PG-18: turtles reach adult shell shapes at about 65% maximum carapace length.

Ontogenetic shell shape changes of turtles are often only documented for individual species. It is currently unclear how shell shape changes during ontogeny across species, if there are common trends, and at what point in ontogeny individuals reach their adult morphology. Inspired by questions of whether some morphologies are too juvenile to be included into macroevolutionary studies of shell shape, we develop ontogenetic shell shape curves based on landmarked 3D shell shapes of turtles. Species-specific allometric shape regressions confirm that turtles show marked ontogenetic shell shape change. Geometric morphometric analysis shows that juvenile turtles have rounded shells, and ontogenetic differentiation between species increases adult turtle disparity. Disparity analysis indicates that juvenile shells across turtle clades are more similar than adult shapes, suggesting an important role of developmental constraints on early turtle shell shape, and possible adaptive post-natal ontogenetic changes that produce the observed adult shell shape disparity. Ontogenetic shell shape curves indicate when turtles converge onto adult morphologies, here quantified as 85% the distance between juvenile shape and maximum size adult shape. This happens at about 65% of the species-specific maximum carapace sizes. Sexual shell shape dimorphism is comparatively low across turtles even in the presence of pronounced sexual size dimorphism. These preliminary results provide guidance for studying shell shape macroevolution, but need to be scrutinized further in the future by data addition. Supplementary Information: The online version contains supplementary material available at 10.1186/s13358-025-00395-0.