Variation in the ilium of North American Bufo (Lissamphibia; Anura) and its implications for species-level identification of fragmentary anuran fossils

The ilium is widely considered to be the single best element upon which to base the identification of fossil anurans when isolated bones are all that are available. I tested the diagnostic utility of continuous and discrete ilial characters for toads of the genus Bufo by evaluating their distribution among an expanded taxonomic sample of 27 extant North American species. Support for species-level identifications and for the identification of traditionally recognized species groups based on ilial characters alone was assessed. Results indicate that no distinctive mor phologies in the ilium of extant North American Bufo are known that permit a species-level identification or the diagnosis of any traditionally recognized species group. Two of the ten included extinct species, B. holmani and B. kuhrei, do contain ilial characters that fall outside the observed range of the examined extant species. The fact that both morphometric and discrete characters fail to support species-level identification of isolated ilia of extant Bufo indicates that identifications should be restricted to higher taxonomic levels until new ilial characters are established. Potentially informative characters should be tested against a comparative sample that reflects clade diversity rather than one that reflects our geographic and/or temporal assumptions. This approach helps to avoid circularity if the identifications are used subsequently in analyses of faunal dynamics.