“Regressed” Macrostomatan Snakes

Abstract On some morphology-based phylogenies of extant snakes the capacity to ingest prey of a diameter larger than the snake's head optimizes as a derived condition of macrostomatan snakes such as boas and pythons. The evolution of macrostomatan jaw mechanics can be traced in the more basal scolecophidian and anilioid snakes, such as blind snakes, thread snakes, pipe snakes, and shield tails. Several recent morphology-based phylogenetic analyses of snake interrelationships including fossil snakes have placed fossil taxa of large body size and/or with a macrostomatan skull structure basal to either all extant snakes, or basal to the Alethinophidia (Anilioidea plus Macrostomata, excluding Scolecophidia). This has led to the characterization of scolecophidians and/or anilioids as “regressed macrostomatans”. These snakes would have lost their macrostomatan feeding capacities in adaptation to a fossorial or secretive mode of life, correlated in some forms such as scolecophidians and uropeltines with miniatur...