Cranial osteology of the North American sand lizards, genus Uma (reptilia iguanidae)

The cranial osteology of the genus Uma is described in detail, interspecific variation is analyzed, and the natural selective forces which produced this variation are discussed. The variation is interpreted with respect to the phylogeny and biogeography of the genus. Statistically significant interspecific variation was found in seventeen of the twenty-one skull characters examined in this study. Based on these characters, the genus Uma has been divided into subgroups: a southern one consisting of U. exsul and U. paraphygus and a northern one consisting of U. notate, U. inornata and U. geoparia. The major source of variation distinguishing these subgroups is in their dentition. This divergence appears to be the result of natural selection operating through diet. Uma inornata is more phenotypically similar to U. scorparia than to U. notate. This relationship is consistent with previous comparative physiological studies of the genus but not with comparative studies of external morphology or behavior. The latter indicate that U. inornata is more similar to U. notate than to U. scoparia. This pattern of variation can be explained in terms consistent with the phylogeny of the genus Uma as proposed by Norris (1958).