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Ammonites are long gone. The last of their coil-shelled, many-tentacled kind disappeared 66 million years ago in one of the worst mass extinctions of all time. The nearly 10-kilometer-wide asteroid that struck the Earth and drew the curtain on the Cretaceous wiped them out, just as it did the flying pterosaurs and non-avian dinosaurs.
And yet, there were survivors. While more than 70 percent of known species went extinct during the disaster, many others survived. The puzzle facing paleontologists now: why did the prolific and long-lived ammonites perish while other marine life—including their distant cousins, the nautilus and squid—persist? After all, ammonites had previously survived three other mass extinctions.
Looking at what made the end of the Cretaceous so catastrophic provides some clues. The previous mass extinctions the cephalopods had managed to weather were gradual perturbations of Earth’s systems, like intense volcanic activity that played out over hundreds of thousands of years. But when the asteroid whapped into the planet at the end of the Cretaceous, the effects were unprecedented. Within the first day, the Earth’s atmosphere heated to oven-like temperatures. Soon after, debris from the impact, and soot from widespread forest fires sparked by the ejecta, started to blot out the sun. Photosynthesis stopped for years, causing a near total collapse of the ecosystem.
Life in the ocean suffered. Most ocean ecosystems rely on photosynthesizers, but the extended night wiped out countless of these prolific autotrophs, nearly reverting the ocean to a bacterial state not seen for a billion years. Whatever survived had to eat other organisms to make it through the dark.
That may have proved a problem for ammonites, which routinely fed on their own offspring.
The asteroid’s impact also released carbon into the atmosphere, turning the ocean acidic—a condition that would have made it harder for baby planktonic ammonites to form shells. If they perished, any surviving ammonite adults would have been deprived of this critical food source, says Amane Tajika, a paleontologist at the American Museum of Natural History in New York.
Before their sudden end, ammonites were flourishing. By the end of the Cretaceous many ammonites had specialized to particular niches—occupying varied depths within the seas—and required comparatively more energy to grow. Ammonites might have been used to ocean resources supporting their needs to rapidly grow shells, and so suffered when those resources became scarce. Their cousin the nautilus instead took the slow-growing, generalist route.
And, as often happens during extinctions, bad luck may have had a hand in the ammonites’ demise.
“Modern squid and their relatives have obscene boom-bust cycles on seasonal, annual, and decadal timescales,” says Kathleen Ritterbush, a paleontologist at the University of Utah. Despite data recorded by scientists and the fishing industry, no one knows what causes these fluxes. “We can’t say what they’re going to do next Tuesday.”
It’s possible that ammonites also followed such volatile cycles. If the asteroid struck during an ammonite bust, “you could tank the smallest ammonite population on their worst day,” Ritterbush says.
Even if paleontologists were able to strap on scuba gear and dive into the Late Cretaceous ocean, finding the reason why ammonites disappeared would still be difficult. Doing it 66 million years after the fact is infinitely more challenging. But the story isn’t just important for understanding a loss. What transpired at the end of the Cretaceous set the foundation of our modern ocean. The end of the ammonites marks the beginning of the ocean as we know it.