The Triassic Period marks the beginning of the Mesozoic Era. Following a mass extinction, new animal groups emerged, including the first dinosaurs. This group would eventually give rise to the largest land animals in the planet’s history. Spanning from 252 to 201 million years ago, the Triassic set the stage for the age of reptiles and witnessed the humble beginnings of these creatures.
The World of the Triassic Period
During the Triassic, Earth’s landmasses were joined into a single supercontinent, Pangaea, surrounded by the Panthalassa ocean. The interior of Pangaea was hot and arid, creating vast deserts in a globally warm world with no polar ice caps. This environment supported resilient plant life rather than lush jungles. Flora consisted largely of ferns, cycads, ginkgoes, and conifers, which provided food and cover for the era’s animals.
General Characteristics of the First Dinosaurs
True dinosaurs first appeared around 233 million years ago as small, lightly-built animals, often comparable in size to a crow or small dog. Many of these initial species were bipedal, giving them an advantage in speed and agility over many contemporary reptiles.
A defining feature that separated dinosaurs from other reptiles was their skeletal structure. Their legs were positioned directly beneath their bodies in an upright stance, rather than sprawling out to the sides like modern lizards. This arrangement allowed for more efficient movement and is a primary characteristic used by paleontologists to identify true dinosaurs.
Notable Triassic Dinosaur Species
Among the earliest known dinosaurs, several species are notable:
- Eoraptor was a small predator discovered in Argentina. Measuring only one to two meters long, it is considered close to the common ancestor of all dinosaurs, with teeth suggesting a diet of insects and small vertebrates.
- Herrerasaurus, from the same region, was larger, reaching up to six meters and making it a top carnivore. Its anatomy shows a mix of primitive and advanced features, providing insight into the early evolution of theropods, the group that later included Tyrannosaurus rex.
- Plateosaurus was an early sauropodomorph that could grow up to 10 meters long. This herbivore used its long neck to browse on vegetation, and fossils found in large groups suggest it may have lived in herds, a behavior common among its later relatives.
- Coelophysis was an agile carnivore found extensively in what is now North America. Thousands of skeletons found together at the Ghost Ranch quarry suggest they gathered in large packs, likely to hunt prey larger than themselves.
Life Alongside Other Reptiles
Despite their evolutionary innovations, dinosaurs were not the dominant land animals during the Triassic. They were relatively rare, living in a world ruled by other reptile groups, primarily the pseudosuchians—the archosaur group that includes modern crocodilian ancestors. These crocodile-line archosaurs, such as the formidable rauisuchians, were the apex predators.
Herbivorous roles were also filled by non-dinosaur reptiles. Armored aetosaurs were common plant-eaters, protected by heavy body armor. In the early part of the period, synapsids—the group often called mammal-like reptiles—were also prevalent. Animals like the tusked, pig-sized Lystrosaurus were abundant across Pangaea just after the Permian extinction.
The Triassic-Jurassic Extinction Event
The Triassic Period ended approximately 201 million years ago with a mass extinction event. While the cause is debated, the leading theory points to massive volcanic eruptions associated with the breakup of Pangaea. These eruptions released enormous amounts of carbon dioxide, leading to rapid climate change and ocean acidification.
This event had a profound impact on ecosystems. Many of the dominant reptile groups, including most pseudosuchians like the aetosaurs and rauisuchians, were wiped out, along with many large amphibians and synapsids. Dinosaurs, however, were among the survivors. The extinction cleared the ecological stage by removing their primary competitors, allowing them to diversify into newly vacant niches. This event paved the way for the subsequent Jurassic Period, the “Age of Dinosaurs.”