Nov. 16, 2004 — Startling new evidence from boreholes drilled into the Chicxulub crater indicate that the great impact there happened hundreds of millennia too early to have been the one that wiped out the dinosaurs.

The discovery comes at a time when some geologists are convinced that the 120-mile-wide Chicxulub crater in Mexico was the "smoking gun" of what killed 70 percent of living species at the Cretaceous-Tertiary (a.k.a. K-T) boundary, 65 million years ago.

Several lines of geological evidence from Chicxulub — presented to a packed meeting room by Princeton University geologist Gerta Keller at this month's annual meeting of the Geological Society of America — make a compelling case for the famous crater having been formed about 300,000 years before the mass die-off 65 million years ago that took 70 percent of life on the planet.

"What Gerta Keller is showing us is that there is reason to doubt," said Spencer Lucas, curator of paleontology and geology at the New Mexico Museum of Natural History and Science. "(The smoking gun) can't be even a 100 years older than the K-T boundary. There is room for inquiry here."

Keller and her colleagues' evidence comes from Yaxcopoil 1, a borehole that was expected to provide final, irrefutable confirmation of Chicxulub's role in the K-T boundary mass extinction. It didn't.

Layers of rocks from the Yaxcopoil 1 borehole are stacked like old newspapers — older as you go down — and tell of the Chicxulub impact with the broken "breccia" rocks. On top of the impact breccia is about two feet of gently-laid-down, thinly layered seafloor mud built up over 300,000 years, Keller said. Those two feet of ho-hum, post-impact mud have the fossils, carbon isotopes and magnetic signal of the late Cretaceous, before the mass die-off, she said.

It's not until 300,000 years later — and two feet higher — that a sharp change in carbon isotopes and changes in microfossils signal the massive K-T extinction event, Keller explained.

Also missing from the Yaxcopoil 1 borehole rocks is any significant iridium signal — the extraterrestrial element that first clued scientists into the fact that an asteroid might have caused the K-T extinctions.

As for what, then, caused the K-T mass extinction: it was probably another asteroid impact combined with intense volcanic activity.

"It might have been a one-two punch," said Lucas.

In fact, he said, many dinosaur researchers suspect that dinos were on the decline before the final mass extinction. Chicxulub might have played a role in "softening" the dinos, he said, after which they may have never quite recovered. The second, still undiscovered, impactor might have been the terminal blow.