The homologies and evolutionary reduction of the pelvis and hindlimbs in snakes, with the first report of ossified pelvic vestiges in an anomalepidid (Liotyphlops beui)

We report the first example of ossified pelvic vestiges in an anomalepidid snake, Liotyplophs beui, and provide a review of the diversity of limb and pelvic elements within Serpentes. We trace the evolution, homology and reduction of the pelvic elements and hindlimbs from the oldest known snakes through to living forms. Evolutionary analysis of the pelvic and limb data shows that the most recent common ancestor of all living snakes (Serpentes) most probably retained all three pelvic elements and rudimentary hindlimbs (femoral spurs). Subsequently, there have been multiple losses of ossified pelvic and hindlimb elements and regaining of ossified pelvic elements. Reduction of the pelvis has followed different routes in the two primary groups of living snakes (scolecophidians and alethinophidians). The single remaining rod-like element in some scolecophidians is the ischium, whereas the single remaining rod-like element in many basal alethinophidians is the pubis. Notably, many basal alethinophidians share a distinctive configuration of cloacal spur (claw), femur and a sizeable pubis, which is likely to be related functionally to the widespread use of the hindlimbs in mating and courtship, rather than the usual representation of the bones as non-functional vestiges.