Trilobites

Trilobites

Trilobite, subphylum of the phylum Arthropoda that includes a large group of extinct marine animals that were abundant in the Paleozoic era. They represent more than half of the known fossils from the Cambrian period (from -543 million years to -490 million years). The trilobite body was generally oval and flat and was divided into three roughly equal sections: a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; a thorax composed of freely articulating segments; and a pygidium (tail) composed of the last few segments fused together with the telson. The name trilobite refers to a pair of furrows along the length of the animal that divided the body into three longitudinal regions. The body was covered by a mineralized shell. Because the dorsal, or upper, shell was thicker than the under shell, it has been the part best preserved in fossil form. Trilobites were abundant inhabitants of the Cambrian and Ordovician (from -490 million years to - 435 million years)geological periods. But they declined thereafter, possibly because they became food for cephalopods and later for fish, and became extinct in the Permian period (245 million years ago). Trilobites are most closely related to the Chelicerates, which include the horseshoe crabs and spiders.

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