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. 2019 Dec 11;14(12):e0225391. doi: 10.1371/journal.pone.0225391

Fig 25. Vertebral length profiles for three stages of archaeocete evolution leading to modern whales.

Fig 25

A, early middle Eocene protocetid Maiacetus inuus, a foot-powered swimmer (GSP-UM 3551; green). B, late Eocene protocetid Aegicetus gehennae, a transitional undulatory swimmer with an elongated body (CGM 60584; gold). C, late Eocene basilosaurid Dorudon atrox, an undulatory swimmer with an elongated body (UM 101215, etc.; blue). D, modern Orcinus orca, an oscillatory tail-powered swimmer (AMNH 34276; gray). Transition from A to B involved elongation of posterior lumbar through anterior caudal vertebrae (Fig 21), reduction of the sacroiliac articulation between the vertebral column and innominate, and loss of fusion between sacral vertebrae. Transition from B to C involved addition of mid-body vertebrae and lumbarization of sacral vertebrae. Transition from C to D involved shortening by reduction in the number of mid-body vertebrae, or, in some cetaceans, shortening of the lengths of individual vertebrae. Symbols are explained in Fig 21.