Scientists discover meteor strike sealed the fate of ammonites

Ammonites basking under the Late Cretaceous sun. Credit: Artwork by Callum Pursall.

Ammonites, those marine creatures with coiled shells that thrived for over 350 million years, met their end alongside the dinosaurs 66 million years ago.

Contrary to some beliefs, ammonites were not already on the decline before this catastrophic event.

New research from the University of Bristol, published in Nature Communications, reveals that their extinction was not a gradual decline but a sudden and unexpected end caused by the same meteor strike that wiped out the dinosaurs.

Dr. Joseph Flannery-Sutherland, the study’s lead author, explains that understanding changes in biodiversity over time is challenging.

The fossil record, which provides clues about the past, is often incomplete and misleading.

Previous researchers might have thought ammonites were in decline because they only analyzed existing fossil data without considering gaps in the record.

To address this, the team created a new database of Late Cretaceous ammonite fossils. They included specimens from museum collections that had not been previously published.

This comprehensive approach helped fill in the gaps and provided a more accurate picture of ammonite biodiversity before their extinction.

Co-author Cameron Crossan, a recent graduate of the University of Bristol’s Palaeobiology MSc program, emphasized the importance of using new sources of specimens.

By doing so, they ensured a clearer understanding of ammonite diversity.

The team then analyzed how ammonite speciation (the formation of new species) and extinction rates varied globally. If ammonites were truly in decline, their extinction rates would have been higher than their speciation rates everywhere.

However, the team found that the balance between speciation and extinction varied across different regions and time periods.

Dr. James Witts of the Natural History Museum in London, the study’s senior author, highlighted that ammonite fossil records are well-sampled in parts of North America.

This led to the mistaken belief that ammonites were struggling, while they were actually thriving in other regions. Their extinction was indeed a chance event, not an inevitable decline.

The team also explored what factors might have influenced ammonite diversity over time.

They considered environmental conditions like ocean temperature and sea level (the Court Jester Hypothesis) and biological processes like predation and competition (the Red Queen Hypothesis).

Their findings showed that the causes of ammonite speciation and extinction were as varied as the rates themselves, depending on where the ammonites lived.

Dr. Corinne Myers of the University of New Mexico, a co-author of the study, pointed out that ammonite diversity was not driven by a single factor like changing temperature. It was more complex and varied by geographic location.

This nuanced understanding challenges the idea of a simple, global explanation for changes in fossil diversity.

Dr. Flannery-Sutherland concluded that paleontologists need to analyze fossil data at regional scales to capture a more accurate picture of how diversity changed over time. This approach reveals how different factors influenced ammonite diversity in different parts of the world, showing a balance between the Red Queen and Court Jester effects.

In summary, the extinction of ammonites was a sudden event caused by a meteor strike, not a gradual decline. Their diverse and thriving populations were abruptly ended, emphasizing the complexity of studying ancient biodiversity.