Raphael Moreno

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.

The Middle–Late Triassic Grabfeld Formation formed in a generally arid, hostile setting with frequent evaporation under alternating sabkha and playa conditions. Here we report evidence of four temnospondyl taxa from the upper part of the sequence (‘Bunte Estherienschichten’), including (1) Metoposaurus sp., (2) a capitosaur, (3) Gerrothorax sp. and (4) Plagiosternum sp. This early Carnian assemblage provides the stratigraphically oldest evidence of metoposaurids and the last report of plagiosternines in the Central European Basin. The stratigraphic age of these strata and the occurrence of Metoposauridae therein sheds new light on the palaeogeographic distribution of the group and their occurrence before the Carnian Pluvial Event.

The Middle Triassic capitosaur Mastodonsaurus giganteus was the largest temnospondyl and the dominating aquatic predator in many European freshwater to brackish ecosystems. It is represented by numerous size classes, which are described and analysed for the first time. The documented size range encompasses specimens between 12–15 mm and 1200 mm in skull length. Early growth stages are restricted to dentaries and interclavicles, whereas juveniles are represented by partial skulls, mandibles, and girdles. The smallest specimens already possessed diagnostic features of the taxon, and small juveniles also shared the dermal ornament with larger specimens. The heavy, disc-shaped intercentra were established early in the juvenile phase. Cranial proportions were remarkably conservative throughout ontogeny, with the orbits proportionately decreasing in size only very moderately, the postorbital skull becoming slightly longer and the occipital margin more concave in the largest forms. Analysis of frequency distributions of M. giganteus in different Lower Keuper deposits in southern Germany reflects habitat preferences in specific phases of its life cycle. The coal-bearing deposit at Gaildorf yielded unusually large specimens with relatively well-ossified appendicular skeletons. In the more common lake shore facies, only adult specimens are present. In turn, juveniles might have dwelled in calmer environments. Smaller lakes were apparently less attractive than larger or deeper water bodies that provided sufficient resources for several temnospondyls, and juvenile specimens have been identified from all of them. The diverse actinopterygian fish fauna provided prey for all growth stages of the large temnospondyl predators.

Temnospondyls had a remarkable worldwide distribution throughout the Triassic at a time of periodic arid climates, and were a stable component of Triassic terrestrial ecosystems. Given the postulated ancestral relationship between temnospondyls and modern lissamphibians it is pertinent to recognize that the group may have exhibited some degree of ecological resilience and adaptability. Despite this higher level of plasticity, temnospondyls might still have been susceptible to environmental and climate changes. Although fossil sites are distributed sporadically, we try to circumvent the present geographical and geological biases by combining actual fossil occurrences with environmental conditions derived from general circulation models. Here, we employ species distribution modelling to examine the palaeogeographic and palaeoclimatic distribution of European temnospondyls during the transition from Middle to Late Triassic. The model shows different suitable areas for temnospondyl habitats that give new potential insights on the biogeographic distribution patterns and dispersal potential. We suggest that the Central European Basin functioned as a focal point for Triassic temnospondyl diversification and spread. Moreover, this paper provides the first application of species distribution modelling for Triassic temnospondyls and aids in understanding their climatic niche occupation and evolution.

Temnospondyl finds rank among the most abundant remains throughout the continental Triassic deposits. In the Central European Basin, they are especially numerous, with diversity peaks in the Röt, Erfurt and Stuttgart formations. Currently, a total of 20 genera and 29 species of temnospondyls is recognized in the German Triassic. The Early Triassic temnospondyl assemblages encompass the five genera Trematosaurus, Sclerothorax, Odenwaldia, Meyerosuchus, and Parotosuchus. Trematosaurus is present with the coeval species T. brauni and T. thuringiensis whereas Parotosuchus is known with the three species P. helgolandicus, P. nasutus and the newly erected P. decumanticus n. sp. from the Black Forest from different time slices. Sclerothorax is known from two widely divergent horizons in the early and late Olenekian of Hesse, whereas Odenwaldia and fragmentarily known Meyerosuchus are from localities in the Black Forest and almost coeval horizons dating late Olenekian. In the Middle Triassic the ten temnospondyl genera Plagiosuchus, Plagiosternum, Megalophthalma, Gerrothorax, Trematolestes, Callistomordax, Eocyclotosaurus, Stenotosaurus, Tatrasuchus and Mastodonsaurus (two species) are recognized. Kupferzellia Schoch, 1997 is a junior synonym of Tatrasuchus Maryanska & Shishkin, 1996. The coeval early Anisian Eocyclotosaurus and Stenotosaurus are morphologically similar but distinct in palate features, whereas the other taxa all fall within the late Ladinian interval of the Erfurt Formation. The lost type and only specimen of “Cyclotosaurus” papilio was most consistent with the genus Eocyclotosaurus. The Late Triassic rock sequence has yielded the six valid genera Plagiosaurus, Gerrothorax, hyperokynodon, Metoposaurus, Capitosaurus and Cyclotosaurus, the latter with the five species C. robustus, C. buechneri, C. ebrachensis, C. posthumus and C. hemprichi. In the Rhaetian, reworked remains may only be referred to indeterminate plagiosaurines and capitosauroids.