Evolutionary Patterns in Advanced Snakes

One prevalent view of phylogenetic events in advanced snakesholds that the fangs evolved along at least two pathways one(e.g. elapids) from ancestors with enlarged anterior and theother (e.g. viperids) from ancestors with enlarged posteriormaxillary teeth. Selective forces driving these changes arepresumed to arise from the increasing advantages of teeth andglands in venom injection. In this paper another plausible viewof these events is proposed. First fangs of both elapids and viperids likely evolved fromreal maxillary teeth. In non-venomous snakes, differences intooth morphology and function suggest that there may be somedivision of labor among anterior and posterior maxillary teeth.Anterior maxillary teeth, residing forward in the mouth likelyserve the biological role of snaring and impaling prey duringthe strike. They are also conical frequently recurved and lacka secretion groove. On the other hand posterior teeth becauseof their geometric position on the maxilla and mechanical advantages,tend to serve as aids in preingestion manipulation and swallowingof prey. They are often blade shaped and occasionally bear asecretion groove along their sides. Although both front andrear maxillary teeth of nonvenomous snakes may be elongatedthis is likely to serve these different functional roles andhence they evolved under different selective pressures. Whenfangs evolved they did so several times independently but fromrear maxillary teeth. In support one notes a) the similar positionpostorbital of venom and Duvernoy s glands b) similar embryonicdevelopment of fangs and rear maxillary teeth c) secretion groovewhen present, is found only on rear teeth and d) similar biologicalroles of some rear teeth and fangs. For ease in clearance ofthe prey during the strike the fangs are positioned forwardin the mouth accomplished in viperid snakes by forward rotationof the maxilla and elapids by rostral anatomical migration tothe front of the maxilla. Second, the adaptive advantage first favoring initial rear toothenlargement likely centered not on their role in venom injectionbut rather on their role in preingestion manipulation and swallowing.However once enlarged, teeth would be preadapted for later modificationinto fangs under selection pressures arising from advantagesof venom introduction. This has implications for the function and evolution of associatedstructures. Besides possibly subduring or even killing of preythe secretion of Duvernoy's gland may be involved in digestionor in neutralizing noxious or fouling products of the prey.The presence or absence of constriction need not be functionallytied to absence or presence of venom injection. The phylogeneticpathways outlined herein were likely traveled several timesindependently in advanced snakes.