Egg Size May Explain Ammonite Extinction After Asteroid Impact

The size of ancient marine creatures' eggs may explain why ammonites vanished while nautiloids survived the asteroid impact that ended the Cretaceous period. Ammonites, a diverse group of shelled cephalopods, disappeared 65.5 million years ago. Nautiloids, another group of shelled cephalopods, survived and exist today.

Researchers compiled a large dataset of Late Cretaceous shelled cephalopod fossils. This dataset included information on body size, egg size, and geographic distribution. Previous theories suggested geographic distribution was a key factor in nautiloid survival. This new study does not support that hypothesis.

Instead, the study suggests egg size played a role. Ammonites laid many tiny eggs, producing small hatchlings. Nautiloids produced fewer, larger eggs and hatchlings. They also had slower metabolic rates, suggesting slower growth.

One hypothesis proposed that large, yolk-rich eggs gave nautilus embryos an advantage. They could develop longer and hatch at a larger size. This might have helped them when marine ecosystems collapsed and food was scarce. However, the study's findings point in a different direction.

The few ammonoid genera that persisted briefly after the extinction event were those with the smallest eggs. This indicates the egg size hypothesis needs revision. Ammonites and nautiloids likely used different strategies to survive the chaotic aftermath of the asteroid impact. Environmental instability, darkness, and collapsing food webs reshaped life on Earth.