The Origin of the Bony Shell of Turtles as a Unique Evolutionary Model in Reptiles

Abstract

Generally, the bony armor of reptiles consists of separate osteoderms which develop in the dermis independently from the internal skeleton. Such a dermal armor is characteristic of ancient pareiasaurs that are closely related to Testudines. On the basis of this fact it has been assumed that the turtle shell is built up as the fusion of the osteoderms with some elements of the internal skeleton. However, this concept is not confirmed by morphogenetic data. During turtle ontogeny the reduction of the trunk myomeres leads to sinking of the vertebrae and ribs into the dermis. The neural and costal plates of the dorsal disk form as the outgrowths of these endoskeletal bones on inside the dermis. There are no independent superficial ossicles over the general plastral bony primordia in the ventral region of the body. Each plastral plate develops only from a single primordium like the clavicles, interclavicle and gastralia of other reptiles. Most likely, Testudines progressed in a unique evolutionary direction. Their bony shell is mainly the result of modification and consolidation of internal skeletal elements. The real osteoderms develop only on the body margins as connections between the dorsal and the ventral discs.