| Amphicticeps | |
|---|---|
 | |
| Holotype skull ofAmphicticeps shackelfordi | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Family: | †Amphicynodontidae |
| Genus: | †Amphicticeps Matthew &Granger, 1924 |
| Type species | |
| †Amphicticeps shackelfordi Matthew & Granger, 1924 | |
| Other species | |
| |
Amphicticeps is an extinct genus of small, weasel-likecarnivoran mammal. It lived inMongolia during theOligocene. The genus was erected in 1924 for the speciesA. shackelfordi on the basis of a well-preserved skull. Historically, the systematic position of this genus has been problematic until more specimens were described decades later.[1]
The type specimen ofAmphicticeps shackelfordi (AMNH 19010) is an almost complete skull. It was discovered during the 1922 field season of the Central Asiatic Expeditions around 2 miles southwest of the Loh campsite inÖvörkhangai Province, Mongolia. This locality is part of theOligocene-agedHsanda Gol Formation. Though several other jaw elements were also collected, these were not figured or assigned toAmphicticeps in the original description as they could not be firmly associated with the type skull.[1] During the 1994 field season of theMongolian Academy of Sciences and theAmerican Museum of Natural History joint expeditions, a naturally associated set of lower and upper jaws ofA. shackelfordi was found, and the previously known jaw elements can now be confidently assigned to this species.[2]
In addition to the type species, two more species ofAmphicticeps were described by Wanget al. in 2005:
A. dorog is currently known exclusively from teeth and jaw elements found in the top of the Tatal Member of theHsanda Gol Formation, Mongolia. The type specimen (MAE SG.9194) is a fragment of the rightmaxilla withpremolars andmolars, and several additional fragments oframi and maxillae are assigned to this species. The specific name is a Mongolian word for 'badger'. This species is believed to be an intermediate form between the less-derivedA. shackelfordi and more derivedA. makhchinus based on the size and characters of its teeth.[2]
The least-known of the three species,A. makhchinus is known only from theholotype specimen (MAE 93–213) consisting of a rightmaxillary fragment with preserved teeth. The specimen was collected from the Tatal Member of theHsanda Gol Formation, Mongolia. The specific name is a Mongolian word meaning 'meat-eater' or 'carnivore', as tooth features indicate this species was the mosthypercarnivorous species in the genus. Based on the premolar labial length,A. makhchinus is the largest known species of theAmphicticeps genus, being 32% larger thanA. shackelfordi and 16% larger thanA. dorog.[2]
Amphicticeps was a small-sized mammal, with the type skull ofA. shackelfordi (the smallest species) measuring 8.7 cm from nasal tip toinion. The skull is strongly built for a carnivoran of this size, with a broad and short rostrum, and moderately sizedcanine teeth. The first upper molar has a postprotocrista, enlarged parastyle and no lingual notch on theentoconid crest. The second upper molar is lingually positioned and reduced, and the third upper molar is either very reduced entirely lost. The upper fourthpremolar has a carnassial notch and thesuprameatal fossa is shallow, these features being primitive compared to the condition seen mustelids.[2]
The classification ofAmphicticeps has historically been problematic. In their initial description of the genus, Matthew and Granger (1924) suggested it to be a "highly progressivemiacid" rather than placing it in any existing carnivoran family.[1] Later authors have assigned it toArctoidea based on dental or cranial features. Schmidt-Kittler (1981) noted the basicranium to have features matching arctoids, and deemed it not to belong inMusteloidea due to its shallow suprameatal fossa and the form of its molars, thus regarding it to be a "basal arctoid" that emerged before the development of Musteloidea.[3] Hunt (1996) proposed thatAmphicticeps may belong in the familyAmphicynodontidae, a classification which was followed by Wang and Qiu (2003).[4][5]
Wanget al. (2005) conducted aphylogenetic analysis findingAmphicticeps to be a basalursoid, being the earliest-diverging member of the clade containing it,Amphicynodon,Ursidae andPhocoidea. Thecladogram results of their phylogenetic analyses are displayed in the cladogram below:[2]
In their description ofEoarctos, Wanget al. (2023) conducted a phylogenetic analysis recoveringAmphicticeps as the earliest-divergingpinniped. Their results are shown in the cladogram below:[6]
All known species ofAmphicticeps hail from theHsanda Gol Formation, which is dated to around 33.4 to 31 million years ago (early Oligocene). This formation is believed to have been deposited in an open,semiaridsteppe environment withplaya lakes andephemeral rivers.[7] In this habitat,Amphicticeps lived alongside a wide variety of small mammals including variousrodents, the leporidDesmatolagus, and the erinaceidPalaeoscaptor.[8][9] Sympatric predators included several species ofHyaenodon, the stenoplesictidsShandgolictis andAsiavorator, the feliformsNimravus andPalaeogale, the amphicynodontidAmphicynodon and the didymoconidsDidymoconus andErgilictis.[10] The largest animal living alongsideAmphicticeps in this formation wasParaceratheriumtransouralicum, a giant hornless rhinocerotoid.[11]
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