Proterosuchia refers to one of the suborders of Thecodont

Thecodont

Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period...

ia; specifically the most primitive and ancestral forms. These were primitive, vaguely crocodile-like, archosaur

Archosaur

Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

s that mostly lived during the Early Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 period.

The name Proterosuchia was coined by Robert Broom

Robert Broom

Professor Robert Broom was a Scottish South African doctor and paleontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow...

 in 1906. In later classifications, several families are included, such as the Proterosuchidae

Proterosuchidae

Proterosuchidae is an early, possibly paraphyletic, assemblage of basal archosauriformes whose fossils are known from the Latest Permian of Russia and the Early Triassic of southern Africa, Russia, China, Australia, and Antarctica...

, Erythrosuchidae

Erythrosuchidae

Erythrosuchidae are a family of large basal archosauromorph carnivores that lived from the later Early Triassic to the early Middle Triassic . Their fossil remains are known so far from South Africa , the Perm region of Russia, and China...

, and Proterochampsidae

Proterochampsidae

Proterochampsidae is a family of archosauriforms. Proterochampsids may have filled an ecological niche similar to modern crocodiles, and had a general crocodile-like appearance. They lived in what is now South America in the Middle and Late Triassic....

.

Under the cladistic system, this is a paraphyletic grade, rather than a natural group, the name is no longer used, although it can be found in many textbooks (up to and including Carroll

Robert L. Carroll

Robert Lynn Carroll is a vertebrate paleontologist who specialises in Paleozoic and Mesozoic amphibians and reptiles.Carroll was an only child and grew up on a farm near Lansing, Michigan...

's Vertebrate Paleontology and Evolution

Vertebrate Paleontology and Evolution

Vertebrate Paleontology and Evolution is an advanced textbook on vertebrate paleontology by Robert L. Carroll, published by WH Freeman. It provides a very detailed technical account of various groups of living and fossil vertebrates....

)

In current phylogenetic understanding, the Proterosuchia constitute the basal Archosauriformes

Archosauriformes

Archosauriformes is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Late Permian...

; that is, the archosauriform groups but excluding the true Archosauria.