The Gavialis-Tomistoma debate: the contribution of skull ontogenetic allometry and growth trajectories to the study of crocodylian relationships

The phylogenetic placement of Tomistoma and Gavialis crocodiles depends largely upon whether molecular or morphological data are utilized. Molecular analyses consider them as sister taxa, whereas morphological/paleontological analyses set Gavialis apart from Tomistoma and other crocodylian species. Here skull allometric trajectories of Tomistoma and Gavialis were contrasted with those of two longirostral crocodylian taxa, Crocodylus acutus and Mecistops cataphractus, to examine similarities in growth trajectories in light of this phylogenetic controversy. Entire skull shape and its two main modules, rostrum and postrostrum, were analyzed separately. We tested differences for both multivariate angles between trajectories and for shape differences at early and late stages of development. Based on a multivariate regression of shape data and size, Tomistoma seems to possess a peculiar rate of growth in comparison to the remaining taxa. However, its morphology at both juvenile and adult sizes is always closer to those of Brevirostres crocodylians, for the entire head shape, as well as the shape of the postrostrum and rostrum. By contrast, the allometric trajectory of Gavialis always begins and ends in a unique region of the multidimensional morphospace. These findings concur with a morphological hypothesis that places Gavialis separate from Brevirostres, and Tomistoma closer to other crocodylids, and provides an additional, and independent, data set to inform on this ongoing phylogenetic discussion.