Trilobites are extinct arthropods, one of the most famous fossil groups outside the dinosaurs. Trilobites appeared right at the dawn of the Cambrian era, 542 million years ago, and went extinct at the end-Permian extinction 250 million years ago, making trilobites iconic of Paleozoic fauna. Early genera include Fallotaspis, Profalloptaspis, and Eofallotaspis. Trilobite fossils are so common that you can buy one on eBay for $10 US Dollars or less. Numerous trilobite fossils have been found in Utah and Morocco, although collectors should be wary, as there is a serious counterfeiting problem with trilobite fossils.
Trilobites were shield-shaped arthropods, especially interesting because they resemble no living arthropods. Trilobites are the only subphlyum of arthropods that went extinct. During the end-Devonian extinction, 364 million years ago, all trilobites except the order Proetida, represented by smaller trilobites, went extinct. Trilobites had trouble managing with the diversification of predators throughout the Devonian. In all, over 17,000 trilobite species have been identified, including later spiny forms, which evolved to cope with increased predation.
Because trilobite fossils are so numerous and well-understood, they serve as an excellent reference fossil, informing research in paleontology, biostratigraphy, and plate tectonics. Because of their diversity, trilobites have been used as a benchmark for speciation rates during the Cambrian explosion, an episode of adaptive radiation during the Cambrian period. Trilobites themselves are a prototypical example of the evolutionary successes of the Cambrian explosion. Trilobites may be basal to all other arthropods. If so, all insects, arachnids, crustaceans, mites, and scorpions evolved from trilobite ancestors.
The trilobite body is divided into three sections: a head, thorax, and tail. In the earliest trilobites, the tail is very short and vaguely pronounced. Trilobites had antennae, and some had horns that resemble those seen on modern beetles. Trilobites filled a variety of ecological niches, and included predators, scavengers or filter feeders. Some only walked on the ocean floor, while others swam and ate plankton. Some even evolved a symbiotic relationship with sulfur-eating bacteria.