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

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 <i>Microraptor</i> 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 <i>Microraptor</i> 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 <i>Microraptor</i> specimens, highlighting the importance of histological analysis.中国东北地区的早白垩世热河生物群（Jehol Biota）产出了种类丰富的带羽毛驰龙科恐龙，拓展了学界对于鸟类起源与飞行演化的认知。然而，相较于该生物群产出的海量化石，基于骨组织学的个体发育参考资料与精细形态学研究仍较为匮乏。本次研究记述了一件产自下白垩统九佛堂组的小盗龙类（microraptorine）新标本。该标本保存有近乎完整的颅后骨骼，结合修订后的骨骼解剖学特征可推断其归属为小盗龙属（Microraptor）。该新标本的股骨长度不足5厘米，是目前热河生物群中已报道的体型最小的驰龙科个体。骨组织学（Osteohistological）分析显示，该个体为不足1岁的幼体。该新标本提供了其他小盗龙属标本中保存不佳的额外形态学细节，涵盖喙骨（coracoid）、髂骨（ilium）及尾部的相关特征。此外，该标本的荐椎与耻骨联合在幼体阶段即已愈合。这一现象提示，在通过骨骼愈合情况评估小盗龙属标本的个体发育阶段时需谨慎，凸显了骨组织学分析的重要价值。