PHYLOGENETIC ANALYSIS OF ONTOGENIC SHAPE TRANSFORMATIONS - A REASSESSMENT OF THE PIRANHA GENUS PYGOCENTRUS (TELEOSTEI)

Despite the potential information that may lie in phylogenetic analyses of ontogenies of body form, few studies have examined methods for extracting and analyzing ontogenetic shape characters. We propose and exemplify a procedure for phylogenetic shape analysis. We use the thin-plate spline decomposed by its partial warps, a method that has several critical advantages over available alternatives. Most notably, shape variables extracted by this method refer to localizable features of the morphology. We demonstrate how these characters can be coded and include them in a phylogenetic analysis of the piranha genus Pygocentrus, using a data set also comprising meristic, myological, and osteological characters. Using ontogenies of these localized shape variables, we have corroborated the monophyly of Pygocentrus. Although we found no new characters corroborating the proposed sister-group relationship of P. nattereri and P. cariba, our characters are all congruent with this hypothesis. Several ontogenetic shape characters serve to diagnose the previously undiagnosed P. nattereri. Independence of ontogenetic shape features is assessed in the same manner as for any other features: by examination of their distributions on the corroborated cladogram. In addition to inspecting associations among characters that changed multiple times, character independence was assessed using the information in the kinds of ontogenetic modifications (gain, loss, reorientation, reversal) and the information in observed development. Most of the geometrically independent features extracted during this study are phylogenetically independent of each other. We also found that region-specific ontogenetic allometries are phylogenetically independent of each other. In addition, localized ontogenetic changes along orthogonal body axes (anteroposterior and dorsoventral in this case) are usually phylogenetically independent. Although these findings of character independence may be specific to this study the method for assessing this independence can be applied generally. Evolution of both spatial and temporal patterns of growth is an inference that depends upon using methods, such as the one employed here, capable of describing the spatial patterning of ontogeny.