Dinos

OrnithischiaWikipedia, the free encyclopedia - Cite This Source


Ornithischia or Predentata is an order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos (?????e???) meaning 'of a bird' and ischion (?s????) meaning 'hip joint'. They are known as the 'bird-hipped' dinosaurs because of their bird-like hip structure, even though birds actually descended from the 'lizard-hipped' dinosaurs (the saurischians). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. They were prey animals for the theropods and were smaller than the sauropods.

Characteristics
The Dinosauria superorder was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division, which has generally been accepted, is based on the evolution of the pelvis into a more bird-like structure (although birds did not descend from these dinosaurs), details in the vertebrae and armor and the possession of a 'predentary' bone. The predentary is an extra bone in the front of the lower jaw, which extends the dentary (the main lower jaw bone). The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.

The ornithischian pubis bone points downward and toward the tail (backwards), parallel with the ischium, with a forward-pointing process to support the abdomen. This makes a four-pronged pelvic structure. In contrast to this, the saurischian pubis points downward and towards the head (forwards), as in ancestral lizard types. Ornithischians also had smaller holes in front of their eye sockets (antorbital fenestrae) than saurischians, and a wider, more stable pelvis. A bird-like pubis arrangement, parallel to the vertebral column, independently evolved three times in dinosaur evolution, namely in the ornithischians, the therizinosauroids and in bird-like dromaeosaurids.

Classification
Taxonomy
Linnaean ranks after Benton (2004),

Order Ornithischia

Family Pisanosauridae
Family Fabrosauridae
Suborder Thyreophora - (armored dinosaurs)

Family Scelidosauridae
Infraorder Stegosauria
Infraorder Ankylosauria
Suborder Cerapoda

Family Heterodontosauridae
Infraorder Ornithopoda

Family Hypsilophodontidae*
Family Hadrosauridae - (duck-billed dinosaurs)
Infraorder Pachycephalosauria
Infraorder Ceratopsia - (horned dinosaurs)

Phylogeny
The ornithischians are divided in the two clades: the first are the Thyreophora and the second the Cerapoda. The Thyreophora include the Stegosauria (like the armored Stegosaurus) and the Ankylosauria (like Ankylosaurus). The Cerapoda include the Marginocephalia (Ceratopsia like the frilled ceratopsidae and Pachycephalosauria) and the Ornithopoda (among which duck-bills (hadrosaurs) such as Edmontosaurus). The Cerapoda are a relatively recent grouping (Sereno, 1986), and may conceivably be identical to (synonymous with) the older group, Ornithopoda: most of these divisions are not true by definition.

Ornithischia|-?Pisanosaurus
`--+-?Fabrosauridae `--Genasauria |--Thyreophora | |--Scutellosaurus | `--Thyreophoroidea | |--Emausaurus | `--Eurypoda | |--Stegosauria | `--Ankylosauromorpha | |--Scelidosaurus | `--Ankylosauria `--Cerapoda |--Stormbergia |--Agilisaurus |--Hexinlusaurus |--Heterodontosauridae `--+--Ornithopoda `--Marginocephalia |--Pachycephalosauria `--Ceratopsia(basal Cerapoda after Butler, 2005)

References

Butler, R.J. 2005. The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho. Zoological Journal of the Linnean Society 145(2):175-218.
Sereno, P.C. 1986. Phylogeny of the bird-hipped dinosaurs (order Ornithischia). National Geographic Research 2(2):234-256.


SaurischiaWikipedia, the free encyclopedia - Cite This Source


Saurischia (from the Greek sauros (sa????) meaning 'lizard' and ischion (?s????) meaning 'hip joint') is one of the two orders/branches of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two great orders, based on their hip structure. Saurischians ('lizard-hipped') are distinguished from the ornithischians ('bird-hipped') by retaining the ancestral configuration of bones in the hip. All carnivorous dinosaurs (the theropods) are saurischians, as are one of the two great lineages of herbivorous dinosaurs, the sauropodomorphs. At the end of the Cretaceous Period, all non-avian saurischians became extinct. This is referred to as the Cretaceous-Tertiary extinction event.

The saurischian lineage diverged from the ornithischians in the late Triassic Period, and retained a three-pronged pelvic structure, with the pubis pointed forward, until some advanced forms in the group Maniraptora reversed this, in parallel with the ornithischian condition. The ornithischians evolved a new hip structure, with the pubis rotating caudally, to become parallel with the ischium, often also with a forward-pointing process, giving a four-pronged structure. This hip structure is similar to that of birds, and so ornithischians are termed 'bird-hipped' dinosaurs, while the saurischians are 'lizard-hipped'. The true bird-hip possessed by modern birds evolved independently in the lizard-hipped theropods in the Jurassic Period, an example of convergent evolution.

While Seeley's classification has stood the test of time, there is a minority theory, first popularized by Robert Bakker in The Dinosaur Heresies that separates the theropods into their own group and places the two great groups of herbivorous dinosaurs (the sauropodomorphs and ornithischians) together in a separate group he named the Phytodinosauria ('plant dinosaurs') (Bakker), or Ornithischiformes (Cooper).

Taxonomy
Classification

Order Saurischia

Eoraptor
Guaibasaurus
Infraorder Herrerasauria
Suborder Sauropodomorpha

Infraorder Prosauropoda
Infraorder Sauropoda
Suborder Theropoda

Infraorder Carnosauria
Infraorder Ceratosauria
Infraorder Deinonychosauria
Infraorder Ornithomimosauria
Infraorder Oviraptorosauria
Additionally, the genera Teyuwasu and Agnosphitys may represent early saurischians, or more primitive non-dinosaurs.

DinosaurWikipedia, the free encyclopedia - Cite This Source


Dinosaurs were vertebrate animals that dominated terrestrial ecosystems for over 160 million years, first appearing approximately 230 million years ago. At the end of the Cretaceous Period, 65 million years ago, a catastrophic extinction event ended the dominance of dinosaurs on land. One group of dinosaurs is known to have survived to the present day: taxonomists believe modern birds are direct descendants of theropod dinosaurs.

Since the first dinosaur fossils were recognized in the early nineteenth century, mounted dinosaur skeletons have become major attractions at museums around the world. Dinosaurs have become a part of world culture and remain consistently popular among children and adults. They have been featured in best-selling books and films, and new discoveries are regularly covered by the media.

The term dinosaur is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although none of these were dinosaurs.

What is a dinosaur?
From the point of view of cladistics (the method of classifying organisms most commonly used by scientists) birds are dinosaurs; but in ordinary speech the word "dinosaur" does not include birds. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur", and "bird" as a synonym for "avian dinosaur" (meaning any animal that evolved from the common ancestor of Archaeopteryx and modern birds). The term "non-avian dinosaur" will be used for emphasis as needed.
Non-avian dinosaurs can be generally described as terrestrial archosaurs with limbs held erect beneath the body, that existed from the Carnian faunal stage of the Late Triassic to the Maastrichtian stage of the Late Cretaceous. This excludes many prehistoric animals that are popularly conceived as dinosaurs. Examples include: marine reptiles like ichthyosaurs, mosasaurs, and plesiosaurs, which were neither terrestrial nor archosaurs; pterosaurs, which were not terrestrial; and Dimetrodon, a Permian animal more closely related to mammals. Dinosaurs were the dominant terrestrial vertebrates of the Mesozoic, especially the Jurassic and Cretaceous. Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a cat, and were generally rodent-sized carnivores of small prey. One notable exception is Repenomamus giganticus, a triconodont weighing between and that is known to have eaten small dinosaurs like young Psittacosaurus.

Dinosaurs were an extremely varied group of animals; according to a 2006 study, over 500 dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1,850, nearly 75% of which remain to be discovered. An earlier study predicted that about 3,400 dinosaur genera existed, including many which would not have been preserved in the fossil record. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Many had bony armor, or cranial modifications like horns and crests. Although known for large size, many dinosaurs were human-sized or smaller. Dinosaur remains have been found on every continent on Earth, including Antarctica. Despite their diversity and dominance, however, as noted, dinosaurs (with the exception of birds) did not spread into aquatic or aerial niches.

Definition
The taxon Dinosauria was formally named in 1842 by English palaeontologist Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were then being recognized in England and around the world. The term is derived from the Greek words de???? (deinos meaning "terrible", "fearsome", or "formidable") and sa??a (saura meaning "lizard" or "reptile"). Though the taxonomic name has often been interpreted as a reference to dinosaurs' teeth, claws, and other fearsome characteristics, Owen intended it merely to evoke their size and majesty.

Distinguishing features
While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common features across a taxonomic group are called synapomorphies.
Dinosaur synapomorphies include an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.

A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. For example, as diapsid reptiles, dinosaurs ancestrally had two pairs of temporal fenestrae (openings in the skull behind the eyes), and as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw. Additionally, several characteristics once thought to be synapomorphies are now known to have appeared before dinosaurs, or were absent in the earliest dinosaurs and independently evolved by different dinosaur groups. These include an elongated scapula, or shoulder blade; a sacrum composed of three or more fused vertebrae (three are found in some other archosaurs, but only two in are found in Herrerasaurus); and an acetabulum, or hip socket, with a hole at the center of its inside surface (closed in Saturnalia, for example). Another difficulty of determining distinctly dinosaurian features is that early dinosaurs and other archosaurs from the Late Triassic are often poorly known and were similar in many ways; these animals have sometimes been misidentified in the literature.

Dinosaurs stood erect in a manner similar to most modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side. Their posture was due to the development of a laterally-facing recess in the pelvis (usually an open socket) and a corresponding inwardly-facing distinct head on the femur. Their erect posture enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles. Erect limbs probably also helped support the evolution of large size by reducing bending stresses on limbs. Some non-dinosaurian archosaurs, including rauisuchians, also had erect limbs but achieved this by a "pillar erect" configuration of the hip joint, where instead of having a projection from the femur insert on a socket on the hip, the upper pelvic bone was rotated to form an overhanging shelf.

Phylogenetic definition
Under phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the most recent common ancestor of Triceratops and modern birds. It has also been suggested that Dinosauria be defined as all the descendants of the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.
There is an almost universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor are related, modern birds are dinosaurs and dinosaurs are, therefore, not extinct. Modern birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.

However, referring to birds as 'avian dinosaurs' and to all other dinosaurs as 'non-avian dinosaurs' is cumbersome. Birds are still referred to as birds, at least in popular usage and among ornithologists. It is also technically correct to refer to dinosaurs as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor. Paleontologists mostly use cladistics, which classifies birds as dinosaurs, but some biologists of the older generation do not.

ArchosaurWikipedia, the free encyclopedia - Cite This Source

Archosaurs (Greek for 'ruling lizards') are a group of diapsid reptiles that is represented today by birds and crocodiles and which also included the dinosaurs.
There is some debate about when archosaurs first appeared. Those who classify the Permian reptiles Archosaurus rossicus and / or Protorosaurus speneri as true archosaurs maintain that archosaurs first appeared in the late Permian. Those who classify both Archosaurus rossicus and Protorosaurus speneri as archosauriformes (not true archosaurs but very closely related) maintain that archosaurs first evolved from Archosauriform ancestors during the Olenekian (early Triassic Period).

Distinguishing characteristics
The simplest and most widely-agreed synapomorphies of archosaurs are:

Teeth set in sockets, which makes them less likely to be torn loose during feeding. This feature is responsible for the name "thecodonts" ("socket teeth"), which paleontologists used to apply to all or most archosaurs.
Preorbital fenestrae (openings in the skull in front of the eyes but behind the nostrils), which reduced the weight of the skull, a useful feature since most early archosaurs had long, heavy skulls, rather like those of modern crocodilians. The preorbital fenestrae (sometimes called anteorbital fenestrae) are often larger than the orbits (eye sockets).
Mandibular fenestrae (small openings in the jaw bones), which may have reduced the weight of the jaw slightly.
A fourth trochanter (ridge for attaching muscles) on the femur. This seemingly insignificant detail may have made the evolution of dinosaurs possible (all early dinosaurs and many later ones were bipeds), and may also be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic Permian-Triassic extinction event.

Archosaur takeover in the Triassic
Mammal-like reptiles were the dominant land vertebrates throughout the Permian, but most perished in the Permian-Triassic extinction event. Lystrosaurus (a herbivorous mammal-like reptile) was the only large land animal to survive the event, becoming the most populous land animal on the planet for a time.
But archosaurs quickly became the dominant land vertebrates in the early Triassic. The two most commonly-suggested explanations for this are:


Archosaurs made quicker progress than mammal-like reptiles towards erect limbs, and this gave them greater stamina by avoiding Carrier's constraint. This is unconvincing since Archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of Lystrosaurus and other mammal-like reptiles.
The early Triassic was predominantly arid, because most of the earth's land was concentrated in the supercontinent Pangaea. Archosaurs were probably better at conserving water than mammal-like reptiles:
Modern diapsids (lizards, snakes, crocodilians, birds) excrete uric acid, which can be excreted as a paste. It is reasonable to suppose that archosaurs (diapsids and ancestors of crocodilians, dinosaurs and birds) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.
Modern mammals excrete urea, which requires a lot of water to keep it dissolved. Their skins also contain many glands, which also lose water. Assuming that mammal-like reptiles had similar features, as argued e.g. in Palaeos , they were at a disadvantage in a mainly arid world. The same well-respected site points out that "for much of Australia's Plio-Pleistocene history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic varanid lizards (Megalania) and land crocs."

Main types of archosaurs
Since the 1970s scientists have classified archosaurs mainly on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot.


The Crurotarsi appeared early in the Triassic. In their ankles the astragalus was joined to the tibia by a suture and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later "crurotarsans" developed fully erect limbs (most notably the Rauisuchia). And modern crocodilians are "crurotarsans" which can walk with their limbs sprawling or erect depending on how much of a hurry they are in.


Euparkeria and the Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus.


The earliest fossils of Ornithodira ("bird necks") appear in the Carnian age of the late Triassic, but it is hard to see how they could have evolved from the "crurotarsans" - possibly they actually evolved much earlier, or perhaps they evolved from the last of the "primitive mesotarsal" archosaurs. Ornithodires' "advanced mesotarsal" ankle had a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement was only suitable for animals with erect limbs, but provided more stability when the animals were running. The ornothodires differed from other archosaurs in other ways: they were lightly-built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornothodires were completely bipedal. The archosaurian fourth trochanter on the femur may have made it easier for ornothodires to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic the ornithodires diversified to produce pterosaurs and dinosaurs.

DiapsidWikipedia, the free encyclopedia - Cite This Source

Diapsids ("two arches") are a group of reptiles that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, tuatara, and possibly even turtles. Under modern classification systems, even birds are considered diapsids, since they evolved from diapsid ancestors and are nested within the diapsid clade. While some diapsids have lost either one hole (lizards), or both holes (snakes), or even have a heavily restructured skull (modern birds), they are still classified as diapsids based on their ancestry. There are at least 7,925 species of diapsid reptile existing in environments around the world today (over 14,600 when birds are included).
Characteristics
The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings (or fenestrae) above and below the eye. This arrangement allows for the attachment of larger, stronger jaw muscles, and enables the jaw to open more widely. A more obscure ancestral characteristic is a relatively long lower arm bone (the radius), compared to the upper arm bone (humerus).
Systematics
Diapsids were originally classified as one of four subclasses of the class Reptilia, all of which were based on the number and arrangement of openings in the skull. The other three subclasses were Synapsida (one opening low on the skull, for the "mammal-like reptiles"), Anapsida (no skull opening, including turtles and their relatives), and Euryapsida (one opening high on the skull, including many prehistoric marine reptiles). With the advent of phylogenetic nomenclature, this system of classification was heavily modified. The Synapsids today are often not considered true reptiles, while the Euryapsida was found to be an unnatural assemblage of diapsids that had lost one of their skull openings. Some studies have suggested that this is the case in turtles as well, and that turtles are actually heavily modified diapsids, which would leave only some prehistoric forms in the Anapsida. In phylogenetic systems, birds (descendants of traditional diapsid reptiles) are also considered to be members of this group.
Well known extinct diapsid groups include the dinosaurs, pterosaurs, plesiosaurs, mosasaurs, and many more obscure lineages. The classification of most of the early groups is fluid and subject to change.

Taxonomy

Subclass DIAPSIDA

Order Araeoscelidia
Order Avicephala
Order Thalattosauria
Order Younginiformes
Superorder Ichthyopterygia (ichthyosaurs)
Infraclass Lepidosauromorpha

Order Eolacertilia
Superorder Lepidosauria (tuatara, lizards, amphisbaenians and snakes)
Superorder Sauropterygia (plesiosaurs and relatives)
Infraclass Archosauromorpha

Order Aetosauria
Order Choristodera
Order Phytosauria
Order Prolacertiformes
Order Pterosauria
Order Rauisuchia
Order Rhynchosauria
Order Trilophosauria
Superorder Crocodylomorpha (crocodiles and extinct relatives)
Superorder Dinosauria

Phylogeny

Diapsida|--Araeoscelida |-?Sphodrosaurus |-?Palacrodon |-?Omphalosaurus
`--+--Avicephala `--Neodiapsida |--Apsisaurus `--Eosuchia |-?Younginiformes `--+-?Claudiosaurus |-?Ichthyopterygia `--Sauria |-?Thalattosauriformes |--Lepidosauromorpha `--Archosauromorpha

ReptileWikipedia, the free encyclopedia - Cite This Source

Reptiles are air-breathing, cold-blooded vertebrates that have scaly bodies as opposed to hair or feathers; they represent an intermediate position in evolutionary development between amphibians and warm-blooded vertebrates, the birds and mammals. They are tetrapods and amniotes whose embryos are surrounded by an amniotic membrane, and members of the class Sauropsida inhabiting every continent with the exception of Antarctica. Today they are represented by four orders:

Crocodilia (crocodiles, gharials, caimans and alligators): 23 species
Sphenodontia (tuataras from New Zealand): 2 species
Squamata (lizards, snakes and amphisbaenids ("worm-lizards"): approximately 7,900 species
Testudines (turtles and tortoises): approximately 300 species
The majority of reptile species are oviparous (egg-laying) although certain species of squamates are capable of giving live birth. This is achieved, either through ovoviviparity (egg retention), or viviparity (offspring born without use of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals with some providing initial care for their hatchlings.

Classification
History of classification
From the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuatara, lizards, snakes, amphisbaenians and turtles, grouped together as the class Reptilia (Latin repere, "to creep"). This is still the usual definition of the term. However, in recent years, many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:


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 synamorphies, as is the proper way. It is instead 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 Reptila are 'non-avian, non-mammalian amniotes'.

By the same token, the traditional class Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustacea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes). If we believe, as some (but not all) zoologists do, that myriapods gave rise to insects, then they should be called Myriapoda*....by this convention Reptilia without an asterisk is synonymous with Amniota, and includes birds and mammals, whereas Reptilia* means non-avian, non-mammalian amniotes.
The terms "Sauropsida" ("Lizard Faces") and "Theropsida" ("Beast Faces") were coined in 1916 by E.S. Goodrich to distinguish between lizards, birds, and their relatives on one hand (Sauropsida) and mammals and their extinct relatives (Theropsida) on the other. This division is supported 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 amphibians as well as early reptiles.

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

This classification supplemented, but was never as popular as, the classification of the reptiles (according to Romer's classic Vertebrate Paleontology) into four subclasses according to the positioning of temporal fenestrae, openings in the sides of the skull behind the eyes. Those divisions were:


Anapsida - no fenestrae
Synapsida - one low fenestra (no longer considered true reptiles)
Euryapsida - one high fenestra (now included within Diapsida)
Diapsida - two fenestrae
All of the above but Synapsida fall under Sauropsida.

Taxonomy
Classification to order level, after Benton, 2004.

Series Amniota

Class Synapsida

Order Pelycosauria*
Order Therapsida

Class Mammalia
Class Sauropsida

Subclass Anapsida

Order Testudines (turtles)
Subclass Diapsida

Order Araeoscelidia
Order Younginiformes
Infraclass Ichthyosauria
Infraclass Lepidosauromorpha

Superorder Sauropterygia

Order Placodontia
Order Nothosauroidea
Order Plesiosauria
Superorder Lepidosauria

Order Sphenodontida (tuatara)
Order Squamata (lizards & snakes)
Infraclass Archosauromorpha

Order Prolacertiformes
Division Archosauria

Subdivision Crurotarsi

Superorder Crocodylomorpha

Order Crocodylia
Subdivision Avemetatarsalia

Infradivision Ornithodira

Order Pterosauria
Superorder Dinosauria

Order Saurischia

Class Aves
Order Ornithischia

Phylogeny
The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by Laurin and Gauthier (1996), presented as part of the Tree of Life Web Project.

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Evolution
Hylonomus is the oldest-known reptile, and was about 8 to 12 inches (20 to 30 cm) long. Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes. Petrolacosaurus and Mesosaurus are other examples. The earliest reptiles were found in the swamp forests of the Carboniferous, but were largely overshadowed by bigger labyrinthodont amphibians such as Proterogynrius. It was only after the small ice age at the end of the Carboniferous that the reptiles grew to big sizes, producing species such as Edaphosaurus and Dimetrodon.The first true "reptiles" (Sauropsids) are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure; but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic) and the archosaurs (today represented by only crocodilians and birds, but also containing pterosaurs and dinosaurs).The earliest, solid-skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals, and are often referred to as mammal-like reptiles, though they are not true members of Sauropsida. (A preferable term is "stem-mammals".)

Systems
CirculatoryMost reptiles have closed circulation via a three-chamber heart consisting of two atria and one, variably-partitioned ventricle. There is usually one pair of aortic arches. In spite of this, because of the fluid dynamics of blood flow through the heart, there is little mixing of oxygenated and deoxygenated blood in the three-chamber heart. Furthermore, the blood flow can be altered to shunt either deoxygenated blood to the body or oxygenated blood to the lungs, which gives the animal greater control over its blood flow, allowing more effective thermoregulation and longer diving times for aquatic species. There are some interesting exceptions among reptiles. For instance, crocodilians have an anatomically four-chambered heart that is capable of becoming a functionally three-chamber heart during dives (Mazzotti, 1989 pg 47). Also, it has been discovered that some snake and lizard species (e.g., monitor lizards and pythons) have three-chamber hearts that become functional four-chamber hearts during contraction. This is made possible by a muscular ridge that subdivides the ventricle during ventricular diastole and completely divides it during ventricular systole. Because of this ridge, some of these squamates are capable of producing ventricular pressure differentials that are equivalent to those seen in mammalian and avian hearts (Wang et al, 2003).
Respiratory
All reptiles breathe using lungs. Aquatic turtles have developed more permeable skin, and some species have modified their cloaca to increase the area for gas exchange (Orenstein, 2001). Even with these adaptations, breathing is never fully accomplished without lungs. Lung ventilation is accomplished differently in each main reptile group. In squamates the lungs are ventilated almost exclusively by the axial musculature. This is also the same musculature that is used during locomotion. Because of this constraint, most squamates are forced to hold their breath during intense runs. Some, however, have found a way around it. Varanids, and a few other lizard species, employ buccal pumping as a complement to their normal "axial breathing." This allows the animals to completely fill their lungs during intense locomotion, and thus remain aerobically active for a long time. Tegu lizards are known to possess a proto-diaphragm, which separates the pulmonary cavity from the visceral cavity. While not actually capable of movement, it does allow for greater lung inflation, by taking the weight of the viscera off the lungs (Klein et al, 2003). Crocodilians actually have a muscular diaphragm that is analogous to the mammalian diaphragm. The difference is that the muscles for the crocodilian diaphragm pull the pubis (part of the pelvis, which is movable in crocodilians) back, which brings the liver down, thus freeing space for the lungs to expand. This type of diaphragmatic setup has been referred to as the "hepatic piston."How Turtles & Tortoises breathe has been the subject of much study. To date, only a few species have been studied thoroughly enough to get an idea of how turtles do it. The results indicate that turtles & tortoises have found a variety of solutions to this problem. The problem is that most turtle shells are rigid and do not allow for the type of expansion and contraction that other amniotes use to ventilate their lungs. Some turtles such as the Indian flapshell (Lissemys punctata) have a sheet of muscle that envelopes the lungs. When it contracts, the turtle can exhale. When at rest, the turtle can retract the limbs into the body cavity and force air out of the lungs. When the turtle protracts its limbs, the pressure inside the lungs is reduced, and the turtle can suck air in. Turtle lungs are attached to the inside of the top of the shell (carapace), with the bottom of the lungs attached (via connective tissue) to the rest of the viscera. By using a series of special muscles (roughly equivalent to a diaphragm), turtles are capable of pushing their viscera up and down, resulting in effective respiration, since many of these muscles have attachment points in conjunction with their forelimbs (indeed, many of the muscles expand into the limb pockets during contraction). Breathing during locomotion has been studied in three species, and they show different patterns. Adult female green sea turtles do not breathe as they crutch along their nesting beaches. They hold their breath during terrestrial locomotion and breathe in bouts as they rest. North American box turtles breathe continuously during locomotion, and the ventilation cycle is not coordinated with the limb movements (Landberg et al., 2003). They are probably using their abdominal muscles to breathe during locomotion. The last species to have been studied is red-eared sliders, which also breathe during locomotion, but they had smaller breaths during locomotion than during small pauses between locomotor bouts, indicating that there may be mechanical interference between the limb movements and the breathing apparatus. Box turtles have also been observed to breathe while completely sealed up inside their shells (ibid).Most reptiles lack a secondary palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged (and protect their brains from getting kicked in by struggling prey). Skinks (family Scincidae) also have evolved a bony secondary palate, to varying degrees. Snakes took a different approach and extended their trachea instead. Their tracheal extension sticks out like a fleshy straw, and allows these animals to swallow large prey without suffering from asphyxiation.
Excretory
Excretion is performed mainly by two small kidneys. In diapsids uric acid is the main nitrogenous waste product; turtles, like mammals, mainly excrete urea. Unlike the kidneys of mammals and birds, reptile kidneys are unable to produce liquid urine more concentrated than their body fluid. This is because they lack a specialized structure present in the nephrons of birds and mammals, called a Loop of Henle. Because of this, many reptiles use the colon to aid in the reabsorption of water. Some are also able to take up water stored in the bladder. Excess salts are also excreted by nasal and lingual salt-glands in some reptiles.
Nervous
The reptilian nervous system contains the same basic part of the amphibian brain, but the reptile cerebrum and cerebellum are slightly larger. Most typical sense organs are well developed with certain exceptions most notably the snakes lack of external ears (middle and inner ears are present). All reptilians have advanced visual depth perception compared to other animals.
There are twelve pairs of cranial nerves.
Reproductive
Most reptiles reproduce sexually, though some are capable of asexual reproduction. All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Tuataras lack copulatory organs, so the male and female simply press their cloacas together as the male excretes sperm. Most reptiles, however, have copulatory organs, which are usually retracted or inverted and stored inside the body. In turtles and crocodilians, the male has a single median penis, while squamtes including snakes and lizards possess a pair of hemipenes. Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion and allantois are present during embryonic life. There are no larval stages of development. Viviparity and ovovivparity have only evolved in Squamates, and a substantial fraction of the species utilize this mode of reprduction, including all boas and most vipers. The degree of viviparity varies: some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, while still others lack any yolk and provide all nutrients via a placenta.

Asexual reproduction has been identified in squamates in six families of lizards and one snake. In some species of squamates, a population of females are able to produce a unisexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids (especially Aspidocelis) and lacertids (Lacerta) In captivity Komodo dragons (varanidae) have reproduced by parthenogenesis.Parthenogenetic species are also suspected to occur among chameleons, agamids, xantusiids, and typhlopids.
Notes