Meteorite impact that wiped out the dinosaurs also meant the end of the ammonites

Scientists have found that the ammonite population was not declining before its extinction.

One of the greatest landmarks in paleontology, these twisted-shelled marine mollusks lived in the world’s oceans for over 350 million years before becoming extinct in the same accident that wiped out the dinosaurs 66 million years ago.

Some paleontologists argue that their demise was inevitable and that ammonite diversity had already declined long before their extinction at the end of the Cretaceous period.

But new research published today in Nature Communications and led by paleontologists at the University of Bristol shows that their fate was not set in stone. Rather, the final chapter of ammonites’ evolutionary history is more complex.

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

“Analyzing the existing fossil record of Upper Cretaceous ammonites as if they represented the entire global history is probably why previous researchers thought they were in long-term ecological decline.”

To solve this problem, the team compiled a new database of Upper Cretaceous ammonite fossils to fill in the sample gaps in their records. “We drew on museum collections to find new sources of specimens, rather than just relying on what had already been published,” said co-author Cameron Crossan, who will graduate from the University of Bristol’s MSc in Palaeobiology in 2023. “This way we could be sure we were getting a more accurate picture of their biodiversity before their complete extinction.”

Using their database, the team then analyzed how ammonite speciation and extinction rates varied in different parts of the world. If ammonites were in decline during the Late Cretaceous, then their extinction rates would have generally been higher than their speciation rates everywhere the team looked. What the team found instead was that the balance between speciation and extinction changed both over geologic time and between different geographic regions.

“These differences in the diversification of ammonoids around the world are a key reason why their history in the Late Cretaceous has been misunderstood,” said lead author Dr James Witts of the Natural History Museum in London. “Their fossil records in parts of North America are very well documented, but looking at just these you might think they were struggling, while in other regions they were actually thriving. Their extinction was really a chance event, not an inevitable outcome.”

To find out what was responsible for the continued success of ammonites in the Late Cretaceous, the team looked at possible factors that might have altered their diversity over time. They were particularly interested in whether their speciation and extinction rates were determined primarily by environmental conditions such as sea temperature and sea level (the court jester hypothesis) or by biological processes such as pressure from predators and competition among the ammonites themselves (the red queen hypothesis).

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

“Paleontologists are often big believers in silver bullets for what causes changes in a group’s fossil diversity, but our work shows that things are not always so straightforward,” concluded Dr Flannery Sutherland. “We cannot necessarily trust global fossil records and need to analyse them at a regional level. This way we can get a much more nuanced picture of how diversity has changed over space and time, including how variations in the balance between the Red Queen and Jester effects have influenced these changes.”

The paper:

“Upper Cretaceous ammonoids reveal that drivers of diversification are regionally heterogeneous” by Joseph Flannery-Sutherland, Cameron Crossan, Corinne Myers, Austin Hendy, Neil Landman and James Witts in Nature Communications.