The Cretaceous period ended with a bang 66 million years ago when an asteroid more than six miles across collided with the Earth off of the Yucatan Peninsula. The resulting prodigious environmental change to land, sea, and atmospheric habitats ultimately led to the famous extinction of the dinosaurs, but also sounded the death knell of other charismatic fossil groups such as ammonite mollusks.
Ammonites are well known for their beautiful, pearlescent coiled shells, and had flourished in the Earth's oceans for much longer than the iconic dinosaurs, having originated more than 350 million years before their extinction.
Some paleontologists, however, have argued that the number of ammonite species, their diversity, was actually declining well before their extinction at the end of the Cretaceous and that their demise was inevitable. To test this, research led by paleontologists at the University of Bristol and UNM Associate Professor Corinne Myers has now shown that this dour fate was not set in stone. Instead, the final chapter in ammonite evolutionary history was more complex.
"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 overcome this issue, the team assembled a new database of Late Cretaceous ammonite fossils to help fill in the sampling gaps in their record. This database utilized not just the fossil records from scientific publications, but also drew heavily from museum collections, including those maintained in UNM's Paleobiology Collections in the Earth and Planetary Sciences Department. Using their database, the team then analyzed how ammonite speciation—formation of distinct new species—and extinction rates varied in different parts of the globe. If ammonites were in decline through the Late Cretaceous, then their extinction rates would have been generally higher than their speciation rates wherever the team looked. Instead, the team found that the balance of 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 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 adds, "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."
So, why were ammonites so successful through the Late Cretaceous? To answer this question, the team looked at potential factors might have caused their diversity to change through time. They were particularly interested in whether their speciation and extinction rates were driven mainly by environmental conditions like ocean temperature and sea level, or by biological processes like pressure from predators and competition between ammonites themselves.
"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, but our work shows that things are not always so straightforward", 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…."
The paper, Late Cretaceous ammonoids show that drivers of diversification are regionally heterogeneous by Joseph Flannery-Sutherland, Cameron Crossan, Corinne Myers, Austin Hendy, Neil Landman and James Witts, was published recently in the journal Nature Communications.
