Figure 1. Convergent evolution of the embryonic limb skeleton in multiple mammal species.

a-b, Phylogeny of (a) mammals and of (b) artiodactyls representing the major groups that have independently lost digits, based on Meredith et al³⁷. Parenthetical lettering references skeletons in accompanying panels. Orange circles indicate an evolutionary incidence of digit loss. Purple circles represent the shift from mesaxonic to paraxonic limbs in basal artiodactyls. Species that sculpt the limb by cell death are highlighted in red, and those that show a restriction of Ptch1 expression are highlighted in green. C Alcian blue and alizarin red stained skeleton of postnatal day 0 three-toed jerboa, D sagitta with the ankle (proximal) at the top. Posterior view (left) highlights the fifth metatarsal (arrow). Anterior view (right) highlights the first metatarsal (arrow head). d, Alcian blue stained skeletons of the approximately 16 dpc five-toed jerboa, A elater, hind foot; e, 30 dpc pig fore foot; f, approximately 16 dpc D sagitta hind foot; g, 50 dpc camel hind foot; h, 34 dpc horse fore foot; c, d, f, scalebar = 0.5 mm. e, g, h, scalebar = 1 mm.