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FYI:

Dinosaurs were vertebrate animals that dominated terrestrial ecosystems for over 160 million years, first appearing approximately 230 million years ago. At the end of the Cretaceous Period, 65 million years ago, dinosaurs succumbed to a catastrophic extinction, which ended their dominance on land. Taxonomists consider modern birds to be the direct descendants of theropod dinosaurs.

Since the first dinosaur was recognized in the 19th century, mounted, fossilized 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 alike. They have been featured in best-selling books and films such as 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 technically none of these were dinosaurs.

Definition 

Triceratops skeleton at the Smithsonian National Museum of Natural History.The taxon Dinosauria was formally named by the English palaeontologist Richard Owen in 1842 as "a distinct tribe or suborder of Saurian reptiles". The term is derived from the Greek words δειν?ς (deinos meaning "terrible", "fearsome" or "formidable") and σα?ρα (saura meaning "lizard" or "reptile"). Owen chose it to express his awe at the size and majesty of the extinct animals, not out of fear or trepidation at their size and often-formidable arsenal of teeth and claws.

Dinosaurs were an extremely varied group of animals; according to a 2006 study, 527 dinosaur genera have been identified with certainty so far, and 1,844 genera are believed to have existed. Some were herbivorous, others carnivorous. Some dinosaurs were bipeds, some were quadrupeds, and others, such as Ammosaurus and Iguanodon, could walk just as easily on two or four legs. Regardless of body type, nearly all known dinosaurs were well-adapted for a predominantly terrestrial, rather than aquatic or aerial, habitat.

Dinosaur synapomorphies

All dinosaurs so far discovered share certain modifications to the ancestral archosaurian skeleton. Although some later groups of dinosaurs featured further modified versions of these traits, they are considered typical across Dinosauria; the earliest dinosaurs had them and passed them on to all their descendants. Such common structures across a taxonomic group are called synapomorphies.

Dinosaur synapomorphies include: reduced fourth and fifth digits on the manus ('hand'), reduced number of digits on the pes (foot) to three main toes, a sacrum (the region of the vertebral column to which the pelvis attaches, composed of three or more fused vertebrae) and an open or perforate acetabulum (hip socket with a hole at its centre). Dinosaurs are unique among all tetrapods in having this perforate acetabulum.

Other shared anatomical features

Scientists generally agree that a variety of other anatomical features were shared by most dinosaurs. These include forelimbs shorter and lighter than hind limbs, an unusual secondary palate that permitted dinosaurs to eat and breathe simultaneously, a relatively straight femur with medially-directed femoral head, two pairs of holes in the temporal region of the skull (i.e. a diapsid skull), rearward-pointing elbows in the front limbs and forward-pointing knees in the hind limbs.

The hip joint arrangement described above allowed an erect stance, in which hind limbs were situated directly beneath the body or 'underslung'. This stance is like that of most mammals today but unlike that of other reptiles, which have a less erect posture and limbs splayed out to either side. The vertical action of the limbs in dinosaurs allowed for more efficient and faster locomotion, compared to the clumsier and slower movement of other 'sprawled' reptiles. It also allowed many types of dinosaurs to become bipedal.

Stegosaurus Taxonomic definition

Under phylogenetic taxonomy, dinosaurs are defined as all descendants of the most recent common ancestor of Triceratops and modern birds. They are divided into Ornithischia (bird-hipped) and Saurischia (lizard-hipped), depending upon pelvic structure. Ornithischian dinosaurs had a four-pronged pelvic configuration, incorporating a caudally-directed (rear-pointing) pubis bone with (most commonly) a forward-pointing process. By contrast, the pelvic structure of saurischian dinosaurs was three-pronged, and featured a pubis bone directed cranially, or forwards, only. Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia, while Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia. It has also been sugg