Trilobites are the first unequivocal arthropods to appear in the fossil record. Their shells at the base of the Cambrian mark the beginning of the Paleozoic Era; they became extinct at the end of the Paleozoic. During these 345 million years, more than 1500 genera, with about 4000 species evolved. Trilobites are named for the 3 longitudinal lobes of their bodies. A raised middle lobe begins at the head and runs down into the tail, and a flatter lobe is present at each side of the middle one. The trilobite body is also divided into 3 transverse sections: the head, the thorax, and the tail. The dorsal, or upper, part of the skeleton was calcified, but most of the lower, or ventral, part including the legs, was not, so most trilobite fossils are dorsal only. The underside of the trilobite skeleton was almost never preserved, since it was not calcified, but a few RARE preservations indicate that there was a pair of antennae on the head, and a series of legs, a pair on each segment.
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