Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs (lizards and kin), and all its descendants. Assuming turtles lie within Sauria, the group can be considered the crown group of diapsids, or reptiles in general. Recent genomic studies and comprehensive studies in the fossil record suggest that turtles are closely related to archosaurs, not to the pre-Saurian parareptiles as previously thought. Sauria includes all modern reptiles (including birds, 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.
|clockwise from top left:
Agkistrodon contortrix (the copperhead, a snake), Dinemellia dinemelli (the white-faced buffalo-weaver, a bird), various extinct ornithischian dinosaurs, Chelonia mydas (the green sea turtle), Anurognathus (an extinct pterosaur), and Alligator mississippiensis (the american alligator, a crocodilian)
The synapomorphies or characters that unite the clade Sauria also help them be distinguished from stem-saurians in Diapsida or stem-reptiles in clade Sauropsida in the following categories based on the following regions of the body.
- 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.
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.
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.
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- 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/
- Schoch, Rainer R.; Sues, Hans-Dieter (24 June 2015). "A Middle Triassic stem-turtle and the evolution of the turtle body plan". Nature. 523 (7562): 584–587. doi:10.1038/nature14472. PMID 26106865. S2CID 205243837.
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