| Protorosaurs | |
|---|---|
 | |
| Fossil specimen ofProtorosaurus speneri,Teyler's Museum | |
 | |
| Skeletal reconstructions of various members ofTanysauria, includingTrachelosaurus fischeri,Dinocephalosaurus orientalis,Tanystropheus hydroides, andTanystropheus longobardicus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauromorpha |
| Order: | †Protorosauria Huxley,1871 |
| Subtaxa | |
Protorosauria is anextinct, likelyparaphyletic group ofbasalarchosauromorph reptiles from the latestMiddle Permian (Capitanian stage) to the end of theLate Triassic (Rhaetian stage) ofAsia,Europe andNorth America. It was named by the Englishanatomist andpaleontologistThomas Henry Huxley in1871 as an order, originally to solely containProtorosaurus. Other names which were once considered equivalent to Protorosauria includeProlacertiformes andProlacertilia.[1]
Protorosaurs are distinguished by their long necks formed by elongatedcervical vertebrae, which haveribs that extend backward to the vertebrae behind them. Protorosaurs also have a gap between thequadrate bones and thejugal bones in the back of the skull near the jaw joint, making their skulls resemble those oflizards.[1] While previously thought to bemonophyletic, the group is now thought to consist of various groups of basal archosauromorph reptiles that lie outsideCrocopoda,[2] though some recent studies have recovered the group as monophyletic.[3] A number of members of Protosauria have been found to belong to a monophyletic group (though not includingProtorosaurus) which was namedTanysauria in 2024.[4]
Protorosauria was considered to be a synonym ofProlacertiformes for many years.[5]
Since 1998, many phylogenetic analyses have found Protorosauria, as used in its widest sense, to be apolyphyletic orparaphyletic taxon.Protorosaurus,Macrocnemus, tanystropheids, and various other protorosaurs are usually placed near the base of Archosauromorpha, whileProlacerta andPamelaria, twoGondwanan Triassic protorosaurs, are now thought to be in a morederived position as close relatives ofArchosauriformes.[6] Mostphylogenetic analyses since 1998 have found a strongly supported clade that includes only the genusProlacerta and theArchosauriformes.[7]
For this reasonProlacerta,Pamelaria, and several other related forms (collectively calledprolacertids) have been removed from Protorosauria. Because the name Prolacertiformes is defined based on the genusProlacerta, the name Protorosauria is used for the remaining group.
Only recently has Protorosauria been defined in aphylogenetic sense as the most inclusiveclade containing taxa such asProtorosaurus,Macrocnemus, andTanystropheus. Analyses, such as Dilkes (1998), Sues (2003), Modesto & Sues (2004), Rieppel, Fraser & Nosotti (2003), Rieppel, Li & Fraser (2008), Gottmann-Quesada and Sander (2009) and Renestoet al. (2010),[7][8][9][10][11] recovered a large Protorosauria, that includesProtorosaurus,Drepanosauridae (and relatives) andTanystropheidae (and relatives). However, some analysis foundProtorosaurus (and sometimes the closely relatedCzatkowiella) to be more advanced[12] or more basal[13] than the node Drepanosauridae+Tanystropheidae, but always more basal thanProlacerta.
Some studies still use the term Prolacertiformes to include prolacertids and traditional protorosaurs, while restricting the term Protorosauria to the smallest clade that includesProtorosaurus,Macrocnemus, andTanystropheus; thus Protorosauria is a true clade, while Prolacertiformes is anevolutionary grade of early archosauromorphs.[14]
Pritchardet al. (2015),[15] Nesbittet al. (2015),[16] Ezcurra (2016)[17] and Spiekman et al., 2021[2] 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 tocrocopods than toProtorosaurus. 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.
The Protorosauria includes the Permian genusProtorosaurus, closely related toCzatkowiella.[18] A wide variety of Permian and Triassic reptiles have been classified within Protorosauria, including the arboreal gliding reptileSharovipteryx and the aquatictanystropheids, which have extremely long necks.
Another enigmatic group of Triassic reptiles, theDrepanosauromorpha, have often been classified as belonging to the Protorosauria.[19]
Pterosaurs have also been proposed as protorosaurs or close relatives of them,[20] although they are now regarded as a morederived group ofarchosaurs.
While Senter (2004) reassigned the bizarre, arboreal drepanosaurids andLongisquama to a group of more primitivediapsids calledAvicephala,[21] subsequent studies failed to find the same result, instead supporting the hypothesis that they were protorosaurs.
The followingcladogram shows the position of Protorosauria among theSauria sensu Sean P. Modesto and Hans-Dieter Sues (2004).[7]
Most recent studies have recovered Protorosauria as a whole as a paraphyletic, cladogram after Spiekman et al. 2021[2]
Although Protorosauria as a whole is often found to be a paraphyletic, a large group of former "protorosaurs" (excludingProtorosaurus) is frequently found to be monophyletic. This clade was given the name "Tanysauria" by Spiekman et al. in 2024.[4]
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