Counting the Fingers of Birds and Dinosaurs

Homologies of the three fingers in birds have been debated for more than a century. Paleontologists have traditionally identified avian digits as I-II-III largely on the basis of phalangeal counts, whereas embryologists number them II-III-IV on the basis of development in the egg. The report by Ann C. Burke and Alan Feduccia (1) is a synthesis of the embryological evidence. They observe that in most amniotes, the first digit to form in the pentadactyl manus is digit IV, which develops a "primary axis." With the use of this developmental constraint, they identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. This conclusion is incompatible with theropod (dinosaur) ancestry of birds because theropod digits are identified as I-II-III, with vestigial fourth and fifth digits apparent in the fossil record. The origin of birds from dromaeosaurid-like theropods is supported by a large suite of synapomorphies, and thus has achieved wide acceptance (2-4). Burke and Feduccia apparently regard these similarities as convergence, not homology.