Around 470 million years ago, life in Earth’s oceans took an important evolutionary step when marine animals began moving into empty seashells for the first time.
According to a new international study led by researchers at the University of Tartu, this simple change created an entirely new habitat and helped shape marine ecosystems that still exist today.
The research, published in Scientific Reports, shows that empty shells became valuable homes for a wide range of sea creatures, providing shelter, protection and a place to feed.
The discovery offers new insight into how small ecological innovations can have a major impact on the evolution of life.
Today, empty shells rarely remain vacant for long. They are often covered by or filled with organisms such as bryozoans, worms and other marine animals that attach themselves to hard surfaces.
However, the researchers found that this was not always the case.
During the Cambrian Period, more than 500 million years ago, empty mollusk shells were left completely unoccupied, even though some animals were already living in naturally sheltered spaces elsewhere on the seafloor.
By examining fossils from around the world, the research team discovered that the first known animals to colonize the insides of empty shells appeared during the Middle Ordovician, about 470 million years ago.
They initially settled inside the large chambered shells of cephalopods, ancient relatives of today’s nautiluses. Later, similar communities expanded into the empty shells of snails and clams.
Over time, this new way of life became increasingly common. By the Late Ordovician, around 450 million years ago, many empty shells were densely occupied by a variety of filter-feeding animals.
These included bryozoans, brachiopods, sponges, graptolites and tube-building organisms known as cornulids. These animals survived by filtering tiny food particles from the surrounding seawater.
The researchers also found that not all shells offered the same living conditions. Large chambered cephalopod shells allowed water to circulate more freely, bringing in oxygen and food while removing waste. As a result, these shells supported the richest and most diverse communities.
In contrast, the tightly coiled shells of snails had much narrower openings, reducing water flow. Animals living inside these shells were generally smaller, and some species that depended on stronger water currents were unable to survive there.
The scientists believe several factors encouraged animals to begin occupying empty shells during the Ordovician. Marine biodiversity was increasing rapidly at the time, creating more competition for living space. Predators were also becoming more common, making sheltered habitats increasingly valuable. At the same time, mollusks were evolving larger shells, providing more room for other organisms after the original owners died.
The oldest examples of shell-dwelling communities have been found in the Baltica region, which includes present-day Estonia. While this may suggest the behavior first evolved there, the researchers note that fossils from this region have been studied more extensively than those from many other parts of the world.
By the end of the Ordovician, similar shell communities had spread across what are now North America, southern China and parts of the ancient supercontinent Gondwana, showing that the innovation had become a global phenomenon.
The study highlights how even a simple change—using empty shells as homes—can transform entire ecosystems. By creating new living spaces, these ancient marine animals helped increase biodiversity and laid the foundation for many of the complex seafloor communities found in today’s oceans.
The study was published in Scientific Reports.
