Marine animals colonized empty shells 470 million years ago, reshaping ocean ecosystems

Half a billion years ago, the ocean floor looked fundamentally different from what we see today. Empty shells lay scattered across the seafloor, waiting. For millions of years, they remained just that—empty. Then, around 470 million years ago, something shifted. Small animals began moving into these abandoned homes, and that simple act of colonization would reshape the architecture of marine life itself.

Researchers from the University of Tartu, working with colleagues across the globe, pieced together this story by examining fossils from multiple continents. What they found was a clear timeline: during the Cambrian Period, more than 500 million years ago, the interiors of mollusk shells sat vacant despite the existence of organisms that could have lived in sheltered spaces. But by the Middle Ordovician—roughly 460 million years ago—the first animals had moved in, initially claiming the large, roomy chambers of nautiloid cephalopods before spreading into the tighter confines of snail and clam shells.

The colonization accelerated. By the Late Ordovician, about 450 million years ago, the insides of many shells had become densely packed communities. Filter-feeders like bryozoans, brachiopods, sponges, and tube-building cornulids crowded these spaces, extracting food particles from the water column. What had been empty real estate became thriving neighborhoods, each shell a miniature ecosystem.

But not all shells were created equal. The shape of the shell determined who could live there and how well they could thrive. The spacious chambers of nautiloids allowed water to circulate freely, delivering oxygen and food while whisking away waste. These shells became the most diverse habitats, supporting the richest communities. In contrast, snail shells with their narrow openings restricted water flow. The animals that settled there grew smaller, and entire groups that required strong currents never colonized them at all. The geometry of a shell, in other words, wrote the rules for who could live inside.

Why did this ecological innovation emerge precisely during the Ordovician? The timing was not coincidental. The period saw an explosion of marine biodiversity, with sessile organisms—those anchored to hard surfaces—diversifying rapidly. Predation pressure was increasing, making shelter more valuable. Mollusk shells themselves were growing larger throughout the era, creating more usable space. Together, these forces created the conditions for a new way of life. An empty shell offered protection from predators and physical disturbance, while providing a hard surface for delicate filter-feeders to attach themselves to.

The oldest evidence of this behavior comes from the Baltica region, which includes present-day Estonia, suggesting this innovation may have first taken hold in those ancient seas. Yet within a few million years, the practice had spread globally. By the end of the Ordovician, similar shell-dwelling communities existed in what is now North America, southern China, and the Gondwana supercontinent. What began as a local adaptation became a worldwide ecological event.

The significance of this shift extends far beyond the Ordovician. The colonization of empty shells demonstrates how seemingly small innovations—a change in behavior, a new use for existing structures—can ripple through entire ecosystems. These modest adjustments to how organisms lived and where they lived helped build the seafloor communities we recognize today. Understanding these ancient ecological turning points offers insight into how life on Earth has repeatedly reinvented itself, and how the most transformative changes often begin with something as simple as moving into an empty home.