For 350 million years, the ammonites ruled the ancient seas. These spiral-shelled creatures, relatives of today's nautilus and squid, survived the most catastrophic mass extinction in Earth's history—the Permian-Triassic event that obliterated 96 percent of all marine species roughly 252 million years ago. Yet somehow, 66 million years after an asteroid slammed into Earth and ended the age of dinosaurs, the ammonites vanished entirely while their cousins, the nautiloids, persisted to this day. It's one of evolution's most tantalizing puzzles, and now researchers at the University of Oxford believe they may finally understand why.
Evolutionary biologist Michael Schmutzer and his colleagues approached the mystery by assembling the largest dataset ever compiled for Late Cretaceous shelled cephalopods—combining published research with fossils long forgotten in museum drawers worldwide, what scientists call "dark data." Their findings, presented at the European Geosciences Union General Assembly in May 2026, challenge a long-held assumption about survival.
Previous research suggested that where these creatures lived determined who made it through the extinction. Schmutzer's analysis doesn't support that idea. Instead, his team turned their attention to egg size—and what they found flipped the hypothesis on its head.
Ammonites produced vast numbers of tiny eggs, releasing microscopic hatchlings into the ocean with only a few surviving to adulthood. Nautiloids, by contrast, invested in fewer, much larger eggs with rich yolks, producing hatchlings that emerged at a more robust size. Scientists had hypothesized that these "premium" nautiloid offspring enjoyed an advantage when ecosystems collapsed and food became scarce.
But here's the twist: the ammonite genera that clung to existence longest after the asteroid struck were those with the smallest eggs—not the largest. This suggests the egg hypothesis contains truth, just not in the way researchers expected. The real story may be one of sheer luck. "Maybe ammonites were just unlucky," Schmutzer told the assembly.
The research underscores how even the most resilient species can fall victim to circumstances beyond adaptation. Ammonites had weathered catastrophe after catastrophe for hundreds of millions of years. Their survival strategies, honed over eons, simply weren't enough against the darkness, collapsing food webs, and chaos that followed the Chicxulub impact. Nautiloids, with their slower metabolisms and larger hatchlings, happened to have traits that buffered them against that particular disaster.
Understanding these ancient survivors matters more than academic curiosity. As humanity navigates its own era of rapid environmental change, these 350-million-year-old fossils remind us that survival is never guaranteed—yet also that life, given enough time, finds ways to endure.