Anchitherium (Mammalia, Perissodactyla, Equidae) from the early Miocene Hiramaki Formation, Gifu Prefecture, Japan, and its implication for the early diversification of Asian Anchitherium

Abstract

An Anchitherium specimen with a nearly complete series of the upper cheek teeth (P2–M3), from the upper part (ca.17–18 Ma) of the Hiramaki Formation, Kani Basin, Gifu Prefecture, Japan, was previously referred to Anchitherium “hypohippoides” nomen dubium. Despite that it provides one of the best examples of significant dental characters of Asian Anchitherium, it has remained undescribed and unprepared until recently. Although a paucity of materials from Asia makes the taxonomy of Asian Anchitherium difficult to assign, comparison showed that the specimen should be reassigned to Anchitherium aff. A. gobiense; it differs from A. aurelianense and is rather similar to A. gobiense from China by virtue of large size and expanded hypostyles. The Japanese Anchitherium also shows distinct features including straight (flattened) ectolophs with narrow mesostyles, rudimentary crochets, and enamel protuberances at the lingual mouth of the median valleys. This combination of accessory features has not been known in Asian Anchitherium and seems to be rarely observed among the diversified European species. The existence of Japanese Anchitherium implies early species diversification in East Asia that predates a greater diversification in Europe associated with the Middle Miocene Climatic Optimum and supports paleogeographical and paleozoological connection to the Asian mainland under a warm and humid climate prior to the formation of the Japanese archipelago (ca. 16.5 Ma).

New record of anchithere (Anchitherium aurelianense) in the Miocene of Eastern Siberia, Russia

“…5-7). Largetoothed anchithere from Japan (Miyata & Tomida, 2010) is slightly different from Tagay form in dental elements. It shows an incipient crochet and enamel cusplet at the mouth of the middle valley, in P2 hypocone is merged with hypostyle, and anterostyle appears.…”

“…The third lower molar from Tagay is more gracile than all specimens described from Kazakhstan (width index of 51.5 against 55.1, 55.9, and 60.1). It is close to East Asian forms (51.4 Gifu; 50.8 Fangxian; 50.0 Inner Mongolia; 50.7 Junggar Basin) (Tleuberdina & Forsten, 2001;Miyata & Tomida, 2010). At the same time the Tagay specimen is quite small.…”

“…Teeth from Kazakhstan anchitheres (m1 or m2 from a series of sites of different age) vary as 17.4 × 12.2 and 19.0 × 12.0; in Mongolian forms (Ulan-Tologoi), 16.6 × 12.5; 15.9 × 12.3; 16.5 × 11.2; in Chinese forms (Fangxian), 18.9 × 14.2 mm (Belyaeva, 1954;Miyata & Tomida, 2010;Tleuberdina & Forsten, 2001). Turkish forms from Pașalar show values 19.2-21.9 × 14.6-17.6 for premolars and 17.6-20.8 × 15.5-15.7 for molars (Forsten, 1990).…”

Anchitheriinae (Perissodactyla, Equidae) from the early middle Miocene locality Gračanica (Bugojno Basin, Bosnia-Herzegovina)

“…The Asian "giant" Anchitheriinae, Sinohippus, differs from the studied material by a larger size, by having relatively high crowns and by lacking lingual cingula on cheek teeth (Salesa et al 2004;Klementiev and Sizov 2015). Likewise, Paranchitherium karpinskii is larger in size and is distinct from the A. hippoides from Gračanica by the presence of cement on cheek teeth (Borissiak 1937(Borissiak , 1945Forsten 1982;Miyata and Tomida 2010;Klementiev and Sizov 2015). The North-American Kalobatippus can be distinguished from the studied material by the presence of a more developed hypoconulid on the m3 as well as shorter and more slender metapodials (MacFadden 2001).…”

“…Among the large Anchitheriinae, the genus Sinohippus differs by an i3 conspicuously smaller than the i1/ 2, a tendency to hypsodonty, more reduced cingula on cheek teeth, a hypoconule without connection to the metaloph on upper cheek teeth, the isolated cusps on the protoloph on P2 and a weaker hypoconulid on the m3 (Salesa et al 2004;Ye et al 2005;Hou et al 2007). Paranchitherium karpinskii is also discriminated by the presence of thin cement and a well developed crochet connecting the protoloph to the metaloph on upper cheek teeth (Borissiak 1937(Borissiak , 1945Forsten 1982;Miyata and Tomida 2010;Klementiev and Sizov 2015). In contrast, it shares several features with Anchitherium such as a strong and triangular hypoconule on the upper cheek teeth, the development of a parastyle, the lingual cingulum on the P2, a twinned metaconid/metastylid column, a distinct hypoconulid on lower cheek teeth and a sharp protoconidal labial crest on the p2 (Abusch-Siewert 1983;MacFadden 2001;Klementiev and Sizov 2015).…”