The discovery of a bone that allows clear gender identification of the Mesozoic bird Confuciusornis sanctus (C. sanctus) is reported in Nature Communications this week. The work also provides evidence for sexual dimorphism in this extinct species.

Fossils of the Early Cretaceous bird C. sanctus are abundant in lake deposits in north-eastern China, some found with long ornamental tail feathers and others without. Specimens of C. sanctus that have ornamental tail feathers are commonly interpreted as male and those without as female, but there has been little evidence to support this conclusively. Anusuya Chinsamy-Turan, Luis Chiappe, and colleagues report the discovery of medullary bone - tissue unique to reproductively active female birds - in a C. sanctus specimen that doesn’t possess ornamental feathers. The authors suggest that it provides evidence that individuals of C. sanctus without ornamental feathers are females.

The work will help to provide insight into the onset of sexual maturity and attainment of adult body size of this and other early birds, as well as revealing the gender.