Esben Horn/10 Tons

Earliest mollusk probably looked like a spiky slug

Today’s mollusks are a wildly diverse bunch, from octopuses and oysters to snails and slugs—a modern miscellany that has made it tough for scientists to envision what the group’s oldest common ancestors looked like. Now, a handful of 480-million-year-old fossils from southeastern Morocco suggests that those progenitors may have resembled a spine-covered slug that wore a small toenail-shaped shell on its head. Paleontologists have placed the creature in a new genus named Calvapilosa (“hairy scalp” in Latin). Complete fossils range from 16.7 to 68.3 millimeters long, but fragments hint the animal may have grown to at least 120 mm, the researchers report today in Nature. The upper part of the creature’s body (artist’s reconstruction at left) was a cloak of flesh covered with tiny plates of chitin that in turn sported hollow spines possibly made of chitin, the same material as the shells of modern-day shrimp. Underneath (right), the creature had a snaillike foot for locomotion and a mouth whose rasplike radula was covered with at least 125 rows of tiny teeth. The fine-grained sandstones that held the Calvapilosa fossils don’t appear to have been exposed to waves or strong currents, so the animal probably lived in deep waters. Calvapilosa wasn’t the oldest creature in its primitive lineage, the researchers note, but it is the best known because fossils of its closest kin have been extremely fragmentary. Previous studies have suggested the oldest mollusks evolved about 535 million years ago. And although some previous studies suggested that the first mollusks didn’t have shells, the new findings hint that the oldest species in the group actually did sport a hardened structure made out of calcium carbonate.