Courtesy Mark RyanKeller and her colleague Thierry Adatte of the University of Lausanne in Switzerland have published yet another paper challenging the prevailing theory that an asteroid was the major cause of the extinction of the dinosaurs at the end of the Cretaceous Period 65 million years ago. The extinction affected over 60 percent of animal species including all of the non-avian dinosaurs.
Actually a number of events were occurring around the end of the Cretaceous including long-term volcanic activity, rapid marine regression, and the infamous asteroid impact. There’s also evidence that some of the dinosaur population was already in decline 10 million years prior to the events. But the scenario is usually played out with the meteor coming late in the sequence and delivering the deathblow to an already weakened eco-system, and paving the way for mammals to take over the ecological gap.
Courtesy Mark RyanThe asteroid impact hypothesis was first proposed in 1980 by Luis Alvarez of the University of California after he and his son Walter discovered a claystone layer rich in the rare-earth element iridium and peppered with shocked quartz in many locations around the world. Iridium is a rare element not commonly found on the Earth’s surface but abundant in meteorites. Shocked quartz was first noticed in sand grains in craters created by nuclear test bombs and later in meteor impact sites. Alvarez hypothesized that the only other possible source for this naturally deposited strata would be from a large extraterrestrial object hitting the Earth. From studying the amount of iridium found in Cretaceous-Tertiary (K-T) boundary formations, he calculated that a bolide about 6 miles across would have been necessary to create that much iridium around the world. But at the time evidence of such an was unknown. It wasn’t until after Luis Alvarez’s death in 1988 that a crater on the Yucatan Peninsula near Chicxulub Mexico became the chief suspect. The Chicxulub impact crater was the right size and the right age fitting all the criteria of a Cretaceous extinction event.
But Keller claims the asteroid came too early to put the coup de grace on dinosaurs or any species for that matter. Her study of spherules in strata at localities in Mexico has convinced her that the asteroid collided with Earth 300,000 years before any mass extinction.
At El Penon, a location very near the impact crater site, Keller and Adatte studied a 30-ft layer of sandstone above the iridium layer that they calculated had been laid down at a rate of about 1-inch per thousand years. This means it took 300,000 years to pile up the entire section of sediment.
Fossils were analyzed on either side of the iridium layer and the researchers found that of 52 species counted below the iridium layer (meaning before the impact), the same 52 species were found above it, meaning the asteroid hadn’t caused any extinction. But at the top of the 30 feet of sandstone overlaying the iridium claystone things were different.
"The mass extinction level can be seen above this interval," Keller says. "Not a single species went extinct as a result of the Chicxulub impact."
The more likely culprit, according to Keller, is India’s Deccan Traps flood basalts which for several million years poured out tremendous amounts of lava and noxious fumes into the atmosphere that would have had put long and tremendous stress on the existing ecosystem. Keller seems to roll out a paper on the subject every year or so in the last decade. We covered some of her Deccan Traps research 2007 which you can read here.
Whatever the case we do know is that non-avian dinosaurs left the planet after the Cretaceous, as none of their fossils have been found above the K-T Boundary. Well, even that doesn’t appear to be the case anymore. Recent dinosaur fossils found in the San Juan Basin in northwestern New Mexico are suspected to come from a stratum that post-dates the Cretaceous extinction. More research needs to be done on the find, but even this wouldn’t be unexpected. Even after the host declares the party’s over, there are always some stragglers who just don’t want to leave.
Keller's paper was published this week in the Journal of the Geological Society.