The development and homology of the chelonian carpus and tarsus

The long‐standing controversies involving the number and homologies of the elements of the carpus and tarsus of turtles are reviewed from a developmental perspective. The analysis is based on a detailed description of the chondrogenesis of the carpus and tarsus in the species Chelydra serpentina and Chrysemys picta. The first stage described is the differentiation of a Y‐shaped chondrogenetic condensation involving the humerus (femur)‐radius/ ulna (tibia/fibula). This stage is followed by the early formation of a series of connected condensations off the distal end of the postaxial element (ulna or fibula). This linear array, which we refer to as the primary axis, comprises the ulnare‐distal carpal 4‐metacarpal 4 in the carpus and the fibulare‐distal tarsal 4‐metatarsal 4 in the tarsus. There are two precondensations that branch off the primary axis. The proximal one will soon form the intermedium while the distal one will generate a digital arch that will give rise sequentially to digits 3‐2‐1, in this order. Digit 5 is not part of the digital arch and forms as an independent condensation.