PASOS: a method for the phylogenetic analysis of shape ontogenies.

We present a novel phylogenetic approach to infer ancestral ontogenies of shape characters described as landmark configurations. The method is rooted in previously published theoretical developments to analyse landmark data in a phylogenetic context with parsimony as the optimality criterion, in this case using the minimization of differences in landmark position to define not only ancestral shapes but also the changes in developmental timing between ancestor-descendant shape ontogenies. Evolutionary changes along the tree represent changes in relative developmental timing between ontogenetic trajectories (possible heterochronic events) and changes in shape within each stage. The method requires the user to determine the shape of the specimens between two standard events, for instance birth and onset of sexual maturity. Once the ontogenetic trajectory is discretized into a series of consecutive stages, the method enables the user to identify changes in developmental timing associated with changes in the offset and/or onset of the shape ontogenetic trajectories. The method is implemented in a C language program called SPASOS. The analysis of two empirical examples (anurans and felids) using this novel method yielded results in agreement with previous hypotheses about shape evolution in these groups based on non-phylogenetic analyses.