Evolution of the Turtle Body Plan by the Folding and Creation of New Muscle Connections

Shelling Turtles In almost all vertebrates, the shoulder girdle (scapula) lies outside the ribs. The turtle is unique in that the carapace, the dorsal part of the shell, which is formed from the ribs, encapsulates the scapula. To understand the origin of the turtle-specific body plan, Nagashima et al. (p. 193 ; see the cover; see the Perspective by Rieppel ) compared chicken, mouse, and the Chinese soft shelled-turtle, Pelodiscus sinensis . Modern embryos were studied via whole-mount immunostaining, three-dimensional reconstructions, and with markers for early skeletal precursors and compared with previously reported fossils. Initially, embryos of the three animals share a common developmental pattern, one that is likely to have been shared with their last common ancestor. This pattern, however, is modified in the turtle by a specific folding of its body wall during embryogenesis. This folding preserves some of the connectivity between skeletal and muscle elements but also produces new connections.