After 350 Million Years of Survival, Egg Size May Help Explain Why Ammonites Didn’t Make It Through the End‑Cretaceous Extinction

Life in the ancient oceans was not easy, but few creatures coped with it better than ammonites. These spiral-shelled animals thrived for more than 350 million years, surviving multiple mass extinctions and spreading into seas around the world. Then, about 66 million years ago, they disappeared.

A new analysis presented at the EGU General Assembly returns to a familiar question: why did ammonites disappear while their relatives, the nautiloids, survived? The researchers compared ammonites to nautiloids, a much less diverse group that survived the same extinction event. The extinction followed an asteroid impact that wiped out non-avian dinosaurs and disrupted marine ecosystems. Their survival may have been influenced by how they reproduced and how these strategies played out during ecosystem collapse.

The ammonites laid large numbers of tiny eggs, releasing small hatchlings into the open sea, where only a few survived. Nautiloids took the opposite approach, producing fewer and larger eggs that hatched into more developed young and probably grew more slowly.

“Having large, yolk-rich eggs allowed nautilus embryos to grow longer and hatch at a larger size, which could have given them an advantage when marine ecosystems collapsed and food was scarce,” Michael Schmutzer, an evolutionary biologist at the University of Oxford, said in a press release.

Learn more: The Permian extinction: life on Earth almost disappeared during the “Great Dying”

Why ammonites disappeared while nautiloids survived

Ammonites were marine relatives of modern squid and octopuses, known for their coiled shells and range of shapes and sizes. At their peak, they evolved into thousands of species. Nautiloids are a simpler, less diverse group of shelled cephalopods, and some still survive today, including the chambered nautilus found in the Indo-Pacific.

They had already survived some of the most extreme events in Earth’s history, including the Permian-Triassic extinction about 252 million years ago, which wiped out about 96 percent of marine species.

“This is a tragic story, as this incredibly diverse group has survived multiple mass extinctions, including the most dramatic mass extinction event in history,” Schmutzer added in the press release.

To revisit the problem, Schmutzer and his colleagues assembled the largest dataset ever on Late Cretaceous shelled cephalopods, combining fossil records with data on body and egg sizes and specimens held in museum collections, some of which had rarely been studied. Some of these fossils are sometimes referred to as “dark data” because they have not been fully analyzed.

An idea focused on geography. Nautiloids lived in larger areas, which may have helped them survive in places where conditions remained more stable after the impact. But this new analysis finds no strong evidence that reach alone makes a difference. Instead, nautiloids appear to have taken a more conservative approach to growth and reproduction.

Egg size may be part of the explanation

The story is not that simple. The few groups of ammonites that survived briefly after the extinction were those that contained the smallest eggs, not the largest. This runs counter to the idea that larger, more developed newborns were the key to survival.

Perhaps what mattered was how each group lived. After the asteroid impact, sunlight faded, temperatures plummeted, and marine food webs collapsed. Entire marine ecosystems destroyed. Traits that once helped ammonites thrive may have suddenly worked against them.

In the end, survival may not have favored the most successful group, but rather those whose life cycle proved adapted to a changing world. Maybe the ammonites were unlucky, Schmutzer said.

The results will be presented at the EGU 2026 General Assembly, which will be held from May 3 to 8.

Learn more: The Late Permian mass extinction explained

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