Steinkern (internal mould) of Maclurites neritoides containing cryptic fauna cornulitids, bryozoans, and graptolites (arrows).
A paper just published in Scientific Reports by an international team led by Olev Vinn of the University of Tartu, Estonia, provides a lovely illustration of the opportunistic nature of evolution.
Some 470 million years ago — almost 470 million years before creationists’ mythical ‘Creation Week’ — a seemingly modest ecological innovation occurred in the oceans: animals began using the sheltered interiors of empty mollusc shells as habitats. This happened during the Middle Ordovician, following the Cambrian radiation, and helped to establish a new ecological niche that remains important in marine ecosystems today.
Creationists like to misrepresent the Cambrian radiation as the sudden appearance of complex, multicellular animals without ancestors. In reality, it was neither instantaneous nor without an evolutionary prehistory. It unfolded over tens of millions of years and grew out of changes already under way among the preceding Ediacaran biota, including increasing mobility, burrowing and ecological interaction.
As animals became more mobile, predation intensified and defensive structures became increasingly advantageous. Mineralised skeletons and shells evolved independently in several lineages, probably under several interacting pressures, including protection, structural support and changing ocean chemistry. Molluscs were among the groups that acquired shells, and their subsequent diversification produced such familiar forms as gastropods, bivalves and shelled cephalopods.
One consequence was the growing accumulation of empty shells on the seabed. Each dead mollusc left behind a hard surface and a sheltered cavity that other organisms could potentially exploit. Yet the researchers found that, throughout the Cambrian, the interiors of mollusc shells and other skeletal remains remained free of encrusting animals, even though organisms capable of living on surfaces inside other cavities already existed.
That changed during the Middle Ordovician, when attached animals began colonising the relatively large, spacious shells of nautiloid cephalopods. They later spread into the empty shells of gastropods and bivalves. No foresight or planning was required: once organisms capable of settling inside these shells gained access to a protected surface supplied with food-bearing water, any heritable tendency that improved their ability to exploit it could be favoured by natural selection.
By the Late Ordovician, about 450 million years ago, the interiors of many empty shells were densely encrusted with bryozoans, brachiopods, sponges, graptolites, cornulitids and other attached animals. Many were suspension-feeders whose survival depended upon a continuing flow of water carrying oxygen and suspended food particles while removing metabolic waste.
The researchers found that shell architecture strongly influenced the abundance and size of these inhabitants, although it had less effect on the broad taxonomic composition of the communities. Water circulated comparatively freely through the spacious chambers of nautiloid shells, allowing them to support abundant and diverse communities. By contrast, tightly coiled gastropod shells with narrow openings had poorer water circulation and generally contained fewer and smaller organisms; some groups that required stronger currents were absent altogether.
The expansion of this new habitat coincided with the Great Ordovician Biodiversification Event, during which marine diversity increased dramatically. The researchers suggest that several factors encouraged its spread: increasing predation made sheltered cavities valuable refuges; sessile, encrusting organisms were becoming more diverse; and mollusc shells were becoming larger, providing more internal space for colonisation.
The oldest known examples come from Baltica, the ancient continent that included what is now Estonia, suggesting that this ecological innovation may have arisen there. However, the researchers caution that this apparent geographical origin could reflect sampling bias, because the Ordovician fossils of Baltica have been studied particularly thoroughly.
How an Empty Shell Became an Evolutionary Opportunity. When a mollusc died, its soft tissues decomposed or were eaten, but its mineralised shell could remain on the seabed. This left a durable hard surface surrounding a sheltered cavity — effectively a small, unoccupied habitat.By the end of the Ordovician, comparable communities were present in Laurentia — including present-day North America — as well as southern China and Gondwana, showing that the occupation of empty shells had become a global ecological phenomenon. It is a fine example of evolution as a continuing process: one evolutionary development alters the environment, creates new opportunities and thereby opens the way for further diversification.
For sessile animals whose free-swimming larvae needed somewhere to settle, an empty shell could offer several advantages:
- A firm attachment surface: Unlike loose sediment, the shell provided a stable foundation upon which an organism could grow.
- Protection from predators: Animals attached inside a shell were less exposed than those living on its outer surface.
- Shelter from disturbance: The cavity offered some protection from strong currents, moving sediment and other physical disruption.
- Less competition: When shell interiors were first colonised, they represented comparatively unused space.
- Access to food and oxygen: Where water circulated freely, it delivered oxygen and suspended food particles while carrying away waste.
However, not every shell provided equally favourable conditions. The large chambers and wide openings of nautiloid shells allowed relatively good water circulation and could support numerous, sometimes comparatively large, inhabitants. Tightly coiled gastropod shells with narrow openings had poorer water exchange and tended to contain fewer and smaller organisms. For a suspension-feeder, a sheltered cavity was useful only if enough water continued to flow through it.
An evolutionary cascade
The occupation of empty shells illustrates how one evolutionary development can create opportunities for further evolution:
- Molluscs evolved mineralised shells.
- As molluscs diversified and became more abundant, empty shells accumulated on the seabed.
- Those shells created a new supply of sheltered, hard surfaces.
- Some encrusting animals began settling inside them.
- Individuals better able to attach, feed and survive within shells could leave more descendants.
- Shell interiors consequently developed their own distinctive ecological communities.
- The addition of this new habitat increased the complexity and biodiversity of marine ecosystems.
The empty shells had not evolved to provide homes for other animals, and the molluscs gained nothing from their occupation after death. It was an unintended ecological consequence of an earlier evolutionary innovation.
Nor did evolution need to anticipate how the shells might eventually be used. Once the opportunity existed, natural selection could favour any heritable variation that helped an organism exploit it. One lineage’s discarded structure had become another lineage’s habitat.
This is how evolution so often proceeds: not according to a plan directed towards a predetermined goal, but through the opportunistic exploitation of whatever environments, structures and resources happen to become available.
The paper in Scientific Reports was accompanied by a news release from the University of Tartu:
Empty seashells became new habitats 470 million years ago
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 mollusc 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.
In modern seas, empty mollusc 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 mollusc 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.
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 to 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 mollusc 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.
Publication:
The age of these fossils is, of course, the most obvious problem for creationists. Animals were colonising empty mollusc shells about 470 million years before their supposed ‘Creation Week’. But even if that inconvenient date could somehow be wished away, the sequence of events preserved in the rocks would remain just as damaging to the creationist account.
The fossil record does not show a completed marine ecosystem appearing all at once, least of all only a few thousand years ago. During the Cambrian, suitable shell interiors existed and cryptic animals lived in other sheltered cavities, yet empty shells remained uncolonised. During the Middle Ordovician, animals began occupying the spacious interiors of nautiloid shells; later they spread into gastropod and bivalve shells; and by the Late Ordovician, complex shell-dwelling communities had become globally widespread. That is a chronological succession of ecological changes, not the instantaneous creation of fixed and fully formed communities.
The distribution of these organisms was also governed by ordinary physical conditions. Spacious shells with good water circulation supplied more oxygen and food and supported larger, more abundant communities. Narrow, tightly coiled shells restricted water exchange and supported fewer and smaller inhabitants. The organisms occurred where their inherited characteristics enabled them to survive, not where some supernatural designer had assigned them to live.
Most revealingly, the new habitat itself was an accidental by-product of earlier evolution. Molluscs did not evolve shells to provide homes for other animals after their deaths. Empty shells simply became available, and organisms capable of exploiting that opportunity gained shelter, attachment surfaces and access to food-bearing water. Evolution then proceeded through the differential survival and reproduction of those variants best suited to the new conditions. There was no plan, no anticipation and no need for either.
Creationism explains none of this: neither the immense age, the ordered appearance of successive communities, their dependence upon earlier evolutionary innovations nor their later spread across the world. It can accommodate the evidence only by inventing additional miracles after the evidence has been discovered — excuses that predict nothing and explain nothing.
The rocks contain no trace of a completed creation; they preserve a history of evolution continually making new opportunities from the remains of what evolved before.
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