The smallest known specimen of Microraptor (Dinosauria: Dromaeosauridae) from the Jiufotang Formation in northeastern China

Abstract

ABSTRACT The Early Cretaceous Jehol Biota in northeastern China has yielded a diversity of feathered dromaeosaurid dinosaurs and expanded our knowledge of avian origin and flight evolution. However, histology-based ontogenetic references and detailed morphological studies remain limited relative to the abundant fossils. Here, a new specimen of microraptorine from the Lower Cretaceous Jiufotang Formation is described. This specimen includes a nearly complete postcranial skeleton and inferred as a Microraptor based on revised osteological characters. The femoral length of the new specimen is less than 5 cm, making it the smallest dromaeosaurid individual reported from the Jehol Biota until now. Osteohistological analysis reveals that this individual was a juvenile that is less than one-year old. The new specimen provides additional morphological details that are poorly preserved in the other Microraptor specimens, including aspects of the coracoid, ilium, and tail. Additionally, the sacrals and pubic symphysis of the new specimen have respectively fused at the juvenile stage. This phenomenon suggests caution should be taken in using skeletal fusion for assessing the ontogenetic stage of Microraptor specimens, highlighting the importance of histological analysis.