Morphospace occupation of temnospondyl growth series: a geometric morphometric approach

Abstract

A morphometric analysis based on 16 landmarks (specific geometric points) identified on the skull roof of different growth stages of various temnospondyls revealed patterns of shape change during ontogeny. Data from 50 individuals from 12 taxa, encompassing euskelians (dissorophoids, eryopids and zatracheids) and stereospondylomorphs were plotted in a morphospace diagram defined by two principal-component axes (PC 1 and PC 2) of a principal-component analysis of the shape variables. The morphospace occupation reflects phylogenetic relationships of the main clades of temnospondyls. All stereospondylomorphs investigated here have similar ontogenetic trajectories with long intervals on PC 1. Eryopids and zatracheids have shortened their trajectories on PC 1 and are separated in morphospace relative to the stereospondylomorphs and dissorophoids. Outgroup comparison with Cochleosaurus bohemicus (Frič, 1876) suggests that a long ontogenetic trajectory with gradual development represents the plesiomorphic condition for temnospondyls. More derived stereospondylomorphs (among the taxa considered here, but excluding groups such as derived rhytidosteids and brachyopoids) increased the length of their trajectories on PC 1 and show a peramorphic pattern, resulting in the long and slender skull of piscivorous predators. Dissorophoids exhibit truncated ontogenetic trajectories and occupy the morphospace region of larval stereospondylomorphs, thus attaining a paedomorphic appearance probably by hypomorphosis. Amphibamid and branchiosaurid dissorophoids have clearly distinguishable larval and postlarval developments, indicating that these forms had evolved a lissamphibian-like type of metamorphosis.