Figure 1: Evolution of ventilatory mechanisms in Amniota.

(a) Plesiomorphic costal ventilation (1) is employed in Lepidosauria (for example, V. exanthematicus) and in taxa with a diaphragm (Mammalia: Hylobates sp.; 2), hepatic piston (Crocodylia: A. mississippiensis; 4) and sternal pump (G. domesticus; 5), but not in turtles (Testudines: C. serpentina), which have a unique abdominal muscle-based mechanism (3). Phylogeny is derived from morphology^4,17,43,44. Molecular results switch the positions of Testudines and Lepidosauria^45,46,47 but do not affect our inferences regarding the evolution of lung ventilation in turtles. (b,c) Digital reconstruction of the skeleton, lungs and hypaxial muscles of C. serpentina (b, dorsal view; c dorsolateral view). Ap, tendinous aponeurosis; Lu, lung; Oa, M. obliquus abdominis; Ta, M. transversus abdominis; Tr, trachea; Tt, M. transversus thoracis.