Thadeosaurus colcanapi (Carroll 1981) Late Permian, ~260 mya, ~60 cm in length, nests between Acerosodontosaurus and Claudiosaurus in the marine branch of the Younginiformes. Tangasaurus and Hovasaurus are sister taxa.
The skull of Thadeosaurus is known only from a crushed juvenile skull that exposes only the palate and skull roof in ventral view. The skull appears to be quite similar to that of Acerosodontosaurus.
The neck was short and small with eight vertebrae. The deep but narrow sculling tail was twice as long as the rest of the body.
The scapula and coracoid were fused only in adults.
Pedal digit V was very long and subequal to IV. Metatarsals II and III aligned with the middle of p1.1.
Many specimens are known including several juveniles.