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11. Nautilus
These marine mollusks have remained largely unchanged for 500 million years. They first appeared in the Late Cambrian period and became a significant group of sea predators during the Ordovician period. They’re the sole living cephalopods (a group that includes octopuses) whose bony body structure is externalized as a shell. And interestingly, they’re much closer to the first cephalopods that appeared 500 million years ago than the early modern cephalopods that appeared about 100 million years later.
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