HIGH-PRECISION AGE CONSTRAINTS ON THE JURASSIC RISE OF FEATHERED DINOSAURS AND EUTHERIAN MAMMALS: U-PB GEOCHRONOLOGY OF THE YANLIAO BIOTA FROM JIANCHANG (WESTERN LIAONING PROVINCE, CHINA)

The renowned Early Cretaceous Jehol Biota consists of extraordinarily well-preserved fossils of feathered theropod dinosaurs, early birds and placental mammals that record the evolutionary diversification of these important faunal groups. However, the earliest feathered dinosaurs, eutherian mammals and transitional forms of pterosaurs occur in the Jurassic continental successions of northeastern China and are part of a distinct assemblage of terrestrial vertebrates, freshwater invertebrates, well-preserved insects and plants known as the Yanliao Biota (and its equivalent Daohugou Biota). Their discovery has greatly expanded our knowledge of the origin of – and biodiversity and palaeobiology in a time before – the Jehol Biota. Fundamental questions nevertheless remain as to the precise correlation of fossil-bearing strata, rates of dinosaur and mammalian evolution, and their relationship to environmental change in deep time, mainly due to paucity of precise and accurate temporal constraints.

Vertebrate fauna belonging to the Yanliao Biota have been recovered and extensively studied from the Jurassic Lanqi Formation in Jianchang, western Lioaning Province of China. Earlier geochronological investigations by the U-Pb SIMS and LA-ICPMS methods on the volcanic and pyroclastic rocks associated with the Lanqi and correlative formations in northeastern China established the first-order synchroneity of the Yanliao and Daohugou biotas as Jurassic assemblages that unequivocally predated the Early Cretaceous Jehol Biota. However, a high-resolution chronostratigraphic framework for the pre-Jehol biotas has thus far been hampered by the limited precision of the in situ radioisotopic dating techniques. Here we present high-precision U-Pb zircon geochronology by the CA-ID-TIMS method from three ash beds interstratified with fossil horizons of the Lanqi Formation at Jianchang (Daxishan section). The results constrain the preserved Yanlio Biota to time intervals between 160.889 Ma, 160.383 Ma and 160.254 Ma with 2σ analytical uncertainties on the order of ± 46 to ± 69 kyr. These data place the Yanliao Biota from Jianchang in the Oxfordian Stage of Late Jurassic, and mark the Daxishan section as the site of Earth’s oldest documented feathered dinosaurs and eutherian mammals.