Developmental patterns and the identification of homologies in the avian hand

Homologies of digits in the avian hand have been debated for 150 years. Cladistic analysis nests birds with theropod dinosaurs. Theropod hands retain only digits I-II-III, so digits of the modern bird hand are often identified as I-II-III, Study of the developing manus and pes in amniote embryos, including a variety of avian species, shows stereotyped patterns of cartilage condensations. A primary axis of cartilage condensation is visible in all species that runs through the humerus into digit IV. Comparison to serially homologous elements of the hindlimb indicates that the retained digits of the avian hand are II-III-IV.