''Thadeosaurus'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the araeosc ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

s from the late

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

Lower Sakamena Formation Lower may refer to: * ''Lower'' (album), 2025 album by Benjamin Booker *Lower (surname) *Lower Township, New Jersey *Lower Receiver (firearms) *Lower Wick Lower Wick is a small hamlet located in the county of Gloucestershire, England. It is sit ...

( Sakamena Group) of Madagascar. The genus contains a single species, ''Thadeosaurus colcanapi'', known from several specimens preserved as natural molds.

Discovery and naming

The generic name, ''Thadeosaurus'', is an

anagram An anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once. For example, the word ''anagram'' itself can be rearranged into the phrase "nag a ram"; which ...

of "'' Datheosaurus''", a

synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...

genus to which fossils of the former were initially referred. The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

, ''colcanapi'', honors J.-M. Colcanap, a French infantry captain and the discoverer of the holotype specimen.

Description

''Thadeosaurus'' was a superficially

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

-like reptile, with a remarkably long tail that comprised about two-thirds of the animal's total length of . It had long toes, especially on the hind legs, and a strong

breast bone The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood ve ...

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), ''

Tangasaurus ''Tangasaurus'' is an extinct genus of aquatic animal, aquatic basal (phylogenetics), basal tangasaurid neodiapsid known from the Late Permian geological timescale, period (late Changhsingian stage) of Tanga Region, Tanga, northeastern Tanzania. ...

'', and '' Datheosaurus'' (now recognized as a

caseid Caseidae are an extinct family of basal synapsids that lived from the Late Carboniferous to Middle Permian between about 300 and 265 million years ago. Fossils of these animals come from the south-central part of the United States (Texas, Oklaho ...

). In his 1981 publication naming ''Thadeosaurus'' and ''

Claudiosaurus ''Claudiosaurus'' (''claudus'' is Latin for 'lameness' and ''saurus'' means 'lizard') is an extinct genus of diapsid reptiles from the Late Permian Sakamena Formation of the Morondava Basin, Madagascar. It has been suggested to be semi-aquatic. ...

'', Carroll noted similarities between ''Thadeosaurus'' and ''

Youngina ''Youngina'' (named after John Young (1823–1900)) is an extinct genus of diapsid reptile from the Late Permian Beaufort Group ('' Tropidostoma''-'' Dicynodon'' zones) of the Karoo Red Beds of South Africa. This, and a few related forms, mak ...

'', but opted to describe it as a 'primitive'

sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosau ...

n—an 'ancestral taxon' to

nothosaur Nothosaurs (superfamily Nothosauroidea) were Triassic marine sauropterygian reptiles. They averaged about in length, with a long body and tail. The feet were paddle-like, and are known to have been webbed in life, to help power the animal when sw ...

s and

plesiosaur The Plesiosauria or plesiosaurs are an Order (biology), order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period (geology), Period, possibly in the Rhaetian st ...

s. In the description of the early Permian reptile ''

Orovenator ''Orovenator'' is an extinct genus of diapsid from Lower Permian ( Artinskian stage) deposits of Oklahoma, United States. It is known from two partial skulls from the Richards Spur locality in Oklahoma. The holotype OMNH 74606 consists o ...

'', the

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

results of Reisz et al. (2011) suggested a close relationship between ''Thadeosaurus'' and ''Youngina'', united in the family

Younginidae Younginidae is an extinct family of diapsid reptiles from the Late Permian. In a phylogenetic context, younginids are near the base of the clade Neodiapsida. The only species definitively a member of the Younginidae is '' Youngina capensis'' fro ...

. These results are displayed in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below: 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 Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The earliest traditionally identified diapsids, the Araeosceli ...

family

Tangasauridae Tangasauridae is an extinct family of diapsids known from fossil specimens from Madagascar, Kenya and Tanzania that are Late Permian to Early Triassic in age. Fossils have been found of numerous specimens of common members of this family such as ...

, as the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the clade formed by ''

Hovasaurus ''Hovasaurus'' is an extinct genus of basal diapsid reptile. It lived in what is now Madagascar during the Late Permian and Early Triassic, being a survivor of the Permian–Triassic extinction event and the paleontologically youngest member of t ...

'' and ''

'', a position also supported by Philip J. Currie in a publication redescribing ''Tangasaurus''. The results of the strict consensus phylogenetic results of Buffa et al. (2025) are displayed in the

below:

References

Diapsids Prehistoric reptile genera Permian reptiles of Africa Fossils of Madagascar Permian Madagascar Fossil taxa described in 1981 {{permian-reptile-stub