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DINOSAUR ART PRINTS

Dinosaurs were the dominant vertebrate animals of terrestrial ecosystems, from the late Triassic period  to the end of the Cretaceous period , when most of them became extinct in the Cretaceous–Tertiary extinction event.

The discovery in 1862 of Archaeopteryx first suggested a close relationship between dinosaurs and birds, as only the fossilized feathers of Archaeopteryx clearly distinguished it from contemporary small predatory dinosaurs such as Compsognathus. Research since the 1970s indicates that theropod dinosaurs are most likely the ancestors of birds; in fact, most paleontologists regard birds as the only surviving dinosaurs and some believe dinosaurs and birds should be put together under one biological class.^[1] Crocodilians are the other surviving close relatives of dinosaurs, and both groups are members of the Archosauria, a group of reptiles that first appeared in the very late Permian and became dominant in the mid-Triassic.

For about the first half of the 20th century, both scientists and the general public regarded dinosaurs as slow, unintelligent cold-blooded animals. However, the bulk of research since the 1970s has supported the view that they were active animals with elevated metabolisms, and often with adaptations for social interactions. This change of view was strongly influenced by evidence of the descent of birds from theropod dinosaurs.

Since the first dinosaur fossils were recognized in the early nineteenth century, mounted dinosaur skeletons have become major attractions at museums around the world. Dinosaurs have become a part of world culture and remain consistently popular among children and adults. They have been featured in best-selling books and films (notably Jurassic Park), and new discoveries are regularly covered by the media.

The term dinosaur is sometimes used informally to describe other prehistoric reptiles, such as the pelycosaur Dimetrodon, the winged pterosaurs, and the aquatic ichthyosaurs, plesiosaurs and mosasaurs, although none of these were dinosaurs.

In colloquial English "dinosaur" is sometimes used to describe an obsolete or unsuccessful thing or person, despite the dinosaurs' 160 million year reign and the global abundance and diversity of their descendants, the birds. This usage became common while dinosaurs were regarded as cold-blooded and sluggish.

Under phylogenetic taxonomy, dinosaurs are usually defined as all descendants of the most recent common ancestor of Triceratops and modern birds.^[4] It has also been suggested that Dinosauria be defined as all of the descendants of the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.^[5] Both definitions result in the same set of animals being defined as dinosaurs, including theropods (mostly bipedal carnivores), sauropodomorphs (mostly large herbivorous quadrupeds with long necks and tails), ankylosaurians (armored herbivorous quadrupeds), stegosaurians (plated herbivorous quadrupeds), ceratopsians (herbivorous quadrupeds with horns and frills), and ornithopods (bipedal or quadrupedal herbivores including "duck-bills"). These definitions are written to correspond with scientific conceptions of dinosaurs that predate the modern use of phylogenetics. The continuity of meaning is intended to prevent confusion about what the term "dinosaur" means.

There is an almost universal consensus among paleontologists that birds are the descendants of theropod dinosaurs. Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural, birds are dinosaurs and dinosaurs are, therefore, not extinct. Birds are classified by most paleontologists as belonging to the subgroup Maniraptora, which are coelurosaurs, which are theropods, which are saurischians, which are dinosaurs.^[6]

From the point of view of cladistics, birds are dinosaurs, but in ordinary speech the word "dinosaur" does not include birds. Additionally, referring to dinosaurs that are not birds as "non-avian dinosaurs" is cumbersome. For clarity, this article will use "dinosaur" as a synonym for "non-avian dinosaur". The term "non-avian dinosaur" will be used for emphasis as needed. It is also technically correct to refer to dinosaurs as a distinct group under the older Linnaean classification system, which accepts paraphyletic taxa that exclude some descendants of a single common ancestor.

Dinosaurs (including birds) are archosaurs, like modern crocodilians. Archosaurs' diapsid skulls have two holes, called temporal fenestrae, located where the jaw muscles attach, and an additional antorbital fenestra in front of the eyes. Most reptiles (including birds) are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no temporal fenestra, are anapsids. Anatomically, dinosaurs share many other archosaur characteristics, including teeth that grow from sockets rather than as direct extensions of the jawbones. 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 crocodylians sprawl out to either side.

Collectively, dinosaurs are usually regarded as a superorder or an unranked clade. They are divided into two orders, Saurischia and Ornithischia, depending upon pelvic structure. 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. Saurischians ('lizard-hipped', from the Greek sauros (σαυρος) meaning 'lizard' and ischion (ισχιον) meaning 'hip joint') retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward.^[19] This basic form was modified by rotating the pubis backward to varying degrees in several groups (Herrerasaurus,^[30] therizinosauroids,^[31] dromaeosaurids,^[32] and birds^[6]). Saurischia includes the theropods (bipedal and mostly carnivores, except for birds) and sauropodomorphs (long-necked quadrupedal herbivores).

By contrast, ornithischians ('bird-hipped', from the Greek ornitheios (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint') had a pelvis that superficially resembled a bird's pelvis: the pubis bone was oriented caudally (rear-pointing). Unlike birds, the ornithischian pubis also usually had an additional forward-pointing process. Ornithischia includes a variety of herbivores. (NB: the terms "lizard hip" and "bird-hip" are misnomers — birds evolved from dinosaurs with "lizard hips".