct genus of large carnivorans belonging to the family Amphicyonidae (known colloquially as "bear dogs"), subfamily Amphicyoninae, from the Miocene epoch. Members of this family received their vernacular name for possessing bear like and dog like features. They ranged over North America, Europe, Asia, and Africa.

In a note dated back to May 16, 1836, French geologist Alexandre Leymerie wrote of a letter in April that he requested from French palaeontologist Édouard Lartet, which provided details of his exploits in palaeontological sites in the French department of Gers, in particular the commune Sansan. Lartet described his finds of fossil taxons that he found within the sites, including "Mastodonte" (species assigned to it were later reclassified to another mammutid Zygolophodon and the gomphothere Gomphotherium), "Dinotherium" (its species eventually reclassified as either Deinotherium or Prodeinotherium), "Rhinoceros" (reclassified as an aceratherine rhinocerotid Hoploaceratherium), and "Palaeotherium" (the referred equid species now known as belonging to an anchitherine Anchitherium). He also recalled finding fossil "deer" species of which he said that the largest ones were the size of extant deer in France while the smallest ones were the size of small antelope. The palaeontologist noted that the "peaceful ruminants" coexisted with a "formidable" large carnivore he provisionally named Amphicyon based on two half jaws and bones that he sent to a museum. He described it as having unilobed incisors and compressed canines similar to raccoons but also a carnivorous molar and its first two tubercles conforming those to dogs. Lartet then stated that the genus's most distinct trait was the existence of a third tubercle at the upper jaw, which was not known in any other carnivore. The genus name appears to be derived from the Ancient Greek terms ἀμφί ("on both sides") and κύων ("dog"), but Lartet did not define the genus's etymology. Despite the initial status of the genus name Amphicyon as nonpermanent, French anatomist Henri Marie Ducrotay de Blainville, a peer who Lartet had regularly discussed his fossil findings with, had sketched mammal skeletons and fossils in 1841, where he recognized the 2 species "Amphicyon major" and "Amphicyon? minor." In 1851, Lartet reviewed the fossil carnivoran genera from Sansan. Among them were Amphicyon, in which it was reconfirmed as a carnivorous mammal the size of extant bears that was discovered in Sansan in 1835. He recalled that its single lobed incisors and its canines of serrated ridges are similar to the raccoon while the molars were similar to that of a dog. He confirmed the fossil specimens along with the third tubercle in the upper jaw (of which he said that it only exists in the extant bat eared fox (then known as "Canis megalotis")) as belonging to the species Amphicyon major. The palaeontologist described it as also having an anatomy of plantigrade locomotion similar to extant bears with few differences in form. Blainville was mentioned as speculating that it must have had a long and very strong tail. The species "Amphicyon minor" was reclassified as a separate genus Hemicyon, which he described as a carnivore larger than a European wolf that was closer in form to a dog than Amphicyon and had dentition similar to mustelids. He also described a newer genus Pseudocyon, which he misidentified as being digitigrade and described as being smaller than Amphicyon and coming closest to canids based on its dentition and bones. All three genera, Lartet said, had canines that retained finely serrated edges, implying that they were some of the top coexisting predators of the Miocene in modern day France.

Amphicyon was a large to very large predator, although the various species differ considerably in size, ranging from moderately sized species such as A. astrei to the huge A. ingens, which was one of the biggest carnivorans of all time. The estimated weight of male A. major is 212 kg, while females are smaller, at only 122 kg, indicating significant sexual dimorphism. The shoulder height of a young female, which has been estimated to have weighed 125 kg, has been reconstructed as 65 cm. As the largest Old World species of the genus, A. giganteus was considerably larger, with females weighing 157 kg and males 317 kg, although they may have grown considerably larger. The mass of several other European species has been estimated craniodental measurements, which generally falls into the range of estimations derived from postcranial remains, although it may slightly overestimate their weight. A. astrei is the smallest species, estimated at 112 kg, while A. laugnacensis and A. lactorensis were somewhat larger, at ~130 kg and 132 kg, respectively. A. olisiponensis is estimated at 147 kg and A. carnutense as 182 kg, while A. eppelsheimensis and A. gutmanni are among the biggest members of the genus, with estimated weights of 225 and 246 kg. Its skeleton showcases a variety of features resembling canids, ursids and felids. Amphicyon possessed a powerful skull, with a long snout and high sagittal crests. The canines are robust, and the posterior molars are enlarged, whereas the anterior premolars are reduced. Its neck is wide, similar to that of a bear. Its postcranial skeleton is stout and robust, with massive, powerful limbs, and mobile shoulder joints as well as flexible wrists. The upper limb bones are comparatively long in comparison to the lower ones, and it did not possess any adaption towards cursoriality. Its posture was more similar to plantigrade taxa such as ursids than to digitgrade ones like felids, and their claws were not retractable. Amphicyon also had a rather flexible back, and a heavy tail, which has been estimated to have possessed as many as 28 caudal vertebrae, and may have been as long as the rest of the spine. Another dental microwear analysis also supports an omnivorous diet for A. giganteus, whose dentition possesses a high number of large pits and several small pits, and notes that it clearly differs from bone crushing taxa such as hyaenas. As both its anterior premolars and posterior molars are reduced, A. olisiponensis may have been more hypercarnivorous than other European species. Other bitemarks referred to the species A. olisiponensis were found on a metapodial belonging to the large anthracothere Brachyodus onoideus. Bite traces on various mammalian long bones from the Early Miocene of Czechia have also been attributed to Amphicyon. As patterned bones have no immediate benefit for feeding, they likely represent evidence of active predation.

Strong sexual dimorphism is present in a variety of species, known from both Eutope and North America, with the males being considerably larger than the females. Although this size difference is present in many amphicyonids, it is more strongly developed in Amphicyon than in Cynelos lemanensis. The males furthermore possess slightly longer and more robust snouts, larger canines and immense sagittal crests. Comparison with other strongly sexually dimorphic carnivorans suggests that Amphicyon was polygynous, with territorial males competing with each other for females during the mating season. This may have contributed to the size increase observed within the genus. Another ichnotaxon associated with Amphicyon is Platykopus maxima from the Hungarian Early Miocene locality Ipolytarnóc. The footprints were attributed to A. major on the basis of their size and short phalanges.

Fossil remains of Amphicyon are most common in Western and Central Europe, where they were discovered in various countries, including France, Germany, Spain and Hungary, but were also found in Bosnia Herzegovina and Turkey. A. astrei is the oldest known species, and may have been the ancestor of the later members of the genus, and is known from the earliest Miocene of France. It has been suggested that there were at least three dispersal events from European Amphicyon into Eastern Asia, with the first one being the ancestors of the North American species, the second one dating to the Early Miocene or earliest Middle Miocene, leading to A. zhanxiangi, and the last one, that of the A. ulungurensis lineage, which occurred slightly later. There was generally no closer affinity between the Chinese amphicyonids and those of the Indian Subcontinent during the middle Miocene. A very large humerus from the Manchar formation indicates that a gigantic species was present in the Siwaliks during the early parts of the Middle Miocene. South East Asian reports include a large incisor from the Aquitanian (~23 21 Ma) of Vietnam, and a species from the Lower Irrawaddy Formation of Myanmar, which is likely closely related to Arctamphicyon. The only definitive African remains of Amphicyon are from Arrisdrift in Namibia, which has variously been dated to 17.5 Ma or 16 Ma, and belong to the species A. giganteus. Others tentatively refer this taxon to the genus Myacyon. The migration of Amphicyon from Eurasia to North America was part of a trans Beringian faunal exchange between the two continents during the Early Miocene. The oldest North American member of the genus is A. galushai, which first appeared between 18.8 and 18.2 Ma. It likely gave rise to the larger A. frendens, which itself was ancestral to the huge A. ingens, which was also the last North American member of the genus, disappearing around 14.2 Ma. This lineage was probably endemic to North America, and is mostly known from the Great Plains, although remains of A. ingens were also discovered in California and New Mexico.