Dinosaurs were reptiles that dominated the earth for 160 million years before being wiped out by a meteor. They gave rise to modern birds and were part of the first wave of biodiversity after the Permian-Triassic extinction event. Dinosaurs are defined as descendants of the common ancestor of Triceratops and modern birds. They are divided into two groups based on hip structure and ranged in size from chicken-sized to 148 feet long and weighing up to 100 tons. The largest dinosaur ever discovered may have been 200 feet long and weighed 220 tons.
Dinosaurs were a group of archosaur reptiles that dominated the earth for 160 million years, from the late Triassic (230 million years ago) to the end of the Cretaceous (65 million years ago), when they were wiped out by a meteor. The only surviving line of dinosaurs gave rise to modern birds. The first dinosaur evolved about 21 million years after the Permian-Triassic extinction event wiped out most life. They were part of the first major wave of biodiversity that followed this cataclysmic event.
Dinosaurs are members of the superorder Dinosauria, which also includes birds. Their closest relatives are pterosaurs, flying reptiles sometimes mistakenly called dinosaurs. Pterosaurs and dinosaurs are part of an unclassified clade of archosaurs called Ornithodira. The only surviving archosaurs other than birds are the Order Crocodilia, which includes crocodiles, alligators, gharials, and caimans.
Officially, dinosaurs are generally defined as all descendants of the common ancestor of the Triceratops dinosaur and modern birds. This common ancestor lived in the Late Triassic, so dinosaurs comprise a huge group. Triceratops and modern birds were chosen because they are somewhat distantly related, so the descendants of their common ancestor are numerous and include all dinosaurs.
The definition based on morphology rather than phylogeny is “terrestrial archosaur reptiles with limbs held erect under the body”. If crocodilians evolved to walk with their limbs erect under their bodies, they would fit this definition.
Dinosaurs are divided into two large groups depending on the structure of the hip: saurischids and ornithischids. These mean “lizard hips” and “bird hips.” Lizard-pelvis dinosaurs include theropods (mostly bipedal carnivores, including Tyrannosaurus rex, and sauropodomorphs (large, long-necked, quadrupedal carnivores). Bird-pelvis dinosaurs included many different herbivores, including hadrosaurids duck-billed”), pachycephalosaurians (“bone heads”), ankylosaurians (armored dinosaurs with club-shaped tails), stegosaurians (armored dinosaurs with pointed tails), ceratopsians (Tceratops and relatives), and various generic.
Dinosaurs ranged in size from the size of a chicken (Compsognathus) to the largest sauropods, which were 30 – 45 meters (100 – 148 feet) long and weighed 60,000 – 100,000 kilograms (60 – 100 tons), if not more. The largest dinosaur ever discovered, Amphicoelias fragillimus, may have reached 60m (200ft) in length, about as long as a 20-story building, although it’s based only on descriptions of a femur and vertebrae that have since crumbled into dust. Bruhathkayosaurus, another sauropod, may have weighed 220 tons, more than a blue whale, although the estimate is disputed.
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