![]() This one is a bulbous round ammonite shell shape, which the researchers refer to as “sphereocone” shaped. (Left) A 3D scan of one of the many types of ammonite shells. And while their fossils may be reminiscent of the chambered nautiluses bobbing around the ocean today, their closest living relatives are squids and octopuses, explains Ritterbush. They were so successful for long periods of Earth’s history that they were considered a “cosmopolitan” group-readily evolving, diverse, and widespread. Since they came onto the scene in the Devonian Period about 400 million years ago, the carnivorous ammonoids ruled the Earth’s superocean, Panthalassa from the safety of their twisted shells. The coiled shell of this now-extinct group of cephalopods, the ammonoids-often referred to colloquially by one of the subgroups, ammonites-is one of the most iconic and common fossils found around the world today. “They were really the deal in cephalopods for a really long time.” ![]() “If you went to cast a line off a pier during the time of the dinosaurs and try to catch something in the ocean, you’d probably come up with some species of ammonite,” says Kathleen Ritterbush, a paleontologist and assistant professor at the University of Utah. They’re fossilized shells of a cephalopod that had dominated the ocean during the age of the dinosaurs for about 300 million years. According to legend, the distinct, coiled-up shape has been attributed to the Christian saint Hilda of Whitby, circa 614-680, who was said to have beheaded and petrified swarms of serpents into “snake stones.”īut these swirling stones are not reptilian. On the coasts of the English countryside, round, whorled stones can be found embedded in rocks and cliffs. "Snake stones" or ancient sea creature? Credit: opacity/ flickr/ CC BY-NC-ND 2.0
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