Evolutionary Patterns in Advanced Snakes

Abstract

One prevalent view of phylogenetic events in advanced snakes holds that the fangs evolved along at least two pathways one (e.g. elapids) from ancestors with enlarged anterior and the other (e.g. viperids) from ancestors with enlarged posterior maxillary teeth. Selective forces driving these changes are presumed to arise from the increasing advantages of teeth and glands in venom injection. In this paper another plausible view of these events is proposed. First fangs of both elapids and viperids likely evolved from real maxillary teeth. In non-venomous snakes, differences in tooth morphology and function suggest that there may be some division of labor among anterior and posterior maxillary teeth. Anterior maxillary teeth, residing forward in the mouth likely serve the biological role of snaring and impaling prey during the strike. They are also conical frequently recurved and lack a secretion groove. On the other hand posterior teeth because of their geometric position on the maxilla and mechanical advantages, tend to serve as aids in preingestion manipulation and swallowing of prey. They are often blade shaped and occasionally bear a secretion groove along their sides. Although both front and rear maxillary teeth of nonvenomous snakes may be elongated this is likely to serve these different functional roles and hence they evolved under different selective pressures. When fangs evolved they did so several times independently but from rear maxillary teeth. In support one notes a) the similar position postorbital of venom and Duvernoy s glands b) similar embryonic development of fangs and rear maxillary teeth c) secretion groove when present, is found only on rear teeth and d) similar biological roles of some rear teeth and fangs. For ease in clearance of the prey during the strike the fangs are positioned forward in the mouth accomplished in viperid snakes by forward rotation of the maxilla and elapids by rostral anatomical migration to the front of the maxilla. Second, the adaptive advantage first favoring initial rear tooth enlargement likely centered not on their role in venom injection but rather on their role in preingestion manipulation and swallowing. However once enlarged, teeth would be preadapted for later modification into fangs under selection pressures arising from advantages of venom introduction. This has implications for the function and evolution of associated structures. Besides possibly subduring or even killing of prey the secretion of Duvernoy's gland may be involved in digestion or in neutralizing noxious or fouling products of the prey. The presence or absence of constriction need not be functionally tied to absence or presence of venom injection. The phylogenetic pathways outlined herein were likely traveled several times independently in advanced snakes.