Ancient fossils show how last mass extinction forever scrambled ocean’s biodiversity

By Stewart Edie for 'The Conversation'

Washington: 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 per cent 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.

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 was forever scrambled as a new ecological landscape emerged. Species that were once dominant struggled, while evolutionary newcomers rose in their place.

The reasons some species survived and others didn’t leave many questions to explore. Those that filtered phytoplankton from the water column suffered some of the highest species losses, but so did species that fed on organic scraps and didn’t rely as much on the Sun’s energy. Narrow geographic distributions and different metabolisms may have contributed to these extinction patterns.

Biodiversity bounces back

Life rebounded from each of the Big Five mass extinctions throughout Earth’s history, eventually punching through past diversity highs. The rich fossil record and spectacular ecological diversity of bivalves gives us a terrific opportunity to study these rebounds to understand how ecosystems and global biodiversity rebuild in the wake of extinctions.

The extinction caused by the asteroid strike knocked down some thriving modes of life and opened the door for others to dominate the new landscape.

While many people lament the loss of the dinosaurs, we malacologists miss the rudists.

These bizarrely shaped bivalves resembled giant ice cream cones, sometimes reaching more than 3 feet (1 meter) in size, and they dominated the shallow, tropical Mesozoic seas as massive aggregations of contorted individuals, similar to today’s coral reefs. At least a few harbored photosymbiotic algae, which provided them with nutrients and spurred their growth, much like modern corals.

Today, giant clams (Tridacna) and their relatives fill parts of these unique photosymbiotic lifestyles once occupied by the rudists, but they lack the rudists’ astonishing species diversity.

Mass extinctions clearly upend the status quo. Now, our ocean floors are dominated by clams burrowed into sand and mud, the quahogs, cockles and their relatives – a scene far different from that of the seafloor 66 million years ago.

New winners in a scrambled ecosystem

Ecological traits alone didn’t fully predict extinction patterns, nor do they entirely explain the rebound. We also see that simply surviving a mass extinction didn’t necessarily provide a leg up as species diversified within their old and sometimes new modes of life – and few of those new modes dominate the ecological landscape today.

Like the rudists, trigoniid bivalves had lots of different species prior to the extinction event. These highly ornamented clams built parts of their shells with a super strong biomaterial called nacre – think iridescent pearls – and had fractally interlocking hinges holding their two valves together.

But despite surviving the extinction, which should have placed them in a prime position to accumulate species again, their diversification sputtered. Other types of bivalves that made a living in the same way proliferated instead, relegating this once mighty and global group to a handful of species now found only off the coast of Australia.

Lessons for today’s oceans

These unexpected patterns of extinction and survival may offer lessons for the future.

The fossil record shows us that biodiversity has definite breaking points, usually during a perfect storm of climatic and environmental upheaval. It’s not just that species are lost, but the ecological landscape is overturned.

Many scientists believe the current biodiversity crisis may cascade into a sixth mass extinction, this one driven by human activities that are changing ecosystems and the global climate. Corals, whose reefs are home to nearly a quarter of known marine species, have faced mass bleaching events as warming ocean water puts their future at risk. Acidification as the oceans absorb more carbon dioxide can also weaken the shells of organisms crucial to the ocean food web.

Findings like ours suggest that, in the future, the rebound from extinction events will likely result in very different mixes of species and their modes of life in the oceans. And the result may not align with human needs if species providing the bulk of ecosystem services are driven genetically or functionally extinct.

The global oceans and their inhabitants are complex, and, as our team’s latest research shows, it is difficult to predict the trajectory of biodiversity as it rebounds – even when extinction pressures are reduced.

Billions of people depend on the ocean for food. As the history recorded by the world’s bivalves shows, the upending of the pecking order – the number of species in each mode of life – won’t necessarily settle into an arrangement that can feed as many people the next time around.

(Stewart Edie is a Research Geologist and Curator of Paleobiology, Smithsonian Institution)