Around 66 million years ago, a cataclysmic asteroid struck Earth, marking the end of the Cretaceous period and bringing about a mass extinction event that wiped out roughly 70% of life on the planet. While the aftermath of this catastrophic impact is well-known for obliterating dinosaurs, the ocean’s response to this calamity is equally intriguing.
Marine ecosystems, too, suffered devastating losses, with species like mosasaurs and ammonites disappearing almost entirely. However, some creatures, particularly marine bivalves, displayed a remarkable and unexpected resilience.
An Unexpected Resilience
Marine bivalves, including clams, oysters, and mussels, have long been known for their ability to thrive in various ecological roles across the world’s oceans. These creatures, which often burrow into the seafloor or attach to underwater surfaces, faced significant losses during the mass extinction. Yet, despite the dramatic decline in species, many bivalves managed to survive. According to paleobiologists, around three-quarters of bivalve species were wiped out, but what’s surprising is that nearly every “mode of life” among them continued to persist in some form.
In a new study published in Science Advances, researchers studied thousands of bivalve fossils to track their survival through the extinction event. The results were startling. Even when 70% of bivalve species were lost, almost all the diverse ecological roles these creatures played within their ecosystems remained intact. “Statistically, that shouldn’t have happened,” the researchers noted.
The Role of Specialized Survival Strategies
What made this survival so unexpected? One crucial factor was the unique ways in which some bivalves interacted with their environments. While many species of marine life rely on sunlight and phytoplankton for nutrition, certain bivalves have adapted to more specialized survival strategies. For instance, some bivalves harbor chemosymbionts and photosymbionts—bacteria and algae that produce nutrients for the bivalves, creating a symbiotic relationship. Other bivalves, like mussels, have developed the ability to anchor themselves to rocks by secreting strong silken threads, making them more resistant to environmental changes.
In contrast, species that depended on sunlight for photosynthetic algae struggled. The dust and debris from the asteroid’s impact likely blocked out the sun, severely disrupting the food chain and affecting many marine species. However, the survival of bivalves with more specialized diets, like those that rely on chemosynthesis, proved that life on Earth could recover in ways not entirely dependent on sunlight.
The Long-Term Effects on Marine Ecosystems
Although many bivalves survived, the aftermath of the extinction event didn’t simply restore life to its former state. The research reveals that while some bivalves found success, others failed to capitalize on the ecological space left open by extinct species. Certain groups, such as the once-dominant rudists—large, reef-building bivalves—were wiped out entirely. Their ecological role in shallow, tropical seas was eventually taken over by giant clams and other species, though these replacements lack the same level of diversity the rudists once held.
In the case of the trigoniid bivalves, a group that built particularly tough shells, survival did not necessarily lead to recovery. Despite making it through the extinction, these species didn’t diversify in the same way others did. Today, only a handful of trigoniid species survive, mostly found off the coast of Australia. The study shows that simply surviving the extinction event didn’t guarantee that these species would dominate the post-extinction landscape. Instead, new species and ecological niches took over.
