New Study Casts Doubt on the Asteroid Strike Theory of Dino Extinction

By Eliza Strickland | April 29, 2009 10:50 am

Chicxulub impactThe enormous meteor that smashed into Mexico’s Yucatan peninsula 65 million years ago didn’t deal a death blow to the dinosaurs, a new study declares. Based on a close examination of sediment layers from that epoch, a team of researchers led by Gerta Keller has previously argued that the Chicxulub impact happened 300,000 years before the mass extinction known as the Cretaceous-Tertiary extinction event. Now, Keller has found supporting evidence that the impact had little immediate effect on the planet’s biome. Says Keller: “It didn’t kill the dinosaurs. In fact, it didn’t cause much damage that we can determine from the geological record” [The Scientist].

Since the 112-mile-wide Chicxulub crater was discovered in 1978, many researchers have come to believe that the massive impact caused clouds of dust to shroud the earth, cooling the planet and killing the dinosaurs along with many other species. But Keller’s new study, to be published in the Journal of the Geological Society, offers a serious challenge to that theory.

The researchers examined sandstone layers at a Mexico site about 750 miles from the crater, focusing on the fossilized plankton species present in the rock directly below and above a thin layer of iridium. The layer of that mineral, which is common in asteroids, marks the time of the impact. They found 52 species of plankton below the line–and above the line after the impact the same 52 species were abundant. “We found not a single species went extinct as a result of the Chicxulub impact,” says Keller. This means the Chicxulub impact couldn’t have been the sole event causing the extinction, she says [Nature blog].

These species samplings are not, of course, conclusive, and plenty of other surveys since 1978 do tie the extinctions closely to the asteroid. But since the new digs were so close to ground zero, the immediate species loss ought to be have been — if anything — greater there than anywhere else in the world. Instead, the animals seemed to escape unharmed [Time]. Still, many researchers are not convinced by Keller’s findings, and some suggest that the ocean could have buffered the impact’s effect on marine species like the plankton that Keller studied, causing a delay of several thousand years (a blink of an eye, in geological time) before extinctions began to occur.

Keller, however, is already moving on to the next question: If an asteroid strike didn’t kill the dinosaurs, what did? Her team believes that some kind of atmospheric haze might indeed have blocked the sun, making the planet too cold for the dinosaurs — it just didn’t have to have come from an asteroid. Rather, they say, the source might have been massive volcanoes, like the ones that blew in the Deccan Traps in what is now India at just the right point in history [Time].

Related Content:
80beats: Forget “The Asteroid”: Could Supervolcanoes Have Killed the Dinosaurs?
DISCOVER: Did an Asteroid Really Dust the Dinosaurs?
DISCOVER: When North America Burned explains how the asteroid could have set a continent on fire

Image: NASA

  • Brian

    On the face of it, this study’s reasoning is not impressive. Not at all.

    Let’s see, the authors want to know whether the impact damaged populations of dinosaurs, so they study… plankton. Yeah, I know, prey species, base of the food chain, and all that. Still not impressive.

    #1. If I were a lifeform at the time, interested in the survival of my species, I’d WANT to be plankton. The oceans have a shot at shielding me from the worst effects of impact. I’m tiny and have trillions of friends. I can be wiped out locally and recolonize from waters far, far away. I have a short life and can evolve quickly under pressure. Being plankton is good under these conditions;

    #2. It’s not just what, or whether plankton species went extinct. It’s relevant what the relative proportions of extant species are. It’s also relevant what the total biomass of plankton is before and after the event.

    #3. My understanding of large scale impacts are that the worst effects are often on land. Major fallout? Worse on land because all that matter has nowhere to go, whereas in the ocean it can at least sink to the bottom. Continental firestorms? Not a problem in the water. Atmospheric poisoning? If you breathe water you’re not in immediate danger.

    #4. Look at large species known to have survived the event. Crocodilians. They live largely aquatic lives, they can binge feed then fast for long periods, and they can go dormant if necessary.

    #5. Do you want to be around, anywhere on Earth, during a Chicxulub scale event? Seriously, does this sound like a good place to be? This doesn’t even pass the laugh test.

  • Jamie

    I have to agree with the previous poster. There have been many extinction events; but from memory they’re all famous for extinguishing macroscopic life, not microscopic.

    The fact that no plankton were rendered extinct is very unimpressive – they had an entire ocean’s worth of population to draw on.

    Further, the fact that plankton are the ONLY lifeform they make reference to to support their theory makes it look even lamer. It really looks like they have a pet theory about the Deccan traps (An idea that’s been around a long time) and are clutching at straws to discredit the asteroid theory.

    More research needed. Find evidence of impact on more significant species and then you’ve got something.

  • amphiox

    Macroscopic life is the unique habitat for probably a majority of the diversity of microscopic life, so in any major extinction of macroscopic life, a substantial portion of microscopic lifeforms would go out with their multicellular hosts, even if it would be hard to find direct evidence of this in the fossil record.

    In the case of the plankton, though, even if the impact decimated them to the point of just a couple bedraggled survivors, so long as the species aren’t wiped out completely, in just a couple centuries populations numbers could easily recover all the way back to pre-impact baseline.

  • Sundance

    The beauty of the impact theory of extinction has always been that (aside from fitting the facts – an impact did occur) it explained why the large, probably warm-blooded creatures became extinct while the smaller things, like lizards, rodents, fungi, and plankton didn’t. It seems to me that this study simply shows that the impact theory successfully explains what it was supposed to.

  • SamIAm

    The extinction of the dinosaurs and other species is related to the excessive volcanic activity that created the Deccan Traps / just as the Siberian Traps was created by fractures in the Earth’s crust following the asteroid impact at the end of the Permian period that led to the Great Extinction.

    Until the scientific evidence is completely in and analyzed, all we can safely say is that both extinctions were the indirect & direct result of large asteroid impacts.

  • Helen

    I have to agree with the first two comments: Keller’s study leaves a lot of elbow room. The Chicxulub meteor event has never been definitively linked to the extinction of the dinosaurs. It’s just the closest match we’ve found, in size and time-period, to what would have been needed to create a planet-wide scorch-mark in the geologic record. Even if Keller disproved Chicxulub’s involvement in this event, she won’t be able to entirely rule out a meteor impact.

  • Skwish

    I’ve always wondered if the impactor might have caused enough acid rain to kill off animals with open nests. This would have weakened the shells of animals like dinosaurs, but not turtles and birds, who cover the eggs one way or another. Not that I’m an expert or anything, just an idea I’ve had.


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