Protorosauria is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

, likely

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

group of basal archosauromorph reptiles from the latest Middle Permian ( Capitanian stage) to the end of the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

( Rhaetian stage) of

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and

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. It was named by the English

anatomist Anatomy () is the branch of morphology concerned with the study of the internal structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old scien ...

and paleontologist Thomas Henry Huxley in 1871 as an order, originally to solely contain '' Protorosaurus''. Other names which were once considered equivalent to Protorosauria include Prolacertiformes and Prolacertilia. Protorosaurs are distinguished by their long necks formed by elongated

cervical vertebrae In tetrapods, cervical vertebrae (: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In saurop ...

, which have ribs that extend backward to the vertebrae behind them. Protorosaurs also have a gap between the quadrate bones and the jugal bones in the back of the skull near the jaw joint, making their skulls resemble those of

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 ...

s. While previously thought to be

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

, the group is now thought to consist of various groups of basal archosauromorph reptiles that lie outside Crocopoda, though some recent studies have recovered the group as monophyletic. A number of members of Protosauria have been found to belong to a monophyletic group (though not including ''Protorosaurus'') which was named Tanysauria in 2024.

Classification

Protorosauria was considered to be a synonym of Prolacertiformes for many years. Since 1998, many phylogenetic analyses have found Protorosauria, as used in its widest sense, to be a

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

taxon. ''Protorosaurus'', '' Macrocnemus'', tanystropheids, and various other protorosaurs are usually placed near the base of Archosauromorpha, while '' Prolacerta'' and '' Pamelaria'', two

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

n Triassic protorosaurs, are now thought to be in a more derived position as close relatives of Archosauriformes. Most

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 ...

analyses since 1998 have found a strongly supported clade that includes only the genus '' Prolacerta'' and the Archosauriformes. For this reason ''Prolacerta'', ''Pamelaria'', and several other related forms (collectively called prolacertids) have been removed from Protorosauria. Because the name Prolacertiformes is defined based on the genus ''Prolacerta'', the name Protorosauria is used for the remaining group. Only recently has Protorosauria been defined in a

sense as the most inclusive

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

containing taxa such as '' Protorosaurus'', '' Macrocnemus'', and '' Tanystropheus''. Analyses, such as Dilkes (1998), Sues (2003), Modesto & Sues (2004), Rieppel, Fraser & Nosotti (2003), Rieppel, Li & Fraser (2008), Gottmann-Quesada and Sander (2009) and Renesto ''et al.'' (2010), recovered a large Protorosauria, that includes '' Protorosaurus'', Drepanosauridae (and relatives) and Tanystropheidae (and relatives). However, some analysis found ''Protorosaurus'' (and sometimes the closely related '' Czatkowiella'') to be more advanced or more basal than the node Drepanosauridae+Tanystropheidae, but always more basal than ''Prolacerta''. Some studies still use the term Prolacertiformes to include prolacertids and traditional protorosaurs, while restricting the term Protorosauria to the smallest clade that includes ''Protorosaurus'', ''Macrocnemus'', and ''Tanystropheus''; thus Protorosauria is a true clade, while Prolacertiformes is an evolutionary grade of early archosauromorphs. Pritchard ''et al.'' (2015), Nesbitt ''et al.'' (2015), Ezcurra (2016) and Spiekman et al., 2021 found that even this definition of Protorosauria, like Prolacertiformes, was an unnatural group of various non-Crocopodan archosauromorphs. These studies found that tanystropheids were archosauromorphs more closely related to crocopods than to ''Protorosaurus''. Nevertheless, Ezcurra noted that archosauromorph systematics required further study, and that phylogenetic support for Protorosauria being a natural group was only barely weaker than the support for the group being unnatural.

Included groups

The Protorosauria includes the Permian genus '' Protorosaurus'', closely related to '' Czatkowiella''. A wide variety of Permian and Triassic reptiles have been classified within Protorosauria, including the arboreal gliding reptile '' Sharovipteryx'' and the aquatic tanystropheids, which have extremely long necks. Another enigmatic group of Triassic reptiles, the Drepanosauromorpha, have often been classified as belonging to the Protorosauria. Pterosaurs have also been proposed as protorosaurs or close relatives of them, although they are now regarded as a more derived group of archosaurs. While Senter (2004) reassigned the bizarre, arboreal drepanosaurids and '' Longisquama'' to a group of more primitive diapsids called Avicephala, subsequent studies failed to find the same result, instead supporting the hypothesis that they were protorosaurs.

Cladogram

The following

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 ...

shows the position of Protorosauria among the Sauria sensu Sean P. Modesto and Hans-Dieter Sues (2004). Most recent studies have recovered Protorosauria as a whole as a paraphyletic, cladogram after Spiekman et al. 2021 Although Protorosauria as a whole is often found to be a paraphyletic, a large group of former "protorosaurs" (excluding ''Protorosaurus'') is frequently found to be monophyletic. This clade was given the name " Tanysauria" by Spiekman et al. in 2024.

References

{{Taxonbar, from=Q133072 Archosauromorpha Paraphyletic groups Taxa named by Thomas Henry Huxley Guadalupian taxonomic orders Cisuralian taxonomic orders Early Triassic taxonomic orders Middle Triassic taxonomic orders Late Triassic taxonomic orders