Morphology, development, and evolution of venom-delivery systems in snakes

The first chapter describes fang development. Elapids, viperids, and some other groups of colubroid snakes have tubular fangs for the conduction of venom into their prey. The literature describing the development of venom-conducting fangs provides two contradictory accounts of fang development. Some studies claim that the venom canal forms by the infolding of a deep groove along the surface of the tooth to produce an enclosed canal. In other works the tubular fang is said to form by the deposition of material from tip to base, so that the canal develops without any folding. This study was undertaken to examine fang development and to account for the disagreement in the literature by determining whether fang formation varies among groups of venomous snakes and whether it differs between embryos and adults. Adult and embryonic representatives of elapids and viperids were examined. All fangs examined, elapid and viperid, embryos and adults, were found to develop into their tubular shape by the addition of material to the basal end of the tooth rather than by the folding inward of an ungrooved tooth to form a hollow fang. In some cases, the first fang that develops in embryonic snakes differs morphologically from all those formed subsequently.; In the second chapter, external morphological development of the monocled cobra, Naja kaouthia, is described between oviposition and hatching. Ten developmental stages are diagnosed according to twelve features. These include fusion of the body wall musculature along the ventral midline, appearance of the endolymphatic ducts, formation of the eyelid, and the appearance of scales on the head and body. Additional observations of the developing skull are made at four of these ten stages, based on cleared and stained heads.; The third chapter reconstructs the evolution of venom-delivery systems in snakes. The Colubroidea represent approximately 2300 of the 2700 species of living snakes and include all venomous taxa. Although many morphological studies of colubroid snakes have been carried out over the last century or so, the phylogenetic relationships within this group are poorly known. In the following study, components of the venom-delivery system were analyzed in the context of two alternate hypotheses of the phylogenetic relationships among colubroid snakes. Results indicate that several major morphological changes occurred early in colubroid evolution: a Duvernoy's gland evolved, posterior maxillary teeth became specialized relative to anterior maxillary teeth, and the attachment of the pterygoideus muscle moved forward to a position associated with the posterior maxillary teeth. These innovations may have enabled the great radiation of colubroid snakes. More recently, three separate lineages of colubroids have independently evolved highly specialised front-fanged venom-delivery systems with large and complex venom glands, venom gland compressor muscles, and tubular fangs. These conclusions held true even when characters were analyzed in the context of two alternate phylogenetic hypotheses for the Colubroidea.