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Orthoceras regulare
Ordovician cephalopod
Nautiloids are an ancient type of cephalopod, the group that includes squid, octopuses, and cuttlefish today. They have been swimming through the ocean for 495 million years, since the late Cambrian and early Ordovician periods. The nautilus is the only kind of nautiloid alive today.
Most modern cephalopods have an internal shell or no shell at all, but early cephalopods had external shells. Many nautiloids had long, straight shells called orthocones. Later forms were bent or coiled. Some nautiloids grew to gigantic sizes (over 20 feet long!), and they were the first large predators in the ocean.
Length: 19 inches