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 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. Most
phylogenetic 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
phylogenetic sense as the most inclusive
clade 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==