(The Center Square) - The Cretaceous period concluded 66 million years ago when an asteroid more than six miles in length collided with the Earth near the Yucatan Peninsula.

The changes to the land, sea, and atmospheric habitats caused dinosaurs to go extinct, scientists have now found that the explosion many times the power of the Hiroshima nuclear bomb was hard on other kinds of animals as well. Among them were 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," a University of New Mexico release said.

Image

Yet, some paleontologists argue that the variety of ammonite species and their diversity was on the decline long before their extinction at the end of the Cretaceous. These paleontologists contest that the demise of ammonites was inevitable.

Researchers led by paleontologists at the University of Bristol and UNM Associate Professor Corinne Myers tested this theory. The researchers concluded that the last chapter of ammonite evolutionary history was more complex than they originally thought.

“Understanding how and why biodiversity has changed through time is very challenging,” lead author Joseph Flannery-Sutherland said. “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.”

The team overcame this issue by creating a new Late Cretaceous ammonite fossil database to address the sampling gaps in its 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," the release said.

If ammonites faced decline through the Late Cretaceous, they would have faced higher extinction rates than their speciation rates wherever the team made observations.

However, the researchers saw that the speciation and extinction balance changed through both geological time and between geographic regions.

“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.” Myers said in the report. “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.”

That said, why did ammonites have so much success through the Late Cretaceous?

The team looked at many potential factors to address this question. They were mostly interested in whether speciation and extinction rates were predominantly driven by environmental conditions, including ocean temperature and sea level, or by biological processes like "pressure from predators and competition between ammonites themselves," the report said.

“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 said. “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 journal Nature Communications recently published 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.