Dinosaurs: A Comprehensive Guide to Extinct Reptiles

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Introduction to Dinosaur

Dinosaur, an extinct reptile that lived on land. Dinosaurs lived from about 230,000,000 to 65,000,000 years ago, during the Mesozoic Era. Fossil skeletons and other fossil remains of dinosaurs have been found on every continent. Dinosaur fossils are especially plentiful in the United States, particularly in Colorado, Montana, Wyoming, New Mexico, and Utah. Other extinct animals that lived at the same time as the dinosaurs include the pterosaurs, which were flying reptiles, and the plesiosaurs and ichthyosaurs, which lived in water.

Dinosaurs varied greatly in size and shape.

Scientists disagree as to whether dinosaurs were, like most other reptiles, cold-blooded or, like birds and mammals, warm-blooded. (Cold-blooded animals have a body temperature that varies with the temperature of their surroundings; warm-blooded animals can maintain a constant body temperature.)

The word dinosaur means terrible lizard, but not all dinosaurs were large and ferocious. Some were about one foot (30 cm) long. The largest dinosaurs were more than 80 feet (24 m) long and weighed more than 80 tons (72,600 kg). About 800 species have been identified.

Dinosaurs of the Jurassic Period lived roughly 150 million to 200 million years ago.

Some dinosaurs lived in herds, foraged for plants, walked on all fours, and were slow and lumbering. Other dinosaurs lived alone, hunted other animals, walked on their hind legs, and were fast and agile. Many fossilized dinosaur eggs have been found. Scientists believe that most types of dinosaurs laid eggs and that some dinosaurs, unlike most reptiles today, defended their recently hatched young and brought food to them.

Some dinosaurs had small, simple brains; others, especially those that hunted, had large, complex brains. Large dinosaurs probably had specialized nerve centers in the pelvic and shoulder regions of the spinal cord to help control the movements of the limbs and tail. Large dinosaurs probably had very powerful hearts and efficient lungs to supply the body with blood and oxygen.

Dinosaurs developed from a group of primitive reptiles, the thecodonts. Herrerasaurus is one of the earliest known dinosaurs, having lived 230,000,000 years ago. Fossil bones, found in Argentina, reveal that it was about six feet (1.8 m) long and weighed about 300 pounds (136 kg). Coelophysis lived about 215,000,000 years ago in what is now New Mexico. It was about eight feet (2.4 m) long and weighed about 50 pounds (23 kg).

Kinds of Dinosaurs

Dinosaurs are divided into two orders---the Saurischia (derived from the Greek for lizard-hipped) and Ornithischia (birdhipped). The saurischians had hips that resembled those of lizards, while the ornithischians' hips resembled those of birds.

Saurischians

are divided into two groups---the theropods and sauropods. With few exceptions, theropods walked on their hind legs and used their forelimbs for grasping. Some theropods were small, others very large. Nearly all were meat-eaters and had long, pointed teeth. Many scientists believe that certain theropods evolved into birds. Examples of theropods are Tyrannosaurus and Allosaurus.

Orinthomimus was a very fast dinosaur of the Cretaceous period.

Tyrannosaurus, or Tyrannosaurus rex, was probably the most ferocious of all dinosaurs. Fifty feet (15 m) long and weighing about 8 tons (7,300 kg), it was the largest meat-eater ever to live on land. Allosaurus was also a ferocious dinosaur. It was about 30 feet (9 m) long.

Tyrannosaurus rex was a theropod fifty feet long, weighing about eight tons.

Sauropods walked on all four feet and had massive legs. Their teeth were suited only for eating soft, juicy plants. Among the sauropods were the largest land-dwelling animals. Examples of sauropods are Apatosaurus, Brachiosaurus, and Diplodocus.

Apatosaurus was for many years called Brontosaurus and is still sometimes called by that name. It was from 60 to 70 feet (18 to 21 m) long with a relatively short body and a long neck and tail. Its weight was probably 30 to 35 tons (27,200 to 31,800 kg). Another giant sauropod, 80 feet (24 m) long, was Brachiosaurus. Unlike other sauropods, it had forelegs larger than its hind legs and the body sloped toward the tail. The nostrils were above the eyes. Brachiosaurus was the largest dinosaur; some probably weighed more than 80 tons (72,600 kg). Diplodocus was longer, but not as heavy as Brachiosaurus. It was about 87 feet (27 m) long.

The brachiosaurus was a huge, herbivorous dinosaur.Ornithischians

were plant-eaters. Some walked on their hind legs, others on all fours. Most ornithischians developed structures for defense against their enemies---primarily the meat-eating dinosaurs. Ornithischians are divided into four groups---the armored, plated, horned, and duck-billed dinosaurs.

Ankylosaurus, a typical armored dinosaur, had a broad, squat body with strong, stumpy legs. It was about 33 feet (10 m) long, 6 feet (1.8 m) wide, and about 4 feet (1.2 m) tall. The back, sides, and tail were covered with rows of raised bony plates. The tail ended in a large, clublike mass of bone that was probably used as a weapon.

Stegosaurus, a plated dinosaur, was more than 25 feet (8 m) long and weighed 2 tons (1,800 kg). It had massive hind legs that were twice the length of the forelegs. Extending down its back was a double row of triangular bony plates standing upright. The largest plates, above the hind legs, were 3 feet (90 cm) high. The plates possibly served as protection or as a means of regulating body temperature by heat exchange with the atmosphere. The tail had four spikes, and was probably used as a weapon.

Stegosaurus was a plated dinosaur more than twenty-five feet long.

Triceratops was the largest of the horned dinosaurs. The largest known specimen was 30 feet (9 m) long and weighed approximately 12 tons (10,900 kg). It had three horns---a short one above the snout and a long one above each eye. The head was protected by a large bony structure that extended back over the neck and shoulders.

Anatosaurus, a duck-billed dinosaur, had an elongated skull with a broad, flattened snout and a toothless beak. It was about 30 feet (9 m) long and 13 feet (4 m) tall and weighed about 4 tons (3,600 kg). Anatosaurus usually walked on all four legs. Fossil remains show that it ate twigs, seeds, and fruits and that it had a thick, leathery hide.

Dinosaurs of the of the Cretaceous Period lived roughly 65 million to 150 million years ago.

Why Dinosaurs Became Extinct

Dinosaurs were extinct for millions of years before humans appeared. There is no complete explanation for why the dinosaurs became extinct. Among the many theories that have been proposed for the extinction are the following: (1) dinosaurs evolved such awkward bodies that they could not breed; (2) infectious diseases wiped out the dinosaurs; (3) small mammals fed on dinosaur eggs; and (4) sea levels dropped, creating land bridges that allowed dinosaurs previously separated to invade each other's territory, upsetting the balanced ecological relationships that existed between species. Most of these explanations fail to consider that the dinosaurs were well adapted to their environment, living successfully for millions of years, and that many other creatures became extinct at about the same time as the dinosaurs.

The most likely explanation for the extinction of the dinosaurs is that there was some type of drastic global change, to which the dinosaurs were extremely vulnerable. Such a change could have resulted from a catastrophe that affected the climate and vegetation, reducing the food supply, or decreased the oxygen concentration of the atmosphere. Some scientists believe that the dinosaurs had already been dying off gradually (possibly for some of the reasons listed in the preceding paragraph) and the catastrophe greatly accelerated their decline.

Scientists have proposed several theories to describe how such a global catastrophe might have occurred, including: (1) an extremely large object from space crashed into the earth, producing an explosion that would have created immense clouds of dust and ignited vast forest fires; (2) massive volcanic eruptions ignited fires and emitted volcanic gases and ash; and (3) a combination of the first two theories---that is, an object from space hit a weak spot in the earth's crust, causing violent volcanic eruptions. The clouds of dust or ash resulting from these catastrophes would have blocked out sunlight for months or years, chilling the land and killing plants. The dinosaurs and certain other species would have died from a lack of food and warmth.

In 1991, scientists discovered evidence in the Yucatn of a very large crater dating from the time the dinosaurs became extinct. The crater, which is now filled in and buried, was probably formed by the impact of a comet or asteroid. Many scientists believe this impact could have produced a global catastrophe leading to the dinosaurs' extinction.