The fate of the ammonites was sealed by the meteorite impact that wiped out the dinosaurs

Ammonites were not in decline before their extinction, scientists have found.

Shelled marine molluscs, one of the great icons of paleontology, thrived in Earth’s oceans for more than 350 million years until they became extinct 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 was greatly reduced before they became extinct at the end of the Cretaceous.

However, new research, published in Nature Communications and led by palaeontologists at the University of Bristol, shows that their fate was not set in stone. Instead, the latest chapter in the ammonite evolutionary story 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 part of the story, but it’s often an unreliable narrator. Diversity patterns may simply reflect sampling patterns, essentially where and when we found new fossil species, rather than actual history biological.

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

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

“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 MSc Paleobiology program at the University of Bristol. “That way we could be sure we were getting a more accurate picture of their biodiversity before their total extinction.”

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

“These differences in the diversification of ammonoids around the world are a crucial part of why their Late Cretaceous history is poorly understood,” said senior author Dr. James Witts of the Natural History Museum, London. “Their fossil record in parts of North America is very well-examined, but if you look at that alone, then you might think they were struggling, when in fact they were thriving in other regions. Their extinction was with truly 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 the rate of species extinction was driven primarily by environmental conditions such as ocean temperature and sea level (the Court Jester Hypothesis), or by biological processes such as predation pressure and competition among the ammonites themselves (the Jester Hypothesis). of the Red Queen).

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

“Paleontologists are often fans of silver bullet stories about what drove 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 rely on global fossil datasets and have to analyze them on a regional scale. That way we can capture a much more nuanced picture of diversity change across space and time , which also shows how the changing balance of the Red Queen versus the effects of the Jester court shaped these changes.”