A team of British and American paleontologists has analysed the fossil record of ammonites (marine mollusks best known as snails) from the Late Cretaceous period (105 to 66 million years ago), a period that some scientists characterise as a period of decline before their complete extinction at the Cretaceous-Paleogene boundary. The study suggests that, far from disappearing before this, ammonites were still going strong around the world during the Late Cretaceous.
Ammonites thrived in the Earth’s oceans for more than 350 million years before becoming extinct in the same chance event that wiped out the dinosaurs 66 million years ago.
Some paleontologists argue that the extinction of ammonites (the last major lineage of ammonoids) was inevitable, and that ammonite diversity declined long before their extinction at the end of the Cretaceous.
But new research suggests that ammonites’ fate is not sealed; rather, the final chapter of their evolutionary history is more complicated.
“Understanding how and why biodiversity has changed over time is extremely difficult,” said Dr Joseph Flannery Sutherland, a palaeontologist from the Universities of Birmingham and Bristol.
“The fossil record tells us part of the story, but it’s often an unreliable narrator.”
“Patterns of diversity may simply reflect sampling patterns — essentially when and where new fossil species were discovered — rather than actual biological history.”
“Analysing the extant Late Cretaceous ammonite fossil record as if it were a complete global story is probably why previous researchers have thought of ammonites as being in a long-term ecological decline.”
To overcome this problem, Dr. Flannery Sutherland and his colleagues have built a new database of Late Cretaceous ammonite fossils to fill sample gaps in the record.
“We used museum collections to provide new sources of specimens rather than relying on what has already been published,” said researcher Cameron Crossan of the University of Bristol.
“By doing this, we’re confident that we’ll get a more accurate picture of the biodiversity before it went extinct.”
The paleontologists used the database to analyze how ammonite speciation and extinction rates changed in different regions of the planet.
If ammonites had declined throughout the Late Cretaceous, their extinction rate would have generally been higher than their speciation rate in every region the team studied.
What the researchers found instead was that the balance between speciation and extinction has shifted through geological time and between different geographic regions.
“These differences in ammonite diversification around the world are an important reason why the story of the Late Cretaceous has been misunderstood,” said Dr James Witts, a palaeontologist at the Natural History Museum in London.
“The fossil record in some parts of North America is very well-sampled, but looking at it alone it might seem like North America was struggling while other parts were thriving.”
“Their extinction was an accident, not a foreseeable event.”
To understand why ammonites continued to thrive throughout the Late Cretaceous, scientists investigated factors that may have changed ammonite diversity over time.
They were particularly interested in whether ammonite speciation and extinction rates were driven primarily by environmental conditions such as ocean temperature and sea level, or by biological processes such as pressure from predators and competition between ammonites themselves.
“We found that the causes of ammonite speciation and extinction were as geographically diverse as their rates,” said Dr. Colin Myers, a paleontologist at the University of New Mexico.
“You can’t look at the whole fossil record and say, for example, that diversity is entirely driven by changes in temperature.”
“In reality it was more complicated and it depended on where in the world they lived.”
Team result Published in a journal Nature Communications.
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JT Flannery Sutherland others2024. Late Cretaceous ammonites show regional heterogeneity in drivers of diversification. Nat Community 15, 5382; doi: 10.1038/s41467-024-49462-z
Source: www.sci.news
