Osteological Observations on New World Bufo

Abstract

Skeletons of 51 recognized New World forms of Bufo have been examined. Five major species groups are recognized on the basis of skeletal characteristics. The monotypic hematiticus group possesses a skull of usual bufonid proportions, moderately extensive dermal ornamentation, an occipital canal, and independent frontoparietals and prootics. The valliceps group includes the majority of other forms outside the United States; it differs from the hematiticus group primarily in the fusion of the frontoparietals with the prootics; three distinct subgroups are recognized. The Andean spinulosus group agrees with the valliceps group in the fusion of the frontoparietals with the prootics, but the extent of dermal ornamentation and of some dermal bones are markedly reduced, with correlated exposure of the occipital groove. The americanus group is limited to the United States and adjacent Mexico and Canada. The skull is distinctly elevated, dermal ornamentation somewhat reduced, the occipital canal partially or completely exposed, and the frontoparietal and prootics are independent. The boreas group, limited to western North America, differs from the americanus group in having typical skull proportions and virtually no dermal ornamentation. The spinulosus group was probably derived from the valliceps group; with this exception, it is unlikely that any one of the major groups gave rise to any other. All could have developed from a common ancestral stock with essential characteristics similar to those of regularis and related African forms. The boreas group may not have arisen directly from such a stock, but be related to B. bufo, which in turn was derived from a regularis-like ancestor. If so, the entry of the boreas group into this hemisphere was probably independent from, and much later than, the entrv of the ancestral stock of the other groups.