The Stereospondyli are a group of extinct temnospondylamphibians. Relative to other early tetrapods (labyrinthodonts), they had simplified backbones, where the whole vertebra was made of a single intercentrum, topped by a neural arch. The whole vertebral structure was rather weak, meaning that most stereospondyls were aquatic. Indeed, the family Plagiosauridae appear to have been wholly aquatic and retained their larval gills in adulthood.
Stereospondyls were the latest-surviving temnospondyl group. With the diversification of crocodile-like archosaurs and an extinction event at the end of the Triassic, most other temnospondyls disappeared. Two families of stereospondyls survived into the Jurassic, and some appear to have survived into the Early Cretaceous in Gondwana. Koolasuchus is the latest known stereospondyl, living during the late Early Cretaceous in what is now Australia in areas that would then have been too cold for crocodiles.
Yates, A. M. & Warren, A. A. (2000), The phylogeny of the 'higher' temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zoological Journal of the Linnean Society 128: 77-121.