Thadeosaurus colcanapi (Carroll 1981) Late Permian, ~260 mya, ~60 cm in length, is the most primitive of the Younginiformes, a clade that included protorosaurs and archosauriforms (including dinos, birds and crocs). Derived from a sister to Claudiosaurusand Petrolacosaurus, Thadeosaurus phylogenetically preceded Acerosodontosaurus.
Distinct from Petrolacosaurus, the skull of Thadeosaurus is known only from a crushed juvenile skull that exposes only the palate and skull roof in ventral view. The ectopterygoid is separate from the palatine.
The neck was long and robust 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 to form one large scapulocoracoid, larger than the pelvis. The manual phalanges were nearly all the same length. The unguals were sharp.
Pedal digit V was very long and subequal to IV. Metatarsals II and III aligned with the middle of p1.1.
Like Mesosaurus, Thadeosaurus had a tail that could have been used for swimming, but the sharp claws indicate it was not a full-time swimmer. Many specimens are known including several juveniles.