Researchers at The University of Texas at Austin have determined that the buried Ames impact structure in Oklahoma formed approximately 370 million years ago during the Late Devonian period, placing the 10-mile-wide crater nearly 100 million years later than its long-held Ordovician timeline and linking it to a major extinction.
Deep beneath the town of Ames, Oklahoma, sits a massive subsurface meteor impact structure that spans roughly 9 to 10 miles in diameter. For decades, the geological community accepted that the crater was forged roughly 467.5 million years ago during the Ordovician Meteor Event, a historic window of heavy bombardment across North America. That long-accepted chronology has now been completely upended by a team of geoscientists who re-examined the site using modern radiometric dating techniques.
Zircon Crystals Reveal a Late Devonian Strike
To uncover the true age of the Ames crater, researchers analyzed zircon crystals extracted from granite at the impact site. Zircon functions as a microscopic geological clock because the mineral traps uranium as it forms, allowing scientists to track radioactive decay into lead. The investigative team analyzed 37 separate measurements.
The findings were published in the journal Meteoritics & Planetary Science. To confirm that the zircon microstructures actually recorded the extreme shock pressures of a meteoric collision rather than later thermal resets, the scientists collaborated with NASA to image the mineral grains using cathodoluminescence and electron backscatter diffraction.
“No matter what technique we used, it was coming back to this younger signal.”
Elizabeth Catlos, associate professor at the University of Texas at Austin Department of Earth and Planetary Sciences
The Ames impact structure is now covered by sedimentary rocks and is also an area of oil and gas extraction.
Why Fossil Evidence Led Scientists Astray for Decades
The previous assignment of an Ordovician age relied entirely on biochronology rather than radioisotopes. Investigators had discovered fossilized teeth from an ancient eel-like creature known as a conodont, along with biological moulds preserved within the structure. Because those organisms lived during the Ordovician period, researchers considered the geological case closed.
The new research offers a different explanation for how those ancient fossils wound up inside a much younger crater. The asteroid impact likely excavated and redeposited older fossils inside the crater millions of years later. Rapid redeposition and burial inside the crater during tsunami backwash and resurge preserved the delicate conodont teeth, even though those organisms had already been dead for millions of years before the impact occurred.
Implications for North American Impact Chronology and Mass Extinction
Removing the Ames structure from the Ordovician period alters how geologists interpret ancient space debris. Previously, the high frequency of Ordovician craters led some researchers to propose that Earth might have been encircled by a Saturn-like ring of asteroid debris. With Ames removed from that dataset, the tally of impacts tied to the Ordovician event drops.
Instead, the revised timeline places the Ames impact near the Frasnian-Famennian mass extinction event, which occurred about 372 million years ago. The chronological overlap places the event alongside other North American impacts from the same era.