CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY CHARLES D. WINTERS / SCIENCE PHOTO LIBRARY
Fossilized trilobites. Rock containing fossilized trilobites, an extinct class of once common marine arthropod. Trilobites lived right through the Palaeozoic era, 590-286 million years ago (mya). Most species were found on the sea floor. Many trilobites had flexible backs so they could roll up into a ball to present only their exoskeletons to attacking predators. These specimens are from the Devonian period (408-360 mya).
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