Patterns of skeletal development and morphological evolution in extant salamanders and Paleozoic branchiosaurids

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The increased interest in development and its role for morphological evolution in recent years has yielded a number of methodologies for the analysis of ontogenetic sequence data and the acquisition of alternative morphologies. Salamanders are classic study animals for the relationships between ontogeny and phylogeny because of their biphasic life history and a high degree of developmental plasticity. In this study, modern methods will be applied for the first time to ontogenetic sequence data of salamanders allowing for the identification of changes within the sequence and their placement in a given phylogeny. Sequence data from various regions of the body (limbs, skull, whole body) and two taxonomic levels (interfamily vs. intrafamily level) will be considered. This approach provides the framework for the interpretation of ontogenetic events with respect to phylogeny, function, and life history. Furthermore, ontogenetic plasticity in the distinct trophic morphotypes of the salamander Ambystoma tigrinum (large, broadheaded cannibal vs. small, “normal” morph) will be investigated to 1) identify events and changes in timing that produce the different morphologies and 2) quantify the shape change throughout ontogeny using geometric morphometric approaches. The results will be compared to a rare dataset from the fossil record, the ossification sequence of the 300 Ma old branchiosaurid Apateon, which shows indications for the possible presence of a trophic polymorphism similar to salamanders.