Wednesday, May 6, 2009

Fossil of the Week

Trilobites are well-known invertebrate fossils, especially plentiful in the New York Devonian surrounding Ithaca. They are arthropods, with segmented bodies and legs, and compound eyes. They were all marine, and mostly predators or scavengers on the seafloor, however, some are believed to have been filter-feeders or even planktonic. Trilobites first appear in the fossil record in the Lower Cambrian, and reached their height in the Ordovician; they gradually declined through the Devonian, finally going extinct in the Permian mass extinction. The fossil record of trilobites was critical to the development of the theory of punctuated equilibrium by Niles Eldredge and Stephen Jay Gould in 1972. The word "trilobite" means three-lobed, and refers to the three-part (right, left and center) body of the trilobite; trilobite bodies also have three main sections: the head shield (cephalon), multiple articulated segments together comprising the thorax, and the tail shield (pygidium). Trilobites often remind people of terrestrial "pill bugs", but their closest living relatives are horseshoe crabs and horseshoe shrimps (Cephalocarida).

Text by Paula Mikkelsen, Photo courtesy J. Casciano

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