The Ammonites’ fate was sealed by a meteor strike that wiped out the dinosaurs

Scientists have discovered that the Ammonites were not in retreat before their extinction.

One of the great icons of paleontology, the coiled-shell marine molluscs flourished in Earth’s oceans for more than 350 million years until they died out during the same random 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 had been declining long before they became extinct at the end of the Cretaceous.

However, new research, published in The nature of communication and led by palaeontologists at the University of Bristol shows that their fate was not set in stone. Instead, the final chapter of ammonites’ evolutionary history is more complex.

“Understanding how and why biodiversity has changed over time is very challenging,” said lead author Dr. Joseph Flannery-Sutherland. “The fossil record tells us something of the story, but it’s often an unreliable narrator. Diversity patterns may reflect sampling patterns, basically where and when we found new fossil species, rather than actual biological history.”

“Analyzing the extant Late Cretaceous ammonite fossil record as if it were a complete, global story is probably why previous researchers thought they were in long-term ecological decline.”

To overcome this problem, the team compiled a new database of Late Cretaceous ammonite fossils to help fill sampling gaps in their record.

“We drew on museum collections to provide new sources of specimens rather than just relying on what had already been published,” said co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s palaeobiology master’s program. to be sure we get a more accurate picture of their biodiversity before their complete extinction.”

Using their database, the team then analyzed how the rate of speciation and extinction of ammonites varied in different parts of the globe. If ammonites were in decline during the Late Cretaceous, then their extinction rate would generally be higher than their speciation rate, wherever the team looked. Instead, the team found that the balance of speciation and extinction has changed both over 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 might think they were struggling, while in other areas they were actually thriving. Their extinction was really a random event, not an inevitable outcome.”

To find out what was responsible for the continued success of ammonites in the Late Cretaceous, the team investigated potential factors that may have caused their diversity to change over time. They were particularly interested in whether their rates of speciation and extinction were driven mainly 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 the ammonites themselves (the red queen hypothesis).

“We found that the causes of ammonite speciation and extinction were as varied geographically as the rates themselves,” said co-author Dr. Corinne Myers of the University of New Mexico. “You couldn’t just look at their overall fossil record and say that their diversity was driven solely by temperature change, for example. It was more complex and depended on where in the world they lived.”

“Paleontologists are often fans of telling the story of what caused changes in a group’s fossil diversity, but our work shows that things are not always so straightforward,” concluded Dr. Flannery Sutherland. “We can’t necessarily trust global fossil datasets and need to analyze them at a regional scale. This way we can capture a much finer picture of how diversity has changed across space and time, which also shows how variations in the balance of Red Queen versus Court Jester effects have shaped these changes.”