A new Rattus species and its associated micromammals from the Pliocene Yangyi Formation in Baoshan, western Yunnan, China

Rattus (sensu stricto) is one of the groups of rodents and is most closely related to human activities. The diversity of extant Rattus species is the highest among rodents, but fossil species are rare, mainly found in Asia during the Late Pliocene to Early Pleistocene. Here, we described a Late Pliocene new species of Rattus based on material from the upper Yangyi Formation in Baoshan county, Yunnan Province, China. The primitive morphological characteristics of fewer molar roots, t12 preserved on M1-2, t9 preserved on M3, and buccal stylids well developed on lower molars indicate that the new Rattus is by far the most primitive form in the Rattus genus. In dental morphology of Rattus, the molars have an evolutionary trend of gradually increasing the number of roots and fusing cusps into an arched ridge, reduction to disappearance of the t7 and t12 on upper molars, and reduction to disappearance of the mesiocentral cusp on m1, the anterior buccal stylid on m1-3 and hypoconid on m3. Discovery of the new Rattus from Baoshan confirms that Rattus probably originated in southern Asia before the Late Pliocene, and also provides new fossil evidence to calibrate the molecular clock of the divergence of Rattus from other murines. In addition, we also reported five associated small mammals: Neotetracus sp. nov., Anourosorex qianensis, Soricidae gen. et sp. indet., Ia io, and Kowalskia sp. Most of them inhabit warm and humid tropical or subtropical montane forests or shrubland environments. The composition of this mammalian assemblage indicates that the paleoelevation and paleoclimate of Baoshan area in the Pliocene are very similar to those of the present.