Abstract

We report new, fossiliferous Paleogene Naran Bulak Formation localities from the central-eastern part of the Nemegt Basin of the Gobi Desert, Mongolia. Early Paleogene localities have been previously identified only in the western half of the Nemegt Basin. The new localities, near the town of Daus, are also noteworthy for their geographical proximity to Ukhaa Tolgod, a Late Cretaceous Djadokhta Formation locality known for its numerous dinosaurs, mammal, and lizard fossils. The Daus section consists of the Zhigden, Naran and Bumban Members of the Naran Bulak Formation at three localities, and mammal and ostracod fossils were discovered in the Naran Member. Noteworthy discoveries are a dentary of the pantodont Archaeolambda cf. planicanina, postcrania of Pantolambdodon, a skull of the gliroid Gomphos, and a partial skull with a worn and damaged dentition provisionally identified as an arctostylopid. Biostratigraphy has been the primary means of dating Paleogene Asian faunas, however, the local fauna from the new localities does not fit easily with established patterns. The Naran Member and Archaeolambda planicanina and the arctostylopid Palaeostylops have typically been allied with the Gashatan Asian Land Mammal Asia (ALMA) and attributed to the latest Paleocene. By contrast, Gomphos has been repeatedly found in the Bumban Member and assigned a Bumbanian ALMA, which has been considered as the earliest Eocene. Pantolambdodon has been reported from middle Eocene Arshatan and Irdinmanhan ALMA beds. The co-occurrence of these taxa in Naran Member beds complicates the temporal interpretation of the new localities and the reliability of broader biostragraphic patterns.