LAUSR

The dorsal part of the turtle shell, or the carapace, is made of modified ribs and the vertebral column. The structure is regarded as an example of evolutionary novelties because the topographical relationships between the ribs and the scapula is reversed as compared to that in other amniotes. To explain this, the folding theory assumes that the turtle-specific body plan was achieved through sequential developmental changes that proceeded in a stepwise manner. This involved the axial arrest of the rib primordia (turtle ribs are confined in the axial part of the embryo), peripheral growth of the carapacial anlage along the embryonic ridge called the carapacial ridge (CR), and inward folding of the ventral body wall to encapsulate the scapula. Survey of fossil records suggested that a lineage of marine reptiles experienced the axial arrest and loss of sternum about 250mya, which is consistent with the molecular clock estimate of the turtle-archosaurian dichotomy. The CR, specific to the turtle embryo, is characterized by turtle-specific expression of genes involved in the canonical Wnt signaling pathway, and functions in the marginal growth of the carapacial primordium, resulting in the fan-shaped pattern of the turtle ribs in the late phase of morphogenesis. Other turtle-specific genes were potentially responsible for the axial arrest of the ribs. A Triassic fossil animal, Odontochelys, assumed to have been in an ancestral lineage of the modern turtle, exhibited a morphology resembling the embryo of modern turtles before the folding. Such an anatomy is consistent that enhancement of the late CR function in late development has led to the acquisition of a round carapace, resulting in encapsulation of the scapula as an evolutionary novelty of the turtle lineage.