SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY
An internal fossil cast of the trilobite, Anebolithus simplicior (Whittard), from the Ordovician period (500 to 435 million years ago), found in the Lower Llanvirn sediments, Builth Wells inlier, Powys, Wales. The trilobites are an extinct group of marine arthropods, which were distributed throughout the prehistoric oceans. Most trilobites lived on the sea bottom, although some burrowed in sediment and a few were swimmers. A. simplicior probably lived in dark, muddy waters. Trilobites first appeared during the Cambrian period (570 to 510 m. y. ) and flourished for more than 200 million years before becoming extinct during the Permian period (280-230 m.y.).
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