Fossils reveal 500 million years of changes in ocean life

We know the oceans are changing. But how have they changed over the last half-billion years? Until now, we had no clear measure of how the total mass of ocean life – not just its variety – has evolved over deep time.

In a first-of-its-kind effort, scientists from Stanford University measured how the biomass of marine organisms – the actual amount of living material in the ocean – has changed over the last 540 million years.

Long-term changes in ocean life

The study shows a long-term trend of rising biomass, with noticeable crashes during major extinction events. This pattern closely tracks with known increases in marine biodiversity, suggesting a deep link between how much life the ocean holds and how varied that life is.

Study lead author Pulkit Singh is a postdoctoral scholar in Earth and Planetary Sciences in the Stanford Doerr School of Sustainability.

“Understanding the amount of biomass is important because it represents key traits about an ecosystem that are not captured by the number of species or even the number of niches that they fill,” said Singh.

“But as we move into the past, our measurements of biomass are very limited, so that was the big gap in biological history we wanted to fill with our study.”

Ocean life changes across history

Getting to this result took years of meticulous work. Singh and his team dug into data from over 7,700 marine limestone samples spanning hundreds of millions of years.

The samples came from more than 100 published studies and Singh’s own data collection. Each rock was cut and polished into slices thin enough for light to pass through, then examined under a microscope.

Using a method called petrographic point-counting, the researchers calculated what percentage of each sample was made up of skeletal remains – shells from animals, certain algae, and single-celled protists.

It’s not just busywork. The amount of shell material in these rocks tells scientists how much living material existed at different points in Earth’s history.

More shells in the ocean generally mean more living things. And more life usually means more energy flowing through ecosystems – a strong signal of planetary health.

The challenge of measuring biomass

Tracking biodiversity through deep time has been part of science since the 19th century. But measuring biomass – the total amount of living material – is a much rarer effort.

Measuring biomass demands massive amounts of data and offers no guarantee of a clear pattern.

“The first quantitative effort to document and graph biodiversity across geological time was made in 1860, but until Pulkit’s paper, there’s never been a corresponding biomass-across-time paper,” noted senior study author Professor Jonathan Payne.

“I’m impressed by his intellectual courage to go and take a chance on a project like this.”

Biomass over millions of years

The earliest rocks examined, from the Cambrian period about 540 million years ago, showed low shell content – under 10 percent on average.

By the Ordovician, starting around 485 million years ago, shell percentages rose. This increase lines up with the Cambrian Explosion, a period when life rapidly diversified.

Early on, sponges were major players. Later, they were overtaken by echinoderms (ancestors of starfish) and arthropods like trilobites.

A gradual increase in biomass

Over the next 230 million years, shell content stayed mostly above 20 percent, aside from big dips during mass extinctions.

One of the sharpest declines came during the Permian-Triassic extinction about 250 million years ago – the “Great Dying.” Shell material dropped to just 3 percent.

Life rebounded after that, although smaller setbacks followed during the end-Triassic and the well-known Cretaceous-Paleogene extinction 66 million years ago.

In our current era – the Cenozoic – shell volume climbed above 40 percent, boosted by creatures like mollusks and corals. “The overall pattern that we were able to capture is that it’s a gradual increase,” Singh said.

Consistent biological patterns

The team knew they had to rule out alternative explanations. Maybe shell-boring predators were less common in the past. Maybe certain environments were overrepresented in the data.

To check, the researchers sorted samples by depth, latitude, and geological setting. The trend held steady.

“The more tests we did and the more we divided our dataset, we realized that these big biological patterns we were seeing stayed over time,” Singh said.

Why did ocean life keep growing?

The answer may come down to diversity. As life evolved to fill more ecological roles, it got better at using available energy. Organisms became more specialized, more interconnected.

That allowed ecosystems to recycle nutrients more efficiently – from sunlight-eating phytoplankton to decomposers returning nutrients to the cycle.

“The overall idea is that there is more food available in ecosystems and because of that, the ecosystems can support more life, there’s more energy available, and that leads to greater abundance expressed in biomass,” Singh said.

Humans are rapidly changing ocean life

The study paints a long view of life’s upward climb, but it ends before the modern age. Today’s oceans face threats from overfishing, pollution, acidification, and climate change.

Researchers agree – we’re already in the middle of a human-caused sixth mass extinction. And if biodiversity keeps falling, we may see long-term drops in biomass too – with effects that could last millions of years.

“From our study’s perspective, modern times are quite complicated given the extent of human activity that’s rapidly altering conditions planetwide, including in the oceans,” said Payne.

“But our findings show that overall biomass is linked to biodiversity and that losses in biodiversity may suppress productivity for geologically meaningful intervals, adding one more argument of why conserving biodiversity is essential for the health of humans and our planet.”

The full study was published in the journal Current Biology.

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