Figure 6.

Hypothesis about the phylogenetic transformation of digit development leading from an ancestral five-digit theropod hand (e.g., Herrerasaurus) to the three-digit hand of Allosaurus and the maniraptorans including living birds. (A) Herrerasaurus has five digits (DI–DV), but only three are functional (DI–DIII, bold) and the remaining are vestigial (DIV and DV, outlined) (see also Fig. 3a). We assume that Herrerasaurus followed the usual amniote pattern in which condensation CI develops into digit DI, etc. (B) The next stage of evolution is the loss of digit DV, as exemplified by Coelophysis (see Figs. 1 and 3b). Coelophysis has three functional digits, DI–DIII, and one vestigial digit, DIV. We assume that this step is because of a secondary loss of digit DV in which condensation CV forms but fails to differentiate into a digit. Condensation CV still forms during hand development of living birds (1) but fails to fully chondrify and is eventually absorbed. Occasionally, a remnant of DV is found in a few individuals of Coelophysis (59), demonstrating that CV is retained from early theropods into living birds. (C) The transition from a four- to a three-digit hand, represented by Allosaurus, eventually leads to the loss of condensation CI, as shown in the development of living birds. During this transition, we hypothesize a frame shift that forces CII in to the developmental trajectory of digit DI, CIII into DII, and CIV into DIII. This hypothesis reconciles the findings of comparative anatomy, which show that the remaining three digits of birds are digits DI, DII, and DIII and the embryological data that demonstrate that these digits develop from condensations CII, CIII, and CIV.