Temporal range: Late Triassic
Cyclotosaurus is an extinct genus of temnospondyl within the family Mastodonsauridae. It was of great size for an amphibian, reaching 3–4.3 m (9–14 ft) in length with an elongated 70 cm (28 in) skull.
German naturalist Eberhard Fraas erected the genus Cyclotosaurus in 1889, with C. robustus (previously Mastodonsaurus robustus) as the type species. Several species are known, mainly from Germany and Poland in Central Europe, as well as East Greenland and Thailand. The relationships between species is unclear. The English zoologist Richard Owen described a species "Labyrinthodon pachygnathus" (Cyclotosaurus pachygnathus) of uncertain affinity in 1842.
The oldest species is Cyclotosaurus papilio, known from a partial skull recovered from the Ladinian (Middle Triassic) age Upper Muschelkalk beds from Baden-Württemberg in Germany. Cyclotosaurus robustus is known from the Carnian (Late Triassic) Schilfsandstein Formation in Stuttgart-Feuerbach in Germany, while C. ebrachensis has been described from the Blasensandstein Formation in Ebrach. Cyclotosaurus intermedius has been described from lacustrine deposits dated to the late Carnian in Krasiejów in southern Poland. It is so named as it has features intermediate between the more ancient C. robustus and more recent C. mordax. Importantly, postcranial material of this species has been recovered, which is unusual this genus. Cyclotosaurus carinidens is known from the Norian (Late Triassic) age Knollenmergel of Halberstadt, and Cyclotosaurus posthumus from the Stubensandstein Formation (Norian) in Pfaffenhofen. A partial skull very similar to C. posthumus has been recovered from the Norian (Late Triassic) Huai Hin Lat Formation near Chulabhorn Dam in Northeastern Thailand.
Cyclotosaurus are thought to have been semi-aquatic carnivores, though feeding strategies likely differed between species.
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