Ammonites became extinct along with the dinosaurs due to the meteorite impact
A new study rewrites the end of these famous cephalopod molluscs, one of the great icons of paleontology.
There was a time when the oceans were abundantly populated by living beings capable of enduring for over 350 million years. These were the ammonites, cephalopod mollusks that, thanks to their extraordinary abundance, have become one of the main symbols of paleontology. However, they disappeared without a trace at the end of the Cretaceous, 66 million years ago, immediately following the same event that wiped out the dinosaurs and more than 75% of all life on Earth. Some paleontologists have claimed that the over 10 km asteroid that caused the fifth great extinction of life on our planet was just the "tip of the iceberg" and that the disappearance of the ammonites was inevitable because their diversity had been in decline for a long time when the catastrophe occurred. But a group of researchers has just discovered that, just before their extinction, ammonites were not in complete decline; on the contrary, the last chapter of their long history was much more complex than previously thought. The study has just been published in 'Nature Communications' (https://www.nature.com/articles/s41467-024-49462-z).
“Understanding how and why biodiversity has changed over time is a great challenge,” says Joseph Flannery-Sutherland, the co-lead author of the article. Fossil records tell us part of the story, but they are often an unreliable narrator. Diversity patterns can simply reflect certain data, essentially where and when we have found new fossil species, but not the true biological history. “So analyzing the fossil records of late Cretaceous ammonites as if they were the entire global story is probably why previous researchers believed they were mired in a long-term ecological decline.”
To clarify the matter, the authors of the article developed a new database of late Cretaceous ammonite fossils that helped fill existing sampling gaps in their records. “We turned to museum collections to add new sources of specimens,” says Cameron Crossan, another co-author of the work, “rather than relying solely on what had already been published. This way, we could be sure to get a much more accurate picture of their biodiversity before their total extinction.”
Using their new database, the team analyzed how the speciation and extinction rates of ammonites varied in different parts of the world. If they were indeed in decline during the late Cretaceous, their extinction rates would generally have been higher than the speciation rates, regardless of where the team looked. Instead, the researchers discovered that the balance between speciation and extinction changed both over geological time and between different geographic regions.
“These differences in ammonoid diversification around the world,” explains James Witts from the Natural History Museum in London and co-lead author of the study, “are a crucial part of why their history during the late Cretaceous has been misinterpreted. Their fossil record in some parts of North America is very well sampled, but if you only look at this, it is easy to think they were in trouble, whereas in reality, they were thriving in other regions. Their extinction was truly a random event and not an inevitable outcome.”
To determine the exact key to the ammonites' continued success during the late Cretaceous, the team examined potential factors that might have caused the change in their diversity over time. The researchers were particularly interested in whether their speciation and extinction rates were primarily driven by environmental conditions such as temperature and sea level or by biological processes like predator pressure and competition among the ammonites themselves. “What we found,” says co-author Corinne Myers from the University of New Mexico, “is that the causes of ammonite speciation and extinction were as geographically diverse as the rates themselves. You cannot simply look at their total fossil record and say their diversity was entirely determined by temperature changes, for example. It was a bit more complex than that, and it depended on where they lived in the world.”
“Paleontologists,” concludes Flannery-Sutherland, “often advocate for miraculous solutions that explain what led to changes in a group's fossil diversity, but our work shows that things are not always that simple. We cannot necessarily rely on global fossil data sets and must also analyze them on a regional scale. This way, we can capture a much more nuanced picture of how diversity has changed over space and time.”
