The Dinosauria is a monophyletic group of diapsids with an origin estimated at about 240 million years ago, during the Late Triassic period. The group is usually defined in terms of shared features among one or more families of archosaurs[5] — a paraphyletic group of diapsids including non-avian reptiles and their most recent common ancestor — but phylogenetic analysis of the dinosaurs has shown them to be members of the Parareptilia. Most of the major clades within the Dinosauria are known from Triassic deposits, but several are known from both earlier and later geologic periods. Dinosaurs continued to exist until the Cretaceous–Paleogene extinction event. The last major extinction event occurred at the Cretaceous–Paleogene boundary (K–Pg), resulting in the extinction of non-avian dinosaurs, pterosaurs, and the egg-laying mammal ornithomimosaurs. Of the remaining clades, only the Avemetatarsalia, Ornithomimosauria, and Marginocephalia survived the K–Pg extinction event, with the Avemetatarsalia and Marginocephalia diversifying afterwards.[5] The Dinosauria were originally defined in 1842 as the most recent common ancestor of Aves and Crocodilia, both descended from a hypothetical primitive archosaur called Scrotum humanum, who in turn was defined in 1840 as a "reptile with gizzard".[6] They were then understood to have evolved from ancient sarcopterygians.[7] Despite the fact that there was no direct transitional form connecting Aves and Crocodilia, there was also no convincing evidence from 1842 to support this definition of dinosaurs as all descendants of "Scrotum humanum".[7] In 1850, Richard Owen renamed dinosaurs to "Dinosauria" and defined them as reptiles with scaly skin, a hip joint which is not in the middle of the body, a single claw on the outer surface of the foot, and a sail on the back, based on the discovery of Omosaurus.[8] Evolution Below is a phylogenetic tree of the major dinosaur groups during the Early and Middle Triassic. The color codes represent when each group evolved: Gorgosaurus, the earliest dinosaur, and its kin were mostly vegetarian, and although very large, they were still only as heavy as a modern moose or cougar.[9] Early in the Triassic, larger and more advanced predators evolved, in particular the tyrannosaurids. An early example is Lythronax, which had a short skull and heavy jaws. The carnivores soon became much larger and diverse, developing into the classic Saurischia and Ornithischia. Dinosaurs diversified from the Lower Triassic into many types, including small, bird-like coelurosaurs and the early relatives of theropods (including the saurischians Dilophosaurus, Scutellosaurus and Eoraptor). These early theropods were small, lightly built animals that may have been piscivores, while the small meat-eaters were called dromaeosaurids. The carnivores gradually became larger and more specialized, acquiring massive beaks with sharp cutting teeth. One particular group of carnivores called Saurischia was particularly abundant. One sub-order of Saurischia were the ornithischians, which were plant-eaters as well as herbivores; in particular, they included the herbivorous ankylosaurs. The armoured dinosaurs (armadillos) appear in the Early Jurassic period, and although usually considered a separate order to dinosaurs are usually considered part of dinosauria in modern classifications. In the Jurassic, sauropodomorphs, theropods, and ceratopsians (the horned dinosaurs) diversified. The sauropods, long the dominant land vertebrates, were the largest animals of the time, and the necks of at least some species (Sauroposeidon in particular) were over 50 feet (15 metres) long. They were fast-growing animals with a long lifespan, but were probably slow-moving.[citation needed] The sauropods were mostly plant-eaters, and were the ancestors of the largest land animals of the next period, the titanosaurs. The most diverse group of sauropodomorphs in the Late Jurassic was the Camarasaurids (which had large teeth in both upper and lower jaws),[10] and the most successful group in the Early Cretaceous was the Euhelopodidae, which had a beak with teeth on both sides, and herbivores.[10] In the Early Cretaceous period, the diversity of theropods continued to increase. In particular, the megalosaurs became the most diverse group of carnivorous dinosaurs, especially in the later Cretaceous. The early to mid-Cretaceous, and particularly the later Cretaceous, were times of increased and more varied carnivory in dinosaurs, with the group going from having a carnivorous origin to being highly abundant in carnivorous forms. Dinosaur evolution in the Late Cretaceous is still poorly understood, and paleontologists are continuing to study it. The first to diverge from other dinosaurs were carnivorous, tooth-less quadrupeds that walked on all fours.[11] There were no living birds and dinosaurs are no more closely related to birds than they are to crocodiles or lizards. Birds, crocodiles, and lizards are all living reptiles. A new definition of dinosaurs was necessary in order to distinguish dinosaurs from living reptiles and to clarify their relationships with living and fossil reptiles. In the 1960s, when a growing number of dinosaur fossils were discovered, attempts were made to create phylogenetic trees to link dinosaurs together into a group. In 1974, Romer, who had studied the Triassic-Jurassic boundary, began working with Donald Henderson on a series of papers titled "A New System of Dinosaur Classification". Their research resulted in a tree that grouped dinosaurs with birds into a unique group.[12] At the time, bird and reptile features could not be distinguished, but since 1984 it has been possible to use statistical analysis to determine that the earliest fossil dinosaurs possessed birdlike characteristics. This is also supported by features like the wrist bone and breast bone present in feathered dinosaurs.[13][14] Dromaeosauridae are the earliest relatives of all modern birds, and they were among the most numerous predatory dinosaurs. By 135.5 to 130.5 million years ago, the family Protoceratopsidae appeared, and were the dominant carnivores in the Cretaceous. Therizinosauroids, which were contemporaneous with them, were even more successful, living all over the world in ancient ecosystems. Early in the Jurassic Period, primitive sauropodomorphs appeared, as well as primitive herbivores with small, leaf-shaped teeth.[15] These primitive sauropodomorphs had the first large-scale animals in the Jurassic period, in both the North American Morrison Formation and the Lufengpithecus of China. Early sauropodomorphs also came in a number of different forms, some being only a few feet (a metre or less) long. Sauropoda is a grouping of dinosaurs that includes the largest animals ever to walk on land. They lived on every continent, from South America to Europe to Asia, Africa, and Antarctica, and in many types of ancient ecosystems, including oceanic, coastal, desert, montane, and boreal. Many sauropod groups are divided into families and subfamilies. Sauropods ranged in size from about 3 meters (10 ft) long to about 50 meters (160 ft), with the largest sauropods being about 35 meters (115 ft) long.[17] Dinosaur groups, especially the sauropodomorphs, became quite diverse during the Late Triassic Period. Most dinosaurs were small, and were often only 2 or 3 meters (6.5–9 ft) in length. Nevertheless, sauropodomorphs had survived and flourished, making them the most numerous land animals of the Late Triassic period. The evolution of ornithischian dinosaurs continued to diversify in the Triassic, and included the first armoured dinosaurs (titanosaurs and ankylosaurs) and the earliest stegosaurians. In the Jurassic Period, the first sauropods appeared. A unique group of theropods appeared in the Early Cretaceous. In the Early Cretaceous Period, the first carnivorous dinosaurs appeared: the maniraptorans, including the Troodontidae, oviraptorosaurs, dromaeosaurids, and some birds. These were all smaller animals than their relatives, and the birds, by this time, were flying, as were a few dromaeosaurids, which would eventually become raptorial feeders. The evolution of dinosaurs in the Late Cretaceous is not well understood. By this time, large, highly specialized, long-necked sauropods had appeared, with the largest examples known being Argentinosaurus and the Argentinian Epachthosaurus. Other groups flourished during the Cretaceous, including various