Amazingly enough, dinosaurs are divided into two orders:  Saurischia and Ornithischia.  Saurischia (lizard-hipped) are characterized as such because of their forward jutting pubic hipbones and their elongated neck bones.  They are divided into two infraorders:  carnivorous (meat-eating) bipedal (walking on two legs) theropods and herbivorous (plant-eating) quadrupedal (walking on four legs) sauropodomorphs.  Ornithischia (bird-hipped) are characterized as such because their pubis (hipbone) is slanted back parallel to another hipbone and they have a predentary bone at the tip of their toothless lower jaw.  They are plant eaters.  There are three infraorders:  the birdlike ornithpods, the armored thyreophorans, and the horned or thick skulled marginocephalians. 

Theropods belong to the Saurischia order.  Theropods (beast feet) were bipedal.  They had long muscular back legs.  They were carnivores (meat eaters).  It is believed that the larger groups hunted alone and used their teeth to attack their prey.  The smaller groups traveled in packs and used their claws to attack their prey.  Theropoda are divided into two infraorders:  Ceratosauria and Tetanurae.

Ceratosauria is the first infraorder.  They ranged in size from small to large.  They had a kinked upper jaw, a crested snout, an unusual vertebrae and birdlike hipbones.  Though relationships are somewhat uncertain, they were divided into eight families:

Noasauridae- was a bipedal that was eight feet long.  They had long switchblade toe-claws.  Fossils were found in Argentina.

Alvarezsauridae (Alvarez lizard)- was a bipedal carnivore.  Fossils were found in Argentina.

Abelisauridae (Abel’s lizard)- was a bipedal that grew to be approximately thirty feet long.  Fossils were found in Argentina.

Velocisauridae (quick lizard)- was a small bipedal with unusual upper foot bones.  Fossils were found in Argentina.

Ceratosauridae-had a horn on top of their noses.  They also had hard brow ridges above their eyes and a line of bony plates going down their backs.  They grew to be twenty feet in length and weighed up to one ton.  Fossils were found in North America and Africa.

Coelophysidae- are believed to have been social creatures.  They grew to be ten feet in length.  Fossils were located in North America.

Halticosauridae (nimble lizard)- was a predatory dinosaur that grew to be eighteen feet in length.  Fossils were found in Europe.

Megalosauridae (great lizard)- was ten feet tall and weighed up to one ton.  Their heads were about three feet long.  They had a pigeon-toed gait and held their tails off the ground when they moved.  They were most likely able to move quickly.  They grew to be about thirty feet long.  Fossils were found in Europe and Africa.

Tetanurae is the second infraorder.  They had tails that were stiffened.  They were divided into two family subgroups:  Carnosauria and Coelurosauria.

Carnosauria families

Allosauridae- was a large bipedal predator.  They had huge jaws with bladelike teeth.  They had bulky bodies with muscular legs and sharp claws.  They grew to be thirty-six feet long and weighed up to two tons.  They were found in North American and Australia.

Itemiridae (Itemir lizard)- was a small predator possibly related to Tyrannosaurus.

Dryptosauridae (wounding lizard)- was a bipedal predator that grew to be about twenty feet in length.  They were found in North America.

Tyrannosauridae (tyrant lizard)- was one of the largest flesh-eating dinosaurs.  They had huge jaws and sharp claws.  They ate large herbivorous dinosaurs.  They grew to be thirty-nine feet in length and weighed up to approximately six tons.  They were found in North America.

Aublysodontidae (blunt tooth)- was a bipedal predator found in North America and China.

Spinosauridae (thorn lizard)- were huge predators with a sail like skin and a bulky body.  They ate fish and other dinosaurs.  They grew to be forty-nine feet long and weighed up to about four tons.  They were found in Africa.

Baryonychidae (heavy claw)- were large predators.  They had long bodies and powerful jaws with sharp teeth.  They ate fish and carrion.  They grew to be thirty-four feet long and weighed up to two tons.  They were found in Africa and Europe.

Coelurosauria families

Ornithomimidae-were lightweight creatures weighing only twenty-eight pounds.  They grew to be about seven feet long.  They ate small vertebrates.  They were found in North America.

Deinocheiridae (terrible claw)- was small but heavily armed with bladelike teeth and large claws.  They ate other dinosaurs.  They grew to be about ten feet long and weighed up to one hundred and thirty pounds.  They were found in North America.

Oviraptoridae- was a bipedal, birdlike predator.  They ate mollusks and eggs.  They had a deep beak with two teeth like bones, strong arms with curved claws and a long tail.  They grew to be seven feet long and weigh about seventy-three pounds.

Caenagnathidae- (recent jaw)- was a small bipedal carnivore that was found in North America.

Elmisauidae (foot lizard)- a ten foot long bipedal with bird like feet.  They were found in North America.

Therizinosauridae- grew to be thirty-nine feet long and was found in Mongolia.

Troodontidae (wounding tooth)- was sharp eyed.  They had hands that could grasp and large hook like claws.  They had long legs with three toed feet.  They ate lizards, mammals and other dinosaurs.  They grew to be about seven feet in length.  They were found in North America.

Dromaeosauridae (running lizard)- a six foot bipedal found in North America.

Archaeopterygidae (ancient wing)- had long slim jaws with sharp teeth.  They also had grasping fingers that protruded from their wings and a long tail.  They ate insects and small vertebrates.  They grew to be two feet in length.  They were found in Europe.

Avimimidae-which was a toothless, longlegged dinosaur resembling a bird

Coeluridae (hollow tail)- a six foot long bipedal found in North America.

Compsognathidae (elegant jaw)- was one of the smallest dinosaurs.  They grew to be no more than five feet long and weighed up to seven pounds.  They were fast moving and preyed on invertebrates and lizards.  They were found in Europe.