Thick successions of middle and late Cenozoic sedimentary rocks occur in the Linxia Basin of China. These deposits comprise an archive recording spatiotemporal patterns of mountain uplift, erosion, basin deformation, and associated changes in the monsoon, as a result of the growth of the northeastern Tibetan Plateau. The Linxia Basin is also world famous for its abundant and diverse vertebrate fossils that shed light on Cenozoic terrestrial ecosystem evolution; however, previous studies of these important fossils have been beset by issues related to commercial excavation and associated difficulties in ascertaining provenance, with many specimens in private collections. Despite years of intensive studies by geologists from Lanzhou University and vertebrate paleontologists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), aspects of basin stratigraphy and chronology still remain controversial. The heart of these controversies often revolves around questions of imprecise fossil provenance and related casual references of fossils as age tie-points, different interpretations of lithostratigraphic units at different localities across the basin, and ultimately different age determinations. In this special issue, entitled Biostratigraphy, Chronostratigraphy and Vertebrate Paleontology of the Linxia Basin, we report the findings of an IVPP project, funded by the Chinese Academy of Sciences, which addresses these controversies, proposes a new age model for basin, and develops understanding of the fossil assemblages. In this introduction to the special issue, we review the tectonic context for basin evolution and chronology as well as lithostratigraphy, paleoenvironments and climates. While issues of provenance remain challenging and will continue to burden Chinese vertebrate paleontologists into the future, our findings shed new light on the vertebrate paleontology of the Linxia Basin, and unique circumstances in which it developed.

Of the many peculiarities that enable the giant panda (Ailuropoda melanoleuca), a member of the order Carnivora, to adapt to life as a dedicated bamboo feeder, its extra "thumb" is arguably the most celebrated yet enigmatic. In addition to the normal five digits in the hands of most mammals, the giant panda has a greatly enlarged wrist bone, the radial sesamoid, that acts as a sixth digit, an opposable "thumb" for manipulating bamboo. We report the earliest enlarged radial sesamoid, already a functional opposable "thumb," in the ancestral panda Ailurarctos from the late Miocene site of Shuitangba in Yunnan Province, China. However, since the late Miocene, the "thumb" has not enlarged further because it must be balanced with the constraints of weight bearing while walking in a plantigrade posture. This morphological adaptation in panda evolution thus reflects a dual function of the radial sesamoid for both bamboo manipulation and weight distribution. The latter constraint could be the main reason why the panda's false thumb never evolved into a full digit. This crude "thumb" suggests that the origin of the panda's dedicated bamboo diet goes back to as early as 6-7 Ma.