Oldest known skeleton of a stem cervid (Artiodactyla, Cervidae) from the Lower Miocene of Japan and implications for the initial dispersal of stem cervids

A fossil deer skeleton, including cranial appendages, of an intermediate-aged individual, as indicated by the wear stage of the M3/m3, was newly discovered from the Lower Miocene Togari Member (ca. 18 Ma) of the Akeyo Formation, Mizunami Group in central Japan and classified as a new species of Procervulus (Cervidae, Artiodactyla, Mammalia), P. mizunami sp. nov . Procervulus mizunami shares several features with basal stem cervids from Europe, such as Procervulus , Heteroprox and Acteocemas , notably dichotomous antlers and distinct internal postprotocristae on the upper molars. The frontal appendages of P. mizunami are more similar to those of Procervulus than to those of Heteroprox or Acteocemas , in that the antlers lack derived apomorphic features around the antler base, such as the basal thickening and accessory structures. It resembles P. dichotomus in pedicle diameter and tooth size, but differs in having a proportionally wider antler base and a shorter pedicle. The frontal appendages of P. mizunami are also similar to those of P. praelucidus , but P. mizunami has larger teeth and a larger pedicle diameter than the latter. The unusually short pedicle is diagnostic of P. mizunami and distinguishes it from all other known species of Procervulus . The postcranial bones of P. mizunami are fundamentally similar to those of P. dichotomus , although the metacarpal is proportionally much shorter than the metatarsal in the former. Basal cervids bearing dichotomous antlers are well documented from the Lower Miocene of Europe, but have not previously been recorded from contemporaneous localities in Asia. The discovery of P. mizunami provides the first evidence that one of the earliest cervids with dichotomous antlers had dispersed into East Asia by ca. 18 Ma. The plesiomorphic characteristics found in the remains of P. mizunami suggest that the earliest cervids rapidly dispersed throughout Eurasia.