Sauria is the clade containing the

most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of archosaurs (such as

crocodilians Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...

, dinosaurs, etc.) and lepidosaurs ( lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely related to archosaurs than to lepidosaurs as part of

Archelosauria Archelosauria is a clade grouping turtles and archosaurs (birds and crocodilians) and their fossil relatives, to the exclusion of lepidosaurs (the clade containing lizards, snakes and the tuatara). The majority of phylogenetic analyses based on m ...

, Sauria can be considered the crown group of

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...

s, or reptiles in general. Depending on the systematics, Sauria includes all modern reptiles or most of them (including

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s, a type of archosaur) as well as various extinct groups. Sauria lies within the larger total group Sauropsida, which also contains various stem-reptiles which are more closely related to reptiles than to mammals. Prior to its modern usage, "Sauria" was used as a name for the suborder occupied by lizards, which before 1800 were considered crocodilians.

Systematics

Recent genomic studiesCrawford, 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. and comprehensive studies in the fossil record suggest that

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s are closely related to archosaurs, not to the pre-Saurian

parareptile Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

s as previously thought. In cladistic analysis of 2018,

Pantestudines Pantestudines or Pan-Testudines is the group of all reptiles more closely related to turtles than to any other living animal. It includes both modern turtles (crown group turtles, also known as Testudines) and all of their extinct relatives (also ...

(turtles and close relatives) were placed within Diapsida but outside of Sauria.

Synapomorphies

The synapomorphies or characters that unite the clade Sauria also help them be distinguished from stem-saurians in

Diapsida Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...

or stem-reptiles in clade Sauropsida in the following categories based on the following regions of the body.Pough, F. H., Janis, C. M., & Heiser, J. B. (2005). Vertebrate life. Pearson/Prentice Hall.Laurin, Michel and Jacques A. Gauthier. 2011. Autapomorphies of Diapsid Clades. Version 20 April 2011. http://tolweb.org/accessory/Autapomorphies_of_Diapsid_Clades?acc_id=465 in The Tree of Life Web Project, http://tolweb.org/ *Cephalad Region **Dorsal origin of temporal musculature **Loss of caniniform region in maxillary tooth row **External nares close to the midline **Postparietal absent **Squamosal mainly restricted to top of skull **The occipital flange of the squamosal is little exposed on the occiput **Anterior process of squamosal narrow **Quadrate exposed laterally **Unossified dorsal process of stapes **Stapes slender *Trunk Region **Sacral ribs oriented laterally **Ontogenetic fusion of caudal ribs **Trunk ribs mostly single headed *Pectoral Region **Cleithrum absent *Pelvic Region **Modified ilium *Limb Region **Tubular bone lost **Entepicondylar foramen absent **Radius as long as ulna **Small proximal carpals and tarsal **Fifth distal tarsal absent **Short and stout fifth or hooked metatarsal **Perforating foramen of manus lost 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.

Phylogeny

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 found '' Eunotosaurus'', usually regarded as a turtle relative, to be only very distantly related to turtles in the clade

Parareptilia Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

. 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 found ''Eunotosaurus'' to be an actual early stem-turtle, though other versions of the analysis found weak support for it as a parareptile. The cladogram below follows the analysis of Li ''et al''. (2018). It places turtles within Diapsida but outside of Sauria (the Lepidosauromorpha + Archosauromorpha clade).

References

{{Taxonbar, from=Q2254408 Reptile taxonomy Tetrapod unranked clades Permian reptiles Extant Permian first appearances Guadalupian first appearances