Thadeosaurus

Thadeosaurus is an extinct genus of diapsid reptiles from the late Permian Lower Sakamena Formation (Sakamena Group) of Madagascar. The genus contains a single species, Thadeosaurus colcanapi, known from several specimens preserved as natural molds.

Thadeosaurus Temporal range: Late Permian, ?Capitanian–Lopingian PreꞒ Ꞓ O S D C P T J K Pg N
Speculative life restoration
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Diapsida
Clade:	Neodiapsida
Family:	†Younginidae
Genus:	†Thadeosaurus Carroll, 1981
Type species
†Thadeosaurus colcanapi Carroll, 1981

Discovery and naming

The generic name, Thadeosaurus, is an anagram of "Datheosaurus", a synapsid genus to which fossils of the former were initially referred. The specific name, colcanapi, honors J.-M. Colcanap, a French infantry captain and the discoverer of the holotype specimen.^[1][2]

Description

Size compared to a human hand

Thadeosaurus was a superficially lizard-like reptile, with a remarkably long tail that comprised about two-thirds of the animal's total length of 60 centimetres (24 in). It had long toes, especially on the hind legs, and a strong breast bone.^[1][3][2]

Classification

The relationships of Thadeosaurus have been debated since its 1981 description. Prior to receiving a name, the fossil material was provisionally referred to Broomia (now recognized as a millerettid^[4]), Tangasaurus, and Datheosaurus (now recognized as a caseid synapsid). In his 1981 publication naming Thadeosaurus and Claudiosaurus, Carroll noted similarities between Thadeosaurus and Youngina, but opted to describe it as a 'primitive' sauropterygian—an 'ancestral taxon' to nothosaurs and plesiosaurs.^[1]

In the description of the early Permian reptile Orovenator, the phylogenetic results of Reisz et al. (2011) suggested a close relationship between Thadeosaurus and Youngina, united in the family Younginidae. These results are displayed in the cladogram below:^[5]

	Captorhinidae Diapsida Araeoscelidia Neodiapsida Orovenator Lanthanolania Tangasauridae Tangasaurus Acerosodontosaurus Hovasaurus Younginidae Thadeosaurus Youngina Claudiosaurus Sauria Archosauromorpha Lepidosauromorpha

In 2025, Valentin Buffa and colleagues thoroughly redescribed the fossil material assigned to Thadeosaurus, and reassessed its phylogenetic position. They identified it as a member of the neodiapsid family Tangasauridae, as the sister taxon to the clade formed by Hovasaurus and Tangasaurus, a position also supported by Philip J. Currie in a publication redescribing Tangasaurus.^[6] The results of the strict consensus phylogenetic results of Buffa et al. (2025) are displayed in the cladogram below:^[2]

Diapsida	Araeoscelidia Varanopidae Neoreptilia Parareptilia Neodiapsida Youngina Tangasauridae Kenyasaurus Acerosodontosaurus Thadeosaurus Tangasaurinae Hovasaurus Tangasaurus Claudiosaurus Weigeltisauridae Weigeltisaurus Coelurosauravus Rautiania spp. Sauria Paliguana Lepidosauria Archosauromorpha