Postcranial morphology and ontogeny of the Middle Triassic plagiosaurid temnospondyl Gerrothorax pulcherrimus and the increased body-flattening of bottom-dwelling plagiosaurs

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Abstract In order to understand the evolutionary modifications of the skeleton in plagiosaurids, a group of highly aberrant, flattened Triassic temnospondyls, a comprehensive morphological and ontogenetic description of the postcranium of the Middle to Late Triassic plagiosaurid Gerrothorax pulcherrimus (Fraas, 1913) is provided. Character evolution towards and within plagiosaurids involved (i) increasing size and bone thickness of the dermal pectoral girdle, (ii) broadening of the trunk, and (iii) successive flattening of the entire body, correlating with the reduced curvature of the ribs, flattening of the scapula and cleithrum as well as of the ilium and puboischium. The humerus and femur were small but well differentiated. The forelimbs probably served to thrust the anterior body away from the ground especially during the feeding strike, whereas the low caudal neural and haemal arches do not suggest a deep tail fin. Although the complete body was covered by dermal armour, some flexibility between the osteoderms allowed for a certain degree of mobility. Its body proportions indicate that G. pulcherrimus was not a particularly able swimmer and its extreme degree of body flattening suggests a rather passive lifestyle as an ambush predator. It inhabited oligohaline to miohaline water bodies where it preferred oxygen-rich littoral environments.

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