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

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.

Limb development in the Pacific giant salamanders, Dicamptodon (Amphibia, Caudata, Dicamptodontidae)

The larvae of Pacific giant salamanders, Dicamptodon (Dicamptodontidae), develop in streams, display precocious limb development, and possess a full complement of digits at hatching. Limb morphogenesis was studied using cleared whole mounts stained with Alcian Blue or by immunohistochemical methods to reveal stages of limb development. Limb morphogenesis in these salamanders conforms to the general caudate pattern, i.e., preaxial dominance and precocious development of an initially isolated digital arch, despite some differences in detail from other salamander taxa. These comparisons support the hypothesis that a fundamental ground plan characterizes limb development in salamanders which transcends adaptive modifications related to ecology and life-history evolution. This ground plan differs from that which characterizes frogs and amniotes, which shows postaxial dominance and a digital arch that arises from basal elements.