Dinosaurs are animals that dominated the terrestrial ecosystem
for over 100 million years. Non-avian dinosaurs became extinct at the
end of the Cretaceous period, 65 million years ago. Knowledge about
dinosaurs comes from both fossil and nonfossil records, including
fossilized bones, feces, trackways, gastroliths, feathers, impressions
of skin, internal organs and "soft tissues." Since the first dinosaur
was recognized in the 19th century, their mounted skeletons have
become major attractions at museums around the world. Dinosaurs have
become a part of world culture and remain consistently popular,
especially among children. They have been featured in best-selling
books and blockbuster films such as Jurassic Park, and new
discoveries are regularly covered by the media. The term is also used
informally to describe any prehistoric reptile, such as the pelycosaur
Dimetrodon, the winged pterosaurs, and the aquatic
ichthyosaurs, plesiosaurs, and mosasaurs, though none of these are
dinosaurs. The on-going dinosaur renaissance began in the 1970s and
was triggered, in part, by John Ostrom's discovery of Deinonychus
-- an active, vicious predator that may have been warm-blooded
(homeothermic), in marked contrast to the prevailing image of
dinosaurs as sluggish cold-blooded reptiles. Vertebrate paleontology
has also become a global science, with major new discoveries in
previously unexploited regions, including South America, Madagascar,
Antarctica, and most significantly the amazingly well-preserved
feathered dinosaurs in China, which have further solidified the link
between dinosaurs and their living descendants, the 9,000+ species of
modern birds. The widespread application of cladistics, which
rigorously analyzes the relationships between biological organisms,
has also proved tremendously useful in classifying dinosaurs, which
are still known from an incomplete fossil record.
What is a dinosaur?The superorder or clade "Dinosauria" was
formally named by the English scientist Richard Owen in 1842. The term
is a combination of the Greek words deinos ("terrible" or
"fearfully great" or "formidable") and sauros ("lizard" or
"reptile"). Contrary to popular perception, the name was chosen to
express Owen's awe at the size and majesty of the extinct animals, not
out of fear or trepidation at their size and formidable arsenal.
Dinosaurs are extremely varied. Some are herbivorous, others
carnivorous; some bipedal, others quadrupedal. (For details on the
various types of dinosaurs, see Classification below.)
There is an almost universal consensus among paleontologists that
birds are the descendants of theropod dinosaurs. Using the strict
cladistical definition that all descendants of a single common
ancestor are related, modern birds are dinosaurs and dinosaurs
are, therefore, not extinct:
Size
Only a tiny percentage of animals ever fossilize, and most of these
remain buried in the earth. As a result, the smallest and largest
dinosaurs will probably never be discovered. Even among those
specimens that are recovered, few are known from complete skeletons,
and impressions of skin and soft tissue are rare. Reconstructing a
skeleton by comparing the size and morphology of bones to those of
similar, better-known species is inexact, and restoring the muscles
and other organs is, at best, educated guesswork.
While the largest and smallest dinosaurs will probably remain unknown,
and comparisons between existing specimens is imprecise, it is clear
that, as a group, dinosaurs were large. By dinosaur standards the
sauropods were gigantic. The smallest sauropods were larger than
anything else in their habitat, and the largest were an order of
magnitude more massive than anything that has ever walked the Earth.
Classification
Main article: Dinosaur classification
Dinosaurs are archosaurs, like modern crocodilians. These are set
apart by having diapsid skulls, having two holes where jaw muscles
attach, called temporal fenestrae. Birds and most reptiles are
diapsids; mammals, with only one temporal fenestra, are called
synapsids; and turtles, with no temporal fenestra, are anapsids.
Dinosaurs also have teeth that grow from sockets (an archosaur
characteristic) rather than as direct extensions of the jaw bones, as
well as various other characteristics. Within this group, the
dinosaurs are set apart most noticeably by their gait. Instead of legs
that sprawl out to the side, as found in lizards and crocodylians,
they have legs held directly under their body.
Many other types of reptiles lived at the same time as the
dinosaurs. Some of these are commonly, but incorrectly, thought of as
dinosaurs: these include plesiosaurs (which are not closely related to
the dinosaurs) and pterosaurs, which developed separately from
reptilian ancestors in the late Triassic.
Dinosaurs are divided into two major orders, the Saurischia
and the Ornithischia, on the basis of hip structure.
Evolution
Dinosaurs split off from their archosaur ancestors during the
Triassic period.
The first known dinosaurs appeared approximately 230 Ma, about 20
million years after the Permian-Triassic extinction event wiped out
about 70 percent of all biological diversity on the planet. A few
lines of primitive dinosaurs diversified rapidly after the Triassic,
and quickly expanded until they filled most of the vacant ecological
niches. During the reign of the dinosaurs, which encompassed the
ensuing Jurassic and Cretaceous periods, nearly every terrestrial
animal larger than 1 m in length (that we know of) was a dinosaur.
The Cretaceous-Tertiary extinction event, 65 Ma at the end of the
Cretaceous, caused the extinction of all dinosaurs except for the line
that had already led to the first birds.
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