The first trilobites
The animal that will define the Paleozoic appears, and immediately spreads worldwide.
- When
- Known to
- ± 2 million years
- Era
- Cambrian, Paleozoic
Trilobites enter the record about 521 million years ago and diversify with extraordinary speed. Their calcite exoskeletons preserve well, they moulted constantly, and they occupied nearly every marine habitat — which makes them the single most useful fossil group for dating and correlating Paleozoic rocks. Over 20,000 species are known. They persist for 270 million years before dying out in the greatest extinction of all.