Illuminating the dark mess of fibers: Application of circular cross polarized light in unravelling the bone tissue structure of the dermal pectoral girdle of Metoposaurus krasiejowensis.

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Current understanding of the histology of the dermoskeleton of tetrapods comes from fossilized and recent remains of skulls, osteoderms, carapace, plastron and other postcranial material which were always investigated using linear cross polarized light (LCPL) microscopy. The pectoral girdle of vast majority of non-amniote tetrapods, including temnospondyls evolved large ventrally located dermal bones- the interclavicle and a pair of clavicles. Despite that, there is a lack of information about the bone tissue structure from these postcranial dermal bones. This study used circular cross polarized light (CCPL) to investigate the bone tissue composition and structure from the pectoral dermoskeleton of Metoposaurus krasiejowensis, a Late Triassic temnospondyl known to have evolved massive pectoral dermal bones which could have played a role in buoyancy control in these aquatic amphibians. This novel technique shines light into the fine structure of interwoven structural fibers (ISF), a common matrix found in ossified dermal tissues, is a mesh of loops and strands of collagen instead of a lattice patterned matrix as described previously by using LCPL in previous studies that dealt with ossified elements of dermal origin. Our result of ISF is achieved by eliminating bone fiber extinction under CCPL visualization. This feature of CCPL also sheds light into the transitional forms between interwoven and parallel-fibered matrices which was never previously observed. This study shows that the historical understanding of histology of bone tissue from skeletal dermal elements is limited not only due to lack of sampling but also due to the limitations of mineralized tissue visualization with LCPL.