The new fossil record of <italic>Stegolophodon latidens</italic> from the Xiaolongtan locality, Yunnan, China, and the discussion on the age of the <italic>Lufengpithecus keiyuanensis</italic>

Abstract

Lufengpithecus keiyuanensis from the Xiaolongtan Coal Mine is the earliest known Miocene fossil hominoids in southern China, yet its chronological context has long been subjected to disputes, in being either of late Middle Miocene or early Late Miocene age. This controversy is largely due to the uncertainty of the exact positions of previously discovered fossils in the outcrops, and the different chronological interpretations for some proboscidean and suid fossils from associated fossil faunal assemblages. Here we report the first discovery of Stegolophodon latidens from Xiaolongtan with an unambiguous stratigraphic provenance, which provides important evidence in constraining the age of Lufengpithecus keiyuanensis . Stegolophodon has a very high evolutionary rate of dental morphology, and exhibits a pronounced evolutionary trend with time: from the pentalophodont m3 of the Early Miocene or early Middle Miocene, to the pentalophodont M3 and hexalophodont m3 of the late Middle Miocene, to the 5.5-lophed (five principal lophs followed by an extreme well-developed cingulum) M3 and hexa- to heptalophodont m3 of the early Late Miocene, and to the hexalophodont M3 and hepta- to octalophodont m3 of the middle Late Miocene. The newly discovered Stegolophodon latidens bears a 5.5-lophed M3 and a heptalophodont m3, the second and subsequent lophs/lophids show a slight chevron structure, and the pretrite mesoconelets shift mesially and fuse with the corresponding pretrite anterior central conules, forming a “Y-shape”. These traits are similar to, or even slightly more derived than those of Stegolophodon from the lower part of the Dhok Pathan Formation (9.3−8.3 Ma) in the Middle Siwaliks of the Indian Subcontinent. The materials described here can all be assigned to Stegolophodon latidens which was originally discovered from the Yenangyaung fauna, the lower Irrawaddy Formation of Myanmar, deemed of early Late Miocene age. Revisions on the other proboscidean and suid materials also support an early Late Miocene age for Xiaolongtan Fauna. The new fossils were discovered from the upper part of the lignite layers, close to the bottom of the greyish marls of the Buzhaoba Formation. Based on the original records in the geological reports of the Xiaolongtan Coal Mine, the descriptions of the previous publications, as well as the accounts of the fossil finders, the Xiaolongtan coal mine possesses only one fossiliferous layer, which locates in the upper part on the Xiaolongtan Formation (lignite layers). Therefore, the new fossils situate possibly in the same layer as Lufengpithecus keiyuanensis that were found in the 1950’s and 1980’s. From the recent palaeomagnetic investigations, the age of this fossil layer should be 11.6/11.7 Ma, close to the boundary between the Middle and the late Miocene; however, seems to be slightly older than Stegolophodon latidens from the Siwaliks. Further investigations should be carried out to resolve this inconsistency.