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
archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avi ...
s (such as
crocodilians,
dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
, etc.) and
lepidosaurs
The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizard
Lizards are a widespread group of squamate reptiles, ...
(
lizards
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia a ...
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, 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 ...
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
Stem or STEM may refer to:
Plant structures
* Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang
* Stipe (botany), a stalk to support some other structure
* Stipe (mycology), the stem of a mushr ...
-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
lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia al ...
s, which before 1800 were considered crocodilians.
Systematics
Recent genomic studies
[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.] 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 ...
s are closely related to archosaurs, not to the pre-Saurian
parareptiles as previously thought. In cladistic analysis of 2018,
Pantestudines (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 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.[
]
References
{{Taxonbar, from=Q2254408
Reptile taxonomy
Tetrapod unranked clades
Permian reptiles
Extant Permian first appearances
Guadalupian first appearances