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An international study led by researchers from the University of Tartu shows that approximately 470 million years ago, a significant ecological change took place in the world's oceans: For the first time, the interiors of empty mollusk shells began to be used as habitats. This marked the emergence of a new ecological niche and was a crucial step in the development of a marine ecosystem similar to today's.
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In modern seas, empty mollusk shells rarely remain unoccupied for long. They are often covered with various attached invertebrates, such as bryozoans and annelids. However, the study shows that during the Cambrian Period, more than 500 million years ago, the interiors of such shells remained completely uninhabited, even though organisms living in sheltered cavities already existed.
A team of researchers, including Olev Vinn, Oive Tinn, Liisa Lang and Mare Isakar from the University of Tartu, analyzed fossils from different parts of the world and found that the first animals to colonize these sheltered habitats appeared in mollusk shells during the Middle Ordovician, or about 460 million years ago. Initially, the relatively large and spacious shells of cephalopods—or nautiloids—were colonized, and later, these small inhabitants also spread into the interiors of snails and clams. Their paper is published in the journal Scientific Reports.
By the Late Ordovician—about 450 million years ago—the interiors of many shells were already densely covered with attached fauna. These were inhabited by animals that filtered food particles floating in the water, such as bryozoans, brachiopods, sponges, graptolites and cornulids, which built tube-shaped shells.
However, not all empty shells provided the same living conditions. The study revealed that although the communities living in the shells of snails, clams and cephalopods were quite similar in terms of species composition, the shape of the shells influenced how numerous the organisms were and how large they grew. Water circulated more freely in the spacious chambers of the nautiloids, supplying the animals with oxygen and food and carrying away metabolic waste. Consequently, it is precisely in these shells that the most diverse fossil communities are found.
In the coiled shells of snails with narrow openings, water exchange was poorer, and the organisms that grew there were mostly smaller; moreover, some animal groups requiring a stronger water current were entirely absent.
Why did this new way of life emerge during the Ordovician?
The adoption of this new habitat coincided with a major increase in Ordovician biodiversity. At that time, marine communities diversified rapidly, and there was a significant increase in the number of sessile organisms encrusting skeletal substrates compared with earlier periods.
Several factors contributed to the spread of this new habitat: increased predation pressure, the rapid diversification of sessile organisms and the enlargement of mollusk shells throughout the Ordovician. The sheltered interiors of the shells provided protection for the animals and open space for sessile fauna to attach to the hard surface. The interior of an empty shell also offered protection to delicate filter-feeders against both predation and physical disturbances.
The oldest known inhabitants of empty shells have been found in the Baltica region, which may indicate that this new way of life first emerged in these seas. However, the researchers emphasize that this result may also be partly due to the fact that fossils from Estonia and other parts of Baltica have been thoroughly studied.
By the end of the Ordovician, similar communities were already present in Laurentia—present-day North America—as well as in southern China and the Gondwana region. For this reason, the authors view the colonization of empty shells as a global ecological event.
The study helps us better understand how ecological innovations have shaped the evolution of life on Earth. Even a seemingly modest change—the adoption of empty shells as habitats—may have had a significant impact on the development of seafloor ecosystems and the growth of biodiversity hundreds of millions of years ago.
More information: Olev Vinn et al, Colonization of empty shells by cryptic fauna: a global event and important ecological innovation in Ordovician benthic ecosystems, Scientific Reports (2026). DOI: 10.1038/s41598-026-62638-5
Provided by Estonian Research Council
This story was originally published on Phys.org.
