The fate of the ammonites was sealed by the fall of a meteorite which wiped out the dinosaurs

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

The coiled-shelled marine mollusks, one of the great icons of paleontology, thrived in Earth’s oceans for more than 350 million years until they were wiped out in the same chance event that wiped out the dinosaurs 66 million years ago.

Some paleontologists have argued that their disappearance was inevitable and that the diversity of ammonites was declining long before their disappearance at the end of the Cretaceous.

However, new research, published in Nature Communications and led by paleontologists from the University of Bristol, shows that their fate was not set in stone. Instead, the latest chapter in the story of ammonite evolution is more complex.

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

“Analyzing existing Upper Cretaceous ammonite fossils as if they were the complete, global story is likely why previous researchers believed these species were in long-term ecological decline. »

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

“We relied on 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 paleobiology master’s program from the University of Bristol. “This way we could be sure to get a more accurate picture of their biodiversity before they become completely extinct.”

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 until the Late Cretaceous, their extinction rates would have generally been higher than their speciation rates, no matter where the team looked. Instead, the team found 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 partly explain why their Late Cretaceous history has been poorly understood,” said lead author Dr James Witts from the Natural History Museum in London. “Their fossils in parts of North America are very well sampled, but if you just look at them like that, you might think they were struggling, while they were thriving in other areas. Their extinction was truly a fortuitous event and not an inevitable consequence. »

To find out what was responsible for the continued success of ammonites during the Late Cretaceous, the team investigated potential factors that could have driven their diversity over time. In particular, they were interested in whether their speciation and extinction rates were driven primarily by environmental conditions such as 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).

“We found that the causes of ammonite speciation and extinction were as geographically varied as the rates themselves,” said co-author Corinne Myers of the University of New Mexico. “You couldn’t just look at all the fossils and say that their diversity was entirely due to changes in temperature, for example. It was more complex than that and depended on where in the world these species lived. »

“Palaeontologists are often fans of miracle stories about what led to 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 trust global fossil datasets and must analyze them on a regional scale. This way we can capture a much more nuanced picture of how diversity has changed spatially. and over time, which also shows how the variation in the balance of the Red Queen versus the Court Jester effects shaped these changes.