The larvae of spadefoot toads exhibit extreme developmental/endocrinological diversity. For example, New World spadefoot toads (Scaphiopus and Spea) have the shortest larval periods known among anurans, and the tadpoles of Old World spadefoot taxa (Pelobates) are among the largest known. To analyze the patterns of this diversity in an evolutionary context, we generated comparable larval growth and development data from 10 of the 11 taxa of spadefoot toads and from one taxon of parsley frog (Pelodytes), the nearest spadefoot toad relative. We found dramatic differences in growth and development among taxa, which indicated that taxon-specific physiology, rather than phenotypic plasticity, underlies larval period diversity. For all eight response variables (development rate, three growth rates, time to forelimb emergence, time to tail resorption, mass at tail resorption, and body length at tail resorption), taxa within genera were similar to each other and were different from taxa in other genera. Larvae of Scaphiopus were small with short larval periods, larvae of Spea were large with short larval periods, larvae of Pelobates were large with long larval periods, and larvae of Pelodytes were small with long larval periods. Even though taxa within the same genus live in different environments, larval growth and development correlated with phylogenetic groupings rather than breeding habitat. Mapping larval data onto a molecular phylogeny indicated that short larval periods, as well as rapid embryonic development and high temperature tolerance, originated within the spadefoot toad family.