New zircon dating has placed the Ames impact structure dating firmly in the Late Devonian, roughly 100 million years later than scientists had previously assumed, raising fresh questions about which ancient mass extinction the crater is actually connected to. The research, led by scientists at The University of Texas at Austin, was published in July in Meteoritics & Planetary Science.
The Ames impact structure lies buried beneath the town of Ames, Oklahoma, concealed under layers of sediment and invisible from the surface. What makes the crater unusual is how it was found at all: according to the Oklahoma Geological Survey, the structure was not discovered until 1991, when seismic exploration revealed its distinctive underground shape. It has since become economically significant as a producer of oil and gas, as well as a subject of scientific interest.
The structure, which stretches roughly 9 miles (15 kilometres) across, had for years been assigned to a well-known cluster of impacts across North America dating to around 467.5 million years ago, a period called the Ordovician Meteor Event. Some researchers have proposed that this concentration of roughly contemporaneous impacts suggests Earth may once have been encircled by a Saturn-like ring of asteroid debris during the Middle Ordovician. The Ames crater was counted among the evidence for that hypothesis.
Ames Impact Structure Dating Overturned by Zircon Evidence
The original dating rested on biological evidence: conodont teeth, preserved from an ancient eel-like animal, found within the impacted rock. Those fossils belonged to organisms from the Ordovician period. The new study’s lead author, Elizabeth Catlos, associate professor at UT’s Department of Earth and Planetary Sciences, said the teeth were almost certainly already millions of years old by the time the asteroid arrived. The impact, she argued, churned up pre-existing material and incorporated the older fossils into the newly formed rock without destroying them.
The team turned instead to zircon crystals extracted from granite that had been altered by the impact. Zircon U-Pb dating, described by co-author Danny Stockli, dean of the Jackson School of Geosciences, as one of the most accurate methods available for events deep in geological time, consistently returned a younger age. Researchers identified a population of zircons dated to about 369.7 million years ago, as reported by The Brighter Side of News, placing the impact in the Late Devonian. ‘No matter what technique we used, it was coming back to this younger signal,’ Catlos said.
To verify that the zircon crystals had actually been subjected to impact conditions rather than ordinary geological processes, the team worked with NASA to image them using cathodoluminescence and electron backscatter diffraction. When zircon undergoes the extreme pressures generated by a meteorite strike, it recrystallises in a characteristic pattern. Those structural changes confirmed the crystals had recorded the collision itself, not some earlier or unrelated event.
A Connection to the Frasnian-Famennian Extinction
The revised age does more than remove the Ames structure from the Ordovician Meteor Event. It places the impact close to the Frasnian-Famennian mass extinction, which occurred around 372 million years ago and wiped out a large proportion of marine life. Whether that proximity is coincidence or something more remains an open question, but Catlos was explicit about the implications for how scientists classify the crater. ‘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.
That matters, according to Catlos, because establishing accurate dates for impacts is essential to the broader debate about what drives mass extinctions: forces from space such as meteor strikes, or processes internal to Earth such as large-scale volcanic activity. Removing the Ames structure from the Ordovician cluster, as Gizmodo noted, also weakens one piece of the evidence previously used to argue for a debris ring around ancient Earth. Stockli called for the same zircon approach to be applied to other impact sites across the continent, to build a more reliable timeline of these events.
The project was initiated by Andrew Parisi, a former graduate student at the Jackson School of Geosciences who graduated in 2018 and has since passed away. Parisi travelled to Oklahoma to obtain the rock core from the Oklahoma Geological Survey and extracted the zircon crystals that underpinned the study. Co-author Michael Brookfield, an affiliated researcher at the school, also died before the paper appeared. Research Professor Sean Gulick and Professor Emeritus Mark Cloos at the Jackson School contributed to the research. The paper carries their names alongside the revised date that changes where a 370-million-year-old crater fits in Earth’s history.
