Any of a group of ovate arthropods (subphylum Trilobita) that came to dominate the seas c. 540 million years ago and became extinct c. 245 million years ago. Trilobites had a chitinous exoskeleton and three body lobes: a raised middle lobe with a lower lobe on each side. The head, thorax, and tail were segmented; each segment bore two appendages. The forwardmost appendages were sense and feeding organs. Most species had two compound eyes, though some were eyeless. Some were predators, others were scavengers, and still others probably ate plankton. Paradoxides harlani, found near Boston, grew to 18 in. (45 cm) long and may have weighed 10 lbs (4.5 kg). Other species were small.
Trilobite—Leslie JackmanNatural History Photographic Agency
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