Introduction to Trilobites

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Trilobites were arthropods with soft bodies and many legs that were positioned under a heavily calcified, segmented shell. They first appeared in the Early Cambrian Period (544 million years ago) and thrived through the Middle and Late Cambrian times before their extinction in the Ordovician period (495 million years ago). In their 49 million year existence, they diversified quickly and produced a fossil record that geologists refer to often.

Arthropods are a phylum of hard-shelled creatures with multiple body segments and jointed legs. Trilobites are a class of this phylum that includes eight orders: Agnostida, Asaphida, Corynexochida, Lichida, Phacopida, Proetida, Ptychopariida, and Redlichiida. The trilobites were very diverse, especially in size, ranging from less than 1 millimeter to 70 centimeters. That's almost two feet long! Although they ranged in size, they all shared a standard three-part body: The cephalon (head), thorax, and pygidium (tailpiece). The name "Trilobite" refers to the lobes of the soft body. The axial lobe is centered between the left pleural lobe and the right pleural lobe. The horseshoe crab of today is perhaps the best example to illustrate the physical appearance of a trilobite.

Trilobites occupied only marine environments. Their niches within the marine environment varied from intertidal and nearshore to deep continental slopes. The majority of trilobites were benthic, or bottom dwellers. They crawled on the sea floor and through reefs. They were able to dig into bottom sediments for food or cover. They were adapted to either carnivorous or herbivorous lifestyles depending upon the species. The herbivores fed on algae, some browsed on corals, sponges, and bryozoans. Some were filter feeders ...

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...the late Permian period. Their 49 million year existence is theorized to be halted by a predator known as Anomalocaris. The half-meter long Anomalocaris was an active swimming predator (proto-anthropoda) that was physically capable of swallowing a trilobite whole but may also have bitten out ragged chunks of their prey. Trilobite fossils have been found with such wounds, supporting the extinction by predation theory.

The Trilobites were an interesting as well as important creature. Their large numbers, varied species, and world-wide occupation have made them an integral piece of the geologic puzzle of time.

Sources:

Stanley, Steven M. Earth System History. New York: W.H. Freeman Co., 1999

http://aloha.net/~smgon/trilobite.htm

http://enchantedlearning.com

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