Autopodial development in the sea turtles Chelonia mydas and Caretta caretta.

The manus and pes were studied using whole-mount and histological preparations of ontogenetic series of Chelonia mydas and Caretta caretta. Patterns of connectivity and sequences of chondrification events are similar to those reported for other turtle species, with respect to both the primary axis and the digital arch. There is no evidence of anterior condensations in the region distal to the radius and the tibia, supporting the hypothesis that the radiale and tibiale are absent in turtles. The three middle metacarpals are the first elements to start ossification in the manus of C. mydas, while ossification has not started in the pes. In the hatchling of C. mydas, most carpals have started ossification, whereas tarsals are mostly still cartilaginous. In C. caretta, the first carpals to ossify are the ulnare and intermedium, followed by the pisiform. Among metatarsals, the fifth hooked metatarsal is the last one to start ossification. The fibulare and intermedium fuse early in chondrogenesis, later becoming the astragalocalcaneum. Ossification in the carpals of C. caretta starts while tarsals are still cartilaginous. The derived autopodial proportions in each autopodium of adults are laid out at the condensation stage, and features that were present in basal turtles are absent at all stages examined (developmental penetrance). In contrast to this, conservatism is expressed in the presence of similar patterns of connectivity during early chondrogenesis, and in the development of overall proportions of the manus versus pes. As in adult anatomy, the development of the autopodium of marine turtles is a mosaic of derived and plesiomorphic features.