Three-toed browsing horse Anchitherium clarencei from the Early Miocene (Hemingfordian) Thomas Farm, Florida

Abstract

Anchitherium clamncei Simpson 1932 is described from the early Miocene (early Hemingfordian, ca. 18 million-year-old) Thomas Farm locality in north central Florida. This rare species of three-toed browsing horse demonstrates diagnostic characters including a reduced m3 hypoconulid heel and relatively robust metapodials, both of which also differentiate Anchitherium from its close relative Kalobatippus. The distinctness of these two genera has been questioned for 85 years. Some workers argue that Anchitherium Meyer 1844 is the senior synonym of Kalobatippus Osborn 1915, with the former genus being distributed throughout Holarctica during the Miocene. Other workers believe that Kalobatippus is restricted to the New World, whereas Anchitherium is restricted to the Old World during the Miocene. In the interpretation presented here, both Kalobatippus and Anchitherium occur as distinct genera in North America, with the former found principally in Arikareean faunas and the latter found principally in Hemingfordian faunas (although A. clarencei extends into the early Barstovian in Florida). New World Anchitherium sensu stricto, such as A. clarencei, has the derived characters shared with Old World species of Anchitherium. The postcranial morphology represented by A. clarencei from Thomas Farm indicates relatively flexible fore- and hind-limbs with functional tridactyly.