A new study utilizing museum collections has revealed that ammonites, iconic spiral-shelled marine mollusks, were not in decline before their sudden extinction alongside non-avian dinosaurs 66 million years ago.

Contrary to previous beliefs, this research, published in Nature Communications and led by paleontologists from the University of Bristol, demonstrates that the fate of ammonites was not predestined by a gradual decline but was a result of the same catastrophic event that wiped out the dinosaurs.

For over 350 million years, ammonites thrived in Earth’s oceans, capturing the fascination of both paleontologists and enthusiasts. Historically, some scientists argued that ammonites were already on a path to extinction due to a long-term decrease in diversity. However, this new study challenges that notion by suggesting that the final chapter of the ammonites’ history is significantly more complex than previously thought.

Dr. Joseph Flannery-Sutherland, the study’s lead author, emphasized the monumental challenge of understanding how and why biodiversity has changed over time. The fossil record often acts as an unreliable narrator, with observed diversity patterns potentially being mere artifacts of sampling patterns rather than true reflections of biological history. Previous studies may have mistakenly analyzed the Late Cretaceous ammonite fossil record as a complete and global narrative, leading to the erroneous conclusion of a long-term ecological decline.

Ammonites basking under the Late Cretaceous sun
Ammonites basking under the Late Cretaceous sun. Credit: Callum Pursall (@cpursall on X)

To address this limitation, the research team from Bristol created a new database of Late Cretaceous ammonite fossils, filling crucial gaps in the fossil sampling record. Cameron Crossan, a co-author of the study and a graduate of the University of Bristol’s MSc program in Paleobiology, highlighted that the team turned to museum collections for new specimen sources instead of relying solely on previously published data. This innovative approach allowed for a more accurate and comprehensive picture of ammonite biodiversity before their total extinction.

Utilizing this expanded database, the team analyzed how ammonite speciation and extinction rates varied across different parts of the world. They discovered that the balance between speciation and extinction shifted over geological time and across geographic regions.

If ammonites had been in decline, their extinction rates would have been consistently higher than their speciation rates in any studied region. However, the findings revealed a dynamic balance, with regions like North America appearing to show a decline due to well-sampled fossil records, while other regions indicated thriving ammonite populations.

Reconstruction of ammonites
Reconstruction of ammonites. Credit: Bramfab / Wikimedia Commons

Dr. James Witts, a co-author from the Natural History Museum in London, emphasized the importance of regional differences in ammonoid diversification for understanding their Late Cretaceous history. He noted that relying on data from well-sampled regions alone could lead to the false impression of a global decline, whereas ammonites were actually prospering in less-studied areas. This finding underscores that their extinction was a fortuitous event rather than an inevitable outcome.

To better understand the factors enabling the continued success of ammonites during the Late Cretaceous, the research team investigated whether speciation and extinction rates were driven primarily by environmental conditions like ocean temperature and sea level (Court Jester Hypothesis) or by biological processes such as predator pressure and competition among ammonites (Red Queen Hypothesis). Dr. Corinne Myers, a co-author from the University of New Mexico, remarked that the causes of ammonite speciation and extinction were as geographically varied as the rates themselves, indicating a complex interplay of factors rather than a single driving force.

Dr. Flannery-Sutherland concluded that paleontologists often favor simplified narratives about what drove changes in fossil diversity within a group. However, this study reveals the complexity of such changes, highlighting the need for regional analyses to capture a more nuanced picture of how diversity evolved over space and time. This approach demonstrates how variations in the balance of the Red Queen and Court Jester effects shaped the history of ammonites, ultimately leading to their sudden extinction in the same catastrophic event that claimed the dinosaurs.

SOURCES

University of Bristol

Flannery-Sutherland, J.T., Crossan, C.D., Myers, C.E. et al. Late Cretaceous ammonoids show that drivers of diversification are regionally heterogeneous. Nat Commun 15, 5382 (2024). doi.org/10.1038/s41467-024-49462-z

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