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518 FOSSIL VERTEBRATES/Mesozoic Amphibians and Other Non-Amniote Tetrapods floor that underplates the occipital region, and a parasphenoid (central strut in the palate) that is extremely narrow Most had poorly ossified postcranial skeletons and small limbs and can be assumed to have been aquatic predators Early in the Late Triassic, the trematosaurids vanish from the fossil record, but two descendant groups appear, the Almasauridae and the Metoposauridae (Figure 3) Both have long skulls like the trematosaurids had, but the snouts are short and the orbits are quite near the front of the head Almasaurids were small animals, no more than a metre in length, whereas metoposaurids grew to m and had extremely flattened skulls The Trematosauroidea appears to have been a basically Laurasian group, with successful forms extending into Gondwana Evidence of the basal Benthosuchidae and primitive trematosaurs such as Thoosuchus and Angusaurus is found only in continental deposits in the Lower Triassic of Russia The more advanced trematosaurids are mostly Early Triassic, but one relict genus, Hyperokynodon, survived up to the Late Triassic in Germany Records of the more advanced trematosaurids are also found in Laurasia from Arizona to Russia, but evidence of two subfamilies exists among all of the major Gondwanan temnospondyl faunas Most are in freshwater deposits, but in Svalbard, a range of trematosaurid fossils is found in unambiguously marine deposits, providing one of the few examples of a marine amphibian fauna The Late Triassic almasaurids and metoposaurids appear to have been restricted to the floodplains associated with the developing rift valley systems across north and central Pangaea, and most fossil materials are found in North America, Europe, North Africa, and India Capitosauroidea Figure Stereospondyli Skull of the primitive tremattosauroid Benthosuchus sushkini, from the Lower Triassic, Russia Specimen at the Natural History Museum, London ß Andrew Milner The capitosauroids (also known as mastodonsauroids) were a large, successful, and comparatively structurally uniform group occurring globally throughout the Triassic, but not surviving into the Jurassic Most adults were 2–3 m long (Figure 4), but individuals in a few taxa (e.g., Mastodonsaurus) grew to a total length of 5–6 m The skull was superficially alligator-like in most genera, with a long and broad snout One capitosauroid family of note is the Cyclotosauridae, a group characterized by having the tympanic (eardrum) region of the skull surrounded by outgrowths of the neighbouring skull bones (Figure 1F) Cyclotosaurids first appear as a minor group in the early Triassic but are seen to be the predominant capitosauroid group in the Late Triassic Mastodonsaurus from the Middle Triassic of Germany is noteworthy as the first Figure Stereospondyli Skeleton of the metoposaurid Buettneria perfecta, from the Upper Triassic, North America Specimen at the American Museum of Natural History, New York ß Andrew Milner
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