c. 200–65 Million BCE

Age of the Dinosaurs

As mass extinctions at the end of the Permian and Triassic periods wreaked havoc on what were before then the dominant classes of vertebrate land animals on Earth, new ecologic and evolutionary niches for survival were opened up to what had previously been relatively minor, or perhaps not-yet-existing, kinds of creatures. Such was the case with the ancient dinosaurs, a class of animals unlike anything the world had seen.

Dinosaurs are amniotic vertebrate reptiles that first appear in the fossil record in the middle Triassic geologic period (around 240 million years ago), as a class of animals that diverged from their contemporaneous archosaur predecessors. Archosaurs were the dominant land predators during the Triassic, but only a few of their classes, including the dinosaurs, remained after the End Triassic mass extinction. Fossils of the oldest-known Triassic dinosaurs indicate that they probably started as relatively small (dog-sized) bipedal predators—certainly not the giant, dominant terrestrial animals they would soon become.

Why did early Jurassic dinosaurs evolve to larger sizes and the dizzying array of body styles preserved in the fossil record? A leading hypothesis is that since many of their early lineages survived the End Triassic mass extinction, they found themselves in a world where the major land predators had been wiped out, and amid a flora and fauna that was undergoing a rapid and prolific recovery. Rather suddenly, dinosaurs became the top competitors for abundant food and prey resources in the Jurassic world. In their heyday, dinosaurs evolved into at least 700 different species. They ranged in size from 2.5-foot-long (75-centimeter) Eoraptor to the 130-foot-long (40-meter) Argentinosaurus, and included what must have been among the most terrifyingly powerful and capable predators to ever live, such as Tyrannosaurus Rex.

The Age of the Dinosaurs ended rather abruptly, however, some 65 million years ago, when a large asteroid is hypothesized to have impacted the Earth and created a climate and food-chain catastrophe that would wipe out most of the dinosaurs, as well as perhaps 75 percent of all other species of life on Earth. One specific class of dinosaurs did survive the End Cretaceous mass extinction, however, and they are still with us today: more than 10,000 species of birds now descend from ancestors that were feathered dinosaurs.

The size and diversity of the dinosaurs, the world’s dominant land-dwelling vertebrate class for more than 135 million years, was stunning. In this artist’s rendering, 13-foot-tall (4 m) plant-eating Diplodocus wade in the shallow waters below flying Pterosaurs.