• geometric morphometrics;
  • operculum;
  • Triassic;
  • Monte San Giorgio;
  • taxonomy


Despite an impressive radiation of more than 30 species in the wake of the end-Permian mass extinction, the taxonomic study of Saurichthys has suffered from a lack of universally diagnostic features and a lack of tested quantitative schemes that can be applied to analyse interspecific morphological differences. In this study, we provide an initial quantitative framework for morphological evolution in Saurichthys by focusing on a single bone, the opercle and exploring patterns of interspecific variability in shape using outline-based geometric morphometrics and linear measurements. For the six species examined, comprising 155 specimens and representatives from the Early, Middle and Late Triassic, our results indicate that interspecific shape differences largely reflect an anterior–posterior dimension decrease (= craniocaudal direction) as the dorso-ventral dimension remains similar. In contrast, intraspecific variability in shape is subtle and spread across the outline of the bone, such that counter-acting dimension differences (increase/decrease) were found to occur along a single margin at oblique axes in several species. Our quantitative scheme, which is widely applicable to other groups, provides a useful description of the broad modes of opercle shape change that may help as a starting framework from which to develop character states for opercle morphology in future study.