What caused the extinction of ammonites? These animals that dominated the planet’s oceans for millions of years and left us their fossils?
Ammonites are very famous fossils, along with trilobites, as they are found everywhere. For millions of years these animals dominated the Earth’s oceans, and then they disappeared. A new study published in the journal Nature Communications, led by paleontologists from the University of Bristol along with a team of international researchers, including Dr. Austin Hendy, Curator of Invertebrate Paleontology at the Los Angeles County Museum of Natural History, finds that instead of declining before its extinction, ammonites were still at their peak worldwide in the Late Cretaceous. Using museum collections, the new study compared their diversity around the world just before extinction, unraveling for the first time the complex evolutionary history of its final chapter.
Ammonites, marine mollusks often distinguishable by their coiled shells, are one of the great icons of paleontology. They flourished in Earth’s oceans for more than 350 million years until their extinction during the same catastrophic 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 long before their extinction at the end of the Cretaceous Period and that their demise was inevitable.
The innovators of swimming
“Ammonites had an incredible evolutionary history. With their formidable shells and powerful tentacles, they innovated in the act of swimming. They could grow to the size of a car or just a few millimeters in diameter. They played equally disparate roles in their ecosystems, from predators near the top of the food chain to plankton filter feeders,” Hendy said..
“Understanding how and why biodiversity has changed over time is very difficult,” said lead author Dr Joseph Flannery-Sutherland. “The fossil record tells us part of the story, but it is often an unreliable narrator. Diversity patterns may simply reflect sampling patterns – essentially where and when we have found new fossil species – rather than actual biological history. Analysing the existing fossil record of Late Cretaceous ammonites as if it were the complete, global story is probably why previous researchers thought they were in long-term ecological decline.”
To overcome this problem, the team assembled a new database of Late Cretaceous ammonite fossils to help fill in sampling gaps in their record. “We used museum collections to provide new sources of specimens rather than simply relying on what had already been published,” said co-author Cameron Crossan, a 2023 graduate of the University of Bristol’s Masters in Palaeobiology program. “In this way, we could be confident we were getting a more accurate picture of their biodiversity before their total extinction.”
Using their database, the team then looked at how ammonite speciation rates – that is, the formation of new, distinct species – and extinction rates varied in different parts of the world. If ammonites were in decline during the Late Cretaceous, then their extinction rates would have generally been higher than their speciation rates wherever the team looked. Instead, the team found that the balance of speciation and extinction changed both over geologic time and between different geographic regions.
“These differences in ammonoid diversification around the world are a crucial part of why their Late Cretaceous history has been misinterpreted,” said lead author Dr. James Witts of the Natural History Museum in London. “Their fossil record in parts of North America is very well sampled, but if you just look at this, you might think they were struggling while they were actually flourishing in other regions. “Its extinction truly was a chance event and not an inevitable outcome.”
Environmental factors or competition
So what caused the continued success of ammonites in the Late Cretaceous? To answer this question, the team looked at possible factors that could have caused their diversity to change over time. They were particularly interested in whether their speciation and extinction rates were driven primarily by environmental conditions such as ocean temperature and sea level, or by biological processes such as predator pressure and ammonite competition.
“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 its total fossil record and say it was driven entirely by changes in temperature, for example. “It was more complex than that and it depended on where in the world they lived.”
“Paleontologists often favor quick-fix narratives to explain changes in a group’s fossil diversity, but our work shows that things are not always so simple,” concluded Dr. Flannery Sutherland.
REFERENCE
Late Cretaceous ammonoids show that drivers of diversification are regionally heterogeneous
Image: Ammonites basking in the sun in the Late Cretaceous. Art by Callum Pursall (@cpursall on X)
