Some snakes are the only vertebrates able to engulf prey with cross-sectional areas several times larger than the area encompassed by the snake’s jaws at peak gape. This ability is conferred by modifying soft tissues ventral to the axial musculoskeletal system for extraordinary extensibility between the mandibles and stomach. Moving large prey into the gut depends on structural decoupling of toothed jaws from the braincase. In all living snakes, kinetic jaws form mobile ratchets. In scolecophidians, transverse maxillary or dentary ratchets have evolved to move small prey into the gut. In alethinophidians, longitudinal palatopterygoid ratchets move the head and body of the snake over the prey. Evidence from extant snakes shows that streptostyly, prokinesis, rhinokinesis and loss of all ventral skeletal elements connected to the axial skeleton were critical to evolution of the upper-jaw ratchet on which macrostomy is based. The existing fossil record gives tantalizing clues that suggest the ancestor of snakes might have been macrostomous. Resolution of this issue will require structural details of the snout, braincase, and toothed ratchets in both ‘basal’ extant snakes and fossils.