Micromammal Fossils from the basal part of the Jiaozigou Formation in Yagou area, Linxia Basin, Gansu Province

摘要: Renewed fieldwork in 2003 produced a rich micromammal assemblage from the basal part of the Jiaozigou Formation in the Yagou area of Linxia Basin. 17 genera distributed among 13 families of three orders were recovered. The micromammal fauna is a typical Oligocene assemblage for Central and Eastern Asia. 14 genera (~82% of the fauna) are common in the Oligocene of Asia. Of them four genera and four species of Eucricetodon are restricted to the Oligocene, one genus (Bagacricetodon) is restricted to Late Oligocene and Glis and Eomyodon made their first appearances in the Late Oligocene. Based on this micromammal composition, the basal part of the Jiaozigou Formation in the Yagou area could be mainly of Late Oligocene in age, which is in accordance with the conclusion based on large mammal fossils. In comparison with the other Late Oligocene micromammal faunas in Central and East Asia, the Yagou Fauna is slightly older than the Ulan III biozone of Nei Mongol and biozone C of Mongolia, because it has two Eocene genera and lacks more advanced genera. This is roughly in accordance with the recent palaeomagnetic interpretation for the Maogou section, where the lower boundary of the Jiaozigou Formation was correlated with Chron C10r (~29 Ma). The presence of large number of xerophilous zapodines, ctenodactylids, cricetids and lagomorphs combined with fossorial Tsaganomys and the lithology of the fossil-bearing deposits, composed of gypsiferous reddish brown mudstone, tend to show a semiarid woodland-shrubland habitat during the Late Oligocene in Yagou area. In Late Oligocene more frequent faunal interchange might have occurred between Asia and Europe (4 genera commonly shared) rather than between Asia and America (only 1 genus shared), partly because of the disappearance of the Turgai Strait.