What is a dinosaur?

imageAlthough the name dinosaur is derived from the Greek for “terrible lizard”, dinosaur were not, in fact, lizards at all. Like lizards, dinosaur are included in the class Reptilia or reptiles one of the five main classes of vertebrata, animals with backbones. However, at the next level of classification, within reptiles, significant differences in the skeletal anatomy of lizards and dinosaurs have led scientists to place these group of animals into two different superorders: Lepidosauria or Lepodosaurs and Archosauria or archosaurs.Classified as  Lepodosaurs are lizards and snakes and their prehistoric ancestors. Included among the archosaurs or “ruling reptiles”, are prehistoric and modern crocodiles and the now extinct thecodonts, pterosaurs and dinosaurs. Palaeontologists believe that both dinosaurs and crocodile evolved in the later years of the Triassic Period (c. 248-208 million years ago), from creatures called pseudosuchian thecodonts. Lizards, snakes and different types of thecodont are believed to have evolved earlier in the Triassic Period from reptiles know as eosuchians.The most important skeletal differences between dinosaurs and other archosaurs are in the bones of the skull, pelvis and limbs. Dinosaur skull are found in a great range of shapes and sizes, reflecting the different eating habits and lifestylesof a large and varied group of animals that dominated life on Earth for an extraordinary 165 million years. However, unlike the skulls of any other known animals, the skulls of dinosaur had two long bones known as vomers. These bones  extended on either side of the head, from the front of the snout to the level of the holes in the skull known as the antorbital fenestra, situated in front of the dinosaur’s orbits or eyesockets.All dinosaurs, whether large or small, quadrupedal or bipedal, fleet-footed or slow-moving, shared a common body plan. Identification of this plan makes it possible to differentiate dinosaur from any other types of animals, even other archosaures. Most significantly, in dinosaurs , the pelvis and femur had evolved so that the hind limbs were held vertically beneath the body, rather than sprawling out to the sides like the limbs of a lizard. The femur of a dinosaurs had a sharply in-turned neck and a ball-shaped head, which slotted into a fully open acetabulum or hip socket. A supra-acetabular crest helped prevent dislocation of the femur. The position of the knee joint, aligned below the acetabulum, made it possible for the whole hind limb to swing backwards and forwards. This unique combination of features gave dinosaurs what is known as a “fully improved gait”. Evolution of this highly efficient methods of walking also development in mammals, but among reptiles it occurred only in dinosaurs.

For the purpose of further classification, dinosaur are divided into two orders: saurischia, or saurischain dinosaurs and Ornithischia or ornithischian dinosaurs. This division is made on the basis of their pelvic anatomy. All dinosaur had a pelvic girdle with each side comprised of three bones: the pubis ,  ilium and ischium. However, the orientation of these bones follows one of two patterns. In saurischian dinosaurs also known as lizard-hipped dinosaur, the pubis points forwards, as is usual in most types of reptile. By contrast, in ornithischian or bird-hipped, dinosaurs, the pubis points backwards towards the rear of the animal, which is also true of birds.

Of the two orders of dinosaurs, the saurischia was the larger and the first to evolve. It is divided into two suborders: Therapoda or therapods and Sauropodomorpha or Sauropodomorphs. The therapods or “beast feet” were bipedal, predatory carnivores. They ranged in size from mighty tyrannosaurus rex, 12m long, 5.6m tall and weighing an estimated 6.4 tonnes, to the smallest know dinosaur, compsognathus, a mere 1.4m long and estimated 3kg in weight when fully grown. The sauropodomorphs or “lizard feet forms”, included both bipedal and quadrupedal dinosaurs. Some sauropodomorphs were carnivorous or omnivorous but later species were typically herbivorous. They included some of the largest and best-known of all dinosaurs, such as Diplodocus, a huge quadruped with an elephant –like body, a long. Thin tail and neck that gave it a total length of 27m, and a tiny head.

Ornithischian dinosaur were bipedal or quadrupedal herbivores. Tey are now usually divided into three suborders: Ornithipoda, thyreopora and marginocephalia. The ornithopods or “bird feet”, both large and small, could walk or run on their long hind legs, balancing their body by holding their tails stiffy off the ground behind them. An example is lguanodon, up to 9m long, 5m tall and weighing 4.5 tonnes. The thyreophorans or “shield bearers”, also known as armoured dinosaurs, were quadrupeds with rows of protective bony spikes, studs or plates along their backs and tails. They included stegosaurus, 9m long and weighing 2 tonnes.

The marginocephalians or “margined heads”, were bipedal or quardrupedal ornithischians with a deep bony frill or narrow shelf at the back of the skull. An example is Triceratops, a rhinoceros- like dinosaur, 9m long, weighing 5.4 tonnes and bearing a prominent neck frill and three large horns.

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