Massive numbers of sharks died abruptly 19 million years ago, new data show. Fossils from sediments in the Pacific Ocean reveal that 90 percent of them vanished. And so far, scientists don’t know why.
“It’s a great mystery,” says Elizabeth Sibert. She led the new study. A paleobiologist and oceanographer, she works at Yale University. That’s in New Haven, Conn. “Sharks have been around for 400 million years. And yet this event wiped out [up to] 90 percent of them.”
Sharks have suffered losses in the past. It started 250 million years ago during the Great Dying. This event marked the end of most large ocean species. Much later, about 66 million years ago, a huge asteroid fell to Earth. It killed off most dinosaurs — and 30 to 40 percent of shark species. After that, sharks enjoyed about 45 million years as the ocean’s top predator. They even survived large climate disruptions, such as an episode about 56 million years ago when global levels of carbon dioxide spiked and temperatures soared.
The newly discovered fossils are a surprising twist in the shark’s story.
Sifting sediment
Sibert sifted through fish teeth and shark scales in the sediment. She worked with Leah Rubin, a student at the College of the Atlantic in Bar Harbor, Maine. Scientists had collected that sediment during various 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 explains.
Sharks’ bodies are mostly cartilage. Unlike bone, cartilage is difficult to preserve as fossils. But sharks’ skin is covered in tiny scales. Each scale is about the width of a human hair follicle. These scales make for an excellent record of past shark abundance. They contain the same hard mineral as sharks’ teeth. Both can turn to fossils in sediments. “And we will find several hundred more [scales] compared to a tooth,” Sibert explains.
What her team discovered was a surprise. From 66 million to about 19 million years ago, the ratio of fish teeth to shark scales held steady at about 5 to 1. Then the ratio took a dramatic turn: 100 fish teeth appeared for each shark scale. The team estimates this change was abrupt — within 100,000 years or so.
That sudden disappearance of shark scales came at the same time as a change in the scales’ shapes. This provides clues about shark diversity.
Most modern sharks have lined grooves on their scales, ones that may help them swim faster. Other sharks’ scales have geometric shapes. The researchers looked at the change in the abundance of various scale shapes before 19 million years ago and then again afterward. This revealed a huge loss in shark diversity. It appears some seven in every 10 shark species went extinct.
And this extinction event was quite “selective,” notes Rubin. After the event, the geometric scales “were almost gone.” And that previous diversity in sharks, she adds, was never seen again. She and Sibert describe their findings June 4 in Science.
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A cautionary tale
An explanation for the massive shark die-off isn’t obvious, Sibert says. “Nineteen million years ago is not known as a formative time in Earth’s history.” Solving the mystery is one question she hopes to answer. She wants to understand how the varied scale shapes might relate to shark lineages. She’d also like to learn what impact the sudden loss of so many big predators might have had on other ocean dwellers.
Answers to those questions could be helpful today. Overfishing and ocean warming in the last 50 years have decreased shark populations by more than 70 percent. This loss of sharks no doubt impacts the ocean’s ecology.
Catherine Macdonald is a marine conservation biologist at the University of Miami in Florida. She sees the study as a cautionary tale. “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,” she notes.
What happens to communities of the ocean’s top predators can be critical signs of those changes. Unraveling how the ocean ecosystem responded to shark losses in the past could help researchers predict what may await us now, Sibert says. “The sharks are trying to tell us something,” she explains, “and I can’t wait to find out what it is.”
Power Words
More About Power Wordsasteroid: A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
Atlantic: One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
biodiversity: (short for biological diversity) The number and variety of species found within a localized geographic region.
biologist: A scientist involved in the study of living things.
carbon: A chemical element that is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.
carbon dioxide: (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
cartilage: (adj. cartilaginous) A type of strong connective tissue often found in joints, the nose and ear. In certain primitive fishes, such as sharks and rays, cartilage provides an internal structure — or skeleton — for their bodies.
conservation: The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.
conservation biologist: A scientist who investigates ways to help preserve ecosystems and especially species that are in danger of extinction.
diversity: A broad spectrum of similar items, ideas or people. In a social context, it may refer to a diversity of experiences and cultural backgrounds.(in biology) A range of different life forms.
ecology: A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem: A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.
expedition: A journey (usually relatively long or over a great distance) that a group of people take for some defined purpose, such as to map a region’s plant life or to study the local microclimate.
extinction: The permanent loss of a species, family or larger group of organisms.
follicle: The cells and other tissues that surround hair at its root.
fossil: Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
marine: Having to do with the ocean world or environment.
mineral: Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).
paleobiologist: A scientist who studies organisms that lived in ancient times — especially geologically ancient periods, such as the dinosaur era.
population: (in biology) A group of individuals from the same species that lives in the same area.
predator: (adjective: predatory) A creature that preys on other animals for most or all of its food.
ratio: The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
sediment: Material (such as stones and sand) deposited by water, wind or glaciers.
shark: A type of predatory fish that has survived in one form or another for hundreds of millions of years. Cartilage, not bone, gives its body structure.
sharks: A family of primitive fishes that rely on skeletons formed of cartilage, not bone. Like skates and rays, they belong to a group known as elasmobranchs. Then tend to grow and mature slowly and have few young. Some lay eggs, others give birth to live young.
species: A group of similar organisms capable of producing offspring that can survive and reproduce.
Citations
Journal: E.C. Sibert and L.D. Rubin. An early Miocene extinction in pelagic sharks. Science. Vol. 372, June 4, 2021, p. 1105. doi: 10.1126/science.aaz3549.
About Carolyn Gramling
Carolyn Gramling is the earth & climate writer at Science News. She has bachelor’s degrees in geology and European history and a Ph.D. in marine geochemistry from MIT and the Woods Hole Oceanographic Institution.
