Trilobites were among the most abundant and diverse arthropods in the oceans of the Paleozoic era. Although they have been described as the cockroaches of the ancient seas, they represent an evolutionary lineage distinct from that of insects; they were more closely related to modern arachnids such as the horseshoe crab and spiders. Besides being diverse and abundant, they have been marvelously preserved as fossils, since they were the first arthropods to have external skeletons (even though just their upper surfaces) reinforced with calcite. They were abundant for 270 million years, becoming extinct only at the Permian extinction. For all four reasons they have been studied extensively by paleontologists as providing a window into how evolution works over long periods of time.

They are called trilobites because of the three lobes, or lengthwise sections, that constitute their bodies behind the head segment. The numerous segments of the middle part of the body were flexible and allowed trilobites to curl up lengthwise, at least a little bit. They had numerous legs, and each leg had a branch of delicate gills which, protected underneath the shell, allowed them to breathe oxygen from the water.

Trilobites had tremendous diversity. More than 5,000 genera have been recognized. They ranged in size from being as big as a dinner plate to as small as a pea. They had many different ways of obtaining food. Some were predators, eating annelid worms, while others were eaten by such larger predators as Anomalocaris during the Cambrian period and mollusks and the earliest fishes during the Ordovician period. Some were scavengers, and some had enormous head brims with which they may have filtered particles of food from the ocean water. They had many different ways of moving about with their two rows of numerous legs. Some scurried about on the seafloor, while others swam at different depths. Some were covered with spines, others were smooth. Most trilobites had eyes made of calcite crystals unlike those of any other animal before or since. Some had clusters of many small eyes (sometimes in fantastic tower-like structures), and some had smaller clusters of large eyes. Some species secondarily lost their eyes during evolution in darkness. . .

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