Saurians
Temporal range: Capitanian–Present,
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)

Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Clade: Sauria
Macartney, 1802
Groups

Sauria is the clade containing the most recent common ancestor of Archosauria (which includes crocodilians and birds) and Lepidosauria (which includes squamates and the tuatara), and all its descendants.[1] 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 diapsids, or reptiles in general.[2] Depending on the systematics, Sauria includes all modern reptiles[3] or most of them (including birds, a type of archosaur) as well as various extinct groups.[4]

Sauria lies within the larger total group Sauropsida, which also contains various stem-reptiles which are more closely related to reptiles than to mammals.[3] 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 studies[5][6][7] and comprehensive studies in the fossil record[8] suggest that turtles 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.[4]

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.[9][10][11]

  • 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.[9]

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.[8]

Diapsida

†Araeoscelidia

Neodiapsida

†Claudiosaurus

†Younginiformes

Sauria

Lepidosauromorpha

 Archosauromorpha 
 Pantestudines 

†Eosauropterygia

†Placodontia

†Sinosaurosphargis

†Odontochelys

 Testudinata 

†Proganochelys

Testudines

†Choristodera

†Prolacertiformes

†Trilophosaurus

†Rhynchosauria

Archosauriformes

(=Archelosauria)

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.[12]

Sauria 
 Ankylopoda 

†Kuehneosauridae

Lepidosauria

Rhynchocephalia

Squamata

 Pantestudines 
†Sauropterygia

†Eosauropterygia

†Sinosaurosphargis

†Placodontia

†Eunotosaurus

†Pappochelys

†Odontochelys

 Testudinata 

†Proganochelys

Testudines

(=Lepidosauromorpha)

Archosauromorpha

The cladogram below follows the analysis of Li et al. (2018). It places turtles within Diapsida but outside of Sauria (the Lepidosauromorpha + Archosauromorpha clade).[4]

†Parareptilia

Eureptilia

†Captorhinidae

†Paleothyris

Diapsida

†Araeoscelidia

†Hovasaurus

†Youngina

crown‑group
Pantestudines

†Acerosodontosaurus

†Claudiosaurus

†Eunotosaurus

†Pappochelys

†Eorhynchochelys

†Odontochelys

Testudinata

Sauria
Lepidosauromorpha

†Kuehneosauridae

†Saurosphargidae

†Eosauropterygia

†Placodus

Lepidosauria

Rhynchocephalia

Squamata

Archosauromorpha

†Trilophosaurus

†Rhynchosauria

†Prolacerta

Archosauriformes

References

  1. ↑ Gauthier, J. A., Kluge, A. G., & Rowe, T. (1988). The early evolution of the Amniota. The phylogeny and classification of the tetrapods, 1, 103-155.
  2. ↑ Simões, Tiago R.; Kammerer, Christian F.; Caldwell, Michael W.; Pierce, Stephanie E. (2022-08-19). "Successive climate crises in the deep past drove the early evolution and radiation of reptiles". Science Advances. 8 (33): eabq1898. doi:10.1126/sciadv.abq1898. ISSN 2375-2548. PMC 9390993. PMID 35984885.
  3. 1 2 Ezcurra, M. D.; Scheyer, T. M.; Butler, R. J. (2014). "The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence". PLOS ONE. 9 (2): e89165. doi:10.1371/journal.pone.0089165. PMC 3937355. PMID 24586565.
  4. 1 2 3 Li, Chun; Fraser, Nicholas C.; Rieppel, Olivier; Wu, Xiao-Chun (August 2018). "A Triassic stem turtle with an edentulous beak". Nature. 560 (7719): 476–479. doi:10.1038/s41586-018-0419-1. ISSN 0028-0836. PMID 30135526. S2CID 52067286.
  5. ↑ Wang, Zhuo (27 March 2013). "The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan". Nature Genetics. 45 (701–706): 701–6. doi:10.1038/ng.2615. PMC 4000948. PMID 23624526.
  6. ↑ 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.
  7. ↑ Jarvis, E.D.; et al. (2014). "Whole-genome analyses resolve early branches in the tree of life of modern birds". Science. 346 (6215): 1320–1331. doi:10.1126/science.1253451. PMC 4405904. PMID 25504713.
  8. 1 2 Lee, M. S. Y. (2013). "Turtle origins: Insights from phylogenetic retrofitting and molecular scaffolds". Journal of Evolutionary Biology. 26 (12): 2729–2738. doi:10.1111/jeb.12268. PMID 24256520. S2CID 2106400.
  9. 1 2 Pough, F. H., Janis, C. M., & Heiser, J. B. (2005). Vertebrate life. Pearson/Prentice Hall.
  10. ↑ Laurin, Michel and Jacques A. Gauthier. 2011. Diapsida. Lizards, Sphenodon, crocodylians, birds, and their extinct relatives. Version 20 April 2011. http://tolweb.org/Diapsida/14866/2011.04.20 in The Tree of Life Web Project, http://tolweb.org/
  11. ↑ 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/
  12. ↑ 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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