The earliest ammonoids on Earth, small organisms called Bacrites, had straight shells. Later ammonites had coiled shells made up of a spiraling series of chambers. They got their name from early Romans who mistook their fossils for ram?s horns (Ammon= a god with ram's horns). Ammonites became common in the seas of the Jurassic (200 million years ago). They were top predators, feeding on fishes, mollusks, arthropods, or other ocean creatures. Ammonites used good eyesight and tentacles to capture prey and feed it into a strong, crushing beak. Their closest living relatives are probably the modern nautiloids, but ancient ammonoids could be wider than a minivan. During the Cretaceous (about 150 million years ago), as ammonoids reached their heyday, some evolved to have shells that were coiled less or not at all. All ammonoids went extinct in the huge extinction event at the end of the Cretaceous (65 million years ago).

Mollusks have inhabited the Earth for at least 540 million years, dating back to the early Cambrian period. The fossil record of mollusks consists mostly of shells. Rarely, it includes mouthparts (radulas) or the trapdoors (opercula) that cover shell openings. The rest of the body is soft and usually does not fossilize. Before the Cambrian, taxa such as Kimberella had many traits expected of early molluscs, but no shells or spines. Cambrian, worm-like animals with spiny scales, such as Wiwaxia, had mollusk-like mouthparts. The earliest agreed-upon mollusk looked like a snail with a single, cap-like shell that curved at the tip. More than 90,000 living species of mollusk have been described, plus another 70,000 species known only from fossils. Because many mollusks live in remote places such as in the bottom sediments of deep ocean, and new species are being discovered at a rapid rate, scientists think that many more species remain to be described.