Figure 7: Evolutionary scenario for the origin of the unique lung ventilation apparatus of turtles.
Phylogeny (left) of stem turtles4,17 showing increasing trunk rigidity towards the crown (Note: the plastron is formed by the fusion of gastralia, interclavicle and clavicles, whereas the carapace is formed by the fusion of ribs, vertebrae, cleithra and ossified osteoderms found along the perimeter of the shell)49. Our results suggest that the early stem lineage of turtles evolved a division of function in which the dorsal ribs took on a purely stabilizing role, whereas abdominal muscles became specialized for ventilating the lungs (right). Homology of the shoulder girdle elements follows Lyson et al.49 Basal amniote redrawn and modified from Carrier50. Clavicle: pink; cleithrum: blue; interclavicle: green; scapula: yellow.
