Dinosaurs are a complex group in the Dinosauria clade of reptiles.

During the Triassic era, between 243 and 233.23 million years ago, they first emerged, although the precise origin and timing of dinosaur evolution is the subject of active study. After the Triassic–Jurassic extinction event 201.3 million years ago, they were the dominant terrestrial vertebrates; their dominance persisted in the Jurassic and Cretaceous periods.

The fossil record reveals that birds are modern feathered dinosaurs, having formed during the Late Jurassic period from earlier theropods, and are the only dinosaur lineage to survive the extinction event of the Cretaceous-Paleogene around 66 million years ago. Therefore, dinosaurs can be split into avian dinosaurs, or birds, and extinct non-avian dinosaurs, all of which are non-bird dinosaurs.

From taxonomic, morphological, and ecological viewpoints, dinosaurs are a complex group of animals. Birds are among the most diverse group of vertebrates, with over 10,700 living species.

Palaeontologists have described over 900 distinct genera and more than 1,000 separate species of non-avian dinosaurs using fossil evidence. Dinosaurs are described by both extant species (birds) and fossil remains on every continent. Before birds were known as dinosaurs during the first half of the 20th century, much of the scientific community assumed dinosaurs to have been slow and cold-blooded. However, most studies performed since the 1970s have shown that dinosaurs were active creatures with elevated metabolisms and various social contact adaptations. Some have been herbivorous, others have been carnivorous.

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While dinosaurs were ancestrally bipedal, quadrupedal species were found in several extinct classes, and some were able to move between these stances. For all dinosaur groups, elaborate display structures such as horns or crests are common, and skeletal modifications such as bony armor and spines have been produced by some extinct groups. While the modern-day remaining avian lineage (birds) of the dinosaurs are typically small because of flight restrictions, many ancient dinosaurs (non-avian and avian) were large-bodied, it is estimated that the largest sauropod dinosaurs attained lengths of 39.7 meters (130 feet) and heights of 18 m (59 ft) and were the largest land animals of all time.

As big, solid bones are more likely to survive until they are fossilized, the assumption that non-avian dinosaurs were uniformly gigantic is based in part on preservation bias. Many dinosaurs, some measuring around 50 centimetres (20 inches) in length, were very small.

In the early 19th century, the first dinosaur fossils were recognized, with Sir Richard Owen having coined the term “dinosaur” in 1841 to refer to these “great fossil lizards” Since then, at museums worldwide, mounted fossil dinosaur fossils have become big attractions, and dinosaurs have been an enduring part of popular culture.

Dinosaurs are generally classified under phylogenetic nomenclature as the group consisting of the most recent common ancestor (MRCA) of Triceratops and modern birds (Neornithes), and all their descendants. Dinosauria was also suggested to be described concerning the Megalosaurus and Iguanodon MRCA since when he recognized the Dinosauria, these were two of the three genera cited by Richard Owen. “Both definitions lead to dinosaurs being described as the same set of animals: “Dinosauria = Ornithischia + Saurischia”.

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Birds are known as belonging to the Maniraptora subgroup, which are coelurosaurs, which are theropods, which are dinosaurs, which are saurischians.

A radical revision of dinosaur systematics was introduced by research by Matthew G. Baron, David B. Norman, and Paul M. Barrett in 2017. As compared to the conventional union of theropods with sauropodomorphs, phylogenetic research by Baron et al. recovered the Ornithischia as being closer to the Theropod than the Sauropodomorpha.

Dinosaurs can generally be defined as archosaurs with hind limbs kept upright beneath the body using one of the above definitions. Other extinct animals are not taxonomically classified as dinosaurs, including pterosaurs, mosasaurs, ichthyosaurs, plesiosaurs, and Dimetrodon, although also popularly conceived as dinosaurs. Pterosaurs, as members of the clade Ornithodira, are distantly related to dinosaurs. Including dinosaurs and pterosaurs, the other groups listed are members of Sauropsida (the reptile and bird clade), except Dimetrodon (which is a synapsid). None of them had the distinctive upright hind limb pose of true dinosaurs.

Also Read, Did you know about the fossils of these dinosaurs?

The dominant terrestrial vertebrates of the Mesozoic Era, especially the Jurassic and Cretaceous periods, were dinosaurs. Other species classes were limited in size and niche; mammals, for instance, seldom surpassed the size of a domestic cat and were usually small prey carnivores of rodent size. They have long been known as an exceptionally diverse group of animals; as of 2018, over 900 genera of non-avian dinosaurs have been described with certainty, and the total number of genera preserved in the fossil record has been estimated at around 1850, almost 75 per cent of which remain to be discovered, and 1124 species by 2016.

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The estimated number of dinosaur species existing in the Mesozoic was 1,543–2,468 in 2016. In 2021, the number of avian dinosaurs (modern-day birds) was estimated at 10,806 species. Some are herbivorous, others are carnivorous, including seed-eaters, insectivores, fish-eaters, and omnivores. Although dinosaurs (as are all modern birds) were ancestrally bipedal, some prehistoric species were quadrupeds, and others, such as Anchisaurus and Iguanodon, could walk on two or four legs just as easily. Popular dinosaur features are cranial alterations including horns and crests, and some extinct species have bony armour.

While considered to be huge, many Mesozoic dinosaurs were smaller or human-sized, and modern birds are usually small in scale. Today, dinosaurs occupy every continent, and fossils indicate that, by the early Jurassic period at least, they had achieved global distribution. Most of the available habitats, from terrestrial to marine, are dominated by modern birds, and there is evidence that certain non-avian dinosaurs (such as Microraptor) could fly or at least glide, and others have semiaquatic habits, such as spinosaurids.

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Dinosaurs may have been put into the category of extinct creatures, but they have left marks on this earth which may never erode.