About 66 million years ago – perhaps on a downright unlucky day in May – an asteroid smashed into our planet.
The fallout was immediate and severe. Evidence shows that about 70% of species went extinct in a geological instant, and not just those famous dinosaurs that once stalked the land. Masters of the Mesozoic oceans were also wiped out, from mosasaurs – a group of aquatic reptiles topping the food chain – to exquisitely shelled squid relatives known as ammonites.
Even groups that weathered the catastrophe, such as mammals, fishes and flowering plants, suffered severe population declines and species loss. Invertebrate life in the oceans didn’t fare much better.
But bubbling away on the seafloor was a stolid group of animals that has left a fantastic fossil record and continues to thrive today: bivalves – clams, cockles, mussels, oysters and more.
What happened to these creatures during the extinction event and how they rebounded tells an important story, both about the past and the future of biodiversity.
Surprising discoveries on the seafloor
Marine bivalves lost around three-quarters of their species during this mass extinction, which marked the end of the Cretaceous Period. My colleagues and I – each of us paleobiologists studying biodiversity – expected that losing so many species would have severely cut down the variety of roles that bivalves play within their environments, what we call their “modes of life.”
But, as we explain in a study published in the journal Sciences Advances, that wasn’t the case. In assessing the fossils of thousands of bivalve species, we found that at least one species from nearly all their modes of life, no matter how rare or specialized, squeaked through the extinction event.
Statistically, that shouldn’t have happened. Kill 70% of bivalve species, even at random, and some modes of life should disappear.
Bivalves had an amazing array of life modes just before the end-Cretaceous mass extinction 66 million years ago. Incredibly, despite the loss of 70% of their species, all but two modes of life survived – Nos. 2 and 10.
Adapted from Edie et al. 2025, Science Advances
Most bivalves happily burrow into the sand and mud, feeding on phytoplankton they strain from the water. But others have adopted chemosymbionts and photosymbionts – bacteria and algae that produce nutrients for the bivalves from chemicals or sunlight in exchange for housing. A few have even become carnivorous. Some groups, including the oysters, can lay down a tough cement that hardens underwater, and mussels hold onto rocks by spinning silken threads.
We thought surely these more specialized modes of life would have been snuffed out by the effects of the asteroid’s impact, including dust and debris likely blocking sunlight and disrupting a huge part of the bivalves’ food chain: photosynthetic algae and bacteria. Instead, most persisted, although biodiversity…
