Guangxicyon
From Wikipedia, the free encyclopedia
| Guangxicyon Temporal range: ?Late Eocene | |
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| Lower jaw | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Family: | †Amphicyonidae |
| Genus: | †Guangxicyon Zhai et al., 2003 |
| Species: | †G. sinoamericanus |
| Binomial name | |
| †Guangxicyon sinoamericanus Zhai et al., 2003 | |
Guangxicyon is a medium-sized, extinct genus of amphicyonid carnivoran, or "bear dog," which inhabited southern China during the Late Eocene. It is notable for being the oldest member of its family known from Asia, and is characterized by a shortened face and atypical, bunodont dentition. It inhabited a subtropical forest shaped by monsoon climate not unlike the one present in the region where it was found today. Only a single species, Guangxicyon sinoamericanus, is known.
The only fossils attributable to Guangxicyon sinoamericanus were originally found in 1983 in Late Eocene rocks from the Baise (also known as Bose) Basin belonging to the Nadu Formation, and include a mandible, a tibia, and a humerus. It was first published as Guangxicynodon sinocaliforniae, though this turned out to be a nomen nudum, before being officially described in 2003. The genus name is a reference to the province of Guangxi, where it was discovered, while the species name honors the joint Sino-American research team that discovered it.[1]
Description
Guangxicyon was a short-faced amphicyonid, with aberrant dentition and a deep horizontal ramus. The depth of the latter below the masseteric fossa is much deeper than that of true canids such as Enhydrocyon, and comparable to that of the later amphicyonid Cynelos. The mandibular symphysis is not fused. While the canine is large, as in other members of the family, the incisors, as well as several of the premolars and molars, are not preserved. However, based on the alveoli, the first two premolars are reconstructed as being single-rooted and reduced. The fourth premolar lacks the posterior accessory cusp usually seen in its relatives. The second and third molar also appear to be reduced and single-rooted. Both premolars and molars lack accessory cups, and are bunodont. Guangxicyon differs from other short-faced amphicyonids and true canids in the simplification and reduction of its cheek teeth, which are blunter, shorter, and less compressed than in taxa such as Brachyrhynchocyon and Enhydrocyon. The reduction of both its anterior (situated towards the front) premolars and posterior (back) molars, as well as all these teeth being single-rooted, makes it distinct among amphicyonids. Typically, the presence of a short face is associated with the loss of at least some teeth. For comparison, the shortening of the snout as seen in Brachycyon is achieved through the crowding of its cheek teeth in regard to the roots of its second premolar and molar, in Enhydrocyon by the loss of the first premolar and sometimes last molar, as well as the oblique reorientation of its remaining premolars, and in Aktaucyon by the reduction of the first upper premolar.[1]
The length of the humerus is estimated to have been 205 mm, and that of the tibia 165 mm. Compared to other amphicyonids, the anatomy of these bones is relatively similar to that of Amphicyon, though more robust and relatively shorter. The distal condyles on the humerus and tibia are broader than in Cynelos and Brachycyon, indicating a less cursorial lifestyle, and a gait more similar to the ancestral condition in amphicyonids.[1]
Based on an equation using its first molar, its body mass has been estimated at roughly 10.6 kg. This makes it larger than most early arctoids such as Amphicynodon, Cephalogale, and Angelarctocyon, though smaller than Lonchocyon.[1][2][3]
