Ordovician Shell Colonization Reveals Early Marine Ecosystem Shift

A new study published in *Scientific Reports* shows that about 470 million years ago, during the Middle Ordovician, small marine animals began to occupy the empty interiors of mollusk shells. This behavior created a previously unused ecological niche and fundamentally altered the structure of early marine ecosystems.

The research, led by scientists at the University of Tartu in Estonia and based on fossils from across the globe, documents a stepwise colonization. Larger cephalopod shells were inhabited first, followed by gastropod and bivalve shells. By the Late Ordovician, the shells were densely packed with diverse fauna such as bryozoans, brachiopods, sponges, graptolites and cornulids, demonstrating how a modest behavioral shift can drive long‑term evolutionary change.