Trilobites

Most trilobites lived in fairly shallow water and were benthic; they walked on the bottom, and probably fed on detritus.
A few, like the agnostids, may have been pelagic, floating in the water column.

Cambrian and Ordovician trilobites generally lived in shallow water.
After the Ordovician period, when many trilobite groups declined or became extinct, the survivors tended to be restricted to deeper water.

Cambrian period:
540 to 500 million years
Ordovician period:
510- 438 million years
Ordovician period:
510 million years