A new multituberculate, Yubaatar qianzhouensis sp. nov.: the first Late Cre-taceous mammal from Ganzhou Basin, Jiangxi Province
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Abstract:
The Multituberculata represents one of the most successful groups of mammals that lived in the Mesozoic and early Cenozoic. More than 10 genera have been reported from North China, but there are still no records of this group from South China. Ganzhou Basin in Jiangxi Province has been reported to be rich in vertebrate fossils, but no mammals have previously been found there. Here, we report a new multituberculate, Yubaatar qianzhouensis sp. nov., from the Upper Cretaceous Hekou Formation of Ganxian County, Ganzhou City, Jiangxi Province, China. This taxon represents the first known multituberculate from South China.The holotype of Yubaatar qianzhouensis preserves the posterior region of the skull and part of the postcranial skeleton. The posterior part of the skull is laterally expanded, giving the skull a square-like shape. A triangular process is formed by the frontals and projects posteriorly between the parietals. The lower jaw is in tight occlusion with the skull, concealing the occlusal surfaces of the teeth. Therefore, the skull was CT scanned, and the teeth were recon-structed using the software Mimics 19.0. The postcranial skeleton includes both forelimbs, the right scapula, and some ribs and lumbar vertebrae. In general, our new specimen is quite similar to Yubaatar zhongyuanensis from the Upper Cretaceous of Henan Province, and can be assigned to Yubaatar based on the following characters: the frontals are pointed posteriorly and contribute to the medial rims of the orbits; the coronoid process is low and short; the angle between the anterior margin of the coronoid and the molar part of the alveolar margin of the mandible is greater than 45°; the molars lack cusp or-namentation; M1 possesses three cusp rows, and the lower molar cusps are nearly uniform in height. However, our new species can also be distinguished from Yubaatar zhongyuanensis by a combination of charac-ters: the m1 cusp formula is 7︰6; the M2 cusp formula is 1︰3︰3; m1 bears a posterobuccal cingulid; the m2/m1 length ratio is smaller than that of Y. zhongyuanensis; and the coronoid process is wedge-shaped and has sharp peaks. Importantly, the skull is wider in this specimen than that of Yubaatar zhongyuanensis, which is regarded as the largest known Mesozoic multituberculate from Eurasia. In addition, three-dimensional (3D) reconstruction methods were used to study the morphology of the specimen’s molars. Our CT sections and 3D reconstructions reveal that most of the molars are damaged, though RM2 is an excep-tion. Furthermore, the 3D reconstructions reveal that Rm2 is present but completely buried in the matrix and make it possible to definitively determine the number of cusps on each molar. To clarify the phylogenetic position of Yubaatar qianzhouensis among multituberculates, we ran a new phyloge-netic analysis based on a recent large data matrix that includes 52 taxa and 130 characters. This analysis resulted in the recovery of 5332 most parsimonious trees, each with a length of 454 steps (CI = 0.452, RI = 0.761). The strict consen-sus tree places Y. qianzhouensis and Y. zhongyuanensis as sister taxa, and Yubaatar as a basal genus within Taeniolabi-doidea. Yubaatar is supported by two synapomorphies: the lingual cusp row of m2 contains 4–5 cusps, and the lingual cusp row of m1 contains 6 or more cusps. Taeniolabidoidea is supported by three synapomorphies, including: on m2, the labial cusp row is about equal in length to the lingual cusp row; the ultimate upper premolar is small relative to M1, and the width/length ratio of M1 is greater than 0.55. The discovery of the new species extends the geographical dis-tribution and increases the diversity of multituberculates during the Late Cretaceous in East Asia. SYSTEMATIC PALEONTOLOGY Class Mammalia Linnaeus, 1758 Order Multituberculata Cope, 1884 Suborder Cimolodonta McKenna, 1975 Superfamily Taeniolabidoidea Granger and Simpson, 1929 Yubaatar Xu, Zhang, Pu et al. 2015 Type species Yubaatar zhongyuanensis Xu, Zhang, Pu et al. 2015 Yubaatar qianzhouensis sp. nov. urn:lsid:zoobank.org:act:7773C559-F1E5-4E57-9DFC-E3E0FAC2 7567 (Fig. 2) Diagnosis no contacts between nasals and parietals; frontals with a posterior process projecting distally in a triangular shape between parietals; anterior part of zygo-matic arches directed transversely; coronoid process orientating parallel to the rest of the outer wall of the dentary; the angle of the coronoid anterior margin to the molar alveolar line on the mandibular body greater than 45°; 6 or more m1 main lingual row cusps; the posterobuccal cingulid on m1; 4–5 m2 lingual row cusps; the conical cusps on lower molars. Etymology The species name is the spelling of the ancient name of Ganzhou City. Material Holotype, CUGW VH101. A skull and partially articulated postcranial skeleton. Locality and horizon Hekou Formation at Gan-xian County, Ganzhou City, Jiangxi Province, China; Upper Cretaceous. Remarks Our new species differs from Yubaatar zhongyuanensis by a suite of characters: the m1 cusp formula is 7︰6; the M2 cusp formula is 1︰3︰3; m1 bears a posterobuccal cingulid; the m2/m1 length ratio is smaller than that of Y. zhongyuanensis, and the coronoid process is wedge-shaped and has sharp peaks.
