graptolites, a once vast and extremely diverse class of marine organisms that thrived during the Ordovician and then, in the extinction event, were very nearly wiped out. To the naked eye, graptolite fossils look like scratches or in some cases tiny petroglyphs. (The word “graptolite” comes from the Greek meaning “written rock”; it was coined by Linnaeus, who dismissed graptolites as mineral encrustations trying to pass themselves off as the remnants of animals.)
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