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Trilobite, an extinct marine invertebrate (animal without a backbone). Trilobites, which lived from about 600 million to 250 million years ago, are the earliest known arthropods (joint-legged animals, such as insects and lobsters). Several thousand species of trilobites have been identified from their fossil remains.

Trilobites were bottom-dwelling animals of worldwide distribution and were once the most numerous animals of the shallow seas. They ranged in length from about 1/4 of an inch (6 mm) to more than two feet (60 cm), but most species were from two to three inches (5 to 7.5 cm) long. Trilobites had compound eyes and breathed through gills.

The body was divided into three vertical parts by two lengthwise furrows and was covered by a hard shell that was molted (shed) periodically. Trilobites had three distinct body regions: head, thorax, and tail. The head, which was covered by a semicircular shield, had one pair of antennae and four pairs of appendages. The thorax was composed of 2 to more than 40 segments, each bearing a pair of limbs. The tail consisted of up to 30 fused segments.

Trilobites make up the subphylum (or class) Trilobita of the arthropod phylum, Arthropoda.