The terms Sauropsida ("lizard faces") and Theropsida ("beast faces") were used

again in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their
relatives on the one hand (Sauropsida) and mammals and their extinct relatives
(Theropsida) on the other. 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, Protosauria ("first lizards") in which he included some animals today
considered reptile-like amphibians, as well as early reptiles.[14]

In 1956, D.M.S. Watson observed that the first two groups diverged very early in
reptilian history, so he divided Goodrich's Protosauria between them. He also
reinterpreted Sauropsida and Theropsida to exclude birds and mammals,
respectively. Thus his Sauropsida
included Procolophonia, Eosuchia, Millerosauria, Chelonia (turtles), Squamata (liza
rds and snakes), Rhynchocephalia, Crocodilia, "thecodonts"
(paraphyletic basal Archosauria), non-avian dinosaurs, pterosaurs, ichthyosaurs,
and sauropterygians.[15]

In the late 19th century, a number of definitions of Reptilia were offered. The
biological traits listed by Lydekker in 1896, for example, include a single occipital
condyle, a jaw joint formed by the quadrate and articular bones, and certain
characteristics of the vertebrae.[16] The animals singled out by these formulations,
the amniotes other than the mammals and the birds, are still those considered
reptiles today.[17]

The first reptiles had an anapsid type of skull roof,

as seen in the Permian genus Captorhinus
The synapsid/sauropsid division supplemented another approach, one that split the
reptiles into four subclasses based on the number and position of temporal
fenestrae, openings in the sides of the skull behind the eyes. This classification
was initiated by Henry Fairfield Osborn and elaborated and made popular
by Romer's classic Vertebrate Paleontology.[18][19] Those four subclasses were:

 Anapsida – no fenestrae – cotylosaurs and chelonia (turtles and relatives)[a]

 Synapsida – one low fenestra – pelycosaurs and therapsids (the 'mammal-like
reptiles')
 Euryapsida – one high fenestra (above the postorbital and
squamosal) – protorosaurs (small, early lizard-like reptiles) and the
marine sauropterygians and ichthyosaurs, the latter called Parapsida in
Osborn's work.
 Diapsida – two fenestrae – most reptiles,
including lizards, snakes, crocodilians, dinosaurs and pterosaurs.

Phylogenetic classifications group the traditional

"mammal-like reptiles", like this Varanodon, with other synapsids, not with extant reptiles
The composition of Euryapsida was uncertain. Ichthyosaurs were, at times,
considered to have arisen independently of the other euryapsids, and given the
older name Parapsida. Parapsida was later discarded as a group for the most part
(ichthyosaurs being classified as incertae sedis or with Euryapsida). However, four
(or three if Euryapsida is merged into Diapsida) subclasses remained more or less
universal for non-specialist work throughout the 20th century. It has largely been
abandoned by recent researchers: In particular, the anapsid condition has been
found to occur so variably among unrelated groups that it is not now considered a
useful distinction.[20]

Phylogenetics and modern definition

By the early 21st century, vertebrate paleontologists were beginning to
adopt phylogenetic taxonomy, in which all groups are defined in such a way as to
be monophyletic; that is, groups which include all descendants of a particular
ancestor. The reptiles as historically defined are paraphyletic, since they exclude
both birds and mammals. These respectively evolved from dinosaurs and from
early therapsids, both of which were traditionally called "reptiles".[21] Birds are more
closely related to crocodilians than the latter are to the rest of extant reptiles. Colin
Tudge wrote:

Mammals are a clade, and therefore the cladists are happy to acknowledge the
traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the
formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand
clade of the Amniota. But the traditional class Reptilia is not a clade. It is just a
section of the clade Amniota: The section that is left after the Mammalia and Aves
have been hived off. It cannot be defined by synapomorphies, as is the proper way.
Instead, it is defined by a combination of the features it has and the features it
lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists
suggest, we could say that the traditional Reptilia are 'non-avian, non-mammalian
amniotes'.[17]
Despite the early proposals for replacing the paraphyletic Reptilia with a
monophyletic Sauropsida, which includes birds, that term was never adopted
widely or, when it was, was not applied consistently.[3]

Bearded dragon (pogona) skeleton on display at

the Museum of Osteology
When Sauropsida was used, it often had the same content or even the same
definition as Reptilia. In 1988, Jacques Gauthier proposed a cladistic definition of
Reptilia as a monophyletic node-based crown group containing turtles, lizards and
snakes, crocodilians, and birds, their common ancestor and all its descendants.
While Gauthier's definition was close to the modern consensus, nonetheless, it
became considered inadequate because the actual relationship of turtles to other
reptiles was not yet well understood at this time.[3] Major revisions since have
included the reassignment of synapsids as non-reptiles, and classification of turtles
as diapsids.[3] Gauthier 1994 and Laurin and Reisz 1995's definition of Sauropsida
defined the scope of the group as distinct and broader than that of Reptilia,
encompassing Mesosauridae as well as Reptilia sensu stricto.[4][22]

A variety of other definitions were proposed by other scientists in the years

following Gauthier's paper. The first such new definition, which attempted to
adhere to the standards of the PhyloCode, was published by Modesto and
Anderson in 2004.[3] Modesto and Anderson reviewed the many previous definitions
and proposed a modified definition, which they intended to retain most traditional
content of the group while keeping it stable and monophyletic. They defined
Reptilia as all amniotes closer to Lacerta agilis and Crocodylus niloticus than
to Homo sapiens. This stem-based definition is equivalent to the more common
definition of Sauropsida, which Modesto and Anderson synonymized with Reptilia,
since the latter is better known and more frequently used. Unlike most previous
definitions of Reptilia, however, Modesto and Anderson's definition includes birds,
as they are within the clade that includes both lizards and crocodiles.[3]

Taxonomy
See also: List of reptiles and List of snakes
General classification of extinct and living reptiles, focusing on major groups.[23][24]
 Reptilia/Sauropsida
 †Parareptilia
 Eureptilia
 †Captorhinidae
 Diapsida
 †Araeoscelidia
 Neodiapsida
 †Drepanosauromorpha (placement uncertain)
 †Younginiformes (paraphyletic)
 †Ichthyosauromorpha (placement uncertain)
 †Thalattosauria (placement uncertain)
 Sauria
 Lepidosauromorpha
 Lepidosauriformes
 Rhynchocephalia (tuatara)
 Squamata (lizards and snakes)
 †Choristodera (placement uncertain)
 †Sauropterygia (placement uncertain)
 Pantestudines (turtles and kin, placement uncertain)
 Archosauromorpha
 †Protorosauria (paraphyletic)
 †Rhynchosauria
 †Allokotosauria
 Archosauriformes
 †Phytosauria
 Archosauria
 Pseudosuchia

 Crocodilia (crocodilians)

 Avemetatarsalia/Ornithodira

 †Pterosauria
 Dinosauria
 †Ornithischia
 Saurischia (including birds (Aves))
Phylogeny
The cladogram presented here illustrates the "family tree" of reptiles, and follows a
simplified version of the relationships found by M.S. Lee, in 2013.
[25]
All genetic studies have supported the hypothesis that turtles are diapsids; some
have placed turtles within Archosauromorpha,[25][26][27][28][29][30] though a few have
recovered turtles as Lepidosauromorpha instead.[31] The cladogram below used a
combination of genetic (molecular) and fossil (morphological) data to obtain its
results.[25]