A new study challenges that view, showing that ammonoids were globally diverse and robust before they went extinct 66 million years ago. This research highlights that their extinction is not an inevitable result, but is influenced by various geographical and environmental factors. Ammonites leap under the late Cretaceous sun. Credit: Artwork by Kalam Purcell
A recent study used museum collections to chart global ammonite diversity before their complete extinction. The findings reveal that ammonites did not decline before they went extinct along with the non-avian dinosaurs 66 million years ago.
A new study published in the journal Natural communicationLed by paleontologists University of Bristol A collaboration with international researchers, including Dr. Austin Hendy, curator of invertebrate paleontology at the Natural History Museum of Los Angeles County, reveals that ammonoids were still strong throughout the late world. CretaceousContrary to popular belief they are declining before becoming extinct.
Made possible by museum collections, the new study compares their diversity around the world just before extinction, revealing for the first time the complex evolutionary history of their final chapter.
Ammonoids, marine molluscs often distinguished by their spiral shells, are one of the great symbols of paleontology. They thrived in Earth’s oceans for over 350 million years until their extinction during the same chance event that wiped out the dinosaurs 66 million years ago. However, some paleontologists have argued that the diversity of ammonites (the last major lineage of ammonoids) was declining before their extinction at the end of the Cretaceous period, and that their extinction was inevitable.
„Ammonites had a fascinating evolutionary history. With their strong shells and powerful tentacles, they innovated the act of swimming. They could grow as large as a car or a few millimeters in diameter. They played equally diverse roles in their ecosystems, from predators at the top of the food web to filtering plankton, ” said Hendy..
Challenges in studying biodiversity
„Understanding how and why biodiversity has changed through time is extremely challenging,” said lead author Dr. Joseph Flannery-Sutherland said. „The fossil record tells us some stories, but it’s often an unreliable narrator. Patterns of diversity reflect patterns of sampling, mainly where and when we find new fossils. species, rather than actual biological history. Analyzing the extant Late Cretaceous ammonite fossil record as a complete, global story, previous researchers may have thought they were in long-term environmental decline.
To overcome this problem, the team assembled a new database of Late Cretaceous ammonite fossils to help fill in the sampling gaps in their record. „We have accessed museum collections to provide evidence of new specimens rather than relying on what has already been published,” said co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s Palaeobiology MSc programme. „This way, we can be sure that we are getting an accurate picture of their biodiversity before their total extinction.”
Using their database, the team analyzed how ammonite genera—creating unique new species—and extinction rates differed in different regions of the world. If ammonites had declined by the Late Cretaceous, their extinction rates would have generally exceeded their extinction rates wherever the group looked. Instead, the team found that the balance of mating and extinction changed over time and between different geographic regions.
„These differences in ammonoid diversification around the world are a major part of why their Late Cretaceous story has been misunderstood,” said senior author Dr James Witts of the Natural History Museum in London. „Their fossil record is very good in some parts of North America, but if you just look at this, you might think they’re struggling when they’re actually thriving in other regions. Their extinction is really a chance event, not an inevitable outcome.”
Environmental factors versus competition
So what accounts for the continued success of ammonites through the Late Cretaceous? To answer this question, the team looked at possible factors that could change their diversity over time. They were particularly interested in whether their rates of inbreeding and extinction were driven mainly by environmental conditions, such as sea temperature and sea level, or by biological processes, such as predator pressure and competition between ammonites.
„We found that the causes of ammonite mating and extinction varied geographically, as did the rates,” said co-author Dr. Corinne Myers of the University of New Mexico. „For example, you can’t look at their entire fossil record and say it’s entirely driven by changing temperatures. It’s more complicated than that and depends on where in the world they live.
„Paleontologists are fans of silver bullet stories for what triggered changes in a group’s fossil diversity, but our work shows that things aren’t always that straightforward,” said Dr. Flannery Sutherland concluded.
Note: Joseph D. Flannery-Sutherland, Cameron T. Crossan, Corinne E. Myers, Austin J.W. Hendy, Neil H. Landman and James D. „Regional Cretaceous ammonoids show drivers of diversification varied regionally” by Witts, 27 Jun 2024, Natural communication.
DOI: 10.1038/s41467-024-49462-z