Definition Triceratops skeleton, Natural History Museum of Los Angeles County Under phylogenetic nomenclature , dinosaurs are usually defined as the group consisting of the most recent common ancestor MRCA of Triceratops and Neornithes , and 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 horridus , Passer domesticus , Diplodocus carnegii , and 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 birds , some prehistoric species were quadrupeds, and others, such as Ammosaurus and Iguanodon , could 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 silesaurids , which 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 diapsids , 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.
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 are found in Herrerasaurus ;  and a perforate acetabulum , or hip socket, with a hole at the center of its inside surface closed in Saturnalia , for 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 therapsids , like cynodonts and rhynchosaurs. Their main competitors were the pseudosuchia , such as aetosaurs , ornithosuchids and rauisuchians , which 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 Rhaetian , respectively,   and the exact date of their extinction is uncertain. These losses left behind a land fauna of crocodylomorphs , dinosaurs, mammals , pterosaurians , and turtles. In the late Triassic and early Jurassic, the continents were connected as the single landmass Pangaea , and 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. Conifers and pteridophytes were the most common plants.
Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek -like organs to keep food in the mouth, and jaw motions to grind food. The earliest part of this time saw the spread of ankylosaurians, iguanodontians , and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America.
In Asia, maniraptoran coelurosaurians like dromaeosaurids , troodontids , and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts , and iguanodontians. A major change in the early Cretaceous, which would be amplified in the late Cretaceous, was the evolution of flowering plants.
At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries , taken to its extreme in hadrosaurids.
In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana , abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores.
Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.
Theropods were also radiating as herbivores or omnivores , with therizinosaurians and ornithomimosaurians becoming common. Some other diapsid groups, such as crocodilians, sebecosuchians , turtles , lizards , snakes , sphenodontians , and choristoderans , also survived the event.
It is often cited that mammals out-competed the neornithines for dominance of most terrestrial niches but many of these groups co-existed with rich mammalian faunas for most of the Cenozoic. Dinosaur classification Dinosaurs belong to a group known as archosaurs , which also includes modern crocodilians.
Within the archosaur group, dinosaurs are differentiated most noticeably by their gait. Dinosaur legs extend directly beneath the body, whereas the legs of lizards and crocodilians sprawl out to either side. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia , while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia.
Anatomically, these two groups can be distinguished most noticeably by their pelvic structure. Saurischia includes the theropods exclusively bipedal and with a wide variety of diets and sauropodomorphs long-necked herbivores which include advanced, quadrupedal groups. Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process.
Ornithischia includes a variety of species which were primarily herbivores.