Limb chondrogenesis in Graptemys nigrinoda (Emydidae), with comments on the primary axis and the digital arch in turtles

Abstract

The early development of skeletal structures of manus and pes was studied using whole-mount, alcian-blue-stained embryos at different stages in an ontogenetic series of Graptemys nigrinoda. 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, except for some potential atavistic occurrences. The anlagen for the fibulare and intermedium were identified, and in some but not all specimens two centralia elements are also present. These elements of the proximal and medial tarsal row fuse into a single cartilaginous structure, which later becomes the astragalocalcaneum. Inconsistencies in the literature about the details of the chondrification sequence of autopodial elements are in part related to real intraspecific and interspecific variation across turtles. The patterns of connectivity, often cited in studies of the limb chondrification sequence of tetrapods, are difficult if not impossible to objectively detect using standard alcian-blue-stained specimens. © 2008 Brill Academic Publishers.