Mandible histology in Metoposaurus krasiejowensis (Temnospondyli, Stereospondyli) from the Upper Triassic of Poland

Recent studies that have systematically augmented our knowledge of dermal bones of the Late Triassic temnospondyl amphibian Metoposaurus krasiejowensis have mostly focused on shoulder girdle elements and the skull. So far, histological data on the mandible are still scant. For the present study, two mandibles have been examined, using 50 standard thin sections. Dermal bones of the mandible reveal a uniform diploë structure, with the external cortex consisting of moderately vascularised, parallel-fibred bone, as well as a distinct alternation of thick zones and thinner annuli. Dense bundles of well-mineralised Sharpey’s fibres are seen in the external cortex over the entire length of the mandible. The trabecular middle region is highly porous and well vascularised, showing small primary vascular canals and more numerous secondary osteons; irregular erosion spaces occur in large numbers as well. The thin and poorly vascular internal cortex consists of parallel-fibred bone. The articular is not a dermal bone in origin, having been formed of a thin layer of avascular cortex and a very extensive, trabecular middle region. In contrast to the dermal bones of the mandible, the articular developed from a cartilaginous precursor, as evidenced by numerous remains of calcified cartilage in the central parts of the bone. Histological variability is extremely high along the mandible, its anterior part being characterised by high compactness and biomechanically good resistance in contrast to the highly porous posterior parts. Distinct variations of bone thickness and degree of bone porosity in specific areas of the mandible, may be due to local differences in biomechanics during feeding. The microstructure of the mandible corroborates a previous study of the active and ambush predation strategy in metoposaurids.

Ornamentation of dermal bones of Metoposaurus krasiejowensis and its ecological implications

Background Amphibians are animals strongly dependent on environmental conditions, like temperature, water accessibility, and the trophic state of the reservoirs. Thus, they can be used in modern palaeoenvironmental analysis, reflecting ecological condition of the biotope. Methods To analyse the observed diversity in the temnospondyl Metoposaurus krasiejowensis from Late Triassic deposits in Krasiejów (Opole Voivodeship, Poland), the characteristics of the ornamentation (such as grooves, ridges, tubercules) of 25 clavicles and 13 skulls were observed on macro- and microscales, including the use of a scanning electron microscope for high magnification. The different ornamentation patterns found in these bones have been used for taxonomical and ecological studies of inter- vs. intraspecific variation. Results Two distinct types of ornamentation (fine, regular and sparse, or coarse, irregular and dense) were found, indicating either taxonomical, ecological, individual, or ontogenetic variation, or sexual dimorphism in M. krasiejowensis. Discussion Analogies with modern Anura and Urodela, along to previous studies on temnospondyls amphibians and the geology of the Krasiejów site suggest that the differences found are rather intraspecific and may suggest ecological adaptations. Sexual dimorphism and ontogeny cannot be undoubtedly excluded, but ecological variation between populations of different environments or facultative neoteny (paedomorphism) in part of the population (with types of ornamentations being adaptations to a more aquatic or a more terrestrial lifestyle) are the most plausible explanations.