Trilobites were the most numerous and successful marine oragnisms of the early Paleozoic Period. They ranged in length from several millimeters to 3 feet. Trilobites had many legs, each with a set of gills and most had multifaceted eyes.
Trilobites are the earliest known animal to possess vision. Some had eyes and some seem to have no eyes at all, while others had sophisticated compound eyes capable of 360 degree vision.
The name 'trilobite" is derived from the three lobes of the body segments. Most trilobites lived in shallow ocean waters and on reefs. Trilobites flourished as swimmers, crawlers and burrowers for some 350 million years. Their skeletons are found in all types of sedimentary rock.
Trilobites appeared some 600 million years ago during the Cambrian period. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago.