Digit Identity and Digit Number
Indirect Support for the Descent of Birds from Theropod Dinosaurs
by Frietson Galis

The evolutionary origin of birds is the subject of heated debate. As more data are collected, a consensus is emerging in favor of a descent from theropod dinosaurs. However, in 1997, important evidence refuting this hypothesis was put forward by Burke and Feduccia. They provided new data that confirmed old claims that the digit identity of the three fingers of the hand in theropods differs from that of birds (digits I, II and III versus II, III and IV), which implies that there is no homology between the first, second, and third digits of the hand of these groups.

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Digit Homology of Birds and Dinosaurs
Accommodating the Cladogram
by Alan Feduccia

The idea that birds are living dinosaurs has gained widespread acceptance, especially in the popular press. However, problems exist, the most serious of which is the mismatch of the digits; whereas theropods have a highly derived hand composed of digits I–II–II, digits II–III–IV occur in birds.

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Response
by Frietson Galis

When hypothesizing evolutionary transitions, scientists attempt to construct the most parsimonious scenario. As Feduccia states, most scientists agree that the scenario in which birds descend from dinosaurs is the most parsimonious. However, this scenario faces one major hurdle – the mismatch of the digits. When we assume that birds are derived from theropod dinosaurs and that the digits of birds are II–III–IV, there are two possible hypotheses: (1) avian ancestors had digits II–III–IV or (2), as was proposed by Wagner and Gauthier, there has been a homeotic shift from digits with identity I–II–III to digits with identity II–III–IV. On the basis of the present fossil record, the second hypothesis is the most probable.

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