FOSSIL VERTEBRATES/Mesozoic Amphibians and Other Non-Amniote Tetrapods 517 Temnospondyls in the Triassic Stereospondyli The stereospondyls comprise one of the largest subgroups of the amphibian group Temnospondyli They were mostly 1- to 2-m-long aquatic and amphibious carnivores filling a broadly crocodile-like niche The ‘stereospondylous’ type of vertebra, after which the group is named, is one in which the anterior central element, the intercentrum, enlarges to form the entire central body (Figure 1A) In fact, this structure characterizes only a few genera (including some of the first to be discovered, hence its original use as a characteristic) One of the most obvious, though not unique, characteristics of the group is that the braincase is broadly sutured to the lateral palatal bones (pterygoids) on either side The stereospondyl radiation had commenced diversification in the Late Permian but is one of the characteristic elements of continental and coastal Triassic faunas There is not full agreement on the classification of the stereospondyls, but three major groups, Rhytidosteoidea, Trematosauroidea, and Capitosauroidea, have recognized by several workers, together with three families of controversial relationships Rhytidosteoidea All of the members included with certainty in the Rhytidosteoidea are restricted to the Late Permian and Early Triassic The basal forms are in the family Lydekkerinidae; these small forms are less than m long and have orbits set in the middle of a blunt alligator-like skull (Figure 1B) They appear to have been amphibious animals The more advanced Rhytidosteidae were more specialized aquatic forms, with small laterally placed orbits and either very triangular skulls (Figure 1C) or flattened rounded skulls This appears to be a fundamentally Gondwanan group, with a few lineages penetrating into Laurasia Most lydekkerinids and rhytidosteids are from the Lower Triassic of South Africa, Madagascar, India, Australia, and Antarctica, with fossil material recently reported from Brazil Three genera are known from Greenland, Svalbard, and North Russia Trematosauroidea Figure Stereospondyli (A) Mastodonsaurus vertebra, demon strating the stereospondyl condition, with an enlarged intercen trum and rudimentary pleurocentra; (B) skull of the lydekkerinid Lydekkerina huxleyi; (C) skull of the rhytidosteoid Peltostega erici; (D) skull of the trematosauroid Lyrocephaliscus euri; (E) skull of the trematosauroid Aphaneramma rostratum; (F) skull of the capito sauroid Cyclotosaurus robustus All reproduced with permission from Schoch RR and Milner AR (2000) Stereospondyli In: Well nhofer P (ed.) Handbuch der Palaăoherpetologie, part 3B Munich: Pfeil The trematosauroids are an entirely Triassic clade of temnospondyls, first appearing after the P–T extinction and last appearing in the mid-Norian The stem of the group is the Benthosuchidae, known largely from fossil material from Russia Benthosuchids were a lineage of crocodile-like temnospondyls with elongate triangular skulls, and the most well-known taxon, Benthosuchus (Figure 2), is known from many hundreds of specimens preserved in three-dimensional form from the Sharzhenga River region This taxon was described fully in 1940 by Bystrow and Efremov in a monograph that still serves as a standard for descriptions of this group The group adaptively radiated in the Early Triassic as the family Trematosauridae The skull shapes within this family vary from an almost isosceles triangle shape (Lyrocephaliscus, from Svalbard; Figure 1D) to an extreme elongation of the snout that results in a gharial-like head (Aphaneramma, also from Svalbard; Figure 1E) All group members are characterized by a distinctive pattern of elongation of the back of the skull roof, small, laterally placed orbits, a braincase