Trilobite fossils. Three fossilised Acaparadoxides briaerus trilobites. These trilobites lived during the Middle Cambrian period (around 540–523 million years ago). The trilobites are an extinct group of marine arthropods with a hard, segmented shell that is divided into 3 sections. Most were bottom dwellers, feeding and living on organic sediment on the ocean floor. Found in Morocco.