Staging criteria for embryos of the spiny softshell turtle, Apalone spinifera (Testudines: Trionychidae).

Previous work describing the embryonic stages of turtle development has not included members of the highly derived trionychid turtles. Staging criteria are described for the spiny softshell turtle (Apalone spinifera) to facilitate comparisons between phylogenetically distant taxa of turtles. Embryonic development in A. spinifera is placed in the context of the widely used sequence of Yntema stages. Novel features are included in the descriptions of staging criteria for Stages 13-26. Comparisons of the development of specific features are made between A. spinifera and other taxa of turtles. Data on the duration of developmental stages at different temperatures and embryo dimensions support the conclusion that morphology-based staging criteria are superior to developmental rate temperature coefficients.