Trilobites

The trilobites are a remarkable group of extinct arthropods that lived in the seas of the Paleozoic era (542–251 million years ago [mya]) and went extinct before dinosaurs dominated the earth. Trilobites were early arthropods with a hard, segmented exoskeleton, and therefore easily fossiliz-able. Although all share a similar body plan, there was a great diversity of size and form. They are grouped in the extinct class Trilobita, which is subdivided into 10 orders, over 150 families, about 5,000 genera, and more than 17,000 described species. with such a diversity of species, it is very probable that the ecological role of trilobites was very extensive. They surely included planktonic, swimming, and crawling forms as well as a varied set of trophic niches (mostly detriti-vores, carnivores, and scavengers). Because of their great diversity and abundance in the fossil record, trilobites are important key taxa in Paleozoic biostratigraphy, chronostratigraphy, paleoenvironmental reconstruction, paleogeogra-phy, and even plate tectonics.

The body plan of all trilobites consists of three main body parts: a cephalon (head), a segmented thorax, and a pygidium (tail). Its name “trilobite” means “three lobed,” but this term does not refer to these three body parts. Instead it points to the fact that they have a long central axial lobe flanked on each side by right and left pleural lobes. Their exoskeleton was composed of calcite and calcium phosphate minerals in a protein lattice of chitin. Many trilobites had compound eyes, in addition to antennae used perhaps for taste and smell.

Trilobites first appeared during the early Cambrian, and flourished throughout the lower Paleozoic. When they first occur in the Cambrian fossil record, they are already highly diverse and geographically dispersed, suggesting a previous evolutionary origin. It is reasonable to assume that the trilobites shared a common ancestor with other arthropods before the Cambrian period. Trilobites have been often included within subphylum Chelicerata, which groups the arachnids (spiders, scorpions, etc.), xiphosurans (horseshoe crabs), eurypterids (sea scorpions), and pynogonids (sea spiders). All of them together with the trilobites seem to share a pre-Cambrian common ancestor, forming the taxon called Arachnomorpha. The ancestors of the trilobites are probably to be found in the arthropod-like, segmented animals called trilobitomorphs, which lived during the Ediacaran period of the Neoproterozoic era, more than 550 [Page 1382]mya. They include fossil taxa like Spriggina and Parvancorina found in the Ediacara Hills (South Australia). These taxa have been classified as ven-dobionts, annelids, or arthropods, but their morphology bears a striking resemblance to that of trilobites, and they therefore may be excellent candidates for ancestors of trilobites.

Except perhaps for the order Phacopida, whose first record occurs in Ordovician sediments, all tri-lobite orders appeared during the Cambrian period. The nine trilobite orders and some of their main generaare: Agnostida (e.g., Agnostus, Condylopige), Redlichiida (e.g., Redlicbia, Paradoxides), Corynexochida (e.g., Corynexocbus, Illaenus, Leiostegium), Lichida (e.g., Lieh as, Odontopleur a, Damesella), Phacopida (e.g., Phacops, Calymene, Dalmanites, Cbeirurus), Proetida (e.g., Proetus, Phillipsia, Cypbaspis, Bathyurus), Asaphida (e.g., Asaphus, Ampyx, Dikelocephalus, Cyclopyge, Ramopleurides), Harpetida (e.g., Entomaspis, Harpes), and Ptychopariida (e.g., Ellipsocepbalus, Ptychoparopsis, Conocoryphe, Elratbia). The first true trilobite is probably contained in the order Redlichiida. The olenellids of the order Redlichiida include some of the most primitive trilobites, such as Olenellus and Fallotaspis. The orders Corynexochida, Lichida, and Ptychopariida evolved from Redlichiida during the Middle Cambrian. The rest of the orders (Asaphida, Harpetida, Proetida, and probably Phacopida) evolved from Ptychopariida during the Middle and Late Cambrian. There are doubts concerning the origin of the order Agnostida. The systematic position of this order within the class Trilobita remains uncertain since ontogenetic evidence suggests they descended separately from crustaceans. However, other specialists support the hypothesis that the agnostids evolved from some group of ptychopari-ids. In spite of the doubts about their true origin and the phylogenetic relationships among their orders, trilobites have become an essential group for estimating the rate of speciation during the evolutionary radiation known as the Cambrian explosion, since they are the most diverse fossil group during this so-significant period of life history.

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