Myacyon

Most material belonging to this genus suggests that Myacyon included large to very large amphicyonids. The dimensions of Myacyon dojambir and the m2 from Napudet approach those of the largest Amphicyon giganteus and the enormous Cynelos jitu, which have been estimated to weigh over 300 kg,^[5][15] although the material from the Samburu Hills^[13] and Beni Mellal is considerably smaller.^[6] M. kiptalami is as large or slightly larger than a lion,^[7] and the remains from Kipsaraman, Bled Douarah and Lothagam suggest a similar size,^[2][10] ?M. peignei is not nearly as large, being smaller than most Early Miocene species of Cynelos, except for C. euryodon,^[4] itself comparable in size to the 42 kg^[16] heavy C. lemanensis.^[2]

It is characterized by its sectorial molars, showing adaptions towards hypercarnivory.^[17] The large m1 is unlike that of any other amphicyonid, possessing a shallow carnassial notch.^[18] It has an elongated trigonid, a strong protoconid, which possesses a strong and tall trenchant anterior crest and a short talonid, with a robust, crested hypoconid and a far distally located entoconid which is small, poorly developed and lacks a crest. The paraconid is indistinct and separated from the protoconid by a very weak notch, which is visible on the buccal margin of the anterior crest, while the metaconid is reduced and situated slightly posteriorly. The short and oblong m2 lacks a paraconid and only possesses a poorly developed metaconid, which is situated at the level of the trenchant protoconid. Its talonid is short and narrower than the trigonid, possessing a strong hypoconid but lacking an entoconid. The trigonids of both teeth have a strong buccal cingulum. The m3 is present, but probably reduced.^[6][12]

The cranial remains of M. kiptalami superficially resembles those of large felids and nimravids, with a short snout, enlarged canines and long diastemata. As in lions, the anterior extremities of its nasals flare outwards and slightly downwards, thereby shifting the nasal notches laterally and widening the nostrils. Its maxillo-premaxillary does not touch the nasals until it has reached the orbital line, which is much further back than in canids. Furthermore, the upper border of the stout premaxilla is lower than in dogs, but similarly projects anteriorly, even if it is less pointed, resulting in the incisors lying in an almost straight line, curved incisor battery of dogs. The incisors have steep roots and long axes and occlusal apices, which point sharply towards the rear. As a result of the outbowing of the maxilla in the region of the carnassial and molar, the maxilla is antero-posteriorly shortened. A further adaption towards shortening the snout is the 90° rotation of the P3 along the part of the maxilla in which it is located. It is tucked behind the parastyle of the carnassial, a position usually be filled by a protocone among other amphicyonids. Above it, a large infraorbital foramen is located. It furthermore possesses a large facial fossa, which lies anterior to the orbit and extends anteriorly and ventrally as a curved furrow immediately behind the canine jugum. As a result of these adaptions, Myacyon had a considerable gape, with canine tips extending 40 mm beneath the occlusal surface of the carnassial and incisors.^[7]

M. peignei is only known from a handful of teeth, although the development of the protoconid shelf on its P4 is a distinct feature of this taxon.^[1]

At Oued Mya, Myacyon coexisted with the equid Hipparion and the rhinoceros Aceratherium, as well as the giraffid Palaeotragus and proboscideans.^[6]

The discoveries at Member D of the Ngorora Formation suggests that it was once covered by open woodland, with fluvial conditions being present.^[19] Here Myacyon was found alongside the immense hyaenodont Megistotherium and its smaller relative Dissopsalis, the lioness-sized hyaena Percrocuta as well as a variety of smaller carnivorans.^[7] A large variety of herbivores are known from this site, among them the proboscidean Deinotherium,^[19] bovids, suids^[20] and the early hippopotamus Kenyapotamus.^[21] Fossils of crocodylians, freshwater turtles and fish have also been found.^[21]

As indicated by palaeosol carbon isotope studies and tooth microwear patterns, Fort Ternan, where ?M. peignei has been found, was covered by open woodland and located adjacent to highlands.^[22] The locality shares many taxons with the Ngorora Formation, among them Afrochoerodon, Climacoceras, Chilotheridium, Gentrytragus, Kipsigicerus, Dorcatherium and Kenyapotamus.^[23] The carnivoran assemblage is also similar, including Percrocuta, Dissopsalis and Tugenictis, with a gigantic hyainailourine as the largest predator.^[1] The hominoid Kenyapithecus is among the few genera unique to this site.^[24]