Unraveling Extinction: Why One Ancient Ocean Creature Thrived While Another Disappeared

Ammonites, the magnificent marine cephalopods that flourished for millions of years before their sudden extinction, remain one of paleontology’s most intriguing mysteries. Despite their extensive evolutionary history, spanning roughly 350 million years and surviving numerous catastrophic events including the devastating Permian-Triassic extinction, ammonites vanished at the close of the Cretaceous period, about 65.5 million years ago. This mass extinction event, famously associated with the demise of the non-avian dinosaurs, also saw the survival of a less diverse but related group of shelled cephalopods—the nautiloids—provoking questions about what factors shaped these divergent fates.

The longstanding question as to why ammonites, with their incredible diversity and adaptability, succumbed to extinction while nautiloids endured, has persisted as an evolutionary enigma. Michael Schmutzer, an evolutionary biologist at the University of Oxford, has dedicated efforts to unraveling this puzzle. His collaborative research, to be showcased at the upcoming European Geosciences Union (EGU) General Assembly, leverages an unprecedented compilation of data on Late Cretaceous shelled cephalopods, integrating previously overlooked fossils and so-called ‘dark data’—specimens hidden in museum collections worldwide.

This comprehensive dataset incorporates diverse parameters, including body size metrics, egg morphology, and geographic distribution patterns. Previous speculation had suggested that differences in geographic ranges were responsible: nautiloids purportedly enjoyed broader or more advantageous habitats, allowing them to weather global ecological upheavals. However, Schmutzer’s analysis challenges this notion, finding little support that spatial distribution significantly influenced survival outcomes. Instead, the evidence points towards reproductive strategies, particularly egg size and development patterns, as critical determinants of resilience through the end-Cretaceous extinction.

Ammonites produced an extraordinary number of minuscule eggs, releasing countless microscopic hatchlings into the marine environment. This reproductive strategy, while providing an evolutionary advantage under stable conditions by increasing