c. 320 Million BCE

Reptiles

The evolution about 375 million years ago of animals capable of living on dry land—
at least temporarily at first—quickly led to the development of specialized sub-species of amphibious tetrapod vertebrates that could exploit newly available niches for food and safety. Among the first of those new classes of animals to appear were the reptiles. Unlike their amphibian ancestors, reptiles developed the ability to lay soft-shelled amniotic (fluid-filled) eggs on dry land, thus avoiding the need to return to the water for reproduction like many other animals had.

Some of the earliest known unambiguous fossils of reptiles are of a now-extinct animal known as Hylonomus (from the Latin for “forest dweller”), a vertebrate tetrapod about 8−10 inches (20−25 centimeters) long that lived in the Pennsylvanian geologic period (about 320 to 300 million years ago) and that would have looked a lot like a small modern-day lizard. Based on fossil finds, Hylonomus appears to have dined on insects and lived among the rotten trees and leaves of forested coastal woodlands.

Permanent life on land presented both opportunities and challenges for early (and modern) reptiles. Early reptiles didn’t have to worry about other predators on land, for example, although once the reptile population grew to fill the niche of available food, reptile species would eventually evolve into predators of one another. Reptiles also had to adapt to dramatically changing temperatures or other environmental conditions on land compared to in the water—a significant concern for cold-blooded animals that could become easy lethargic prey in cold temperatures, or while exposed to the sunshine to warm themselves up.

Despite the challenges, reptiles would continue to successfully adapt to life on land in terms of evolutionary developments, and would eventually become the largest animals ever to roam the land. Among the descendants of the first reptiles such as Hylonomus were the ornithischian (“bird-hipped”) and saurischian (“lizard-hipped”) dinosaurs, turtles, tortoises, crocodiles, alligators, snakes, lizards, and—yes—birds. Tens of thousands of modern reptilian species have been cataloged, but tens of thousands more have gone extinct in the past—many from slow natural-selection forces, but many (such as the ancient dinosaurs) in dramatic and rather sudden extinction events caused by meteorite impacts, volcanic eruptions, or other large-scale climate-changing events.

SEE ALSO Snowball Earth? (c. 720–635 Million BCE), Cambrian Explosion (c. 550 Million BCE), First Land Plants (c. 470 Million BCE), First Animals on Land (c. 375 Million BCE), The Great Dying (c. 252 Million BCE), Triassic Extinction (c. 200 Million BCE), Deccan Traps (c. 66 Million BCE), Dinosaur-Killing Impact (c. 65 Million BCE), Arizona Impact (c. 50,000 BCE)

According to the fossil record, early reptiles such as Hylonomus probably didn’t look very different from modern lizards like this one.