Dinosaurs dominated the earth for more than 160 million years, from the late Triassic period to the end of Cretaceous period. The name 'Dinosaur' was derived from the Greek words, 'denios', meaning terrible or powerful, and 'sauros', meaning lizard. The term was coined by the famous English biologist Sir Richard Owen in 1842. On the basis of the structure of their hips or pelvic bones, they are generally divided into two orders, namely lizard-hipped or Saurischian dinosaurs, and bird-hipped or Ornithischian dinosaurs. They can also be classified as carnivorous or meat-eating, herbivorous or plant-eating, and omnivorous or both plant and meat-eating dinosaurs. Here is an account of the different types of dinosaurs, that once thrived on earth.
Acrocanthosaurus
Acrocanthosaurus was a carnivorous bipedal i.e. two-footed dinosaur, that mainly lived in present North America about 125 to 100 million years ago, during mid Cretaceous period. Its fossil was first discovered in Oklahoma, Texas. It usually grew up to a length of 40 feet and weighed 2.4 tons. The distinguishing feature of this dinosaur was the large neural spine on the vertebrae, extending from the neck to the upper tail, which was presumed to act as a reservoir of fat and temperature regulator. Its forelimbs were strongly built, but stiff and short. The head was long and narrow, while the tail was heavy and long. This heavy tail helped in balancing the weight of the head and the body.
Albertosaurus
Albertosaurus mainly inhabited North America during the late Cretaceous period, approximately 76 to 74 million years ago. The first specimen of this animal, a partial skull was found in 1884 near Red Deer River in Alberta, Canada. The forelimbs of this meat-eating bipedal dinosaur were very small with two fingers, while the hindlimbs were long with four-toed foot. The weight of this 26 feet long dinosaur was 3 tons. It had a massive head with sharp teeth. The name Albertosaurus was given by an American geologist and paleontologist, Henry Fairfield Osborne.
Allosaurus
Allosaurus, the huge predator lived in the late Jurassic and early Cretaceous period, about 154 to 135 million years ago in North America. However, few fossil specimens were also discovered in Africa and Australia. The name was derived from the Greek words, 'allos', meaning strange or different, and 'saurus', meaning large dinosaurs.
Allosaurus was generally 30 feet long, though some of them grew up to 39 feet. This bipedal dinosaur had a large head with sharp teeth and a short neck. Hindlimbs were large and powerful, while the forelimbs were comparatively small and three-fingered. The weight of the body was counterbalanced by a heavy, long tail. They were supposed to hunt in groups and were at the top of the food chain. It is believed that they used to hunt not only the large herbivorous dinosaurs, but even the predators or the carnivorous dinosaurs.
Gigantosaurus
Gigantosaurus could grow up to a length of 43 feet, and the largest of them were estimated to be 6.2 tons. Gigantosaurus inhabited Argentina, almost 93 to 89 million years ago during the late Cretaceous period. The fossil of this huge animal was discovered in 1993 by Ruben Carolini in southern Argentina. The animal was supposed to have a good sense of smell, as a well-developed olfactory region was found in the brain.
Tyrannosaurus rex
This bipedal carnivorous dinosaur with a massive skull and long, heavy tail lived in North America and Asia in the late Cretaceous period, about 93 to 89 million years ago. The average length of this huge animal was 39 feet, however the largest specimen found till now is 42 feet long. The average height was estimated to be 20 feet, while the average weight was 7 tons. Before the discovery of Gigantosaurus, it was thought to be the largest dinosaurs.
Tyrannosaurus was not a fast runner, and hence thought to be mainly a scavenger. Forelimbs were small, but strongly built, and the hindlimbs were the longest among theropod dinosaurs in comparison to body size. Two-clawed fingers were present in the hindlimbs, while remnant of a third finger could also be detected. The skull was wide at the rear, but narrow at snout. The jaw contained curved large teeth, the largest being 30 cm in length, and was the largest for any carnivorous dinosaur.
Spinosaurus
Spinasourus or spine lizards inhabited the earth almost 95 to 70 million years ago. According to previous estimation, its average length was 39 feet and weight was 7 tons. However, recent studies have shown that, it was a giant animal that weighed about 9 tons with a length of 51 feet, which makes it the largest known carnivores. The distinguishing feature of this dinosaur was the presence of 6 feet long, fin-shaped spine on its back, which was supposed to assist in temperature regulation. Its skull was 1.75 meter long, one of the largest skull of any carnivore.
Achelousaurus
Achelousaurus was believed to be the transitional form between the ceratopsians (horned dinosaurs) and the hornless ones. It lived in North America during Cretaceous period about 83 to 74 million years ago. The average length of this animal was about 20 feet, while the weight was 6 tons. The first fossil specimen was discovered in 1995 by Scott Sampson in Montana, and was named after Achelous, the deity of Greek mythology. The characteristic feature of Achelousaurus are, a parrot-like beak, a rough boss or circular projection on the snout and behind the eyes and a long bony-neck frill with two horns.
Alamosaurus
Alamosaurus was a massive herbivorous dinosaur that inhabited North America and present day southwestern part of United States during late Cretaceous period, between 70 to 65 million years ago. The average weight of this animal was estimated to be 33 tons, and it usually grew up to a length of 69 feet. The first fossil was found in 1992 by Charles W. Gilmore. Its neck and tail were remarkably long.
Apatosaurus
Apatosaurus was a plant eater, about 70 feet long, that lived in the late Jurassic period almost 157 to 146 million years ago in western North America. The average weight of this giant animal was estimated to be 33 tons. Earlier, it was thought that due to its heavy weight, it could not support its weight on land, and hence remained submerged in water. However, recent discoveries do not support this conviction. The most distinguishing and interesting feature of this animal was the large long-neck and the long whip-like tail, which was used for defense. Its forelimbs were slightly larger than the hindlimbs, and the skull was comparatively small.
Brachiosaurus
Brachiosaurus lived on earth during the late Jurassic period and even in the early Cretaceous period almost 156 to 145 million years ago. It was 75 feet long, 41 feet tall and weighed around 89 tons. It was so long that, it could reach for the foliage at the top of the trees. It is one of the largest as well as heaviest land animals for which a complete skeleton exists.
Diplodocus
The first fossil of this dinosaur was found in 1877 by S.W. Williston. The name 'Diplodocus' was derived from the Greek words, 'diploos', meaning double and 'dokos', meaning beam. The characteristic feature was the double-beamed chevron bones present in the undersurface of the tail. It lived in North America during Jurassic period about 145 million years ago. Legs of Diplodocus were very strong, and the forelimbs were slightly shorter than the hindlimbs. Head was small as compared to the size of the body.
Dinosaurs became extinct from the face of Earth about 65 million years ago. But, scientists think that many animals have descended from dinosaurs through the evolutionary process. Due to the similarity between theropod dinosaurs and birds, dinosaurs are thought to be direct ancestors of birds. Many biologists are of the opinion that they should be grouped under one biological class. Crocodiles are also regarded to be another close relative of dinosaurs, that are still thriving on the planet Earth.
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