Sauria is theclade ofdiapsids containing themost recent common ancestor ofArchosauria (which includescrocodilians andbirds) andLepidosauria (which includessquamates and thetuatara), and all its descendants.[2] Since most molecular phylogenies recover turtles as more closely related to archosaurs than to lepidosaurs as part ofArchelosauria, Sauria can be considered thecrown group of diapsids, orreptiles in general.[3] Depending on the systematics, Sauria includes all modern reptiles[4] or most of them (includingbirds, a type of archosaur) as well as various extinct groups.[5]
Sauria lies within the larger total groupSauropsida, which also contains variousstem-reptiles which are more closely related to reptiles than to mammals.[4] Prior to its modern usage, "Sauria" was used as a name for thesuborder occupied bylizards, which before 1800 were considered crocodilians.
Sauria was historically used as a partial equivalent forSquamata (which contains lizards and snakes).[6] The redefinition to cover the last common ancestor of archosaurs and lepidosaurs was the result of papers by Jacques A. Gauthier and colleagues in the 1980s.[7]
Genomic studies[8][9][10] and comprehensive studies in the fossil record[11] suggest thatturtles are closely related to archosaurs as part of Sauria, and not to the non-saurianparareptiles as previously thought.
The synapomorphies or characters that unite the clade Sauria also help them be distinguished from stem-saurians inDiapsida or stem-reptiles in cladeSauropsida in the following categories based on the following regions of the body.[12][13][14]
However, some of these characters might be lost or modified in several lineages, particularly among birds and turtles; it is best to see these characters as the ancestral features that were present in the ancestral saurian.[12]
The cladogram shown below follows the most likely result found by an analysis of turtle relationships using both fossil and genetic evidence by M.S. Lee, in 2013. This study foundEunotosaurus, usually regarded as a turtle relative, to be only very distantly related to turtles in the cladeParareptilia.[11]
The cladogram below follows the most likely result found by another analysis of turtle relationships, this one using only fossil evidence, published by Rainer Schoch and Hans-Dieter Sues in 2015. This study foundEunotosaurus to be an actual early stem-turtle, though other versions of the analysis found weak support for it as a parareptile.[15]
The cladogram below follows the analysis of Liet al. (2018). It places turtles within Diapsida but outside of Sauria (the Lepidosauromorpha + Archosauromorpha clade).[5]
^Crawford, Nicholas G., et al. "More than 1000 ultraconserved elements provide evidence that turtles are the sister group of archosaurs." Biology letters 8.5 (2012): 783–786.
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