Researchers have discovered that ammonites did not experience a decline before their extinction.
These marine molluscs with coiled shells were one of palaeontology’s great icons and lived in Earth’s oceans for over 350 million years until they died during the same chance occurrence that wiped out the dinosaurs 66 million years ago.
Some experts in palaeontology have argued that their disappearance was unavoidable and that the diversity of Ammonites was diminishing long before they became extinct at the conclusion of the Cretaceous period.
However, new findings published in Nature Communications and led by palaeontologists at the University of Bristol demonstrate that their destiny was not predetermined. Instead, the final chapter in the evolutionary history of Ammonites is more complex.
“Understanding how and why biodiversity has changed through time is very challenging,” said lead author Dr 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.
“Analysing 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.”
In order to address this problem, the team compiled a new collection of Late Cretaceous ammonite fossils to help address the gaps in their data.
“We drew on museum collections to provide new sources of specimens rather than just relying on what had already been published,” said co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s Palaeobiology MSc program. “This way, we could be sure that we were getting a more accurate picture of their biodiversity prior to their total extinction.”
Utilizing their database, the team proceeded to examine the variations in the rates of ammonite speciation and extinction in different regions of the world. If the extinction rates of the ammonites were higher than their speciation rates during the Late Cretaceous, it would indicate a decline in their population. However, the team discovered that the balance between speciation and extinction changed both through geological time and between different geographic regions.
“These differences in ammonoid diversification around the world is a crucial part of why their Late Cretaceous story has been misunderstood,” said senior author Dr 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, you might think that they were struggling while actually flourishing in other regions. Their extinction really was a chance event and not an inevitable outcome.”
In order to determine the reasons behind the ongoing prosperity of ammonites during the Late Cretaceous period, the research group examined various factors that could have influenced changes in their diversity over time. Their focus was primarily on whether changes in speciation and extinction rates were primarily influenced by environmental factors such as ocean temperature and sea level (known as the Court Jester Hypothesis) or by biological mechanisms such as predation pressure and competition among the ammonites themselves (known as the Red Queen Hypothesis).
“What we found was that the causes of ammonite speciation and extinction were as geographically varied as the rates themselves,” said co-author Dr Corinne Myers of the University of New Mexico. “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, but our work shows that things are not always so straightforward,” Dr Flannery Sutherland concluded. “We can’t necessarily trust global fossil datasets and need to analyze 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.”
Journal reference:
- Joseph T. Flannery-Sutherland, Cameron D. Crossan, Corinne E. Myers, Austin J. W. Hendy, Neil H. Landman & James D. Witts. Late Cretaceous ammonoids show that drivers of diversification are regionally heterogeneous. Nature Communications, 2024; DOI: 10.1038/s41467-024-49462-z