Ammonoids appear
Coiled cephalopods evolve, and give geology one of its finest tools for dating rock.
- When
- Known to
- ± 3 million years
- Era
- Devonian, Paleozoic
Ammonoids branch off from the nautiloids in the Early Devonian, evolving intricately folded internal partitions that let them build thinner, lighter shells. They evolve rapidly, disperse widely and are abundant, which makes them superb index fossils for the next 300 million years. They survive three mass extinctions before finally dying out with the dinosaurs.