A brief history of dinosaurs

 Dinosaurs were a successful group of animals that emerged between 240 million and 230 million years ago and came to rule the world until about 66 million years ago, when a giant asteroid slammed into Earth. During that time, dinosaurs evolved from a group of mostly dog- and horse-size creatures into the most enormous beasts that ever existed on land.

Some meat-eating dinosaurs shrank over time and evolved into birds. So, in that sense, only the non-avian dinosaurs went extinct. (For the purposes of this article, "dinosaurs" will refer to non-avian dinosaurs, unless otherwise stated.)

During the roughly 174 million years that dinosaurs existed, the world changed greatly. When dinosaurs first appeared in the Triassic period (252 million to 201 million years ago), they roamed the supercontinent of Pangea. But by the time the asteroid hit at the end of the Cretaceous period (145 million to 66 million years ago), the continents were in approximately the same place they are today.

The oldest unequivocal dinosaur fossils, dating to about 231 million years ago, are from Ischigualasto Provincial Park in northwestern Argentina, and include the genuses Herrerasaurus, Eoraptor and Eodromaeus. Scientists are still debating whether Nyasasaurus, a genus found in Tanzania that dates to about 240 million years ago, is also an early dinosaur or a dinosauromorph, a group that includes dinosaurs and their close relatives, said Steve Brusatte, a paleontologist at the University of Edinburgh in Scotland. 

Related: Photos: Unearthing dinosauromorphs, the ancestors of dinosaurs

Whenever they first appeared, the dinosaurs' unique anatomy set them apart from other animal groups. Dinosaurs are archosaurs, a clade (different groups of animals that share a common ancestor) that includes crocodilians, pterosaurs, dinosaurs and birds. The archosaurs emerged after the end-Permian extinction about 252 million years ago. Over time, some archosaurs, including dinosauromorphs, adapted an upright posture, meaning they had legs under their bodies, rather than out to their sides. 

"Sprawling is all well and good for cold-blooded critters that don't need to move very fast. Tucking your limbs under your body, however, opens up a new world of possibilities," Brusatte wrote in "The Rise and Fall of the Dinosaurs: A New History of a Lost World" (William Morrow, 2018). As archosaur evolution progressed, dinosauromorphs gained long tails, big leg muscles and extra hip bones that enabled them to move quickly and efficiently, Brusatte wrote. 

Dinosaurs were likely warm blooded. Their unique anatomy, such as their strong necks and upright posture, gave them advantages over other animal groups.  (Image credit: UNM Biology)

Some dinosauromorphs evolved into dinosaurs. The differences between the two are small, but dinosaurs' anatomy offered increased benefits, including arms that could move in and out, neck vertebrae that could support stronger muscles than before, and a joint where the thigh bone meets the pelvis, Brusatte wrote.