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. We emphasize that chondrogenetic foci often form as "branches" off existing condensations. This well-defined pattern of connectivity is used to establish unambiguous homologies and allows comparisons with other vertebrates. We conclude that preaxial elements such as the radiale and tibiale are absent in the turtles examined and probably in all turtles. The observed proximal elements that form in the anterior region of the limb and that are often homologized as radiale or tibiale have clear connections to the postaxial elements. For this reason we argue that these elements should be homologized as centralia. Therefore, we find two chondrogenetic condensations in the tarsus and three in the carpus, which we consider centralia. They subsequently fuse with neighboring elements in a complex pattern. We also describe the variable presence of a late-developing element in the tarsus of Chelydra, which, to our knowledge, has never been described. We propose this element to be an atavistic pretarsale. Comparison of the chondrogenetic pattern in turtles with those described in the literature for other vertebrates indicates that there are two invariant patterns in all tetrapods with the exception of the urodeles. These are (1) the primary axis and (2) the digital arch.