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

Despite the potential information that may lie in phylogenetic analyse s of ontogenies of body form, few studies have examined methods for ex tracting and analyzing ontogenetic shape characters. We propose and ex emplify a procedure for phylogenetic shape analysis. We use the thin-p late 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 th e morphology. We demonstrate how these characters can be coded and inc lude them in a phylogenetic analysis of the piranha genus Pygocentrus, using a data set also comprising meristic, myological, and osteologic al 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 thi s hypothesis. Several ontogenetic shape characters serve to diagnose t he previously undiagnosed P. nattereri. Independence of ontogenetic sh ape 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 m ultiple times, character independence was assessed using the informati on in the kinds of ontogenetic modifications (gain, loss, reorientatio n, reversal) and the information in observed development. Most of the geometrically independent features extracted during this study are phy logenetically independent of each other. We also found that region-spe cific ontogenetic allometries are phylogenetically independent of each other. In addition, localized ontogenetic changes along orthogonal bo dy axes (anteroposterior and dorsoventral in this case) are usually ph ylogenetically independent. Although these findings of character indep endence may be specific to this study the method for assessing this in dependence can be applied generally. Evolution of both spatial and tem poral patterns of growth is an inference that depends upon using metho ds, such as the one employed here, capable of describing the spatial p atterning of ontogeny.