Ontogeny of Cranial Morphology in Extant Crocodilians and Its Phylogenetic Utility

The degree to which ontogenetic data could facilitate the understanding of phylogenetic relationships has long been a subject of contention in evolutionary biology. Known occurrences of paedomorphosis have invalidated strict adherences to the recapitulationist theory and the biogenetic law. Nevertheless, the extent to which patterns of ontogenetic morphological changes are phylogenetically informative remains to be tested. Here I use the extant members of Crocodylia to investigate the phylogenetic information contained in (1) the allometric trajectories describing the ontogenetic changes in cranial morphology; and (2) ontogenetically variable characters (OVCs), or shape variables that undergo significant changes in relative position through ontogeny. Using three-dimensional landmark-based geometric morphometric methods, I digitized the crania of ten crocodilian species and quantified the morphological changes associated with growth. For the first study, the shape data were used to construct allometric trajectories for each sampled species to test whether the similarities in the orientation of these trajectories correlate with phylogenetic relatedness. Crucial to this study was the availability of a time-calibrated molecular phylogeny that provided phylogenetic reconstructions independent from morphological data, with which the phylogenetic signal of these trajectories could be tested. A suite of methods was employed to test the phylogenetic signal, including (1) the K-statistic; (2) a likelihood ratio test based on Pagel's lambda; (3) permutational regression analysis on trajectory and phylogenetic distances; (4) topological comparison between the phenogram constructed from a clustering method and the molecular phylogeny; and (5) a Mantel test. All tests produced non-significant results and showed an overall lack of phylogenetic signal, indicating that these allometric trajectories have little phylogenetic information. Interestingly, the topology of the phenogram constructed from the clustering algorithm also differs markedly from the topology of published morphological tree, which suggests that the underlying signal in these trajectories is largely uncorrelated with similarities in adult cranial morphologies. The results of this study counter the assumption that patterns of morphological changes that occur throughout ontogeny contain significant phylogenetic signal and give caution to the use of ontogenetic data for phylogenetic inference. The second study introduces a new class of ontogenetic data for describing ontogenetic changes that could be used in a phylogenetic framework. Ontogenetically variable characters (OVCs) were constructed based on the significance of the correlation between shape and size variables. A character matrix was constructed according to the presence and absence of OVCs. Optimization of implied character state changes on molecular and morphological trees suggests that OVCs, on average, are equally informative for either phylogenetic reconstruction. In contrast, parsimony analysis on the OVC character matrix produces trees that broadly support published molecular trees. These observations indicate substantial potential utility of OVCs as phylogenetic characters, but improvements to the construction of OVCs and further examinations are needed to justify the direct incorporation of OVCs in phylogenetic analyses.