New specimens of anchitheriine from the Linxia Basin, Gansu and evolution of East Asian anchitheriines

Abstract

Anchitheriinae are an extinct subfamily of the Perissodactyla family Equidae, the same family which includes modern horses, zebras and donkeys. Anchitheriinae are the most poorly studied of the Equidae. The distribution of anchitheriines in China has been recorded in Shanxi, Gansu, Inner Mongolia, Xinjiang, Hubei and Jiangsu in the early to middle, and the latest Miocene. Recently, some specimens of Anchitheriine have also been found in the Linxia Basin, Gansu. A specimen from the Wangjiashan locality was found to be a large-sized individual with complex morphology characteristic. The specimen was identical to the reported material of Sinohippus robustus from the same locality. Both specimens were very large size, had a distobuccal crest on the labial wall of p2, and displayed a deep and narrow ectoflexid perpendicular to major axis of the tooth. Based on these observations, the new specimen was identified as S. robustus . Specimens from the Laogou locality have been identified as Anchitherium gobiense in brief reports of the local fauna. Identification of the specimens was confirmed based on the complexity of diagnostic feature complexity such as medium size and characteristics of the lower cheek tooth row that progressively reduced from the middle to the ends. A detailed morphological analysis of these samples was performed on East Asian anchitheriines and compared to specimens from China and Japan. These observations confirmed a previous hypothesis that “ Anchitherium aurelianense ” from Fangshan, Nanjing, were more similar to Anchitherium gobiense , and should be considered as a different lineage from the true Anchitherium aurelianense found in Europe. The specimen from Fangxian, Hubei, was identified as Anchitherium gobiense from a previous research study. However, the size of the specimen was significantly larger than the Xinjiang specimens, and the holotype and the paratype specimens from Tunggur of Inner Mongolia. Two forms with obviously different sizes were also observed amongst the Tunggur specimens. The Fangxian specimen, and the larger from Tunggur were temporally attributed as Anchitherium gobiense type B. Based on morphological differences, Anchitherium aff. gobiense from the Kani Basin, Japan, are different from true Anchitherium gobiense and other forms found in Eurasia and considered to be Anchitherium hypohippoides in the present paper. The Japanese form has a less concave labial wall on the upper cheek tooth, which is a very primitive feature usually seen in ancestral forms found in North America, such as genus Kalobatippus. Eurasian anchitheriines were derived from North American Kalobatippus , and rapidly separated into the European and East Asian clades after they dispersed into Eurasia. The Japanese species Anchitherium hypohippoides, is the most primitive form, and different evolutionary stages were present in the Chinese clade over time. These stages are represented by “ A. aurelianense ” of Fangshan, A. gobiense group (including A. gobiense s. s. and Type B) and Sinohippus . Large species derived from Sinohippus occurred suddenly in China in the latest Miocene, showed gaps in both chronological and morphological transition. The origin of Sinohippus is unclear, but is a likely relative to a large form of Anchitherium sampelayoi in Europe. The first occurrence of latter come from Spain in about 11 Ma. An evolutionary trend was present in the East Asian anchitheriine clade, in which the body size of anchitheriines was firstly reduced and then increased. This is a normal phenomenon in Eurasian Equidea, such as Hipparion and Equus , and correlated to the demand of the animal itself for immigration, and the pattern of change in the climate and environment. Transcontinental immigration often depends on the connection of land bridges caused by drops in the sea level which eventually occur as a result of climate cooling. Large body size is advantageous in having resistance to cold and allows very long-distance immigration. In contrast, reduced body size can help to avoid fierce competition against other forms in similar niches and often results from warmer climates and increased richness of the community. With further improvement in the environment, the body size of anchitheriines may increase due to the improved availability of resources.