What really happened to the Dinosaurs?

Not the kind of pet you’d like to spend a Sunday afternoon with at the park. (Image credit: Will [CC BY-SA 2.0] via Flickr)
There’s something that immediately captures the imagination when one thinks of giant reptiles roaming the earth upon which we now live. They may not get quite as much attention as adorable videos of kittens or puppies on YouTube, but the success of movies such as Jurassic Park clearly show that dinosaurs have a way of engaging with society and causing us to wonder what life would have been like 65 Million years ago (Ma). But how much do we actually know about the demise of the dinosaurs? What brought about their extinction, and how sure are we really?

The second largest Mass extinction ever

Go back just 30 or 40 years and the debate about what killed off the dinosaurs was still raging. After thriving for over 135 million years, suddenly the dinosaurs, along with almost 80% of species on the earth, became extinct. Occurring 65 Ma, this event defined the end of the Cretaceous Era and beginning of the Tertiary Era, and is known as the K-T boundary. Scientists have long argued about the cause(s) of this mass extinction, with the two favoured theories being massive volcanism or an extra-terrestrial impact. Other factors such as climate change and ocean anoxia have also been proposed, although the general consensus is that these may have been only contributing factors, rather than the primary cause of the extinction.

A bow-tie tektite formed by rotation and cooling of molten rock as it traveled through the air following a large impact. (Image Credit: Mike Beauregard [CC BY 2.0] via Flikr)
A bow-tie tektite formed by rotation and cooling of molten rock as it traveled through the air following a large impact. (Image Credit: Mike Beauregard [CC BY 2.0] via Flikr)

Evidence for a giant asteroid

Whilst it is known that there was massive volcanism occurring 65 Ma in India, it was the giant impact theory which became the favoured theory in 1980. This followed detailed studies of a thin, clay-like layer of sediments found globally at precisely the K-T boundary in the geological record. This layer was found to contain an extremely high concentration of the element Iridium, an element which is extremely rare on earth, but has been found in high concentrations in asteroids. The scientists inferred that a worldwide Iridium layer at precisely the K-T boundary could have only been produced by the vaporisation of an asteroid upon impact with the earth. A few years after this initial discovery, another group of scientists found evidence of both shocked quartz and microtektites contained within the very same layer. These are both well reported phenomenon in geology which occur with large impacts.

Where did the asteroid hit?

The only question remaining was where this impact occurred. Whilst you might think a crater from such a significant event would be easy to find, 65 million years of sediments have been building up on the surface of the earth since impact, and no such crater had ever been found. At least, that was the case until 1991 when a team of scientists doing geophysical surveys in the Gulf of Mexico identified a gravity anomaly matching the expected dimensions for a crater produced by the K-T boundary impact. Named the Chicxulub crater, this feature was dated using boreholes to drill through the overlying sediments. This confirmed the age of the crater to be 65 Million years old, placing it right on the K-T boundary, and providing what was thought to be the final piece of evidence confirming the giant impact theory.

This traquil island off the coast of Mexico may look good now, but you sure wouldn’t have wanted to be here 65Ma. (Image Credit: Georgia Popplewell [CC BY-NC-ND 2.0] via Flikr)

End of the story?

However, a paper published in 2004 found that based on their dating methods, the Chicxulub impact actually predates the K–T boundary by about 300 000 years. All of a sudden, questions were again being asked about where the asteroid hit, and the cause of the K-T mass extinction was again being debated. Many people still believe that the Chicxulub crater is indeed the impact site, whilst an alternative crater site in India has been discovered and proposed as a possible location. Some scientists are now even questioning the giant impact theory altogether, proposing that high Iridium concentrations can be sourced from the deep earth, and carried to the surface in the lava extruded from volcanoes. Intriguingly, the largest ever mass extinction, the Permian-Triassic mass extinction, is thought to be a result of massive volcanism in Siberia, and links between mass extinctions and volcanism are relatively frequent throughout the geological record.

There’s always more to learn

Science is an ever changing field, in which today’s theories can soon become tomorrow’s history lesson. New results and discoveries cause old theories to be re-examined, modified and sometimes thrown out all together. So what exactly killed off the dinosaurs? Unfortunately we still can’t be sure.

6 Responses to “What really happened to the Dinosaurs?”

  1. tcouper says:

    Even the names for some of the Australian-specific dinosaurs are great Toni. The Ozraptor and Austrosaurus to name a few. Also on your other point, for a single species (humans) to usher in a new geological era would be amazing if it weren’t characterised by the mass extinctions of flora and fauna as you say. No living creature has had such an impact on the Earth before.

  2. asjones says:

    Hi Lakvin,
    Indeed there is some speculation and debate over the P-T mass extinction as well, and some scientists think that it too could have been caused by an asteroid impact, or even that an asteroid impact triggered the massive volcanism. What we do know is that there was about a million years of fairly continuous volcanism which produced enough lava to cover up to 7 million km^2, which is a pretty massive area, almost enough to cover the wholoe of Australia (~7.6 million Km^2). As you can imagine, volcanism of this magnitude over a prolonged period would have had serious implications for life on earth, and would have had the ability to very rapidly affect the climate of the Earth. The biggest problem when trying to answer questions such as these is that it happened so long ago, 250 million years in the case, and so finding conclusive evidence is very difficult.

  3. brema says:

    Interesting topic! Have always been fascinated by ‘what happened to the Dinosaurs’ ever since I was a kid. Do you think we’ll ever get past speculation and come to an agreement? Or is the nature of investigating something that happened so far in the past inherently speculative?

    Great post, Abe.

  4. tonih says:

    Did you know that there was a whole bunch of Australia-specific dinosaurs? I want them to write a Jurassic Park about them.

    Also, we’re heading into one of the biggest flora and fauna extinction that the world’s ever seen. The culprits? Us. And that’s one of the main arguments for entering into a new time era – the Anthropocene. Kinda cool but also really very sad.

  5. Lakvin says:

    This is quite interesting. I always thought it was scientific fact that the extinction of dinosaurs was caused by an asteroid impact.
    Is there a similar debate over the Permian-Triassic extinction as well or is it relatively clear that it was caused by the volcanic eruptions in Siberia?

  6. Sarah Webber says:

    I love the uncertainty that remains on this question. It suggests that life as we know it could all be wiped out at any moment by an asteroid impact, or a world-engulfing volcanic explosion…. if global warming doesn’t do us in first. Kind of makes you live for the moment.