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.

Dinosaurs 8.2 of 10 on the basis of 2835 Review.