Trilobites, meaning "three-lobed," were the first multiple celled animals with a hardened exoskeleton to exist on earth. They dominated the shallow seas of the Cambrian period 570 million years ago and steadily declined in numbers until they became extinct 350 million years later.

Whatever their size, all trilobite fossils have a similar body pl...

Trilobites, meaning "three-lobed," were the first multiple celled animals with a hardened exoskeleton to exist on earth. They dominated the shallow seas of the Cambrian period 570 million years ago and steadily declined in numbers until they became extinct 350 million years later.

Whatever their size, all trilobite fossils have a similar body plan, being made up of three main body parts: a cephalon (head), a segmented thorax, and a pygidium (tail piece). Trilobites were among the first of the arthropods, a phylum of hard-shelled creatures with multiple body segments and jointed legs (although the legs, antennae and other finer structures of trilobites only rarely are preserved). They constitute an extinct class of arthropods, the Trilobita, made up of nine orders, over 150 families, about 5000 genera, and over 15,000 described species. New species of trilobites are unearthed and described every year, making trilobites the single most diverse group of extinct organisms.

The smallest known trilobite species is just under a millimeter long, while the largest include species from 30 to 70 cm in length (roughly a foot to two feet long!). Most trilobites are about an inch long, and part of their appeal is that you can hold and examine an entire fossil animal and turn it about in your hand. Collectors are especially interested in their eyes, made up of numerous lenses packed sided by side.

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