Osteohistology of enantiornithine birds from the Lower Cretaceous Xiagou Formation

We describe the osteohistology of five enantiornithine bird specimens from Lower Cretaceous Xiagou Formation deposits of the Changma locality in northwestern Gansu Province, China. Samples were taken from the femora of: three specimens of Avimaia schweitzerae (IVPP V25371, IVPP V31956, and GSGM-04-CM-007), Qiliania graffini GSGM-04-CM-006, and Novavis pubisculata IVPP V31957. The objective of this study is to describe intrageneric variation (in Avimaia), and intertaxonomic variation among enantiornithine birds coexisting in an ecosystem. All five specimens have a femoral cortex composed mainly of parallel fibered bone with relatively low vascularity. All three Avimaia specimens have 2–3 vascular canals, and asymmetrical growth marks, indicating cortical drift. In Qiliania there are eight longitudinal vascular canals, five of which are concentrated in one region of the cortex. Although the gross anatomy of the skeleton and fusion of compound elements indicates morphological maturity, neither growth marks, an outer circumferential layer (OCL), nor an inner circumferential layer (ICL) are present. The femur of Novavis has some regions of a woven parallel complex and a higher level of vascularity relative to the other specimens (14 longitudinal channels present). Although this specimen is morphologically immature based on gross anatomy, the femur has a well-developed OCL and ICL. These results emphasize the enantiornithine offset between morphological maturity and osteohistological maturity. Development of the OCL appears to be decoupled from morphological maturity, in some cases forming before the skeleton has fully fused, and in others well after. The specimens are similar in size but vary considerably in the number of growth marks present, from none to two. This suggests either developmental plasticity and diverse growth strategies and, complicates attempts to interpret relative age and growth stage in enantiornithines.