Exploring the paleobiology of ammonoids (Cretaceous, Antarctica) using non-invasive imaging methods

Abstract

Based on medical computed tomography (CT) data, herein I document the ontogenetic changes in conch morphology of Maorites densicostatus and M. seymourianus. For this purpose, 20 specimens were measured in 30° steps starting from a shell diameter of 10 mm, to obtain the ontogenetic trajectories of several conch parameters. Results show that four morphological parameters, aperture height, whorl height, whorl width, and umbilical width have a low variation and can be easily modeled. In contrast, derived indices and whorl expansion rates show a high variation, and their ontogenetic trajectories can be categorized in two stages: the first stage is identified by a rapid morphological change (Perlatum stage), followed by a second stage showing a stable conch morphology (Gibbosum stage). Furthermore, the ontogenetic trajectories revealed that in adulthood, M. densicostatus exhibits similar morphologies to juveniles and sub-adults of M. seymourianus. This phenomenon could indicate a possible sizebased heterochronic shift (paedomorphosis) as a possible evolutionary mechanism.