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    In the wake of history’s deadliest mass extinction, ocean life may have flourished

    Following the most severe known mass extinction in Earth’s history, vibrant marine ecosystems may have recovered within just a million years, researchers report in the Feb. 10 Science. That’s millions of years faster than previously thought. The evidence, which lies in a diverse trove of pristine fossils discovered near the city of Guiyang in South China, may represent the early foundations of today’s ocean-dwelling ecosystems.

    The conventional story was that the ocean was kind of dead for millions of years after this mass extinction, says paleontologist Peter Roopnarine of the California Academy of Sciences in San Francisco, who was not involved in the research. “Well, that’s not true. The ocean [was] very much alive.”

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    The Great Dying, or Permian-Triassic mass extinction, occurred around 251.9 million years ago, at the end of the Permian Period, after a series of massive volcanic eruptions (SN: 12/6/18).

    “The oceans warmed significantly, and there’s evidence for acidification, deoxygenation [causing widespread dead zones], as well as poisoning,” says Roopnarine. “There [were] a lot of toxic elements like sulfur entering into parts of the ocean.”

    Life in the seas suffered. More than 80 percent of marine species went extinct. Some researchers have even proposed that entire trophic levels — castes in an ecosystem’s food web — may have vanished.

    Figuring out how long life took to fully recover in the wake of all that loss has been challenging. In 2010, researchers studying fossils from the Luoping biota in China proposed that complex marine ecosystems fully rebounded within 10 million years. Later, other fossil finds, such as the Paris biota in the western United States and the Chaohu biota in China, led scientists to suggest that marine ecosystems reestablished themselves within just 3 million years.

    Then in 2015, a serendipitous discovery narrowed the gap again. Paleontologist Xu Dai, then an undergraduate student at the China University of Geosciences in Wuhan, was studying rocks from the early Triassic during a field trip near the city of Guiyang, when he split open a piece of black shale. Within the rock, he discovered a surprisingly well-preserved fossil of what would later be identified as a primitive lobster.

    The arthropod’s immaculate condition sparked a series of return trips. From 2015 to 2019, Dai, now at the University of Burgundy in Dijon, France, and his colleagues uncovered a bricolage of fossilized life: Predatory fish as long as baseball bats. Ammonoids in swirled shells. Eel-like conodonts. Early shrimps. Sponges. Bivalves. Fossilized poo.

    Fossilized coelacanths (one shown here), a type of bony fish, are the largest macrofossils yet found in the Guiyang biota.Dai et al/Science 2023

    And the prizes kept coming. Both under and within the Guiyang biota, Dai and his colleagues discovered beds of volcanic ash. An analysis of the amounts of uranium and lead in the ash revealed that the Guiyang biota contained fossils from roughly 250.7 to 250.8 million years ago (SN: 5/2/22). The dating was further supported by the types of fossils found and by an analysis of the different forms of carbon in the rocks.

    Finding a potpourri of life of this age suggests that marine ecosystems rebounded quickly after the Great Dying, within just 1 million years or so, Dai says.

    Alternatively, it may indicate that the extinction event failed to wipe out entire trophic levels, says paleontologist William Foster from the University of Hamburg in Germany, who was not involved in the study. “You have this really environmentally stressful world, but some former marine ecosystems survive.”

    Regardless, it seems clear that these ecosystems were hardy. Due to the motion of tectonic plates, the community preserved in the Guiyang biota was located in the tropics during the early Triassic. At that time, the temperature of the sea surface was nearly 35⁰ Celsius, and past research had suggested many organisms may have migrated away to escape the heat. But, the discovery of the Guiyang biota challenges that, Foster says. Sea creatures “are tolerating it somehow, they’re adapting.”

    According to Dai, the fossils may be evidence that the roots of today’s marine ecosystems took hold shortly after the Great Dying. “These groups are related to modern fish, lobsters and shrimps, their ancestors,” he says. “The oldest time we can find similar seafood to today is [in the time of] the Guiyang biota.”

    But Roopnarine is skeptical. It remains to be seen exactly how the Guiyang biota connects to today’s ecosystems, he says. The fossil assemblage could represent an ephemeral collective of life rather than a stable community, he adds, pointing out that ammonoids and conodonts went extinct.

    Further work will help resolve the many questions unearthed with the Guiyang biota, Dai says. He and his colleagues plan to head back into the field this summer for the first time since 2019. When asked if he’ll be keeping his eyes peeled for another lobster, he responds: “Of course.” More

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    Megatooth sharks may have been higher on the food chain than any ocean animal ever

    Whenever paleontologist Dana Ehret gives talks about the 15-meter-long prehistoric sharks known as megalodons, he likes to make a joke: “What did megalodon eat?” asks Ehret, Assistant Curator of Natural History at the New Jersey State Museum in Trenton. “Well,” he says, “whatever it wanted.”

    Now, there might be evidence that’s literally true. Some megalodons (Otodus megalodon) may have been “hyper apex predators,” higher up the food chain than any ocean animal ever known, researchers report in the June 22 Science Advances. Using chemical measurements of fossilized teeth, scientists compared the diets of marine animals — from polar bears to ancient great white sharks — and found that megalodons and their direct ancestors were often predators on a level never seen before.

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    The finding contradicts another recent study, which found megalodons were at a similar level in the food chain as great white sharks (SN: 5/31/22). If true, the new results might change how researchers think about what drove megalodons to extinction around 3.5 million years ago.

    In the latest study, researchers examined dozens of fossilized teeth for varieties of nitrogen, called isotopes, that have different numbers of neutrons. In animals, one specific nitrogen isotope tends to be more common than another. A predator absorbs both when it eats prey, so the imbalance between the isotopes grows further up the food chain. 

    For years, scientists have used this trend to learn about modern creatures’ diets. But researchers were almost never able to apply it to fossils millions of years old because the nitrogen levels were too low. In the new study, scientists get around this by feeding their samples to bacteria that digest the nitrogen into a chemical the team can more easily measure.

    The result: Megalodon and its direct ancestors, known collectively as megatooth sharks, showed nitrogen isotope excesses sometimes greater than any known marine animal. They were on average probably two levels higher on the food chain than today’s great white sharks, which is like saying that some megalodons would have eaten a beast that ate great whites.

    “I definitely thought that I’d just messed up in the lab,” says Emma Kast, a biogeochemist at the University of Cambridge. Yet on closer inspection, the data held up.

    The result is “eyebrow-raising,” says Robert Boessenecker, a paleontologist at the College of Charleston in South Carolina who was not involved in the study. “Even if megalodon was eating nothing but killer whales, it would still need to be getting some of this excess nitrogen from something else,” he says, “and there’s just nothing else in the ocean today that has nitrogen isotopes that are that concentrated.”

    “I don’t know how to explain it,” he says.

    There are possibilities. Megalodons may have eaten predatory sperm whales, though those went extinct before the megatooth sharks. Or megalodons could have been cannibals (SN: 10/5/20).  

    Another complication comes from the earlier, contradictory study. Those researchers examined the same food chain —  in some cases, even the same shark teeth — using a zinc isotope instead of nitrogen. They drew the opposite conclusion, finding megalodons were on a similar level as other apex predators.

    The zinc method is not as established as the nitrogen method, though nitrogen isotopes have also rarely been used this way before. “It could be that we don’t have a total understanding and grasp of this technique,” says Sora Kim, a paleoecologist at the University of California, Merced who was involved in both studies. “But if [the newer study] is right, that’s crazy.”

    Confirming the results would be a step toward understanding why megalodons died off. If great whites had a similar diet, it could mean that they outcompeted megalodons for food, says Ehret, who was not involved in the study. The new findings suggest that’s unlikely, but leave room for the possibility that great whites competed with — or simply ate — juvenile megalodons (SN: 1/12/21). 

    Measuring more shark teeth with both techniques could solve the mystery and reconcile the studies. At the same time, Kast says, there’s plenty to explore with their method for measuring nitrogen isotopes in fossils. “There’s so many animals and so many different ecosystems and time periods,” she says. 

    Boessenecker agrees. When it comes to the ancient oceans, he says, “I guarantee we’re going to find out some really weird stuff.” More

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    Something mysteriously wiped out about 90 percent of sharks 19 million years ago

    About 19 million years ago, something terrible happened to sharks.

    Fossils gleaned from sediments in the Pacific Ocean reveal a previously unknown and dramatic shark extinction event, during which populations of the predators abruptly dropped by up to 90 percent, researchers report in the June 4 Science. And scientists don’t know what might have caused the die-off.

    “It’s a great mystery,” says Elizabeth Sibert, a paleobiologist and oceanographer at Yale University. “Sharks have been around for 400 million years. They’ve been through hell and back. And yet this event wiped out [up to] 90 percent of them.”

    Sharks suffered losses of 30 to 40 percent in the aftermath of the asteroid strike that killed off all nonbird dinosaurs 66 million years ago (SN: 8/2/18). But after that, sharks enjoyed about 45 million years of peaceful ocean dominance, sailing through even large climate disruptions such as the Paleocene-Eocene Thermal Maximum — an episode about 56 million years ago marked by a sudden spike in global carbon dioxide and soaring temperatures — without much trouble (SN: 5/7/15).

    Now, clues found in the fine red clay sediments beneath two vast regions of Pacific add a new, surprising chapter to sharks’ story.

    Sibert and Leah Rubin, then an undergraduate student at the College of the Atlantic in Bar Harbor, Maine, sifted through fish teeth and shark scales buried in sediment cores collected during previous research expeditions to the North and South Pacific oceans.

    “The project came out of a desire to better understand the natural background variability of these fossils,” Sibert says. Sharks’ bodies are made of mostly cartilage, which doesn’t tend to fossilize. But their skin is covered in tiny scales, or dermal denticles, each about the width of a human hair follicle. These scales make for an excellent record of past shark abundance: Like shark teeth, the scales are made of the mineral bioapatite, which is readily preserved in sediments. “And we will find several hundred more denticles compared to a tooth,” Sibert says.

    Researchers sorted fossil shark scales, or denticles, into two main types: those with linear striations (left) and those with geometric shapes and with no striations (right). Following the shark extinction event 19 million years ago, the geometric denticles all but disappeared from ocean sediments.E.C. Sibert and L.D. Rubin/Science 2021

    The researchers weren’t expecting to see anything particularly startling. From 66 million years ago to about 19 million years ago, the ratio of fish teeth to shark scales in the sediments held steady at about 5 to 1. But abruptly — the team estimates within 100,000 years, and possibly even faster — that ratio dramatically changed, to 100 fish teeth for every 1 shark scale.

    The sudden disappearance of shark scales coincided with a change in the abundances of shark scale shapes, which give some clues to changes in biodiversity. Most modern sharks have linear striations on their scales, which may offer some boost to their swimming efficiency. But some sharks lack these striations; instead, the scales come in a variety of geometric shapes. By analyzing the change in the different shapes’ abundances before and after 19 million years ago, the researchers estimated a loss of shark biodiversity of between 70 and 90 percent. The extinction event was “selective,” says Rubin, now a marine scientist at the State University of New York College of Environmental Science and Forestry in Syracuse. After the event, the geometric scales “were almost gone, and never really showed up again in the diversity that they [previously] did.”

    There’s no obvious climate event that might explain such a massive shark population shift, Sibert says. “Nineteen million years ago is not known as a formative time in Earth’s history.” Solving the mystery of the die-off is at the top of a long list of questions she hopes to answer. Other questions include better understanding how the different denticles might relate to shark lineages, and what impact the sudden loss of so many big predators might have had on other ocean dwellers.

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    It’s a question with modern implications, as paleobiologist Catalina Pimiento of the University of Zurich and paleobiologist Nicholas Pyenson of the Smithsonian National Museum of Natural History in Washington, D.C., write in a commentary in the same issue of Science. In just the last 50 years, shark abundances in the oceans have dramatically declined by more than 70 percent as a result of overfishing and ocean warming. The loss of sharks — and other top marine predators, such as whales — from the oceans has “profound, complex and irreversible ecological consequences,” the researchers write.

    Indeed, one way to view the study is as a cautionary tale about modern conservation’s limits, says marine conservation biologist Catherine Macdonald of the University of Miami, who was not involved with this study. “Our power to act to protect what remains does not include an ability to fully reverse or undo the effects of the massive environmental changes we have already made.”

    Populations of top ocean predators can be important indicators of those changes — and unraveling how the ocean ecosystem responded to their loss in the past could help researchers anticipate what may happen in the near future, Sibert says. “The sharks are trying to tell us something,” she adds, “and I can’t wait to find out what it is.” More

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    Climate change helped some dinosaurs migrate to Greenland

    A drop in carbon dioxide levels may have helped sauropodomorphs, early relatives of the largest animal to ever walk the earth, migrate thousands of kilometers north past once-forbidding deserts around 214 million years ago.
    Scientists pinpointed the timing of the dinosaurs’ journey from South America to Greenland by correlating rock layers with sauropodomorph fossils to changes in Earth’s magnetic field. Using that timeline, the team found that the creatures’ northward push coincides with a dramatic decrease in CO2, which may have removed climate-related barriers, the team reports February 15 in Proceedings of the National Academy of Sciences.
    The sauropodomorphs were a group of long-necked, plant-eating dinosaurs that included massive sauropods such as Seismosaurus as well as their smaller ancestors (SN: 11/17/20). About 230 million years ago, sauropodomorphs lived mainly in what is now northern Argentina and southern Brazil. But at some point, these early dinosaurs picked up and moved as far north as Greenland.
    Exactly when they could have made that journey has been a puzzle, though. “In principle, you could’ve walked from where they were to the other hemisphere, which was something like 10,000 kilometers away,” says Dennis Kent, a geologist at Columbia University. Back then, Greenland and the Americas were smooshed together into the supercontinent Pangea. There were no oceans blocking the way, and mountains were easy to get around, he says. If the dinosaurs had walked at the slow pace of one to two kilometers per day, it would have taken them approximately 20 years to reach Greenland.

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    But during much of the Late Triassic Epoch, which spans 233 million to 215 million years ago, Earth’s carbon dioxide levels were incredibly high — as much as 4,000 parts per million. (In comparison, CO2 levels currently are about 415 parts per million.) Climate simulations have suggested that level of CO2 would have created hyper-arid deserts and severe climate fluctuations, which could have acted as a barrier to the giant beasts. With vast deserts stretching north and south of the equator, Kent says, there would have been few plants available for the herbivores to survive the journey north for much of that time period.
    Previous estimates suggested that these dinosaurs migrated to Greenland around 225 million to 205 million years ago. To get a more precise date, Kent and his colleagues measured magnetic patterns in ancient rocks in South America, Arizona, New Jersey, Europe and Greenland — all locales where sauropodomorphs fossils have been discovered. These patterns record the orientation of Earth’s magnetic field at the time of the rock’s formation. By comparing those patterns with previously excavated rocks whose ages are known, the team found that sauropodomorphs showed up in Greenland around 214 million years ago.
    Vertebrate fossils from the Late Triassic have been found at a number of sites around the world, some of which are marked (black dots) on this map showing how the continents were arranged about 220 million years ago. New dating of rocks at sites in South America and Greenland pinpoint when long-necked dinosaurs known as sauropodomorphs migrated north.Dennis Kent and Lars Clemmensen
    That more precise date for the sauropodomorphs’ migration may explain why it took them so long to start the trek north — and how they survived journey: Earth’s climate was changing rapidly at that time.
    Around the time that sauropodomorphs appeared in Greenland, carbon dioxide levels plummeted within a few million years to 2,000 parts per million, making the climate more travel-friendly to herbivores, the team reports. The reason for this drop in carbon dioxide — which appears in climate records from South America and Greenland — is unknown, but it allowed for an eventual migration northward.
    “We have evidence for all of these events, but the confluence in timing is what is remarkable here,” says Morgan Schaller, a geochemist at Rensselaer Polytechnic Institute in Troy, N.Y., who was not involved with this study. These new findings, he says, also help solve the mystery of why plant eaters stayed put during a time that meat eaters roamed freely.
    “This study reminds us that we can’t understand evolution without understanding climate and environment,” says Steve Brusatte, a vertebrate paleontologist and evolutionary biologist at the University of Edinburgh, also not involved with the study. “Even the biggest and most awesome creatures that ever lived were still kept in check by the whims of climate change.” More