New study reveals that ammonoids thrived until their sudden extinction

A new study challenges the view that ammonites were in decline before their extinction 66 million years ago, showing instead that they remained diverse and robust across the globe. The research highlights that their extinction was driven by diverse geographic and environmental factors, rather than being an inevitable outcome. Ammonites basking in the Late Cretaceous sun. Image credit: Artwork by Callum Pursall

A new study has used museum collections to map the global diversity of ammonites before their complete extinction. The results reveal that ammonites were not in decline before they went extinct alongside the non-avian dinosaurs 66 million years ago.

A new study published in the journal Nature CommunicationsLed by paleontologists at University of Bristol In collaboration with international researchers including Dr. Austin Hinde, curator of invertebrate paleontology at the Natural History Museum of Los Angeles County, it reveals that ammonoids were still strong worldwide during the Late Cretaceous. Cretaceous periodContrary to popular belief, their numbers were in decline before they became extinct.

Thanks to museum collections, the new study was able to compare the diversity of these animals across the world just before extinction, revealing the complex evolutionary history of their final chapter for the first time.

Ammonites, marine mollusks often characterized by their coiled shells, are one of the great icons of paleontology. They thrived in Earth’s oceans for more than 350 million years until they went extinct in the same cataclysmic event that wiped out the dinosaurs 66 million years ago. However, some paleontologists argue that the diversity of ammonites (the last major lineage of ammonites) was in decline long before their extinction at the end of the Cretaceous period and that their extinction was inevitable.

“Ammonites have had an amazing evolutionary history. With their massive shells and powerful claws, ammonites have been inventive swimmers. They could grow to the size of a car or just a few millimeters in diameter. Ammonites have played equally diverse roles in their ecosystems, from predators near the top of the food web to filter feeders on phytoplankton,” Hinde said..

Challenges in the Study of Biodiversity

“Understanding how and why biodiversity has changed over time is very difficult,” said lead author Dr Joseph Flannery-Sutherland. “The fossil record tells us some of the story, but it is often an unreliable narrator. Patterns of diversity can reflect sampling patterns, essentially where and when we find new fossils.” Class“Analyzing the Late Cretaceous ammonite fossil record as if it were the complete global story is likely why previous researchers believed it was in a state of long-term environmental decline.”

To overcome this problem, the team compiled a new database of Late Cretaceous ammonite fossils to help fill sampling gaps in their records. “We used museum collections to provide new sources of specimens rather than relying solely on what had already been published,” said co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s Masters in Palaeobiology. “This way we can be sure we get a more accurate picture of their biodiversity before they went completely extinct.”

Using their database, the team then analyzed how the rates of ammonite species formation and extinction varied in different parts of the world. If ammonites were in decline during the Late Cretaceous, their extinction rates would have generally been higher than species formation rates wherever the team looked. Instead, the team found that the balance of species formation and extinction shifted across geological time and between different geographic regions.

“These differences in the diversity of ammonoids around the world are a key part of why their story in the Late Cretaceous is so poorly understood,” said Dr James Witts, lead author of the study from the Natural History Museum in London. “Their fossil record is very well sampled in parts of North America, but if you looked at this alone you might think they were struggling when in fact they were thriving in other areas. Their extinction was an accidental event rather than an inevitable outcome.”

Environmental factors vs. competition

So what was responsible for the continued success of ammonites during the Late Cretaceous? To answer this question, the team looked at potential factors that might have caused their diversity to change over time. They were particularly interested in whether the rates of species emergence and extinction were driven primarily by environmental conditions such as ocean temperature and sea level or by biological processes such as pressure from predators and competition between ammonites.

“What we found is that the reasons for the emergence and extinction of ammonites were as geographically diverse as the rates themselves,” said study co-author Dr. Corinne Myers of the University of New Mexico. “You can’t just look at their overall fossil record and say it was driven entirely by temperature change, for example. It was much more complex than that and depended on where they lived in the world.”

“Paleontologists are often fond of narratives that describe the factors that led to changes in the fossil diversity of a group, but our work shows that things are not always that simple,” concludes Dr Flannery-Sutherland.

Reference: “Late Cretaceous ammonoids show that diversification drivers are regionally heterogeneous” by Joseph T. Flannery-Sutherland, Cameron D. Crossan, Corinne E. Myers, Austin J. W. Hinde, Neil H. Landman, and James D. Witts, June 27, 2024, Nature Communications.
DOI: 10.1038/s41467-024-49462-z