Ammonoids are an extinct lineage of marine molluscs. Surprisingly, these ancient creatures are more closely related to modern coleoids (squid and cuttlefish) than to modern nautiloids. Ammonites are excellent index fossils, and geologists often rely on their presence for biostratigraphy. The earliest ammonites appeared in the Devonian (400 million years ago), and they flourished until 66 million years ago, up until the Cretaceous-Paleogene extinction. Despite the fossilized shell being very common in old sedimentary rocks, little is known about the creature that inhabited the ammonite shell. Scientists have no idea how many tentacles ammonites possessed, or the shape of the soft body. Although the shell is well preserved during the fossilization process, the soft body of the ammonite would have degraded and not been preserved.
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