Dinosaurs belong to the clade Dinosauria which is within the clade Archosauromorpha.
Pterosaurs are distantly related to dinosaurs, being members of the clade Ornithodira. The other groups mentioned are, like dinosaurs, members of Sauropsida the reptile and bird cladewith the exception of Dimetrodon which is a synapsid.
Definition Triceratops skeleton, Natural History Museum of Los Angeles County Under phylogenetic nomenclaturedinosaurs are usually defined as the group consisting of the most recent common ancestor MRCA of Triceratops and Neornithesand all its descendants.
In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy; this approach requires that, for a group to be natural, all descendants of members of the group must be included in the group as well.
Birds are thus considered to be dinosaurs and dinosaurs are, therefore, not extinct. Norman, and Paul M. Barrett in suggested a radical revision of dinosaurian systematics.
Phylogenetic analysis by Baron et al. They resurrected the clade Ornithoscelida to refer to the group containing Ornithischia and Theropoda. Dinosauria itself was re-defined as the last common ancestor of Triceratops horridusPasser domesticusDiplodocus carnegiiand all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.
Using one of the above definitions, dinosaurs can be generally described as archosaurs with hind limbs held erect beneath the body.
Other groups of animals were restricted in size and niches; mammals, for example, rarely exceeded the size of a domestic cat, and were generally rodent-sized carnivores of small prey. While dinosaurs were ancestrally bipedal as are all modern birdssome prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodoncould walk just as easily on two or four legs.
Cranial modifications like horns and crests are common dinosaurian traits, and some extinct species had bony armor. Although known for large size, many Mesozoic dinosaurs were human-sized or smaller, and modern birds are generally small in size.
Dinosaurs today inhabit every continent, and fossils show that they had achieved global distribution by at least the early Jurassic period. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical for Dinosauria; the earliest dinosaurs had them and passed them on to their descendants.
Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.
Some of these are also present in silesauridswhich Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others.
However, because they are 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 diapsidsdinosaurs ancestrally had two pairs of temporal fenestrae openings in the skull behind the eyesand as members of the diapsid group Archosauria, had additional openings in the snout and lower jaw.
These include an elongated scapulaor shoulder blade; a sacrum composed of three or more fused vertebrae three are found in some other archosaurs, but only two are found in Herrerasaurus ;  and a perforate acetabulumor hip socket, with a hole at the center of its inside surface closed in Saturnaliafor example.
Dinosaurs may have appeared as early as million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.
The terrestrial habitats were occupied by various types of archosauromorphs and therapsidslike cynodonts and rhynchosaurs. Their main competitors were the pseudosuchiasuch as aetosaursornithosuchids and rauisuchianswhich were more successful than the dinosaurs.
Rhynchosaurs and dicynodonts survived at least in some areas at least as late as early-mid Norian and early Rhaetianrespectively,   and the exact date of their extinction is uncertain.
These losses left behind a land fauna of crocodylomorphsdinosaurs, mammalspterosauriansand turtles. In the late Triassic and early Jurassic, the continents were connected as the single landmass Pangaeaand there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores.
Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.Anapsids include extinct organisms and may, based on anatomy, include turtles.
However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles. Pterosaurs lived among the dinosaurs and became extinct around the same time, but they were not dinosaurs.
Physical characteristics. Given the large number of different types of pterosaurs. Dinosaurs belong to the clade Dinosauria which is within the clade Archosauromorpha. belong to the clade Ornithodira which would eventually become modern birds and is known to feature the following characteristics (among others): Supra-acetabular crest (Fig.
1) Diatryma is a large, extinct, predatory bird that exhibits a typical bird. Saurischia Seeley, Subgroups Saurischia (/ s ɔː ˈ r ɪ s k i ə / saw-RIS-kee-ə, meaning "reptile-hipped" from the Greek sauros At the end of the Cretaceous Period, all saurischians except the birds became extinct in the course of the Cretaceous–Paleogene extinction grupobittia.comm: Animalia.
Elaborate display structures such as horns or crests are common to all dinosaur groups, and some extinct groups developed skeletal modifications such as bony armor and spines.
The large sizes of some dinosaur groups, Additional evidence for attacking live prey is the partially healed tail of an Edmontosaurus, a hadrosaurid dinosaur; Kingdom: Animalia. But lepidosaurs, like snakes and lizards, are more similar to each other (except in the characteristics that all reptiles share) than are lizards and dinosaurs.
At the same time archosaurs, like dinosaurs and crocodilians, share more similar characteristics with each other than either of them do with lizards.