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, approximately 65 million years ago, a catastrophic extinction event ended dinosaurs' dominance on land. One group of dinosaurs is known to have survived to the present day: taxonomists consider modern birds to be direct descendants of theropod dinosaurs.
Since the first dinosaur fossils were recognized in the 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 alike. They have been featured in best-selling books and films, 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.
Distinguishing features of dinosaurs
While recent discoveries have made it more difficult to present a universally agreed-upon list of dinosaurs' distinguishing features, nearly all dinosaurs discovered so far 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 an elongated crest on the humerus, or upper arm bone, to accommodate the attachment of deltopectoral muscles; a shelf at the rear of the ilium, or main hip bone; a tibia, or shin bone, featuring a broad lower edge and a flange pointing out and to the rear; and an ascending projection on the astragalus, one of the ankle bones, which secures it to the tibia.[6]
A variety of other skeletal features were shared by many dinosaurs. However, because they were either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies. Such shared features include a diapsid skull bearing two pairs of holes in the temporal region; holes in the snout and lower jaw (two characteristics shared by other archosaurs); loss of the skull's postfrontal bone; a long neck incorporating an S-shaped curve;[7] an elongated scapula, or shoulder blade; forelimbs shorter and lighter than hind limbs, coupled to asymmetrical hands; a sacrum composed of three or more fused vertebrae; and an acetabulum, or hip socket, with a hole at the center of its inside surface.[8]
The open, or "perforate", hip joint described above had significant implications for dinosaur movement and behavior. Most notably, it allowed dinosaur hind limbs to be "underslung", or situated directly beneath the animals' bodies; this, in turn, allowed dinosaurs to stand erect in a manner similar to modern mammals, but distinct from most other reptiles, whose limbs sprawl out to either side.[9] Vertical limb configuration also enabled dinosaurs to breathe easily while moving, which likely permitted stamina and activity levels that surpassed those of "sprawling" reptiles.
Source: Wikipedia.Org
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