The fate of the Ammonites was sealed by a meteor impact that wiped out dinosaurs

Scientists have discovered that ammonite species did not decline before their extinction.

The coiled-shell marine mollusks, one of paleontology’s greatest icons, thrived in Earth’s oceans for more than 350 million years until they became extinct during the same accidental event that wiped out the dinosaurs 66 million years ago.

Some paleontologists have argued that their demise was inevitable and that the diversity of ammonites declined long before their extinction at the end of the Cretaceous.

New research published in Nature communication and led by paleontologists from the University of Bristol, shows that their fate was not set in stone. Instead, the final chapter in the evolutionary history of ammonites is more complex.

“Understanding how and why biodiversity has changed over time is a major challenge,” said lead author Dr Joseph Flannery-Sutherland. “The fossil record tells us part of the story, but it is often an unreliable narrator. Patterns of diversity may simply reflect patterns of sampling, essentially where and when we have found new fossil species, rather than actual biological history.

“By analyzing the existing fossil record of Late Cretaceous ammonites as if it were a complete, global story, previous researchers likely thought the animals were in a long-term ecological decline.”

To solve this problem, the team compiled a new database of late Cretaceous ammonite fossils to fill in the gaps in their data.

“We used museum collections to provide new sources of specimens rather than just relying on what had already been published,” says co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s Palaeobiology MSc programme. “This way we could be sure we were getting a more accurate picture of their biodiversity before they became completely extinct.”

Using their database, the team then analyzed how the speciation and extinction rates of ammonites 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 their speciation rates, wherever the team looked. What the team found instead was that the balance of speciation and extinction changed, both through geologic time and between different geographic regions.

“These differences in ammonoid diversification around the world are a crucial part of why their Late Cretaceous story has been misunderstood,” said lead author Dr. James Witts of the Natural History Museum in London. “Their fossil record in parts of North America is very well sampled, but if you look at this alone you would think they were struggling, while in other regions they were actually thriving. Their extinction was really a chance event and not an inevitable outcome .”

To find out what was responsible for the continued success of ammonites during the Late Cretaceous, the team looked at possible factors that could have caused their diversity to change over time. They were particularly interested in whether their speciation and extinction rates were determined primarily by environmental conditions such as ocean temperature and sea level (the Court Jester hypothesis), or by biological processes such as predator pressure and competition among ammonites themselves ( the Red Queen hypothesis).

“What we found was that the drivers of ammonite speciation and extinction were as geographically varied as the rates themselves,” said co-author Dr. Corinne Myers of the University of New Mexico. “You can’t just look at their total fossil record and say that their diversity, for example, was entirely determined by changing temperatures. It was more complex than that and depended on where in the world they lived.”

“Paleontologists are often fans of wonderful stories about what drove changes in a group’s fossil diversity, but our work shows that things are not always that simple,” concluded Dr. Flannery Sutherland. “We cannot necessarily rely on global fossil datasets and need to analyze them at a regional scale. In this way we can get a much more nuanced picture of how diversity changed over space and time, which also shows how variation in the balance of Red Queen versus Court Jester effects shaped these changes.”