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

ABSTRACT

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.

ACKNOWLEDGMENTS

We thank Y. M. Hou from the IVPP for his help with the CT-scans and X. C. Zhu from the IOZ for assistance with extant specimen examination at the National Animal Collection Resource Center. Yu Ji screen-washed the fossiliferous matrix and sorted the concentrates. We also appreciate the constructive comments from the editors and reviewers. The work was supported by the National Natural Science Foundation of China (42072209, 41888101, 41988101, 41625005), Second Tibetan Plateau Scientific Expedition and Research Program (STEP) (Grant No. 2019QZKK0705) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB26000000, XDA20070203).

SUPPLEMENTARY FILES

Supplementary Data 1: (1) supplementary table about Rattus fossils reported in the world; (2) supplementary idealized figure of murine left molars and 95% confidence ellipses about length and width of selected Rattus; (3) supplementary phylogenetic analysis results.

Supplementary Data 2: morphological matrices analyzed with TNT.

DISCLOSURE STATEMENT

No potential conflict of interest was declared by the authors.

Additional information

Notes on contributors

Meijing Chang

MJC, QL, and XJN designed the project. MJC conducted phylogenetic analysis and drafted the manuscript. CXZ and XPJ performed fieldwork and collected materials. QL, XJN, and CXZ supervised the project. All authors edited the manuscript.

Chunxia Zhang

MJC, QL, and XJN designed the project. MJC conducted phylogenetic analysis and drafted the manuscript. CXZ and XPJ performed fieldwork and collected materials. QL, XJN, and CXZ supervised the project. All authors edited the manuscript.

Xueping Ji

MJC, QL, and XJN designed the project. MJC conducted phylogenetic analysis and drafted the manuscript. CXZ and XPJ performed fieldwork and collected materials. QL, XJN, and CXZ supervised the project. All authors edited the manuscript.

Qiang Li

MJC, QL, and XJN designed the project. MJC conducted phylogenetic analysis and drafted the manuscript. CXZ and XPJ performed fieldwork and collected materials. QL, XJN, and CXZ supervised the project. All authors edited the manuscript.

Xijun Ni

MJC, QL, and XJN designed the project. MJC conducted phylogenetic analysis and drafted the manuscript. CXZ and XPJ performed fieldwork and collected materials. QL, XJN, and CXZ supervised the project. All authors edited the manuscript.