May 16 2017

The Impact That Killed the Dinosaurs

asteroid impactI have loved science as long as I can remember, partly because scientists have the best stories to tell. The stuff that actually happened is usually far more interesting than any fantasy, and has the added bonus of being real. Reality is complex, dramatic, and interesting. Reality is also endlessly surprising.

Lucas wanted to evolve Star Wars. I always thought that was an interesting idea, adding a new layer to the film medium. It would love to see a great director do that well, not only updating special effects, but evolving the story as the culture evolves. Unfortunately, most of the changes Lucas made were crap, watering down character arcs and adding blandness. Some of the extra CG was OK.

Science stories, however, seem to always just get more interesting. That is because the universe has already written the entire script from the beginning, and we are just peeling back the narrative layers one by one. As the story gets deeper, it all makes sense because it has to. Plot twists are never contrived, because they were baked in from the beginning. 

The K-Pg Extinction Event

Think about our understanding of the history of life on Earth and what an interesting story that is. Let’s focus on the dinosaurs – the first description of a dinosaur fossil was in 1824, by William Buckland, who described a megalosaurus (giant lizard). People had probably discovered dinosaur bones previously, including a 1676 discovery of a giant thigh bone, but in the 19th century scientists starting putting together that there was a previous age when giant reptiles ruled the Earth.

These giant reptiles are not around today, so something must have happened to them. Over the next century scientists floated many ideas, all speculative, and none backed by solid evidence. Then, in the 1970s, Luis and Walter Alvarez discovered a thin layer of iridium at what was then known as the K-T boundary (now referred to as the K-Pg boundary) about 66 million years ago. This layer separates the Cretaceous period from the Paleogene period (and also the Mesozoic from the Cenozoic era).

The iridium in this layer is of an isotope that is mainly found in meteorites, not in the earth, so the suggestion here is that a massive meteor containing iridium hit the Earth right at the K-Pg boundary.

That is also the point in geological time when there was a mass extinction, killing 75% of species on Earth including all of the dinosaurs (by which I mean the non-avian dinosaurs, for you cladists). The implication is hard to avoid – the asteroid impact killed the dinosaurs. Scientists estimate that the asteroid was 10 km in diameter. Its impact likely threw up a massive cloud of dust that blocked the sun for months at least, breaking the first link in the food chain and causing ecosystems to collapse. Being large was a particular disadvantage during this time, and few tetrapods over 25 kg survived (only some turtles and crocodilians).

The impact hypothesis was further confirmed when the crater was discovered in the Yucatan peninsula. This not only confirmed the impact, but also provides further analysis of the extinction event. Paleontologists now know where ground zero was, and this has an effect on the fossil layers. Close to the impact, the layers were jumbled by the direct force of the impact. On the opposite side of the planet the dust layer was very thin and therefore provides low resolution for dating fossils relative to each other and the impact.

However, there is a sweet spot in between, where the layers are preserved and thick enough to see clearly where fossils are relative to the impact. When you examine the K-Pg boundary in this sweet spot distance from the impact it is clear – there are dinosaurs right up to the impact, and not beyond. The K-Pg boundary is the extinction event.

But wait, there’s more

The story of the impact and the death of the giant lizards is a good one. It captures the imagination. Further, there is solid evidence to support it. It therefore may be a little surprising to learn that there is a real and enduring controversy over whether or not the dinosaurs were really killed off by the impact.

Here is the problem – the dinosaurs were already going extinct before the asteroid hit. Surveys of the number of dinosaur species and genera show a steady decline leading up to the K-Pg boundary. There is still some controversy about this conclusion as well, with some paleontologists thinking that the decrease is an artifact of the record. The probability of finding a specimen right up to the boundary layer decreases with the rarity of the species, so species should statistically drop off as you get closer to the boundary. This is certainly part of the picture, but it does also seem that dinosaurs were in decline.

However, this doesn’t mean they were going extinct. You can’t just extrapolate trends like that. They may have rebounded in species numbers going forward if the asteroid didn’t hit.

There is also evidence that the  climate was changing, partly due to volcanic activity at the Deccan traps. This is a massive volcano complex in modern day India. They spewed sulphur and dust into the atmosphere and would have made the dinosaurs very unhappy. Some scientist claim that the Deccan traps were killing off the dinosaurs anyway, and the asteroid, at most, was the coup de grace.

But now it seems that the activity at the Deccan traps were not only occurring at the same time as the asteroid impact, the impact itself dramatically increased volcanic activity. It now seems that the asteroid impact and the volcanic activity at the Deccan traps were a “one-two punch” that caused the mass extinction.

I personally disagree with scientists who downplay the role of the impact in the mass extinction. Clearly life was on hard times because of increased volcanic activity, but it also seems clear that the asteroid impact was more than a punctuation mark – it had a major role to play in the mass extinction. Further, it is entirely possible and even probable that the non-avian dinosaurs would have survived even until today without the impact.

Now for a plot twist

There is a new wrinkle in the story. The BBC is reporting (mainly to promote their upcoming series on the topic, but whatever) that scientists have recently discovered that the location of the impact at the K-Pg boundary may have been critical to its deadly effects.

Drilling into the impact crater, geologists have found that it contains a lot of the mineral gypsum (calcium sulfate dihydrate), which contains a large amount of sulphur. When the asteroid hit, therefore, it threw up a large amount of sulphur which was important to the deadliness and density of the resulting dust layer. If that dust layer were not as dense, then light would have returned more quickly, and more species may have survived until the food chain was reestablished.

Gypsum forms by evaporation of sedimentary rock, and so occurs where there is shallow water or where there has been water in the past.

What all this may mean, therefore, is that the precise location of the impact, in the shallow seas of the Yucatan peninsula, may have been critical to its deadliness. If the asteroid struck the deep ocean, it would have had less of an impact on the underlying rocks and threw up less dust. If it struck the middle of a continent, there would have been less gypsum and less sulphur.

A shallow sea was therefore perhaps the deadliest place to strike – too little water to cushion the impact, but enough to allow for high levels of gypsum and therefore sulphur. This further means that if the asteroid struck shortly before or after it actually did the rotation of the Earth would have changed the location of the impact.

This is all a reminder of the contingent nature of history and how fantastically improbable any particular outcome is. Of course, something had to happen, and whatever did happen would be incredibly improbable. Anyone looking back at events would find their own existence less likely than winning the lottery. In a way we have all won the cosmic lottery.

That does not mean that a supernatural force was at work or that we exist by design. Someone had to win. We are just the lucky few who get to exist out of all the possible alternatives that don’t.

Somewhere in the nonexistent probabilities of alternate realities there is a dinosaur happily munching trees because an asteroid was an hour late to the Earth 66 million years ago (maybe).


21 responses so far

21 thoughts on “The Impact That Killed the Dinosaurs”

  1. Ian Wardell says:

    I recorded that BBC programme. Will probably watch it later.

    I was just thinking. Given that crows and certain other birds are so incredibly intelligent, might there have been some super intelligent dinosaurs? Perhaps even a dinosaur civilisation? Presumably if there had been such a civilisation we wouldn’t be able to detect it now.

  2. No dinos had anywhere near the encephalization necessary for civilization. So it would have had to be an entirely unknown clade of dinosaurs, which is highly unlikely. There could also be remnants of such a civilization. If, for example, they smelted metals and made alloys, that would survive.

  3. BaS says:

    Smelting metals, hmm… Dinosaur armor! That would be awesome!

  4. Sarah says:

    It’s improbable that there were intelligent dinosaurs at the time, but it’s not impossible that if they had survived, we might see intelligent ones today.

  5. Sarah says:

    Of course when I say “we” I don’t mean humans – we’d probably not be here.

  6. jwadamson says:

    So Steve is saying that the smaller dinosaurs were jousting and riding around in full metal armor on the larger dinosaurs. Neat. j/k (Poe’s law)

  7. BBBlue says:

    How much competition is there among scientists with competing theories on this topic? I read this sort of thing and I think to myself, gee, that is interesting, but I really don’t care which factor had the greatest effect, it’s enough just to know what the probable contributing factors were.

    Do scientists really get all worked up over the volcanic activity versus asteroid impact debate or is that mostly a product of popular media exaggeration?

  8. They really get worked up about it. The media largely ignore the debate, ironically, and just assume the asteroid impact is the universally accepted answer. When a new study comes out, they just go along with whatever it says without putting it into context.

  9. Johnny says:

    “There could also be remnants of such a civilization. If, for example, they smelted metals and made alloys, that would survive.”

    Is that really the case? I’ve seen hypothetical scenarios about “What is all humans disappeared?” (see vidoe below), and just within millennias, most of the remains of our civilization will be gone. A hypothetical dinosaur civilization would have existed possibly hundreds of millions of years ago, and might not have ever reached the technological level to make plastics or glass (those of our inventions that it will take the longest time for nature to get rid off).

    What Would Happen If Humans Disappeared?:

    (To clarify in case anyone thought otherwise: I don’t believe in the existence of a dinosaur civilization.)

  10. Doug Gordon says:

    “Its impact likely THROUGH up a massive cloud of dust”? Should be “threw”, right? Yikes, science articles with typos make me question whether they’re real or not. Often these kinds of errors are a harbinger of junk science. Author – please correct.

  11. Creeping Malaise says:

    “Its impact likely through up a massive cloud of dust”

    Er – don’t you mean *threw*?

  12. bachfiend says:

    Creeping malaise,

    I missed that one. I was going to mention ‘coup the grace’ but I thought it too pedantic. Perhaps the sentence started as ‘its impact through a massive cloud of dust’ and after ‘through’ was typed the preposition became an intransitive verb, requiring a preposition.

  13. Creeping Malaise says:

    bachfiend –

    My mother worked as a proofreader. Nitpickery is in my blood 😀

    (Incidentally, I’m a huge fan of old Johann myself)

  14. Thanks for the corrections – done.
    But seriously, Doug – this is a blog. I’m busy and my time is a zero sum game. Typos do not call into question the content of a science blog.

  15. Sarah says:

    Coup de Grace is especially bad as a D&D player who has referenced the word on the show 😀

  16. Dan Dionne says:

    Per Johnny’s comment, questioning whether artifacts from a hypothetical lost dino civilization could last over 66 million years, alloys would definitely be a good candidate. The book The World Without Us, by Alan Weisman, claims that the very last human artifacts to exist on earth would be made of bronze. Apparently bronze’s oxidation layer protects it better than any other metal, and bronze statues could last right up until the expanding sun swallows the earth.

    Bronze isn’t naturally occurring (as far as I can tell), so if scientists found a lump of bronze older than 66 million years, that would be a stunning and unnerving specimen. I don’t think it’s particularly likely, but it’s fun to imagine.

  17. locutusbrg says:

    And the winner of pedantic grammarians of the month award goes to…. I let that be 🙂 In the interest of fostering peace and love.

    The question that always niggles at me. If there were small non avian dinosaurs why are they the only large animal group without any small survivors. Small reptiles made it and they had to have the same sensitivity. Small marine animals. The short answer is the ornithicians couldn’t evolve along with the avian sauricians or is that too simplistic.

  18. bachfiend says:


    The reason why small mammals and small reptiles survived the K-Pg extinction event is obvious. Rebecca pointed it out on a ‘science or fiction’ segment on SGU many years ago, regarding the fact that small burrowing mammals and reptiles were able to survive the extinction better than other animals because they were able to hide away and gain shelter.

    It’s obvious, and I’m surprised no one has mentioned it already.

    Rebecca noted the proof of its truth, and it’s absolutely brilliant. I’m really, really surprised that it hasn’t revolutionised the study of palaeontology and Rebecca hasn’t been invited to Stockholm.

    Rebecca noted that burrowing animals must have survived better because fossils are almost always found underground.

    Brilliant, isn’t it?

  19. Gabor Hrasko says:

    I really enjoyed this theroetical play about what would have happened if the meteor had arrived an hour later and had hit deep ocean. Than I realized that one hour would in fact save the Earth totally, as the meteor would miss it totally . Anyhow that was not the main point, I know.

  20. zorrobandito says:

    Thank you for updating us on the status of this controversy, which I wasn’t following particularly closely (being more of a fan of the Cambrian, myself). I’m glad the impact theory is winning out. Very satisfying in a blow-stuff-up sort of way! Dramatic!

    We are certainly fortunate that the smartest surviving dinosaurs are parrots, crows and ravens! They are much more charming than Tyrannosaurus as well as being less dangerous.

  21. Steve Pusser says:

    “Gypsum forms by evaporation of sedimentary rock, and so occurs where there is shallow water or where there has been water in the past.” Should be evaporation of seawater–a sedimentary record with lots of evaporites records multiple incursion and evaporation events in a closed basin, such as what lies under the Mediterranean. The same thing happened many times at the proto-Atlantic made its way into the irregular ocean basin opening up between America and Euro-Africa.

    “A shallow sea was therefore perhaps the deadliest place to strike – too little water to cushion the impact, but enough to allow for high levels of gypsum and therefore sulphur.”

    What matters is how many evaporites are in the rock where it hits. If an asteroid impacted today in Texas’s Permian Basin, I’d expect it to also blow out lot of sulfur. The high sea levels at the end of the Cretaceous meant that just about all of the continental sedimentary basins in the world were undersea at that time, but undersea or on dry land, the sulfur would still be bad.

    Mexico’s El Chichon volcano’s 1980 eruption also was very sulfur-rich and affected world climate; this is attributed to its magma rising through the same type of evaporites and assimilating the element as it went.

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