The Role of Venom Delivery Systems in Snake Evolution

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Recent studies regarding the multiple origin of front-fanged venom delivery systems prompt a reconsideration of the role of venom delivery in ophidian history. Snakes are derived from lizards, which swallow large prey items by an inertial feeding sequence. Early snakes were poorly adapted for inertial feeding, but they may have taken advantage of their relatively rigid cranial structure to stun and immobilize their prey. Most henophidian snakes kill their prey by constriction, thereby involving the axial skeleton and musculature in both locomotion and feeding. Powerful constriction seems to preclude rapid locomotion. Henophidians dominated snake faunas until the mid-Tertiary, when caenophidian snakes suddenly underwent extensive radiation. The Duvernoy's gland, a serous gland occurring in most living colubrid snakes, may have provided the mechanism for uncoupling locomotor and feeding activities by providing a venomous secretion for the immobilization of prey. Thus, caenophidians were able to develop more rapid locomotor systems, which were especially advantageous during the mid-Tertiary expansion of open habitats. Duvernoy's gland is regarded as a characteristic of colubroid snakes. Although the gland is lost in some taxa (including certain constricting colubrids), in others it is hypertrophied and associated with additional morphological features in an integrated front-fanged venom delivery system. The multiple origin of the latter condition is not surprising, given the ancestral nature of the venom delivery system among colubroid snakes.

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