Dinosaurs are a
varied group of animals of the clade Dinosauria. They first emerged during the
Triassic period, 231.4 million years ago, and were the main terrestrial
vertebrates for 135 million years, from the commencement of the Jurassic (about
201 million years ago) till the end of the Cretaceous (66 million years ago),
when the Cretaceous–Paleogene annihilation event led to the annihilation of
most dinosaur groups at the close of the Mesozoic Era.
The fossil record points
that birds grew from theropod dinosaurs throughout the Jurassic Period and, as
a result, they are deemed to be a subgroup of dinosaurs by many archaeologists.
Some birds survived the extinction event that took place 66 million years before,
and their descendants maintain the dinosaur lineage to the current day.
