Geological age of the Yokawa Formation of the Kobe Group (Japan) on the basis of terrestrial mammalian fossils

Abstract   The age of the Yokawa Formation of the Cenozoic Kobe Group distributed in Hyogo Prefecture of western Japan based on mammalian fossils is discussed. Two fossil dental specimens of terrestrial mammals discovered from the lowest part of the Yokawa Formation in the Sanda area are described. These two fossils described here are: (i) a right mandibular fragment with p2–m3 of Bothriodon sandaensis sp. nov. (selenodont anthracotheriid artiodactyl), which appears to be the most primitive among the species of the genus; and (ii) right m1–m3 of cf. Hyrachyus sp. (primitive rhinocerotoid perissodactyl). In the lower part of the Yokawa Formation, Zaisanamynodon (amynodontid perissodactyl) was previously reported. The morphology ('evolutionary stage') of B. sandaensis is indicative of the latest Middle to Late Eocene, that of cf. Hyrachyus sp. is indicative of the Early to Middle Eocene, and Zaisanamynodon is indicative of the Late Middle to Late Eocene. Therefore, the fossil mammals of the Yokawa Formation indicate an latest Middle Eocene ( ca . 38 Ma) correlation for the lower part of the formation, as a working hypothesis. Although the resolution of the geological age based on these mammalian fossils is relatively low compared to that based on marine index fossils, this result is concordant with the recent radiometric correlation of the lower part of the Yokawa Formation in the Sanda area.     

A review of fossil rhinoceroses from the Neogene of Myanmar with description of new specimens from the Irrawaddy Sediments

Four genera and one indeterminate genus (total eight species) of fossil rhinoceroses (Mammalia; Perissodactyla; Rhinocerotidae) are recognized from the Neogene of central Myanmar. In the early Miocene, most area of central Myanmar were under the shallow marine condition, and no rhinocerotid remain has been documented yet. During the middle to late Miocene, the rhinocerotid remains are commonly found and are represented by “Diceratherium” naricum, Brachypotherium perimense, Brachypotherium fatehjangense and an indeterminate rhinocerotid. In the latest Miocene, these archaic rhinoceroses became extinct. In the late Neogene, the extant genera, Rhinoceros (late Miocene to Pleistocene) and Dicerorhinus (Plio-Pleistocene) first appeared in Myanmar. They appear to have dispersed to the Island Southeast Asia from the continental Asia during the early Pleistocene to middle Pleistocene when the eustatic sea level became low remarkably.