Hope in fossilized fish bits
Abundant life in ancient seas sparks optimism for one paleontologist
Estimated reading time: 7 minutes
Fifty million years ago, Earth was a hothouse. Global temperatures surged to 80°F (27°C), a blistering 13 degrees hotter than today’s worst heatwaves, while carbon dioxide levels reached 2,000 parts per million—well beyond what’s considered doomsday levels today.
As the planet’s fever spiked, so too did the ocean’s temperature. Enough to make the Arctic Ocean a reasonably swimmable 60°F (15°C).
Yet, despite these conditions, life in the Eocene Ocean thrived. It was a place where the first cetaceans, with their hind limbs, snouty skulls, and thrashing tails, left their terrestrial ties for the sea. Where tiny prehistoric fish like Knightia and Enchodus propelled past vibrant reefs and tapestries of seagrass and kelp. And where, perhaps most remarkably, open-ocean fish and shark populations increased to staggering levels.
“This was a period of extreme warmth, yet there was this incredible abundance of marine life that crept up as the ocean got hotter and hotter,” said Elizabeth Sibert, an assistant scientist at WHOI who studies how marine life responded to climate change in deep time.
We’re standing at a workbench in her Paleo-FISHES lab as she pulls up a plot on her laptop that correlates time, water temperature, and a proxy of fish and shark abundance. On it, clusters of red and black dots representing fish and sharks, respectively, grow from sparse to dense as she finger-traces them upward along the Y axis.
“Isn’t that wild?” she asked.
Yeah, the idea of abundant life in a hotter ocean does seem kind of wild. It runs counter to most modern-day predictions suggesting that fish stocks will tank as the ocean continues to warm. It’s a particularly unexpected correlation considering the basic principles of primary productivity: warming occurs, the ocean becomes stratified, and fewer nutrients reach the surface where most productivity happens.
But the limited upwelling is just one problem. In some cases, ocean warming can restrict the availability of iron in seawater; in others, it can create oxygen “dead zones” that make certain parts of the ocean uninhabitable.
Our modern-day understanding of ocean warming today, however, doesn’t necessarily relate to how marine life responded to temperature spikes 50 million years ago.
An artist’s rendition of marine life in the Eocene Ocean. (Painting by David Miller for the Florida Museum of Natural History)
To get a read on ocean life in the early Eocene, Sibert looks to rare deep-sea sediments from the middle of the South Pacific Gyre—about as far away from land as you can get. The samples were taken during ocean drilling cruises between 1968 and 2024 as part of the International Ocean Discovery Program (IODP). During these expeditions, long chains of interconnected, 30-meter (98-foot) core barrels were drilled down as far as hundreds of meters below the seafloor. “I think of it like poking really deep holes in the ocean floor with what look like massive straws,” Sibert said. Some of the cores retrieved from these drilling cruises contain ichthyoliths, a fancy name for fossil fish teeth and shark scales. They trickled down to the seafloor when Australia was still attached to Antarctica, Drake’s Passage didn’t exist, and water from the Atlantic flowed uninterruptedly to the Indian Ocean via the Tethys Sea.
Sibert has the world’s largest collection of these primordial fish bits—around 200,000 of them. The “prized samples” as she referred to them, have been remarkably well preserved due to their enamel coatings. But since they’re thinner than human hair, finding them in the sediment is painstaking work, according to Sibert.
“It takes a lot of hard work and patience to process these samples,” she said. “But if you do, you get all these spectacular stories about fish and sharks and their interactions with climate through time.”
Unearthing ancient clues
The first step in processing the samples is drying the sediment and washing it over a sieve to extract silt-like granules. Sibert and her team occasionally use heavy liquid to "float" off excess material, or they apply a special dye to the granules that turns the teeth pink. The remaining material is then combed through using a small paintbrush under a microscope. When a fish tooth or shark scale is spotted, it’s picked from the sample with the toothbrush tip and placed onto an archival cardboard slide. The slide is then viewed with advanced digital microscopy, which allows the researchers to visualize fine details that are impossible to see on a standard microscope.
“This is just really fun to play with,” she said excitedly as she loaded a tray under the scope. “It’s absolutely the coolest thing ever.”
She’s not wrong. On the system’s LCD screen, an image appears of a black circle with a series of white dots scattered inside. It’s a top-down view of some ancient remnants under low magnification. “But this is only half the fun,” she said as she began to zoom in. Suddenly, much larger, crisper images of the individual microfossils fill up the screen. A shark scale with linear ridges comes into view, then a pointy prehistoric fish tooth.
Sibert entered a command on her keyboard and suddenly the entire image turns three dimensional. The cool factor’s high—it’s like having Google Street View on a tiny grid of fossils. But it’s not just bells and whistles; the view allows Sibert to measure the sizes and shapes of each fossil so she can see patterns in the samples and classify things into distinct morphological groups. She likens the shape of the fish teeth to ice cream cones.
“A colleague of mine teases me that all I do is look at teeny tiny ice cream cones all day long,” she said. “But the thing is, the more you look at these tiny little ice cream cones, the more little differences you notice, and those differences are indicative of the different fish that made those teeth”
Surviving the heat
But how did fish and shark communities survive the blazing Eocene Ocean in the first place? Ciara Willis, a marine biologist in WHOI’s Marine Predators Group, thinks there could be a few potential explanations. She said that in a warmer ocean, cold blooded fish and sharks are better able to compete with their warm-blooded counterparts, giving them enough of a competitive advantage to access more prey and grow their population sizes.
Another possibility, she said, is plain old adaptation.
“Generally, when we talk about the geological past, a relatively fast ‘spike’ of some physical variable can result in a relatively stable system on biologically relevant (i.e, multi-generational) timescales,” said Willis. “So, some Eocene fish and sharks may have had sufficient time to adapt.”
Thermal tolerance, however, is only part of the story. Sibert says a lot of other things had to go right at the base of the marine food web for life to flourish in the Eocene—like enough nutrients getting to the surface ocean so plankton can feed and become the right kind of prey for larger animals. “If any step along that way breaks down, the entire food web breaks down and the fish don’t have enough food to sustain them,” she said.
Sibert has been collaborating with Greg Britten, an assistant scientist at WHOI who builds models of marine food webs and ecosystem dynamics, to tease out why there were more fish during this period of extreme warmth. He wonders if perhaps the warmer ocean temperatures made marine microbes more efficient at recycling nutrients, which may have helped sustain productivity at the surface.
“Recycling processes are temperature dependent, so one hypothesis is that when the ocean got really warm fifty million years ago, microbes may have been recycling fast enough to keep nutrients at the surface and not allowing them to sink,” he said.
Bits of hope
Today, the vast majority of predictions say that as climate continues to warm, ocean productivity will tank. Which is one reason why Sibert’s work offers a fundamentally new perspective on how marine ecosystems may respond to global warming. Having seen such a strong positive response in the geological record, she’s hopeful that in the longer term, there could be a similar ecosystem response to ocean warming in the future.
WHOI assistant scientist Elizabeth Sibert. (Photo by Daniel Hentz, © Woods Hole Oceanographic Institution)
“Just knowing that warmer oceans have the potential to support more fish is hopeful,” said Sibert. “It says to me that marine ecosystems are remarkably resilient and that we haven’t crossed an irreversible tipping point yet. We do need to slow the speed of what we’re doing to the planet—the rate of changes in global temperatures today is unprecedented. But I think what we can learn from the past is that marine ecosystems have hope and can come out on the other side of this.”
This research was funded in part by the International Ocean Discovery Program, and an award from the National Science Foundation and the US Science Support Office for Scientific Ocean Drilling, OCE 1450528, Subaward 64B (GG009393-04).
MEET THE AUTHOR Evan Lubofsky
Evan Lubofsky is a science writer and editor at Woods Hole Oceanographic Institution. Read more
