The timeline of dinosaur natural history is divided into three eras: Triassic, Jurassic, and Cretaceous.
Triassic Period (252-201 million years ago):
- 252 million years ago: Dinosaurs first appear in the fossil record.
- 245 million years ago: Dinosaurs become the dominant land animals.
- 230 million years ago: The first true mammals appear.
- 225 million years ago: The first true birds appear.
Jurassic Period (201-145 million years ago):
- 201 million years ago: The first stegosaurs appear.
- 190 million years ago: The first true ceratopsians appear.
- 175 million years ago: The first true sauropods appear.
- 150 million years ago: The first true ornithopods appear.
Cretaceous Period (145-66 million years ago):
- 145 million years ago: The first true hadrosaurs appear.
- 100 million years ago: The first true ankylosaurs appear.
- 80 million years ago: The first true pachycephalosaurs appear.
- 66 million years ago: The dinosaurs become extinct.
