arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. The largest trilobites were more than 70 cm long and may have weighed as much as 4.5 kg.

The first appearance of trilobites in the fossil record defines the