Fig. 7.

Summary of results. (A) Variants in phalanx proportions remain within a predictable range, from a large-to-small gradient in walkers to equal-sized in perchers to distal elongation of the penultimate phalanx in raptors. Walking and perching specialist phenotypes (W, P) are connected in morphospace by a line, where different phenotypes along the line show a trade off of functions (27). Three functions lead to a full triangle whose vertices are W, P, and the raptor specialist phenotype, R. Joint positions are determined in sequence as the digit grows. Distal elongation may be caused by the inhibition of the joint-initiation signals. (B) Hypothesis of origins of modules in the digit. Phalanges in early tetrapods vary as one module, whereas derived tetrapods have two independently varying modules. MT, metatarsal; ph, phalanx.