An artist's rendering shows hatchling nothosaurs heading for the safety of water as a hungry but terrestrial Ticinosuchus attacks near a lagoon in ancient Switzerland. Nothosaurs lived during the mid- and late Triassic period and were among the earliest reptiles to take to the sea. Because nothosaurs may have had to come ashore to lay eggs, the eggs and hatchlings would have been vulnerable to Ticinosuchus. Yet once the hatchlings reached deeper waters, they were safe—for the moment.

An artist's depiction shows a Herrerasaurus lurking in a forest in what is now Argentina as smaller animals hide in the undergrowth. Herrerasaur fossils are among the oldest ever discovered, dating back about 228 million years to the mid-Triassic.

An artist's depiction captures the burst of new life that occurred in the early- to mid-Triassic period. Massive extinctions at the end of the preceding Permian period allowed the plants and animals that survived to grow and diversify relatively free of competition and predators. These conditions gave rise to the dinosaurs, pterosaurs, and early crocodilians.

Chunks of petrified wood lay strewn in Arizona's Petrified Forest National Park. Most petrified trees here are of the species Araucarioxylon arizonicum, a towering conifer that grew in the humid, tropical lowlands of this region during the middle Triassic period, about 220 million years ago. Trees that fell here often landed in deep rivers, where they were quickly buried by sediment. Lack of oxygen inhibited decay, and heat and pressure over millennia turned the wood into solid quartz colored by impurities like iron, carbon, and manganese.

This Triassic-era fish is among the thousands of fossils uncovered in the mountains and karst formations of China's Guizhou Province. Much of this region was under a shallow ocean during the Triassic period, and sediments there teem with the ancient remains of fish, dinosaurs, and marine reptiles.

This middle-Triassic reptile found in Switzerland measures only about 9 inches (23 centimeters) in length. Called Pachypleurosaurus edwardsi, these primitive amphibious reptiles were nothosaurs, among the first reptiles to venture into the shallow seas of the Triassic.

Ichthyosaurs, like this one found in Guizhou Province, China, evolved in the early Triassic, nearly 250 million years ago. Early specimens were reptilian in form—in fact, "ichthyosaur" means "fish-lizard"—but quickly evolved a dolphinlike shape. They went on to become one of the top ocean predators and thrived for 160 million years before going extinct in the mid-Cretaceous, about 25 million years before the dinosaurs died out. (NGC)