Amphicyon

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, Eurasia, and Africa.

In a note dated back to May 16, 1836, French geologist Alexandre Leymerie wrote 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 may come from Ancient Greek ἀμφί (amphí), meaning "both", and κύων (kúon), meaning "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 even greater sizes. 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. The North American species of the genus show a considerable size increase over the course of their evolution, with the earliest one, A. galushai, being estimated at 187 kg, whereas A. frendens was considerably larger, at 432 kg. Finally, the terminal North American species, A. ingens, was among the largest of all amphicyonids, with an estimated body mass of 550 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.

The diet of Amphicyon has proven difficult to reconstruct, as its dentition possesses both crushing and shearing functions. It has been proposed, on the basis of dental wear patterns and morphology, that European species of this genus were bone-crushing mesocarnivores. One study argued that A. longiramus was hypercarnivorous, as the relative grinding area of its lower molars is similar to that of carnivorous canids, whereas another suggested that the North American species of the genus were omnivores. A dental microwear analysis of A. major recovers the diet of this species as mesocarnivorous, similar to red foxes, consuming meat as well as plants and hard items, which presumably included bone. 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.

As it lacked the adaptations for rapid acceleration, Amphicyon seems to have hunted quite unlike lions and tigers, which approach their prey very closely, before overtaking it after a quick burst of speed. However, as even modern pursuit predators such as wolves stalk and ambush their prey, it is likely that Amphicyon did the same. It has been proposed that it pursued its prey for longer distances, and at a speed notably slower than modern wolves. After catching up to its victim, it was likely able to immobilize it with its powerful forelimbs. Its postscapular fossa indicates a well-developed subscapularis minor muscle, which fixes the shoulder joint, and prevents the head of the humerus from being dislocated by the struggles of a prey animal trying to break free. The anatomy of its humerus also supports this, as it showcases the presence of a strong pronator teres muscle, and thereby pronation of the forearm, and powerful flexors of digits and wrists, which are integral to the prey-grasping ability of both extant bears and big cats. Indeed, the trochlea of its humeral condyle is shallower than that of a tiger, suggesting that the pronation/supination of its forearms might have been even greater than in large felids, although it likely lacked the ability of cats to retract their claws. Its small infraorbital foramina indicates that it lacked the well-developed vibrissae of cats, which provides them with the sensory information needed to place a precise killing bite. Therefore, it may have killed its prey by tearing open the preys ribcage, as thylacines did, or by biting into its neck to sever major blood vessels. Just like modern predators, it likely did not target its preys abdomen, as wounds in that area do not kill quickly. As the elongation of its distal limb segments was more similar to that of the solitary tiger than to the social lion, Amphicyon was likely solitary as well. Due to its comparatively slow maximum speed and lack of rapid acceleration, it is unlikely that Amphicyon preyed on cursorial ungulates. However, it has been proposed that its pursuit capabilities were suited to chase mediportal ungulates, such as merycoidodontids and rhinoceroses. A specimen of the rhinoceros Prosantorhinus douvillei was discovered with bitemarks corresponding to those of A. giganteus, although it remains unclear if this was the result of active predation or merely scavenging of remains. 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 Europe 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.