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(Gr. treis, tria, trion: three; lobos: lobe) The most primitive arthropod class, known from more than 3900 fossil species. Inhabitants of Palaeozoic seas, the trilobites appeared first in the early Cambrian, had their widest distribution and greatest diversity in the Cambrian and Ordovician Periods, and became extinct in the Permian. The body was divided into three regions: an anterior cephalon, comprising at least five, fused segments; a mid-body or thorax, with a varying number of segments; and a hind region or pygidium. All three regions were divided by a pair of furrows running the length of the body, giving a trilobite appearance (i.e. a median or axial lobe, flanked on either side by a lateral lobe). The mouth was situated in the middle of the central surface of the cephalon. Paired gill-bearing limbs were attached to the membranaceous, pleural skeleton. X-ray studies show the eyes to have resembled the compound eyes of living arthropods (see Trilobite Eye). Trilobites ranged in size from 0.5 mm long planktonic (see Plankton) forms to those nearly 1 m in length; most species were 3 to 10 cm long. There were 9 orders: Redlichiida, Agnostida, Naraoiidae, Corynexochida, Lichida, Phacopida, Ptychopariida, Asaphida and Proetida.


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The Infraphylum Trilobita is a member of the Subphylum Arachnomorpha. Here is the complete "parentage" of Trilobita:

The Infraphylum Trilobita is further organized into finer groupings including:


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Trilobites (, /'tr?l?ba?t/; meaning "three lobes") are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (526 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, almost all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years. [more]

At least 2,615 species and subspecies belong to the Class Trilobita.

More info about the Class Trilobita may be found here.


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Last Revised: August 24, 2012
2012/08/24 19:59:29