Reconstructing the major skull myology of the giant stem-amphibian Mastodonsaurus: A case study for early tetrapods.

The Devonian fish-to-tetrapod transition and the subsequent conquest of land involved multiple amphibious stem and early tetrapod lineages, yet how these animals reduced their dependence on aquatic environments remains uncertain. Feeding is central to this transition, as early tetrapods likely moved from predominantly aquatic suction feeding to jaw-based prey prehension and processing. Although recent functional approaches-including morphometrics, functional morphology and computational modelling-offer new perspectives, their interpretive strength depends on realistic soft-tissue inputs, particularly cranial musculature. Here, we reconstruct the principal cranio-mandibular (skull) musculature of the Middle Triassic capitosaur temnospondyl Mastodonsaurus giganteus to provide an explicit anatomical foundation for future functional and modelling studies of stem and early tetrapods and to refine ecological interpretations for this species. We formulate alternative muscle hypotheses within an extant phylogenetic bracket (EPB) framework that explicitly reflects uncertainty in temnospondyl placement (temnospondyl versus lepospondyl origin of lissamphibians). We developed hypotheses from a literature review and digital dissections of contrast-enhanced soft-tissue μCT datasets from selected extant bracket taxa and outgroups and assessed them against osteological correlates in M. giganteus. Our reconstruction indicates a more differentiated adductor system than commonly assumed, with subdivision of the adductor mandibulae externus and internus complexes and an inferred intramandibular component consistent with a cartilaginous sesamoid ('cartilago transiliens') functioning as a force-transmitting pulley. We further provide osteological evidence for previously unrecognised pterygoideus components, suggesting additional jaw-closing capacity and a potential contribution to rapid gape formation. The 'tongue' in M. giganteus was likely comparatively weak and less flexibly movable, while as a stereospondyl, it possessed a comparatively strong neck and specialised anterior ribs. Collectively, these traits are consistent with a mainly crocodile-like bite-and-hold prehension ecology and likely indicate kinetic-inertial feeding. Our muscle reconstruction further suggests a more balanced jaw-opening/closing system that may have enabled compensatory suction during gape formation.