By the time trilobites first appeared in the fossil record, they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton, they left an extensive fossil record, with some 17,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology, and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.
Trilobites had many lifestyles; some moved over the sea bed as predators, scavengers, or filter feeders, and some swam, feeding on plankton. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.
The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to some . Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis spp. (western USA), Hupetina antiqua (Morocco) and Serrania gordaensis (Spain). All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location.
All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far (Lemdadella), occurs almost at the same time as the earliest Olenellina, however, implying that trilobite origin lies before the start of the Atdabanian, but without leaving fossils. Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on the origin of trilobites.