Biostratigraphy and palaeoenvironment of the dinosaur-bearing sediments in Lower Cretaceous of Mazongshan area, Gansu Province, China

This paper discusses the lithostratigraphy of the Xinminbao Group in the Mazongshan area of Gansu Province, northwestern China, and the correlation of its biota. The Xinminbao Group was deposited in a fluviolacustrine setting in Mesozoic graben basins under a semi-arid, subtropical climate. The fossil sites concentrated in the lower part of the group have yielded dinosaurs, mammals and other vertebrates that are comparable to the Aptian–Albian vertebrate assemblages that have been found in Mongolia and Siberia. Based on their invertebrate and vertebrate fossils, sediments of the lower part of the Xinminbao Group are considered to be Late Barremian–Aptian in age, although the middle and upper parts could be as young as Albian. The megafossil plants and conchostracans are comparable to those of the Yixian and Jiufuotang Formations in eastern China (Barremian–Aptian). The palynoflora is also comparable to that of the upper Lower Cretaceous elsewhere in northern China, although the presence of two angiosperm taxa, Asteropollis and Tricolpites, suggests that the main fossil sites are Albian. These biostratigraphic data for the Mazongshan area provide a new temporal calibration for the terrestrial biotas of the Lower Cretaceous of central Asia, where most of the known dinosaur-bearing sediments lack precise palynological, radiometric and magnetostratigraphic dating.