Metamorphosis of Cranial Design in Tiger Salamanders (Ambystoma tigrinurn): A Morphometric Analysis of Ontogenetic Change

While ontogenetic analyses of skull development have contributed to our understanding of phylogenetic patterns in vertebrates, there are few studies of taxa that undergo a relatively discrete and rapid change in morphology during development (metamorphosis). Morphological changes occurring in the head at metamorphosis in tiger salamanders (Ambystoma tigrinurn) were quantified by a morphometric analysis of cranial osteology and myology to document patterns of change during metamorphosis. We employed a cross-sectional analysis using a sample of larvae just prior to metamorphosis and a sample of transformed individuals just after metamorphosis, as well as larvae undergoing metamorphosis. There were no differences in external size of the head among the larval and transformed samples. The hyobranchial apparatus showed many dramatic changes at metamorphosis, including shortening of ceratobranchial 1 and the basibranchial. The subarcualis rectus muscle increased greatly in length at metamorphosis, as did hypobranchial length and internasal distance. A truss analysis of dorsal skull shape showed that at metamorphosis the snout becomes wider, the maxillary and squamosal triangles rotate posteromedially, and the neurocranium shortens (while maintaining its width), resulting in an overall decrease in skull length at metamorphosis. These morphometric differences are interpreted in light of recent data on the functional morphology of feeding in salamanders. Morphological reorganization of the hyobranchial apparatus and shape changes in the skull are related to the acquisition of a novel terrestrial feeding mode (tongue projection) at metamorphosis. Metamorphic changes (both internal and external) that can be used to judge metamorphic condition are discussed. Analyses of ontogeny have contributed greatly to our understanding of form and function in the vertebrate skull. The analysis of vertebrate skull development has provided numerous characters for phylogenetic analysis (e.g., de Beer, '37; Goodrich, '30), has greatly increased our understanding of the developmental pathways producing the skull (Hall, '&2a,b; Alberch and Alberch, '81; Hanken and Hall, '&I), and has allowed scaling relationships for components of head design to be quantified (Albrecht, '78; Cochard, '85; Shea, '83, '84; Strauss, '84; Strauss and Fuiman, '85). However, despite many studies of cranial development in vertebrates, little is known about taxa that undergo a relatively discrete and rapid morphological change during ontogeny. Most ontogenetic analyses of head design focus on organisms that exhibit reasonably smooth, continuous modification of form during ontogeny: organisms that metamorphose are not often studied. Species that metamorphose are often of special interest because the rapidity with which changes occur in both form and function provides an experimental opportunity to examine major changes in