Ammonite extinction at the end of the dinosaur era was not inevitable

Ammonites had their time on Earth cut short, a new study suggests.

While previous research had suggested these shelled animals were already on the path to extinction when they were wiped out, a wider analysis reveals that ammonites were thriving in many areas of the world.

In fact, lots of new species were evolving in the Late Cretaceous. A meteorite strike 66 million years ago put an abrupt end to this, as the ammonites went extinct alongside dinosaurs, pterosaurs and as much as 78% of all life.

Dr James Witts, a Natural History Museum scientist who was the senior author of the new paper, says that the story of the ammonites is a cautionary tale for researchers relying on parts of the fossil record to make broad statements about global change.

“These differences in ammonite diversification around the world is a crucial part of why their Late Cretaceous story has been misunderstood”, James says. “Their fossil record in parts of North America is very well sampled, but if you looked at this alone then you might think that they were struggling, while they were actually doing very well in other regions.

“Their global extinction really was a chance event and not an inevitable outcome.”

The findings of the study were published in the journal Nature Communications.

How successful were the ammonites?

The ammonites were the last representatives of an ancient group called the ammonoids that first appeared around 450 million years ago. They’re instantly recognisable for their distinctive, spiral shells, though some species grew into a variety of different shapes.

The ammonoids were a type of mollusc known as a cephalopod, making them a distant relative of squid and octopuses. For over 350 million years, the ammonoids were a major part of life in the seas and have been found as fossils all over the world.

Despite their success most groups were extinct by the Jurassic, around 200 million years ago. From this point onwards, only the ammonites were left to continue the lineage.

Over the next 130 million years, the ammonites managed to weather a series of extinction events caused by everything from oxygen-depleted oceans to climate change. However, some research had previously found that by the end of the Cretaceous Period ammonites seemed to be in a long-term, unstoppable decline.

The scientists behind the current study weren’t convinced by this narrative. They wanted to make sure that these patterns were truly representative of the whole world, and not affected by biases in the fossil record.

“The fossil record can often be an unreliable narrator,” explains lead author Dr Joseph Flannery-Sutherland. “Patterns of diversity can just reflect patterns of sampling, essentially where and when we have found new fossil species, rather than actual biological history.”

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

To reveal this true picture, the research team assembled a huge new database of ammonite fossils from the Late Cretaceous. As well as previously published results, they drew on ‘dark data’ from specimens in understudied museum collections to fill gaps in previous analyses and account for differing rates of ammonite discovery and fossil preservation around the world.

Their analysis found that rates of speciation and extinction varied in different regions over time. This suggests that ammonites were simply experiencing natural selection as they had been for millions of years.

A tale of queens and jesters

Having found that the ammonites were not on the path to extinction, the team wanted to find out why the molluscs continued to be so successful in the Late Cretaceous. In particular, they focused on two key concepts used to explain why new species form and others go extinct.

The Red Queen hypothesis is the first, relating to a quote from Lewis Carroll’s Through the Looking Glass. In the novel, the Red Queen says that “…it takes all the running you can do, to keep in the same place.” The hypothesis suggests that in order to survive, all species have to keep evolving to outcompete their predators and competitors.

An alternative is the Court Jester hypothesis which instead suggests that environmental factors, like climate change, drive speciation and extinction.

The team found that both the Red Queen and Court Jester appeared to be at play with the ammonites. Sea level rise caused extinction rates to increase, as did cooling oceans. Meanwhile, high ammonite diversity appeared to limit the number of new species evolving as different ecological roles were filled.

The interactions between these factors changed in different areas of the world, showing that it’s impossible to discuss global impacts without looking at the individual effects in different regions.

“Palaeontologists are frequently fans of silver bullet narratives for what drove changes in a group’s fossil diversity, but our work shows that things are not always so straightforward”, Joseph adds. “We can’t necessarily trust global fossil datasets and need to analyse them at regional scales.”

“This way we can capture a much more nuanced picture of how diversity changed across space and through time, which also shows how variation in the balance of Red Queen versus Court Jester effects shaped these changes.”

Though it may have been evolution as usual for the ammonites just before the end of the Cretaceous, they were unable to adapt to as devastating an event as a meteorite strike. While some ammonites may have hung on for a very short period after the impact, the entire group would be extinct by 65 million years ago.