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Richard Fortey writes in his book, Life: An Unauthorised Biography, "A common perception of geological history is that the world moved rapidly from a kind of greenish primeval soup through to the dinosaurs, which then (mysteriously) became extinct leaving the world free to be populated by people wearing skins over one shoulder and carrying clubs." (p.242).
Dinosaurs seem to have a fascination about them, like monster stories from Greek mythology, and the knowledge among the public about some species from the dinosaur families exceeds by far knowledge about some existing modern life forms.
Dinosaurs occupy a particular niche in the public mind; the very word conjures up images of gigantic fearsome beasts that ruled the Earth long ago, reigning for some 160 million years, and then mysteriously vanishing with only their titanic bones as evidence of their existence.
Dinosaurs were animals that evolved into many sizes and shapes.
Dinosaurs were quite diverse, and while one person will associate
dinosaurs with a long-necked animal (sauropod), others will think
of a large, fierce meat-eater like Tyrannosaurus rex. The
term "dinosaurs", or the scientific version "Dinosauria", describes
a diverse group of animals with widely different modes of living.
Sir Richard
Owen used this term in 1842 to describe these "fearfully great
reptiles", specifically Megalosaurus, Iguanodon,
and Hylaeosaurus, the only three dinosaurs known at the
time.
The creatures that we normally think of as dinosaurs lived from late in the Triassic period (about 248 million years ago) until the end of the Mesozoic era (about 65 million years ago). Today they are survived by their descendants, the birds.
When did
the dinosaurs first appear on Earth?
The oldest dinosaur types are known from rocks in Argentina and
Brazil and are about 230 million years old. The most primitive of
these types, Eoraptor, was a small meat-eating dinosaur.
Because Eoraptor's skeleton shows some advanced skeletal
features, older dinosaurs may yet be found. An article about one
very old dinosaur can be found at: http://news.bbc.co.uk/1/hi/sci/tech/1352871.stm
Not everything
big and dead is a dinosaur.
Often books written about dinosaurs for a popular audience include
animals such as mammoths, mastodons, plesiosaurs, ichthyosaurs, Pteranodon and
the sail-backed Dimetrodon. These animals were not dinosaurs
but mammals (mammoths and mastodons) and their ancestors (Dimetrodon),
archosaurs (like pterosaurs), or the marine plesiosaurs and ichthyosaurs,
which were distantly related to the dinosaurs. Dimetrodon
was an early relative of the ancestors of mammals.
Dinosaurs are a
specific subgroup of the archosaurs
, a group that includes crocodiles and birds. They came in a range
of sizes, from animals about the size of a turkey up to the enormous
sauropods e.g. Brachiosaurus.
Dinosaurs were land reptiles that evolved an upright gait similar
to that of mammals, i.e. their legs were straight and held directly
under the body, perpendicular to the ground and supporting the weight
of the body so that they could walk or run more efficiently. Other
reptiles and modern lizards have a sprawling gait.
Dinosaurs,
the long-lived giants?
The reason why some dinosaurs evolved into such giants is not well
understood. No modern animals except whales are even close in size
to the largest dinosaurs. Whales, the largest animals ever, can
achieve their great size only because the water buoys them up against
gravity. There were advantages to being a giant on land: a large
body size could offer some protection against predators, and giant
herbivores could have enormously long guts, which will have allowed
maximal digestion and absorption of their often low-quality food.
Their size could have helped to regulate internal body temperature,
and many authors believe that the dinosaurs used mass homeothermy
to allow them to achieve a high and relatively constant body temperature.
Mass homeothermy is when animals "use" their mass as a means of
achieving a fairly constant body temperature. The idea being that,
once you are warm, the bigger you are, the slower you cool. A bigger
size could have let them reach new sources of food (browsing in
treetops, as giraffes do today). Probably some combination of these
factors was involved. The key to their size lies probably in their
environment. During the Jurassic when gigantic sauropods appeared
like the mighty Brachiosaurus,
the plains were probably a great mixture of vegetation (flowering
plants had not evolved at this time) and a large feeding platform
may have been the best design to exploit a variety of different
tough plants at different heights.
Dinosaur lifespans probably varied in length from tens of years to hundreds of years and would have been determined in part by their body size and in part by their type of metabolism. Their possible maximum age can be estimated from the maximum lifespans of modern reptiles (i.e. 66-year lifespan of the common alligator Alligator mississippiensis). A now-extinct Black Seychelles Tortoise (Geochelone (Aldabrachelys) sumeirei), which was an adult when captured, lived a record 152 years in captivity (1766-1918) and had an accidental death. These estimates, based on lifespans of cold-blooded animals, would be too long if dinosaurs had metabolisms more similar to modern birds and mammals; there is good evidence that at least some dinosaurs were warm-blooded.
Not all dinosaurs
lived at the same time.
Different dinosaurs lived at different times. Despite the movies like Fantasia and Jurassic Park, no Stegosaurus ever saw a Tyrannosaurus, because Tyrannosaurus wasn't alive for another 80 or so million years.
By the end of the Palaeozoic Era mammal-like reptiles began to be replaced by the dinosaurs who were the dominant animals for 170 million years spanning over the periods of the Triassic, the Jurassic and the Cretaceous.
Triassic dinosaurs included: Coelophysis, Postosuchus,
Placerias
and Plateosaurus
and many more.
Jurassic dinosaurs included: Diplodocus, Brachiosaurus,
Stegosaurus, Allosaurus
and many more.
Cretaceous dinosaurs included: Iguanodon, Tyrannosaurus Rex,
Parasaurolophus
and many more.
Dinosaurs are
not extinct.
Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous Period), after living on Earth for about 165 million years. The dinosaurs' long period of dominance certainly makes them a great success story in the history of life on Earth. However, based on features of the skeleton, most people studying dinosaurs also consider birds to be dinosaurs. Rather than speak of "dinosaurs" and birds as discrete, separate groups, they refer to the traditional, extinct animals as "non-avian dinosaurs" and birds as birds, or avian dinosaurs. It would be incorrect to say that dinosaurs are completely extinct, because they have left living descendants in the form of cockatoos, and cassowaries, just as modern vertebrates are still vertebrates even though their Cambrian ancestors are long extinct.
A dinosaur Lesson prepared for Teachers and their students on:
http://www.sedl.org/scimath/pasopartners/dinosaurs/welcome.html
 Reference
Books
Bakker, R. (1986) The Dinosaur Heresies; Penguin
Fortey R.(1998): Life: An Unauthorised Biography; London, Flamingo.
Haines T. (1999): Walking with dinosaurs, London, BBC Worldwide.
 Reference
Websites
Dinosaurs: Facts and Fiction: http://pubs.usgs.gov/gip/dinosaurs/
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