Osteology and skeletal development of Apalone spinifera (Reptilia: Testudines: Trionychidae).

Despite considerable attention that other groups of reptiles have received, few descriptions of the development and sequences of chondrification and ossification of the entire skeleton of turtles exist. Herein, the adult skeleton of the spiny softshell turtle, Apalone spinifera (Testudines: Trionychidae), is described; this description forms a basis of comparison for the embryonic skeleton and its ontogenesis. Descriptions are made on the basis of cleared and double-stained embryos and dry skeletal postembryonic specimens. The embryonic chondrocranium of A. spinifera is described and compared to those of Emys orbicularis and Caretta caretta, the sequence of chondrification of fore- and hindlimbs are compared with published descriptions of Chelydra serpentina and Chrysemys picta, and the sequence of ossification of elements is compared with those of C. serpentina, Lacerta vivipara, and Alligator mississippiensis. In A. spinifera, the first elements that ossify (Stage 17) are associated with the dermatocranium and mandible, followed by elements of the dermal skull table, lower jaw, and dermal elements of the plastron. In A. spinifera, the sequence of chondrification of limb elements is similar to that of C. serpentina; however, the sequence of ossification varies greatly among Apalone, Chelydra, Lacerta, and Alligator.