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| Dinosaur |
Owen distinguished dinosaurs from other
prehistoric reptiles by their upright rather than sprawling legs and by the
presence of three or more vertebrae supporting the pelvis, or
hipbone. Dinosaurs are classified into two orders according to differences in
pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or
bird-hipped dinosaurs. Dinosaur bones occur in sediments that were deposited
during the Mesozoic Era, the so-called era of middle animals, also
known as the age of reptiles. This era is divided into three periods: the
Triassic (251 million to 200 million years ago), the
Jurassic (200 million to 145 million years ago), and the
Cretaceous (145 million to 65 million years ago).
Historical references to dinosaur bones may
extend as far back as the 5th century bc. Some scholars think that Greek
historian Herodotus was referring to fossilized dinosaur skeletons
and eggs when he described griffins—legendary beasts that were part
eagle and part lion—guarding nests in central Asia. “Dragon bones” mentioned in
a 3rd century ad text from China
are thought to refer to bones of dinosaurs.
The first dinosaurs studied by
paleontologists (scientists who study prehistoric life) were
Megalosaurus and Iguanodon, whose partial bones were
discovered early in the 19th century in England. The shape of their bones
indicates that these animals resembled large, land-dwelling reptiles. The teeth
of Megalosaurus, which are pointed and have serrated edges, indicate that
this animal was a flesh eater, while the flattened, grinding surfaces of
Iguanodon teeth indicate that it was a plant eater. Megalosaurus
lived during the Jurassic Period, and Iguanodon lived during the early
part of the Cretaceous Period. Later in the 19th century, paleontologists
collected and studied more complete skeletons of related dinosaurs found in New
Jersey. From these finds they learned that Megalosaurus and
Iguanodon walked on two legs, not four, as had been thought.
In the late 19th and early 20th centuries, as
the science of paleontology grew and the search for dinosaur
remains was extended around the world, new kinds of dinosaurs were discovered.
Hundreds of different varieties of dinosaur have been identified from bones
found on all of the continents as well as on the islands of Greenland,
Madagascar, and New Zealand.
Dinosaurs belong to a group of advanced
reptiles called archosaurs, which appeared late in the
Permian Period. Archosaurs survive today in the form of
crocodiles and birds. In addition to dinosaurs,
extinct archosaurs included pterosaurs (flying reptiles) and a
number of other types of reptiles formerly grouped together under the name
thecodonts. During the Triassic, two distinct groups of archosaurs
evolved, one related to crocodiles and the other related to dinosaurs, birds,
and pterosaurs.
Scientists were once confused about the early
ancestors of dinosaurs because some of the archosaurs related to crocodiles
developed a number of dinosaur-like features, including walking upright on two
hind legs. This phenomenon is known as parallel evolution, in which
animals that are not directly related come to resemble each other because they
have similar life styles or diets. New fossils have allowed paleontologists to
better recognize the true ancestors of dinosaurs. The ancestors of dinosaurs
also walked on two hind legs, but had S-shaped necks; simple, hingelike ankle
bones; and other distinctive features.
Fossil evidence of the earliest dinosaurs
dates from about 230 million years ago. This evidence, found in
Madagascar in 1999, consists of bones of an animal about the size
of a kangaroo. This dinosaur was a type of saurischian and was a member of the
plant-eating prosauropods, which were related to ancestors of the giant,
long-necked sauropods that included the Apatosaurus. Prior to this
discovery, the earliest known dinosaur on record was the Eoraptor,
which lived 227 million years ago. Discovered in Argentina in 1992, the
Eoraptor was an early saurischian, 1 m (3 ft) long, with a primitive
skull.
Scientists have identified remains of a few
small dinosaurs representing ornithischians dating from the end of the Triassic
Period before about 200 million years ago. By the middle of the Jurassic Period,
around 180 million years ago, most of the basic varieties of saurischian and
ornithischian dinosaurs had appeared, including some that far surpassed modern
elephants in size. Dinosaurs had evolved into the most abundant large animals on
land, and the dinosaurian age had begun.
Earth’s environment during the dinosaurian
era was far different than it is today. The days were several minutes shorter
than they are today because the gravitational pull of the sun and the moon have
over time had a braking influence on Earth’s rotation. Radiation from the Sun
was not as strong as it is today because the Sun has been slowly brightening
over time.
Other changes in the environment may be
linked to the atmosphere. Carbon dioxide, a gas that traps heat from the Sun in
Earth’s atmosphere—the so-called greenhouse effect—was several
times more abundant in the air during the dinosaurian age. As a result, surface
temperatures were warmer and no polar ice caps could form.
The pattern of continents and
oceans was also very different during the age of dinosaurs. At the
beginning of the dinosaurian era, the continents were united into a gigantic
supercontinent called Pangaea (all lands), and the oceans formed a vast
world ocean called Panthalassa (all seas). About 200 million years ago,
movements of Earth’s crust caused the supercontinent to begin slowly separating
into northern and southern continental blocks, which broke apart further into
the modern continents by the end of the dinosaurian era.
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As a result of these movements of Earth’s
crust (see Plate Tectonics), there was less land in
equatorial regions than there is at present. Deserts, possibly produced by the
warm, greenhouse atmosphere, were widespread across equatorial land, and the
tropics were not as rich an environment for life forms as they are today. Plants
and animals may have flourished instead in the temperate zones north and south
of the equator.
The most obvious differences between
dinosaurian and modern environments are the types of life forms present. There
were fewer than half as many species of plants and animals on land during the
Mesozoic Era than there are today. Bushes and trees appear to have provided the
most abundant sources of food for dinosaurs, rather than the rich grasslands
that feed most animals today. Although flowering plants appeared during the
dinosaurian era, few of them bore nuts or fruit.
The animals of the period had slower
metabolisms and smaller brains, suggesting that the pace of life was relatively
languid and the behavior patterns were simple. The more active animals—such as
ants, wasps, birds, and mammals—first made their appearance during the
dinosaurian era but were not as abundant as they are now.
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