Dinosaurs were a group of animals that first appeared during
the Triassic period
of the
Mesozoic Era,
perhaps around 230 million years ago.
They evolved from early
Archosaurs
(see dinosaur family tree)
and went on to become the most successful land animals for the
next 165 million years ago.
At some point during this era
(probably during the
Jurassic period),
the
first birds
evolved from dinosaurs.
Then, suddenly, about 65 million years ago,
all the dinosaurs (except birds which are technically a group of
living dinosaurs),
became
extinct.
The original definition of the taxon Dinosauria
was made by the British paleontologist,
Sir Richard Owen,
in 1842.
Owen
used the name "Dinosauria" (which means "terrible lizards")
to refer to a specific group of extinct reptiles, of which a number of
examples had then be found.
Today, building on
Owen's
work, dinosaurs are defined based on a number of common skeletal features.
The most
important of these being an erect posture (similar to that of modern
mammals), which allowed dinosaurs to breathe more easily while
moving, and probably assisted dinosaurs in living an active lifestyle.
(It should be noted, a different type of erect posture, known as
"Pillar-erect" seems to have independently evolved in other
groups of
Triassic
reptiles - Aetosaurs and Rauisuchians).
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