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Dinosaur is the name of large extinct reptiles of the Mesozoic Era,

during which they were the dominant land animals on Earth. The term was

proposed as a formal zoologic name in 1842 by the British anatomist Sir Richard

Owen, in reference to large fossil bones unearthed in southern England. The

various kinds of dinosaurs are classified in two formal categories, the orders

Saurischia and Ornithischia, within the subclass Archosauria.

The first recorded dinosaur remains found consisted of a few teeth and

bones. They were discovered in 1882 in Sussex, England, by an English doctor,

Gideon Mantell, who named them iguanodon. About the same time, other fossil

teeth and bones were found near Oxford, England, by Rev. William Buckland.

These were named Megalosaurus. Thousands of specimens have since been

discovered nearly worldwide.

Different types of dinosaurs varied greatly in form and size, and they

were adapted for diverse habitats. Their means of survival can only be

identified from their fossil remains, and some identifications are in dispute.

They ranged in weight from 4 to 6 lb., in the case of the compsognathus, and up

to 160,000 lb., in the case of the brachiosaurus. Most dinosaurs were large,

weighing more than 1,100 lb., and few weighed less than 100 lb. Most were

herbivores, but some saurischians were carnivorous. The majority were four-

footed but some ornithischians and all carnivores walked on their hind legs.

Always classified as reptiles, dinosaurs have traditionally been assumed

to have been reptilian in their physiology, cold-blooded, and ectothermic. In

recent years several different lines of evidence have been interpreted as

indicating that dinosaurs may have had warm blood and high rates of metabolism,

comparable to birds and mammals. Evidence supporting this view includes upright

posture and carriage; mammallike microscopical structure of bones; skeletal

features suggestive of high activity; and specialized food-processing dentitions

and low ratios of dinosaurian predators to prey animals, both suggesting high

food requirements. The evidence is not conclusive--all the facts can be

alternatively explained--but some dinosaurs may have been endothermic.

The reproductive means of most dinosaurs is as yet unknown. Fossil eggs,

attributed to one of the horned dinosaurs and a sauropod, have been discovered

in Mongolia and France. Fragments that are presumed to be of dinosaur eggs have

also been found in Brazil, Portugal, Tanzania, and in the United States,

Colorado, Montana, and Utah. In Montana, Utah, and Alberta, Canada, fossils of

unhatched dinosaur eggs have been discovered. This evidence indicates egg-

laying reproduction in dinosaurs, like most modern reptiles. A few scientists

believe that some dinosaurs may have given birth to living young, but no

conclusive evidence has yet been found to support this.

The two orders of dinosaurs are distinguished by numerous features, the

most diagnostic being the arrangement of the three bones of the pelvious. In

saurischians, these bones were arranged in a triradiate pattern similar to that

of modern crocodilians and lizards; the term Saurischia means lizard hip. The

ornithischian pelvis was usually rectangular or tetraradiate; hence the name,

which means bird hip.

During the 140-million-year reign of the dinosaurs, many new varieties

evolved and older kinds died out. Not all kinds became extinct at once; but

the last of the dinosaurs disappeared at the end of the Cretaceous. Many other

animal kinds died out at about the same time, including the ichthyosaur,

mosasour, plesiosaur, flying reptile, and a variety of lower organisms. What

brought about such widespread extinction among so many different kinds of

organisms is not known; it must, however, have involved major changes in the

environment. Their extinction has been attributed to many causes, including

cosmic radiation, exploding supernova, world-wide fluctuations in sea level,

acid rain caused by volcanic activity, climatic change, and continental drift.

Independent evidence indicates that sea levels did fall and temperatures dropped

at the end of the Mesozoic Era, a time when continents were drifting apart and

new mountain ranges were rising. Although none of these conditions is likely to

have been solely responsible for the extinction of the dinosaurs, collectively

they may have been important.

Whatever the cause, the dinosaurs are now gone. In a way, however, they

may remain. That is, many paleontologists consider birds almost certainly to

have evolved from some small bipedal dinosaur during the Jurassic. If so, the

children of the dinosaurs still exist today.

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