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FOSSIL VERTEBRATES/Mesozoic Amphibians and Other Non-Amniote Tetrapods 519 Figure Stereospondyli Skull of the plagiosaurid Gerrothorax rhaeticus, from the Upper Triassic, Germany Specimen at the Staatliches Museum fur Naturkunde in Stuttgart ß Andrew Milner Figure Stereospondyli (A) Reconstruction of the capito saurid Paracyclotosaurus davidi, from the Middle Triassic, New South Wales, Australia; (B) reconstruction of the chigutisaurid Siderops kehli, from the Lower Jurassic, Queensland, Australia Reproduced with permission from (A) Watson, DMS (1958) A new labyrinthodont (Paracyclotosaurus) from the Upper Trias of New South Wales Bulletin of the British Museum (Natural History) Geological Series 3, 233 263; (B) Warren AA and Hutchinson MN (1983) The last labyrinthodont? A new brachyopoid (Amphibia, Temnospon dyli) fron the early Jurassic Evergreen Formation of Queensland, Australia Philosophical Transactions of the Royal Society, London Series B 303, 62 archaic fossil amphibian to be recognized and described (in 1828) Capitosauroidea may have originated in Gondwana, where the most primitive genus, Watsonisuchus, is known to have existed, but much of the record of this group suggests a uniform presence across Pangaea The only subgroup that may be local in distribution is the Mastodonsauridae (sensu stricto), which has not yet been recognized outside Eurasia Enigmatic Paedomorphic Families Three other families of temnospondyls represent very specialized aquatic forms that have apparently evolved by paedomorphosis (maintaining the larval body form throughout growth to a large size) The relationships between these three groups (Plagiosauridae, Chigutisauridae, and Brachyopidae) are controversial and they are treated here as three distinct groups of uncertain affinities The Plagiosauridae were one of the strangestlooking groups of temnospondyls; they were extremely flat, having a very wide and shallow gape, massive orbits (Figure 5), and vertebrae suggestive of a vertically undulating back, and they were covered in a chain-mail-like armour both dorsally and ventrally The branchial (gill) arches were large and wellossified and they clearly had a large internal gill apparatus Most were less than 50 cm long, but one large fossil specimen from Svalbard is at least m long They are believed to be a paedomorphic lineage evolved from a more terrestrial stereospondyl group, and two quite different terrestrial genera, Peltobatrachus and Laidleria, have both been proposed as the terrestrial relatives of the plagiosaurs Almost all plagiosaurids are known from Eurasia and Greenland with some fragments having been found in Thailand and Australia The Chigutisauridae were large stereospondyls with semicircular skulls bearing a suction–gulping feeding apparatus and small, anteriorly placed orbits (Figure 4B) They may be paedomorphic relatives of the Rhytidosteidae All Triassic chigutisaurids are from the Upper Triassic of Gondwana The Brachyopidae are similar (Figure 6), but much of the resemblance may be convergence and they have some similarities to the Tupilakosauridae (see later), which are certainly not stereospondyls Most Triassic brachyopids are from Gondwana, with one problematic genus (Virgilius) from Arizona The chigutisaurids and brachyopids occur at low taxonomic diversity through their history Other Temnospondyls A few non-stereospondyl lineages of temnospondyls are known to have survived the P–T event and persisted as relicts for a short period in the Lower
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