Trilobites
Trilobites rank among the most important of early animals. Our knowledge of them has been gained from the study of their fossils, usually impressions left of their shells after burial in sediment which subsequently hardened into rock. They appeared abruptly in the early part of the Cambrian Period, and came to dominate the Cambrian and early Ordovician seas. A prolonged decline then set in before they finally became extinct in the Permian Period, about 250 million years ago.
Trilobites belonged to the phylum Arthropoda—like crustaceans, spiders and insects. They are divided into three parts from head to tail (head, thorax and tail) and from side-to-side (the central axis and two lateral lobes). The word 'trilobite' refers to the side-to-side partition, not the head, thorax and tail.
The picture shows how trilobites from different periods lived in the sea.
In shallow waters amongst the bivalves 1, crinoids 2, algae 3 and gastropods 4, the faunas were large, but of low diversity, e.g. Flexicalymene 5.
Intermediate faunas were more diverse— Selenopeltis 6, Geragnostus 7, Chasmops 8 and Remopleurides 9 lived alongside brachiopods 10 and nautiloids 11.
In the depths of the sea, the trilobites were small and blind with moderate diversity— Ampyx 12, Tretaspis 13 and Shumardia 14, with Pricyclopyge 15 swimming above.
Trilobites have long attracted popular as well as scientific attention. The Pahvant Ute Indians of Utah wore specimens of the Middle Cambrian trilobite Elrathia kingii around their necks as lucky charms. It is reported that they were used to guard against sickness and being shot.