Back Logged Paleo Post

Back in 1997, when I was still a freshman in high school, Alan Feduccia wrote a paper on the homology of the digits in the bird limb claiming that embryonic development showed the digits in the hands of birds to be II-III-IV (index, middle and ring finger) rather than I-II-III as seen in Theropod evolution. Pivotal though it is, I’ve only just now gotten around to reading it. His argument was based on the observation that the condensation of cartilage in the limb consistently followed the pattern of going through the humerus, ulna and digit IV. His sample included a chicken, cormorant, turtle and an alligator. In all of them he saw a central axis of cartilage condensation and called them homologous. He neglected to his own a priori assumption of that if condensation goes through digit IV in animals that have digit IV, that it must go through digit IV in animals about which the presence of digit IV is uncertain. Assumed homology to digit IV negates the point of his study. Consequentially, in birds, Feduccia saw the temporary vestige of digit V posterior to IV and absolutely no sign of digit I at the other end. Had he seen a center of ossification for digit I, then I’d accept his results.

A big problem with his methodology was that the only groups that he used for comparison each had 5 digits on the hand. Furthermore, the evolution of the hand and foot of Theropod dinosaurs identical as Feduccia makes it sound. The hand in theropods is I-II-III (which we can tell from a strong reduction and eventually loss of IV & V in primitive theropods) while the main digits on the foot are II-III-IV (since we can see the outer digits 1 & V be reduced to due claws). In the foot of the chick embryo, one does see the remnants of digits I & V, which is why it’s okay to say that the axis of cartilage condensation goes through digit IV in the foot. However, I suspect that the axis in the hand switched from IV to III early in the evolution of theropods (Ornithischians don’t follow this pattern and sauropods keep all of their digits). To substantiate or refute the possibility of the axis of condensation moving, one might compare the embryology to the fossil of horses, sloths, tapirs, guinea pigs or some other digit reduced animal.
Feduccia further argues that “a variation of this pattern wherein the primary axis runs through digit III, would eliminate any phylogenetic significance from the morphological and molecular similarities in amniote limb development. If such a condition could be demonstrated, patterns of limb development would have to be decoupled from phylogeny, and this stereotypic pattern of development accepted as convergence.” This is just nonsense. The axis of condensation has to go somewhere, and it would look identical whether it goes through digit IV or III. It’s much more parsimonious to assume that the digit IV axis is basal and that the digit III condition evolved once and is just a synapomorphy of birds and theropods.