Fig. 6. Origin, evolution, and development of the distal carpals/tarsals during the fin-to-limb transition.

The ulnare/fibulare articulates directly with most of the proximal radials in tetrapodomorph sarcopterygians Eusthenopteron, Tiktaalik, and Elpistostege, with some postaxial digits in stem tetrapods Acanthostega and Ichthyostega and with some postaxial distal carpals/tarsals in crown tetrapods. Digital arch mesopodials have an independent evolutionary history and developmental trajectory in the autopodium, as evidenced by the opposite sequences in the loss of distal carpals/tarsals and corresponding digits in tetrapod groups. Distal carpals/tarsals at the postaxial side are lost earlier than digits in early tetrapods (e.g., Acanthostega) but later than digits in modern tetrapods (e.g., Onychodactylus) when the postminimus is considered as a vestigial digit (see main text). Preaxial dominance facilitates establishing a one-to-one relationship between distal mesopodials and digits in modern tetrapods by promoting the loss of the postaxial distal carpals/tarsals and their associated supernumerary digits in early tetrapods. Interpretations of the radials and distal axial elements along the metapterygial axis follow (10, 11). Neoceratodus from (6, 11); Sauripterus, Eusthenopteron, and Tiktaalik from (9); Elpistostege from (10); Acanthostega and Ichthyostega from (7, 11); Tulerpeton from (70); Whatcheeria from (68); Greererpeton from (64); Proterogyrinus from (65); Balanerpeton from (63); Archegosaurus from (61); Acheloma from (36); and Onychodactylus from CAS 26711.