Maglia, A.*; Buchholz, D.R.: Heterochrony and patterns of osteogenesis in pelobatoid frogs

Change in developmental timing is commonly cited as the source of morphological evolution in anurans (and vertebrates in general). Despite the warnings of Fink (1982), very few studies suggesting heterochrony as the cause of variation in anuran morphology have tested their hypotheses in reference to a phylogeny. Primarily, this is because to do so, two things must be available—a well-supported hypothesis of relationships, and detailed developmental information for the taxa in question. Herein, we examine ontogenetic series of four species of Pelobatoidea, a group for which there are well-supported phylogenetic hypotheses. A total of 177 cleared-and-double stained specimens representing postembryonic ontogenetic series (Gosner Stages 30�46) of lab-reared Spea multiplicata, Scaphiopus couchii, Pelobates syriacus, and Pelodytes ibericus, was examined for patterns of osteogenesis (including timing of onset of ossification and degree of ossification at metamorphosis). Although S. couchii and P. syriacus are similar in number of ossified elements and amount of ossification at metamorphosis, the time needed to ossify is remarkably different (P. syriacus = 23 days; S. couchii = 9 days). Moreover, S. multiplicata and P. ibericus both metamorphose 41 days from hatching and resemble one another in pattern and degree of cranial ossification, but the time to onset of ossification of elements is greater in P. ibericus (20 days) than in S. multiplicata (11 days). These and other heterochronic variations in osteogenesis of the four taxa will be discussed within the framework of a phylogenetic hypothesis. Issues regarding the examination of laboratory-raised versus wild-caught specimens also will be reviewed.