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Trilobites are any extinct marine arthropod of the defunct subphylum Trilobitomorpha. Trilobitomorpha currently has only a single class, "Trilobita," and as such, the two terms are interchangeable. The first trilobites appeared at the beginning of the Cambrian Period, of the Paleozoic Era, 542 million years ago, and the last genus died out at the end of the Permian, during the Permian/Triassic Extinction event, 250 million years ago.

Trilobites are named so for the the way that their axial lobe divides the body into three sections: right and left pleural lobes that flank the central axial lobe. Some genera have a very pronounced division, such as Phacops, others display no lobing, such as Isotelus. A popular misbelief claims that the "three lobes" refers to the way the body is divided into three regions, the head, or cephalon, the thorax, and the tail region, or pygidium. Like many arthropod groups, the trilobites had a single pair of pre-oral antennae (in that, the base of the antennae were before the mouth, or hypostome). Unlike other most other arthropods, trilobites did not have oral appendages such as mandibles, pedipalps or chelae. Food, thus, was apparently simply stuffed into the mouth, or hypostome, which was essentially a muscular hole.

It is commonly thought that the majority of trilobites were bottom-dwelling mud-grubbers, alternatively sifting through mud, or scratching out worms from their burrows. This is because most known species of trilobites were obviously benthic, such as Olenellus, Phacops, and Isotellus, coupled with trace-fossils of trilobite tracks that showed the maker pausing for a moment to dig a shallow pit before continuing. Some species, however, were apparently planktonic, with a tendency towards elongate, streamlined bodies with huge, rounded eyes, such as Opipeuterella, and Carolinites, or having long, splayed rib-like pleural lobes, with to prevent sinking with, like Deiphon.

Trilobita contains ten orders, though two of which, the Agnostida and the Nektaspida, are in dispute.












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