The Cretaceous period concluded dramatically 66 million years ago when an asteroid over six miles wide collided with Earth near the Yucatan Peninsula. This event triggered significant environmental changes across land, sea, and atmospheric habitats, leading to the extinction of dinosaurs and other notable fossil groups such as ammonite mollusks.
Ammonites, known for their distinctive coiled shells, had thrived in Earth's oceans for much longer than dinosaurs, originating more than 350 million years before their extinction. Some paleontologists have suggested that ammonite species diversity was already declining before their extinction at the end of the Cretaceous. However, recent research led by paleontologists from the University of Bristol and UNM Associate Professor Corinne Myers challenges this notion.
“Understanding how and why biodiversity has changed through time is very challenging,” said lead author Joseph Flannery-Sutherland. “The fossil record tells us some of the story, but it is often an unreliable narrator. Patterns of diversity can just reflect patterns of sampling, essentially where and when we have found new fossil species, rather than actual biological history. Analyzing the existing Late Cretaceous ammonite fossil record as though it were the complete global story is probably why previous researchers have thought they were in long-term ecological decline.”
To address this issue, the team created a new database of Late Cretaceous ammonite fossils to fill gaps in their records. This database included fossil records from scientific publications and museum collections like those in UNM’s Paleobiology Collections in the Earth and Planetary Sciences Department. The team analyzed how ammonite speciation—the formation of new species—and extinction rates varied globally. Contrary to prior beliefs, they found that these rates fluctuated over time and across different regions.
“These differences in ammonoid diversification around the world are a crucial part of why their Late Cretaceous story has been misunderstood,” said senior author James Witts of the Natural History Museum, London. “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 flourishing in other regions. Their extinction really was a chance event and not an inevitable outcome.” Corinne Myers added, “Our results also strongly demonstrate the importance of maintaining museum collections as a unique record of the history of life. Without these collections, our picture of biodiversity over time is woefully incomplete.”
To understand why ammonites were successful during the Late Cretaceous, researchers examined factors influencing their diversity changes over time. They explored whether environmental conditions like ocean temperature and sea level or biological processes such as predator pressure and competition among ammonites drove speciation and extinction rates.
“What we found was that the causes of ammonite speciation and extinction were as geographically varied as the rates themselves,” said co-author Myers. “You couldn’t just look at their total fossil record and say that their diversity was driven entirely by changing temperature, for example. It was more complex than that and depended on where in the world they were living.”
“Palaeontologists are frequently fans of silver bullet narratives for what drove changes in a group’s fossil diversity,” Flannery-Sutherland concluded. “But our work shows that things are not always so straightforward. We can’t necessarily trust global fossil datasets; we need to analyze them at regional scales to capture a much more nuanced picture of how diversity changed across space and through time.”
The paper "Late Cretaceous Ammonoids Show That Drivers Of Diversification Are Regionally Heterogeneous" by Joseph Flannery-Sutherland et al., was recently published in Nature Communications.