Dinosauria

Early Dinosaurs

243–201 Ma

vault species

million years

What is an Early Dinosaur?

Early Dinosauria encompasses the basal lineages that arose in the Middle and Late Triassic — before the great split into Saurischia and Ornithischia was fully established. These include the herrerasaurids, among the earliest large predatory dinosaurs, and a range of small, agile forms that give us the closest look at the dinosaur common ancestor.

First dinosaur

Nyasasaurus — possibly ~243 Ma (Middle Triassic)

Herrerasauridae

Earliest large predators, ~231–228 Ma

End-Triassic event

~201 Ma — cleared the way for dinosaur dominance

Key region

Ischigualasto Formation, Argentina

Common ancestor

Small, bipedal, likely omnivorous or insectivorous

Evolution & History

Dinosaurs did not begin as the dominant animals of their world. When they first appeared in the Middle Triassic around 243 million years ago, they were a minor element of ecosystems ruled by crurotarsans — the archosaur lineage that would eventually give rise to crocodilians. Early dinosaurs succeeded by being fast, bipedal, and ecologically flexible at a time when the end-Triassic extinction was about to devastate their competition.

The herrerasaurids — Herrerasaurus and Staurikosaurus from South America — represent the earliest large predatory dinosaurs. They appear in the fossil record fully formed at ~231 Ma, already bipedal and carnivorous, with a distinctive sliding jaw joint that may have provided a secondary grip. Their exact position in the dinosaur tree has been debated for decades: some analyses place them as the earliest saurischians, others as a basal lineage outside both major groups. This uncertainty itself is scientifically significant — herrerasaurids are so primitive that the features defining Saurischia and Ornithischia are not yet clearly drawn.

The Late Triassic extinction at 201 Ma was a pivotal moment. Crurotarsans were devastated; the surviving dinosaurs rapidly diversified to fill the vacated niches. Within a few million years of this boundary, the major dinosaur lineages — theropods, sauropodomorphs, ornithischians — had established themselves as the dominant large vertebrates of every terrestrial ecosystem on Earth. The early dinosaurs of the Triassic had won.