Temporal range: Guadalupian–Carnian
|Fossil specimen of Protorosaurus speneri, Teyler's Museum|
Protorosauria is an extinct group of archosauromorph reptiles from the latest Permian (Changhsingian stage) to the early Late Triassic (Carnian stage) of Asia, Europe, North America. It was named by the English anatomist and paleontologist Thomas Henry Huxley in 1871 as an order. Other names that are for the most part equivalent to Protorosauria include Prolacertiformes and Prolacertilia. Protorosaurs are distinguished by their long necks formed by elongated cervical vertebrae, 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 lizards.
Whether or not protorosaurs represent a monophyletic group (i.e. a distinct evolutionary grouping within Archosauromorpha) is uncertain. Only recently has Protorosauria been defined in a phylogenetic sense as the most inclusive clade containing taxa such as Protorosaurus, Macrocnemus, Tanystropheus. Since 1998, many phylogenetic analyses have found Protorosauria as used in its widest sense to be a polyphyletic or paraphyletic taxon. Protorosaurus, Macrocnemus, tanystropheids, and various other protorosaurs are usually placed near the base of Archosauromorpha while Prolacerta and Pamelaria, two Gondwanan Triassic protorosaurs, are now thought to be in a more derived position as close relatives of Archosauriformes. For this reason Prolacerta, Pamelaria, and several other related forms (collectively called prolacertids) have been removed from Protorosauria. Some recent 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).
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 called Drepanosauridae has usually 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.
Protorosauria was considered to be a synonym of Prolacertiformes for many years. However, most 21st century studies on the phylogeny of "prolacertiformes" indicate that the group as traditionally conceived is polyphyletic; while most prolacertiformes form a clade of basal archosauromorphs, Prolacerta itself is closely related to more derived archosauriforms.
Most phylogenetic analyses since 1998 have found a strongly supported clade that includes only the genus Prolacerta and the Archosauriformes. Protorosaurus and all other traditional prolacertiforms were recovered in more basal position, usually forming a single clade. Because the name Prolacertiformes is defined based on the genus Prolacerta, the name Protorosauria is used for the remaining group. Most of these 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 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. The following cladogram shows the position of Protorosauria among the Sauria sensu Sean P. Modesto and Hans-Dieter Sues (2004).
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.
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