LAURIE O'KEEFE / SCIENCE PHOTO LIBRARY LAURIE O'KEEFE / SCIENCE PHOTO LIBRARY
Marine animals became common 550 million years ago and sea life evolved hard skeletons, such as internal or external shells. Common invertebrates of these early reefs included trilobites, brachiopods (lampshells), bivalves, cephalopods (Orthoceras), crinoids (sea lilies), sponges and early corals.
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