The Triassic Period: Dawn of the Dinosaurs

The Triassic Period lasted from approximately 252 to 201 million years ago, opening immediately after the catastrophic Permian-Triassic extinction event — the worst mass extinction in Earth's history — that wiped out roughly 96 percent of all marine species and 70 percent of terrestrial vertebrate species. The world the Triassic dinosaurs inherited was ecologically devastated and would take millions of years to recover.

All of the world's landmasses were joined into the supercontinent Pangaea during most of the Triassic. This vast single continent stretched from pole to pole, and its interior was far from any ocean — meaning much of it was a scorching, seasonally arid desert. The climate was generally hotter and drier than today, with no polar ice caps and dramatically higher atmospheric carbon dioxide concentrations. Rain fell mostly around the coastal margins, and interior regions cycled between seasonal wet and dry conditions, much like a modern monsoon climate on a global scale.

The dominant land animals at the start of the Triassic were not dinosaurs but a diverse array of archosaurs — the broader group that includes both dinosaurs and crocodilians. Archosaurs split early in the Triassic into two main lineages: Pseudosuchia (the line leading to modern crocodilians) and Avemetatarsalia (the line leading to dinosaurs, pterosaurs, and eventually birds). For much of the early and middle Triassic, the pseudosuchians were actually more diverse and ecologically dominant. Creatures like the large, heavily armored aetosaurs and the predatory rauisuchians occupied the ecological niches that would later be filled by large dinosaurs.

True dinosaurs first appear in the fossil record around 230 to 240 million years ago, during the Middle and Late Triassic. The oldest definitively identified dinosaurs include Eoraptor lunensis and Herrerasaurus ischigualastensis, both discovered in the Ischigualasto Formation of Argentina. These early dinosaurs were relatively small — Eoraptor was roughly the size of a large dog, weighing about 10 kilograms — and were bipedal carnivores or omnivores. They were by no means the dominant animals of their time.

Eoraptor is particularly fascinating because it appears to sit very near the base of the dinosaur family tree. It had the characteristic dinosaur ankle structure, with an upright posture and legs positioned directly beneath the body — a key innovation that set dinosaurs apart from many other archosaurs. This erect posture is thought to have given early dinosaurs an energetic advantage, allowing for greater sustained activity than sprawling reptiles.

The Triassic-Jurassic extinction event, occurring around 201 million years ago, changed everything. This mass extinction event — likely triggered by massive volcanic eruptions associated with the opening of the Central Atlantic Magmatic Province as Pangaea began to fragment — eliminated most of the pseudosuchians and other large archosaurs that had competed with dinosaurs. In the ecological vacuum that followed, dinosaurs rapidly diversified and expanded. The Triassic was the proving ground; the Jurassic would be their triumph.

Plants during the Triassic were dominated by gymnosperms — seed-bearing plants without flowers. Conifers, cycads, ginkgoes, and tree ferns covered the landscape, forming the forests that fed the first herbivorous dinosaurs. Flowering plants had not yet appeared. The seas were recovering from the end-Permian extinction, gradually restoring marine ecosystems that would eventually include the great marine reptiles of the Jurassic and Cretaceous.

The Triassic thus set the stage for everything that followed. The ecological and evolutionary pressures of this period — intense competition, climatic extremes, and a series of extinction pulses — shaped dinosaur anatomy, physiology, and behavior. The survivors that emerged into the Jurassic were ready to build one of the most spectacular radiations in vertebrate history.