Dinosaurs evolved 230 million years ago from archosaurs, after the Permian-Triassic extinction event. They became dominant due to better strategies for coping with arid environments, and quickly represented 50-90% of terrestrial fauna. The two main types, theropods and sauropods, occupied most ecological niches and became progressively larger.
Dinosaurs evolved about 230 million years ago, splitting off from their archosaur cousins in the Early Triassic, 21 million years after the catastrophic Permian-Triassic extinction event that wiped out most life on Earth. Like other archosaurs, dinosaurs are characterized by socketed teeth, which make them less prone to snapping off when feeding, distinctive holes in the skull, and a special crest for attaching muscles to the femur.
Before the Permian-Triassic extinction event, archosaurs were a relatively obscure group of reptiles. In the Late Permian, the land was dominated by therapsids, ancient relatives of mammals. Most therapsids did not survive the end-Permian extinction, or the arid world that followed, very well. This has left several important ecological niches open, which archosaurs have begun to exploit. Many scientists believe the reason for the rise of archosaurs and dinosaurs has to do with better strategies for coping with arid environments. For example, archosaurs can release uric acid – a byproduct of metabolism found in urine – as a paste rather than a liquid, while conserving water. The lack of glandular skin would also have been an advantage.
What is thought to be the common ancestor of all dinosaurs, Eoraptor, a 1-meter-tall bipedal predator, evolved about 230 million years ago. Dinosaurs were once thought to be a paraphyletic group, a group not made up of a common ancestor and all of its descendants, but most scientists have since come to the conclusion that all dinosaurs are descended from a common ancestor. Before the Carnian, around 215 million years ago, dinosaurs were relatively rare, representing only 1-2% of terrestrial fauna. At the Carnian border, however, many of the therapsids and other archosaurs that survived the end-Permian extinction died out, leaving everything wide open for dinosaurs. Quickly, dinosaurs came to represent at least 50%, if not 90%, of all individual terrestrial fauna.
At the end of the Triassic, the age of the dinosaurs had begun. The two main types of dinosaurs, theropods and sauropods, are diverse enough to occupy most of the major ecological niches. Over time, they became less primitive, with some groups, such as sauropods, becoming progressively larger, eventually weighing more than today’s blue whale. During most of the Mesozoic, any animal larger than a meter was a dinosaur of some kind.
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