New Materials Of Beavers (Castoridae,Rodentia,Mammalia) From The Middle Miocene Of Northern Junggar Basin,Xinjiang

The family Castoridae is a monophyletic group of rodents that originated in the Late Eocene of North America(White River Formation in Wyoming),and has been widely found in the Early Oligocene of Asia(Kazakhstan;Xinjiang,China;etc.)and Europe(Belgium;France;etc.)They are characterized by a weak postorbital process,highcrowned cheek teeth(hypsodont),long tubular external auditory canal.The family now contains a single genus and two extant species.However,extinct beavers had a higher diversity of more than 27 genera with at least 100 species and occupied different niches,such as river-corridor habitats or open-habitats.For the extinct species,complete skull,mandible and postcranial bones are rare,and most of them are represented by the isolated teeth.The occlusal morphology of castorid cheek teeth varies at different wear stages and has led to taxonomic misidentification and incorrect classification.Here,10 fossil castorid teeth were described from the Halamagai Formation(the middle Miocene)along the Ulungur River in the northern Junggar Basin,Xinjiang.These teeth are assigned to Steneofiber depereti and Anchitheriomys tungurensis,respectively.Among them,six teeth can be referred to Steneofiber depereti based on the convex and smooth enamel surface of the lower incisor,simple occlusal morphology,and "V-shaped" notch in the mesial side of unworn p4.The other four teeth belong to Anchitheriomys tungurensis characterized by the large size and complex occlusal surface: the metafossettid merges with the mesofossettid in p4,intensively crenulated enamel land in the occlusal surface when cheek teeth are slightly-worn.Computed Tomography scanning technology(CT)was employed to reconstruct the ontogenetic variation of cheek teeth of these two taxa,and the occlusal morphology of different levels was restored.The results show:(1)the occlusal morphology of the heavily worn stage in Steneofiber depereti(Xinjiang specimens)and S.saiamensis are different.In Steneofiber depereti,the buccal hypoflexid of lower teeth was closed before the lingual parafossettid/metafossettid/mesofossettid disappeared while in S.saiamensis the hypoflexid remained in the heavily worn stage.(2)the occlusal morphology of Steneofiber in the “medium-heavily wear stage” can be used for the identification.The occlusal morphology of Anchitheriomys at the “slightly to medium wear stage” is more useful for identification compared to the heavily worn stage.The similarity of the occlusal morphology at the heavily worn stage of Steneofiber? zhaotungensis(Zhaotong,Yunnan),Monosaulax sp.(Yuanmou,Yunnan)and Steneofiber siamensis suggest that they may belong to the same clade in east Asia.In this study,we also compared our species with Steneofiber castorinus from SaintGérand-le-Puy,France(early-middle Miocene,MN2),S.sesri from Ulm(MN2),Germany,S.depereti from Loire Basin,France and Hambach 6C,Germany(MN3-5),and S.siamensis from Thailand.We considered that the big size and the unique location of S.depereti from Xinjiang are very important.Combined with the fact that the crown morphology of Anchitheriomys tungurensis is more complex than that of Anchitheriomys sp.(Middle Miocene,Sa Tai Member,13.0-12.4Ma)from Thailand.The evolutionary level of these two taxa supports the previous conclusion that the age of the Halamagai Formation is MN6.This report increases the diversity of fossil castorids in Xinjiang,expands the geographical distribution of Steneofiber,and suggests that the Middle Miocene warming may have driven some Steneofiber and Anchitheriomys to migrate from Europe to Asia.