The Turtle Evolution: A Conundrum in Vertebrate Evo-Devo

Abstract

Because of their unique morphology, turtles have raised profound questions as to their evolutionary origin. In striking contrast to the body plan of other tetrapods, the shoulder girdle of turtles sits inside the rib cage, which comprises the dorsal shell, or carapace. By this topological change of the skeletal elements, the carapace has been regarded as an example of evolutionary novelty that violates the ancestral body plan of tetrapods. In this chapter, we first overview the phylogenetic positioning of turtles, and then review how turtles evolved their unique body plan. In brief, three points have been clarified by recent studies. (1) Turtles have birds/crocodilians (or archosaurians) affinity of evolutionary origin. (2) During embryogenesis, the turtle also establishes the vertebrate basic body plan, as in other vertebrates, followed by the late developmental stages of generating turtle-specific structures, such as folding of the lateral body wall to make the apparent inside-out topology of shoulder girdle against ribs. (3) One of the causal factors of folding appears to be the concentric growth of carapacial margin, which involves an ancestral gene expression cascade in a new location. These reports allow us to hypothesize the stepwise, not necessarily saltatory, evolution of turtles, consistent with the recent finding of a transitional fossil animal, Odontochelys, that did not have the carapace but already possessed the plastron.