Beneath the small town of Ames, Oklahoma, lies a meteor crater nobody can see. It stretches for miles underground, buried under layers of sediment, and for years geologists believed they knew roughly when it formed.

They were out by nearly 100 million years.

Researchers at The University of Texas at Austin have now dated the impact to about 370 million years ago, in the Late Devonian, rather than about 467 million years ago in the Ordovician. The work was published in July in the journal Meteoritics & Planetary Science, according to the university.

The original clue

The old date rested on one kind of evidence: fossils. Researchers had found the teeth of conodonts, ancient eel-like animals, preserved in the rock. The fossils came from animals that lived during the Ordovician, so the crater was given an Ordovician age.

That placed Ames among a cluster of North American impacts dated to around 467.5 million years ago, an episode known as the Ordovician Meteor Event. So many impacts appear to date from that time that some researchers have proposed Earth may once have been circled by a Saturn-like ring of asteroid debris.

But fossils can be misleading witnesses. Lead author Elizabeth Catlos, associate professor at UT's Department of Earth and Planetary Sciences, said the teeth were probably already millions of years old when the asteroid struck. The impact likely churned up older material and mixed the fossils into the rock, the university said.

A clock inside a crystal

The new evidence came from zircon, a mineral that forms tiny crystals in rocks such as granite.

Geologists value zircon because it works as a natural clock. The crystals contain traces of uranium, which decays into lead at a steady, well-known rate. When zircon forms it takes in uranium but almost no lead, so the lead found inside it later has built up through that decay. Measuring how much uranium has turned to lead shows how much time has passed. The method is called uranium-lead, or U-Pb, dating.

The team dated zircon crystals taken from granite that had been altered by the impact. The answer kept pointing the same way.

"No matter what technique we used, it was coming back to this younger signal," Catlos said.

One question remained: had these particular crystals really been through the collision? Working with NASA, the team imaged them using two techniques, cathodoluminescence and electron backscatter diffraction. When zircon experiences the extreme conditions of an impact, it recrystallises in a distinctive way, and those changes can be detected.

Danny Stockli, dean of UT's Jackson School of Geosciences and a co-author of the study, said zircon U-Pb dating is one of the most accurate ways to determine when events happened deep in Earth's past.

"These small crystals allow us to go back in time and learn about the major changes to Earth's ancient landscapes," Stockli said.

What the new age means, and what it doesn't

The revised date does two things. First, it shows that Ames could not have formed during the Ordovician Meteor Event.

Second, it places the impact close in time to the Frasnian-Famennian mass extinction, which occurred about 372 million years ago and wiped out a large proportion of marine life.

That is a striking coincidence of timing, but it is not evidence that the impact caused the extinction, and the university's account of the research does not claim that it did. The point is about getting the timeline right.

Catlos said more precise dates are essential for understanding how Earth has changed. One open question is whether extinctions were driven mainly by forces from space, such as meteor impacts, or by processes within Earth, such as massive volcanic activity.

"With this research, we're basically taking a major pawn out of the Ordovician Meteor Event and dumping it into the Frasnian-Famennian event, and saying, 'This is where this impact belongs,'" she said.

Stockli hopes the approach will be used elsewhere. "It would be great to do this for more of the meteor impact sites across the continent so we could get a more accurate timeline for these major events," he said.

The student who started it

The project was begun by Andrew Parisi, a former Jackson School graduate student who graduated in 2018 and has since died. According to the university, he travelled to Oklahoma to collect the Ames rock core from the Oklahoma Geological Survey, extracted the zircon crystals and helped to date them.

Co-author Michael Brookfield, an affiliated researcher at the school, also died before the paper was published.