Dinosaur

Dinosaurs
From top left to bottom right: Tyrannosaurus, Diplodocus, Stegosaurus, Parasaurolophus, Protoceratops, and Deinonychus.
Kingdom	Animalia
Phylum	Chordata
Class	Reptilia
Superorder	Dinosauria
Orders	†Ornithischia Saurischia

Dinosaurs were terrestrial vertebrate animals of the clade and superorder Dinosauria. Dinosaurs first appeared approximately 230 MYA in the middle Triassic period; non-avian dinosaurs became extinct 65.4 MYA while birds, or avian dinosaurs, survive to the present.

Dinosaurs were the dominant terrestrial vertebrates for 135 million years, from the beginning of the Jurassic (about 200 million years ago), after the Triassic–Jurassic extinction event eliminated most of their archosaursian competitors, until the end of the Cretaceous (about 65 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of all non-avian dinosaurs at the close of the Mesozoic era.

Birds are a division of the dinosaur order Theropoda, which makes the group Theropoda the second-largest known order of vertebrates after Perciformes.

Etymology

The taxon Dinosauria was formally named in 1842 by paleontologist Richard Owen Sir 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 δεινός (deinos, meaning "terrible," "potent," or "fearfully great") and σαῦρος (sauros, meaning "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.

Evolutionary history

Origins

Artist's impression of Euparkeria.

The dinosaurs' exact anscestors are unknown as there is a large gap between the early and middle Triassic in which few fossil reptiles are found. Later, several small, dinosaur-like archosaurs appear in the fossil record, many of which are candidates for the anscestors of dinosaurs. A well-known example is Euparkeria, a small reptile that may have been bipedal. This is a characteristic primarily found in theropods and small ornithopods, and the popular television series Prehistoric Earth depicted Euparkeria as the anscestor of dinosaurs. However Euparkeria is more likely to be simply an evolutionary cousin due to the different shape of the hip bones and the shape of the skull. Other candidates include Marasuchus and Lagosuchus.

pEarly dinosaurs

The first fully recognizeable dinosaurs are saurischians from the Late Triassic period native to South America. Of these, the oldest is Eoraptor, a cat-sized theropod-like dinosaur with short front mimbs, large eyes, and sharp teeth. From the same time and approximately the same place is another theropod-like dinosaur, Herrerasaurus, which was larger than Eoraptor and possibly more advanced. Although Eoraptor and Herrerasaurus have traditionally been put into families of early theropods, they may be more basal and been the common anscestors of theropods and sauropods.

Artist's depiction of Coelophysis.

Later in the Triassic, dinosaurs increased in numbers. They also began to spread across the world. At the time, the continents were connected as one, which allowed dinosaurs and other animals to spread quickly. Early European dinosaurs included the prosauropods Plateosaurus and Thecodontosaurus, as well as the early theropod Liliensternus. Early North American dinosaurs included the theropod Coelophysis, which is fairly well-known due to its appearence in Walking with Dinosaurs.

During the Triassic, dinosaurs lived alongside various other types of large reptiles, including therapsids, rhynchosaurs, and other archosaurs. The first dinosaurs were small compared to their descendants; the largest Triassic dinosaurs were the prosauropods. Theropods rarely grew larger than the size of a human, and other groups such as ornithopods were virtually nonexistent. The end of the Triassic period is marked by an extinction event that resulted in the destruction of most of the dinosaurs' competitors and allowed the dinosaurs to diversify.

Increasing diversity

In the early Jurassic period, dinosaurs began to spread and diversify. They began to rival their reptilian competitors for dominance, and took on a variety of ecological niches. The largest dinosaurs in the early Jurassic were the prosauropods, such as Massospondylus and Lufengosaurus, which may have given rise to true sauropods. Theropods remained small for the most part, but larger theropods did eventually evolve, such as Dilophosaurus, which may have fed on prosauropods and other dinosaurs.

Artist's impression of Scutellosaurus.

At the same time, two new important dinosaur groups were evolving. The first was the ornithopods, which at first consisted only of tiny, lightweight animals such as Lesothosaurus and Heterodontosaurus. The second group was the thyreophorans, among the first of which were the basal Scutellosaurus and Scelidosaurus. These animals likely evolved armour for protection from predators, but their descendants would build on the use of armour as a defense.

In the mid-Jurassic, the first truly large dinosaurs, such as Barapasaurus and Cetiosaurus appear. These relatively basal sauropods, while small in comparison to the giants to come, were still far larger than any modern terrestrial animal and likely inhabited a similar ecological niche to later sauropods. Perhaps in response to the evolution of these large herbivores, a number of theropods began to increase sharply in size during the middle Jurassic. The largest predators of the time were the megalosaurids, a group that included the earliest tetanuran theropods. Megalosaurids appear to have been the primary terrestrial predators throughout much of the middle Jurassic, but declined sharply and possibly went extinct by the end of the period*. Also during the middle-Jurassic, the earliest carnosaurs appear. While they were initially small, the carnosaurs would ultimately produce some of the largest theropods, and become a relatively diverse group. 

In culture

Public enthusiasm for dinosaurs first developed in Victorian England, where in 1854, decades after the first scientific descriptions of dinosaur remains, the famous dinosaur sculptures were unveiled in London's Crystal Palace Park. The Crystal Palace dinosaurs proved so popular that a strong market in smaller replicas soon developed. In subsequent decades, dinosaur exhibits opened at parks and museums around the world, ensuring that successive generations would be introduced to the animals in an immersive and exciting way.

Dinosaurs' enduring popularity, in its turn, has resulted in significant public funding for dinosaur science, and has frequently spurred new discoveries. In the United States, for example, the competition between museums for public attention led directly to the Bone Wars , during which a pair of feuding paleontologists made enormous scientific contributions.

The popular preoccupation with dinosaurs has ensured their appearance in media. Beginning in 1852 with a passing mention in Charles Dickens' Bleak House, dinosaurs have been featured in large numbers of fictional works. Sir Arthur Conan Doyle's 1912 book The Lost World, the iconic 1933 film King Kong, 1954's Godzilla and its many sequels, the best-selling 1990 novel Jurassic Park by Michael Crichton and its 1993 film adaptation are just a few notable examples of dinosaur appearances in fiction. Authors of general-interest non-fiction works about dinosaurs, including some prominent paleontologists, have often sought to use the animals as a way to educate readers about science in general.

Notes

1. Although the last definitive megalosaurid is known from Tithonian rocks (~148 MYA), the Campanian-age theropod Quilmesaurus has been proposed as a megalosaurid, indicating possible survival of the group until the K-T extinction event.