Study Of A New Microraptorine Specimen From The Lower Cretaceous Of Western Liaoning, China

The Early Cretaceous deposits of western Liaoning and its surrounding region, northeastern China have yielded numerous well-preserved fossils of feathered dinosaurs. Among the feathered dinosaurs, Microraptorinae is the most known as possessing a series of bird-like skeletal features, and long pennaceous feathers that formed aerodynamic surfaces on the arms and tail, but also on the legs. Its discovery provides important evidence and information about the origins of bird and bird flight, and the evolution of deinonychosaurian dinosaurs. Later studies suggest that the microraptorinae, including eight genera, locates at basal location of dromaeosauridae. Here we report a new microraptorine specimen(PMOL-AD00055) from the Jiufotang Formation of Jianchang County, Liaoning Province, China. The specimen is nearly completely preserved, and only the posterior skull and the anterior dorsal vertebrae are missing. It is judged to be an adult individual based on the fusion degree of compound bones. The new specimen has a whole length of about 90 cm. It displays the synapomorphies of dromaeosauridae: dorsal and ventral margins of dentary subparallel; prezygapophyses and chevron of caudal vertebra elongated to form rod-like structure; coracoid with a thick subglenoid fossa; pedal phalanx II-1 shorter than IV-1, and large and curved ungual of pedal digit II, etc. It also shares some derived features with other microraptorines: pubis with a lateral tubercle at the middle; long and slender metatarsi with subarctometatarsalian condition. The new specimen can be distinguished from other microraptorines by the following characters: maxillary process of premaxilla being longer than twice the length of premaxillary rostrum and extended more posteriorly than its nasal process; straight ventral margin of maxillary process of premaxilla without obvious depression; maxillary fenestra large and ovoid; large pleurocoel on the dorsal vertebrate; metacarpal I long, for about 47% length of metacarpal II. The preliminary study and phylogenetic analysis suggest that the new specimen(PMOL-AD00055) likely represent a new microraptorine taxon. The new specimen also preserves some unknown features in other known microraptorines. These features will provide new morphological information of Microraptoria.This article reviews the recent studies of microraptorinae, summarizing, discussing and modifying the diagnosis of the known genera and species; summarizing the important osteologic and feather structures for the origin of the bird and flight. This article also analyses the function of the morphologic characters relative to the flight, the result supports “arboreal hypothesis” for the origin of flight.