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Sauropsida
Sauropsida
("lizard faces") is a group of amniotes that includes all existing birds and reptiles as well as their fossil ancestors and other extinct relatives. Large land animals are either in this group or in its sister group, Synapsida, which includes mammals and their fossil relatives. This clade includes Parareptilia
Parareptilia
and other extinct clades. All living sauropsids are members of the sub-group Diapsida, the Parareptilia
Parareptilia
clade having died out 200 million years ago. The term originated in 1864 with Thomas Henry Huxley,[1] who grouped birds with reptiles based on fossil evidence.

Contents

1 History of classification

1.1 Huxley and the fossil gaps 1.2 Sauropsids redefined 1.3 Detailing the reptile family tree 1.4 Cladistics and the Sauropsida

2 Evolutionary history 3 Phylogeny 4 References

History of classification[edit] Huxley and the fossil gaps[edit]

Benjamin Waterhouse Hawkins' 1855 reconstruction of a Dicynodon
Dicynodon
as a turtle-like creature.

The term Sauropsida
Sauropsida
("lizard faces") has a long history, and hails back to Thomas Henry Huxley, and his opinion that birds had risen from the dinosaurs. He based this chiefly on the fossils of Hesperornis
Hesperornis
and Archaeopteryx, that were starting to become known at the time.[2] In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrate classes informally into mammals, sauroids, and ichthyoids (the latter containing the anamniotes), based on the gaps in physiological traits and lack of transitional fossils that seem to exist between the three groups. Early in the following year he proposed the names Sauropsida
Sauropsida
and Ichthyopsida
Ichthyopsida
for the two latter.[1] Huxley did however include groups on the mammalian line (synapsids) like Dicynodon
Dicynodon
among the sauropsids. Thus, under the original definition, Sauropsida
Sauropsida
contained not only the groups usually associated with it today, but also several groups that today are known to be in the mammalian side of the tree.[3] Sauropsids redefined[edit] By the early 20th century, the fossils of Permian
Permian
synapsids from South Africa had become well known, allowing palaeontologists to trace synapsid evolution in much greater detail. The term Sauropsida
Sauropsida
was taken up by E.S. Goodrich in 1916 much like Huxley's, to include lizards, birds and their relatives. He distinguished them from mammals and their extinct relatives, which he included in the sister group Theropsida (now usually replaced with the name Synapsida). Goodrich's classification thus differs somewhat from Huxley's, in which the non-mammalian synapsids (or at least the dicynodontians) fell under the sauropsids. Goodrich supported this division by the nature of the hearts and blood vessels in each group, and other features such as the structure of the forebrain. According to Goodrich, both lineages evolved from an earlier stem group, the Protosauria ("first lizards"), which included some Paleozoic
Paleozoic
amphibians as well as early reptiles predating the sauropsid/synapsid split (and thus not true sauropsids).[3] Detailing the reptile family tree[edit] In 1956, D.M.S. Watson observed that sauropsids and synapsids diverged very early in the reptilian evolutionary history, and so he divided Goodrich's Protosauria between the two groups. He also reinterpreted the Sauropsida
Sauropsida
and Theropsida to exclude birds and mammals respectively, making them paraphyletic, unlike Goodrich's definition. Thus his Sauropsida
Sauropsida
included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata[4] (lizards and snakes), Rhynchocephalia, Crocodilia, "thecodonts" (paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs, and sauropyterygians.[5] This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate
Vertebrate
Paleontology[6]) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Since the advent of phylogenetic nomenclature, the term Reptilia
Reptilia
has fallen out of favor with many taxonomists, who have used Sauropsida
Sauropsida
in its place to include a monophyletic group containing the traditional reptiles and the birds. Cladistics and the Sauropsida[edit]

Sauropsida
Sauropsida
and the traditional class Reptilia
Reptilia
superimposed on a cladogram of Tetrapods, showing the difference in coverage

The class Reptilia
Reptilia
has been known to be an evolutionary grade rather than a clade for as long as evolution has been recognised. Reclassifying reptiles has been among the key aims of phylogenetic nomenclature.[7] The term Sauropsida
Sauropsida
had from the mid 20th century been used to denote all species not on the synapsid side after the synapsid/sauropsid split, a branch-based clade. This group encompasses all now-living reptiles as well as birds, and as such is comparable to Goodrich's classification, the difference being that better resolution of the early amniote tree has split up most of the Goodrich's "Protosauria", though definitions of Sauropsida
Sauropsida
essentially identical to Huxley's (i.e. including the mammal-like reptiles) are also forwarded.[8][9] Some later cladistic work has used Sauropsida
Sauropsida
more restrictively, to signify the crown group, i.e. all descendants of the last common ancestor of extant reptiles and birds. A number of phylogenetic stem, node and crown definitions have been published, anchored in a variety of fossil and extant organisms, thus there is currently no consensus of the actual definition (and thus content) of Sauropsida
Sauropsida
as a phylogenetic unit.[10] Some taxonomists, such as Benton (2004), have co-opted the term to fit into traditional rank-based classifications, making Sauropsida
Sauropsida
and Synapsida
Synapsida
class-level taxa to replace the traditional Class Reptilia, while Modesto and Anderson (2004), using the PhyloCode standard, have suggested replacing the name Sauropsida
Sauropsida
with their redefinition of Reptilia, arguing that the latter is by far better known and should have priority.[10] Evolutionary history[edit] Main article: Evolution
Evolution
of reptiles

Mesozoic
Mesozoic
sauropsids: the dinosaurs Europasaurus
Europasaurus
and Iguanodon, and the early bird Archaeopteryx
Archaeopteryx
perched on the foreground tree stump.

Sauropsids evolved from basal amniotes stock approximately 320 million years ago in the Paleozoic
Paleozoic
Era. In the Mesozoic
Mesozoic
Era (from about 250 million years ago to about 66 million years ago), sauropsids were the largest animals on land, in the water, and in the air. The Mesozoic
Mesozoic
is sometimes called the Age of Reptiles. Sixty-six million years ago, the large-bodied sauropsids died out in the global extinction event at the end of the Mesozoic
Mesozoic
era. With the exception of a few species of birds, the entire dinosaur lineage became extinct; in the following era, the Cenozoic, the remaining birds diversified so extensively that, today, nearly one out of every three species of land vertebrate is a bird species. Phylogeny[edit] The cladogram presented here illustrates the "family tree" of sauropsids, and follows a simplified version of the relationships found by M.S. Lee, in 2013.[11] All genetic studies have supported the hypothesis that turtles are diapsid reptiles; some have placed turtles within archosauriformes,[12][13][14][15][16][11] though a few have recovered turtles as lepidosauriformes instead.[17] The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results.[11]

Sauropsida

unnamed

†Parareptilia

†Millerettidae

unnamed

†Eunotosaurus

†Hallucicrania

Lanthanosuchidae
Lanthanosuchidae

†Procolophonia

Procolophonoidea
Procolophonoidea

Pareiasauromorpha
Pareiasauromorpha

Eureptilia

Captorhinidae
Captorhinidae

Romeriida

†Paleothyris

Diapsida

Araeoscelidia
Araeoscelidia

Neodiapsida

†Claudiosaurus

Younginiformes
Younginiformes

Sauria

Lepidosauromorpha

†Kuehneosauridae

Lepidosauria

Rhynchocephalia
Rhynchocephalia
(tuatara and their extinct relatives)

Squamata
Squamata
(lizards and snakes)

Archosauromorpha

Choristodera
Choristodera

Prolacertiformes
Prolacertiformes

Trilophosaurus
Trilophosaurus

Rhynchosauria
Rhynchosauria

Archosauriformes
Archosauriformes
(crocodiles, birds, and their extinct relatives)

 Pantestudines 

Eosauropterygia
Eosauropterygia

Placodontia
Placodontia

†Sinosaurosphargis

†Odontochelys

Testudinata

†Proganochelys

Testudines
Testudines
(turtles and tortoises)

References[edit]

^ a b Huxley, T.H. (1863): The Structure and Classification of the Mammalia. Hunterian lectures, presented in Medical Times and Gazette, 1863. original text ^ Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46-138 original text w/ figures ^ a b Goodrich, E.S. (1916). "On the classification of the Reptilia". Proceedings of the Royal Society of London. 89B (615): 261–276. doi:10.1098/rspb.1916.0012.  ^ http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118199 ^ Watson, D.M.S. (1957). "On Millerosaurus and the early history of the sauropsid reptiles". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 240 (673): 325–400. doi:10.1098/rstb.1957.0003.  ^ Romer, A.S. (1933). Vertebrate
Vertebrate
Paleontology. University of Chicago Press. , 3rd ed., 1966. ^ Gauthier, .A., Kluge, A.G & Rowe, T. (1988). The early evolution of the Amniota. Pages 103–155 in Michael J. Benton (ed.): The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst. Ass. Spec. Vol. 35A. Clarendon Press, Oxford. ^ Laurin, M. & Gauthier, J.A. (1996). Amniota, Mammals, reptiles (turtles, lizards, Sphenodon, crocodiles, birds) and their extinct relatives. Version 01 January 1996. The Tree of Life Web Project. ^ Pearse, A.S. (ed, 1947): Zoological Names: a List of Phyla, Classes, and Orders. Prepared for Section F, American Association for the Advancement of Science. Second edition. Durham, North Carolina, U.S.A., pp. 1-22 ^ a b Modesto, S.P.; Anderson, J.S. (2004). "The phylogenetic definition of Reptilia". Systematic Biology. 53 (5): 815–821. doi:10.1080/10635150490503026. PMID 15545258.  ^ a b c Lee, M. S. Y. (2013). " Turtle
Turtle
origins: Insights from phylogenetic retrofitting and molecular scaffolds". Journal of Evolutionary Biology. 26 (12): 2729–2738. doi:10.1111/jeb.12268. PMID 24256520.  ^ Mannen & Li 1999 ^ Zardoya & Meyer 1998 ^ Iwabe et al. 2004 ^ Roos, Aggarwal & Janke 2007 ^ Katsu et al. 2010 ^ Lyson et al. 2012

v t e

Extant chordate classes

Kingdom Animalia (unranked) Bilateria Superphylum Deuterostomia

Cephalochordata

Leptocardii (lancelets)

O l f a c t o r e s

Urochordata (tunicates)

Ascidiacea
Ascidiacea
(sea squirts) Appendicularia (larvaceans) Thaliacea
Thaliacea
(pyrosomes, salps, doliolids)

Craniata (Vertebrates + Myxini) (fish + Tetrapods)

Agnatha
Agnatha
(jawless fish)

Cyclostomata

Myxini (hagfish) Hyperoartia
Hyperoartia
(lampreys)

Gnathostomata (jawed vertebrates)

Chondrichthyes
Chondrichthyes
(cartilaginous fish: sharks, rays, chimaeras)

Osteichthyes (bony fish)

Actinopterygii
Actinopterygii
(ray-finned fish)

Sarcopterygii (lobe-finned fish)

Actinistia
Actinistia
(coelacanths)¹

R h i p i d i s t i a

Dipnoi (lungfish)¹

T e t r a p o d a

Amphibia (amphibians)

A m n i o t a

Synapsida

Mammalia (mammals)

Sauropsida (withal Diapsida)

Lepidosauria

Rhynchocephalia
Rhynchocephalia
(tuatara)² Squamata
Squamata
(scaled reptiles)²

Archelosauria

Testudines
Testudines
(turtles)²,³

Archosauria

Crocodilia
Crocodilia
(crocodilians)² Aves (birds)

¹subclasses of Sarcopterygii ²orders of class Reptilia
Reptilia
(reptiles) ³traditionally placed in Anapsida italic are paraphyletic groups

Taxon identifiers

Wd: Q329457 EoL: 4657519 Fossilworks: 13535

.