The small, immature brachyopid stereospondyl Platycepsion wilkinsoni from the Early or Middle Triassic of Gosford, New South Wales, is redescribed from a developmental point of view. Whereas the sutural contact between postorbital and parietal suggested by earlier authors cannot be confirmed, a new autapomorphy, the posterior process of the interclavicle with a slightly concave posterior margin, is recognized. A further apomorphic state may be the probable presence of five pairs of ossified ceratobranchials. Soft-tissue preservation of external gills demonstrates that Platycepsion wilkinsoni represents a true larva, forming the first evidence of a larval stage in stereospondyls. Morphology and developmental stage of Platycepsion wilkinsoni indicates that brachyopids conserved the plesiomorphic ontogenetic pattern seen in most Paleozoic temnospondyls, including the early ossification of the dermal skull roof and delayed ossification of the postcranium. The strongly developed ornament, the widened lateral lines, and the well-ossified exoccipitals and stapes suggest an even faster ossification and differentiation of the skull compared to Paleozoic temnospondyls and might represent a general stereospondyl characteristic. In the vertebral column, only the poorly differentiated, still separate neural arches are ossified, whereas the centra remained cartilaginous. This implies that the plesiomorphic pattern of vertebral development was conserved in Mesozoic stereospondyls, irrespective of whether the adult vertebral morphology was rhachitomous, stereospondylous, or diplospondylous. The low degree of ossification of the postcranium, its slow differentiation, and the delayed development of the scapulocoracoid indicate an aquatic mode of life, and the strongly ossified skull and hyobranchial apparatus suggest an emphasis on powerful suction feeding.

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