A Survey of Adaptive Relationships of Dentition to Diet in the North American Iguanidae

Dental morphology of poikilothermal tetrapods is frequently discussed in paleontological literature. It is important partly because many fossil forms are known chiefly from teeth and skull fragments, and partly because of the hypothesis that dental morphology is strongly influenced by selective factors operating through diet, and should therefore provide information regarding habits of the forms in question. Direct evidence of the selective influence of diet upon dentition can be found, obviously, only among living forms. Of living poikilothermal tetrapods, lizards appear to be the best subjects for a study of diet and dentition, because of their relatively great diversity of dental structure and of diet, and also because of their ready availability. Dietary studies of lizards occur with moderate frequency in the literature, and are discussed in a subsequent section of the present paper. Studies of lizard dentition, on the other hand, are exceedingly rare, and most are concerned with fossil forms. Consideration of the relation of diet to dentition appears to be confined to one species, Varanus niloticus, which, however, has been discussed rather intensively (Lonnberg, 1903; Schmidt, 1919; Cowles, 1930); no comparable investigation of other lizards has yet been made. The present study is an attempt to remedy this situation, by comparing diets and dentitions of several species of lizards. The chief aim of this comparison is to determine, in general, whether lizard dentition shows adaptive response to selective factors operating through diet, or whether its evolution is so intimately related to that of other characters not directly influenced by diet, that the effect of diet on dentition cannot be discerned. A corollary of this aim is to determine directions of any adaptive trends of lizard dentition, which may be interpreted as being due to selection through dietary factors. All of the lizards chosen for investigation are North American members of the family Iguanidae. These forms are ideal subjects for four reasons. First, their comparatively well-understood taxonomy defines the North American iguanids (exclusive of Anolis) as a closely-related phylogenetic group; in general, morphological basis for this taxonomy rests upon other than dental characters. Closeness of phylogenetic relationship suggests that the genetic substrate of dentitional morphology is relatively homogeneous; the closer the relationship between lizards being compared, the more readily can dentitional differences between them be ascribed to different diets (if, of course, diets are different). The second reason for selecting the North American iguanids for study is that they exhibit a considerable variety of dentitions. The third reason is that members of the group are readily available in numbers, and the