Dinosaur classification is frequently varying, as new matter is exposed and old matter is reexamined and reevaluated. New insights are often being found, and paleontologists again and again oppose on the consequence of one or a different piece of information.Dinosaurs were terrestrial reptiles that walk by an upright position. Their distinctive hip arrangement caused their legs to attach out below their bodies, and not sprawl away from the region (as with other reptiles). When dinosaurs first grow from more prehistoric archosaurs, they were bipedal (they walked on two legs). Much soon, various dinosaur groups returned to a four-legged position, most having back legs much better than their front legs. There were many special types of dinosaurs, conflicting in size and weight, locomotion (two- or four- legged walk, fast or slow speed), diet (meat or plants), and just about every other physiological and anatomical quantify.

Dinosaurs were reptiles from the Mesozoic Era. They are classified as follows:

• Kingdom Animalia (animals)
• Phylum Chordata (having a hollow nerve chord ending in a brain)
• Class Archosauria (diapsids with socket-set teeth, etc.)
• Subclass Ornithodira (dinosaurs and pterosaurs)
• Superorder Dinosauria (dinosaurs)
• Order Saurischia and Ornithischia (based on hip structure)

The dinosaurs are classified on the order or size of their hip(Pelvic) structure.They are:

• the "Lizard-Hipped" or Saurischian dinosaurs
• The "Bird-Hipped" or Ornithischian dinosaurs.

LIZARD-HIPPED DINOSAURS:

These types of Dinosaurs had a hip structure similar to that of lizards. Oddly enough, these "Lizard-Hipped" dinosaurs were the ancestors of the birds.

Other anatomical features that characterize saurischians include an opening below the nostrils (called the subnarial foramen), an elongation on some neck vertebrae (resulting in long necks), a big thumb, an extra, wedge-shaped ankle bone (the astragalus), and many other distinctive features.The oldest known dinosaurs are Saurischians from the mid-Triassic period, about 230 million years ago.Saurischians included both meat and plant eaters.

Examples:

Theropods - bipedal carnivores like T. rex, Velociraptor, Compsognathus, and Deinonychus
Sauropodomorphas - quadrupedal herbivores like Apatosaurus, Brachiosaurus, and Diplodocus.

BIRD-HIPPED DINOSAURS:

These type of Dinosaurs had a hip structure that is similar to that of birds (but did not lead to the birds).They also had beak-like bone in the lower jaw in front of the teeth, called a predentary. Other anatomical differences include a narrow palebral (the bone that crosses the outside of the eye socket), and bony (ossified) tendons above the sacral area of the vertebrae.
The Ornithischian dinosaurs were vegetarians and included plated, armored, and horned dinosaurs.

Examples:

Ornithopods - Hadrosaurs , Hypsilophodontids, Iguanodontids, Fabrosauria .
Marginocephalia - Ceratopsians , Pachycephalosauria.
Thyreophora - Stegosaurids, Ankylosaurids.