Jul 28 2014

Dinosaur Extinction Revisited

Sometimes I just have to indulge my childhood fascination with all things dinosaur. Actually, paleontology in general is one of my favorite subjects – reconstructing an utterly alien past, including incredible and strange-looking beasts.

And of course, one of the most fascinating aspects of the dinosaurs is that, after 165 million successful years on earth, they suddenly went extinct 66 million years ago. What could cause such a catastrophe? When I was in grade school, the textbooks still contained an outdated (and ridiculous) answer – they ran out of food.

Now most people know that the dinosaurs were wiped out by an asteroid, and the event is frequently depicted in movies and popular culture. It is less well known, however, that scientists are still debating this issue.

There is no question that a large meteor slammed into the earth 66 million years ago and caused devastation. We even found the crater, Chicxulub in the Gulf of Mexico. There is also evidence of shocked quartz from the impact, and a layer of iridium from the meteor itself. This event marks the K-T boundary (now called the K-Pg boundary) between the cretaceous and tertiary periods.

At this boundary all the large vertebrates: dinosaurs, plesiosaurs, mosasaurs, and pterosaurs, went extinct, as did many species of plankton, tropical invertebrates, and reef dwellers. Mammals, birds, insects, and most plants did fine, however.

Some paleontologists are still uncertain if the meteor was the sole factor in the K-Pg extinction. When counting fossils it seems that dinosaur species were in decline prior to the meteor impact. Perhaps the impact was just the coup de grace, and the dinosaurs were already on their way out. Others feel the apparent decline is just an artifact of the fossil record, or that the decline was incidental and did not significantly contribute to extinction.

This all brings me to a recent paper in Biological Reviews which addresses this controversy. In the abstract Brusatte et. al. argue:

Non-avian dinosaurs went extinct 66 million years ago, geologically coincident with the impact of a large bolide (comet or asteroid) during an interval of massive volcanic eruptions and changes in temperature and sea level. There has long been fervent debate about how these events affected dinosaurs. We review a wealth of new data accumulated over the past two decades, provide updated and novel analyses of long-term dinosaur diversity trends during the latest Cretaceous, and discuss an emerging consensus on the extinction’s tempo and causes. Little support exists for a global, long-term decline across non-avian dinosaur diversity prior to their extinction at the end of the Cretaceous. However, restructuring of latest Cretaceous dinosaur faunas in North America led to reduced diversity of large-bodied herbivores, perhaps making communities more susceptible to cascading extinctions. The abruptness of the dinosaur extinction suggests a key role for the bolide impact, although the coarseness of the fossil record makes testing the effects of Deccan volcanism difficult.

That’s right – we have to call dinosaurs,  “non-avian dinosaurs” now, because birds are in the dinosaur clade. That’s fine and appropriate, for technical writing, but in non-technical communication, everyone knows exactly what you mean when you refer to “dinosaurs.”

The Deccan volcanism refers to the Deccan traps located in India, representing a massive volcanic feature. Around the time the bolide hit, there is evidence that the Deccan traps were volcanically active (perhaps 30 times as active as the Hawaiian volcanoes are today), and this activity was producing climate change and perhaps even poisoning the atmosphere.

This is the major alternate theory to the cause of the dinosaur extinction, they were on their way out due to the massive environmental disruption caused by volcanic activity in the Deccan traps. Other environmental changes include temperature changes and sea level changes. The authors summarize the current controversy:

Although evidence for an end-Cretaceous impact is unequivocal (Schulte et al., 2010), doubts remain because other severe changes occurred in Earth systems at or near the end of the Cretaceous: intensive volcanism (Courtillot & Renne, 2003; Chenet et al., 2009), temperature oscillations (Li & Keller, 1998; Barrera & Savin, 1999; Huber et al., 2002; Wilf et al., 2003; Grossman, 2012; Tobin et al., 2012), and sea-level fluctuations (Miller et al., 2005). It has been argued that each of these factors may be the primary cause of dinosaur extinction, that their sum resulted in the extinction, or that a bolide impact finished off the dinosaurs after a multi-million-year period of stress triggered by one or more of these changes (Archibald, 1996, 2011).

This recent review of evidence comes to the conclusion that the species decline was not global or sustained, and therefore was not the major cause of the K-Pg extinction, which clearly correlates with the meteor impact. But, the authors conclude that the minor decline in the number of plant species may have made the entire dinosaur ecosystem more vulnerable to collapse when the meteor did hit.

So they are coming down on the notion that the meteor finished off the large vertebrates, who were already under stress, even though the species decline was minor.

One of the points of contention is how to interpret the fossil evidence. Scientists can look at different parts of the world and different fossil beds to gain windows into the diversity of dinosaurs at moments in time. The question is, are there biases in the fossil evidence that falsely make it appear as if the dinosaurs were in serious decline. The authors conclude from existing evidence:

None of these studies has supported a global decline in diversity occurring across all dinosaur groups. However, some evidence has supported Campanian–Maastrichtian declines in the richness of ornithischians (Barrett et al.2009; Upchurch et al.2011) and theropods (Barrett et al.2009; but not Upchurch et al.2011), but not sauropodomorphs.

They go on to do their own updated analysis, concluding:

The new subsampling analyses provide no evidence for a progressive Campanian–Maastrichtian decline in total dinosaur species richness at either the global or North American scales. However, finer-grained analyses support a decline in the species richness of North American ornithischians, but not theropods. This ornithischian decline occurs from the late Campanian to the early Maastrichtian, and ornithischian diversity remains low during the late Maastrichtian.

So it seems that the bottom line here is that there is no significant evidence for major decline in dinosaurs prior to the meteor impact. However, the authors did find a local decline in one type of dinosaur – North American ornithischians. They argue that this decline in plant eaters could have contributed to the ultimate extinction of the dinosaurs due to the meteor. This, however, seems like a small speculation at the end of a fairly solid conclusion for no significant decline.


The entire paper is interesting reading, and spends a great deal of time reviewing the current fossil evidence for the diversity of dinosaurs and their ultimate extinction. It’s actually a good primer for anyone interested in this debate (although it gets a bit technical in places, most of it is very accessible).

Nature News reporting on this study focuses on what I consider to be a very minor part of the paper, the notion that perhaps the slight decline in plant-eating dinosaurs contributed to their extinction from the meteor impact. I fear that this is the angle the press will take – the meteor hit at just the wrong time, what bad luck.

Reading the paper, however, it seems to me the primary conclusion is that the dinosaurs were not in significant decline, and they were indeed wiped out by the meteor which impacted at the Chicxulub crater 66 million years ago.


22 responses so far

22 thoughts on “Dinosaur Extinction Revisited”

  1. BillyJoe7 says:

    Hmmm…I’ve always remembered it as 65 million years…maybe I’ve been rounding off all this time (:

  2. Bruce says:

    “I fear that this is the angle the press will take – the meteor hit at just the wrong time, what bad luck.”

    Unfortunately the BBC already ran with it:

    “‘Bad luck’ ensured that asteroid impact wiped out dinosaurs”


  3. tmac57 says:

    So,does this let the Sinclair Oil Co. off the hook?

  4. steven johnson says:

    Does the assumption that a climax community has a greater diversity of species really have sufficient evidence to support it? The notorious example of the crown of thorns starfish comes to mind.

    Is it even true that a climax community is really stable over significant time periods, as in a fraction of geological time? Evolution in populations is more or less invisible over short time periods but variation and differential reproduction and genetic drift cannot be stopped.

    It sometimes seems as if genera are imagined to be a family and the mulitiplication of species is like having a large, i.e., successful, family. Is that really the notion, or is there another rationale for studying species diversity as markers of success?

    It seems to me that the true marker for evolutionary success is large numbers. I know it is difficult enough to assess the fossil record for diversity. But isn’t looking at diversity in this context something like being a drunk who looks under the street lamp for his keys?

  5. carassius says:

    Dang-it…I’ve always been pleased with myself for remembering, and citing whenever possible, the K-T boundary. I’m now to believe that they’ve renamed it the K-Pg boundary/event.

    Please tell me what ‘Pg’ stands for so I can begin to work it into my memory?

  6. carassius says:

    To answer my own question, Pg = paleogene. Sheesh.


  7. LC says:

    It was a controlled demolition. Any fifth grader can see that.

  8. ‘Climax community’ = no comment

  9. the devils gummy bear says:

    Brontosauruses are thin on one end, much much thicker in the middle, and then thin again at the far end. Paleontology is a crazy faith based pseudoscience. An argument from past dirt is a fallacy.

  10. Creeping Malaise says:

    The Chicxulub crater shows clear evidence of squibs, because I say so.

  11. ferrousbueller says:

    @The Other John Mc

    In my hometown we even have a hall devoted to it:


  12. grabula says:

    “‘Climax community’ = no comment”

    I’ve been trying to get into one of these

  13. grabula says:

    Seems to make some sense though I haven’t gone through the complete paper as of yet. A community begins to take stress from ongoing climactic/geological changes. They may not have had to been on a tipping point to meet their end with the asteroid/meteor strike.

  14. rezistnzisfutl says:

    Well, I’ve had my time going in circles with Mlema. That’s why I don’t have much patience with it anymore. Nothing has changed, and if you look at old blog entries on this subject you’ll see how often we went in circles. It’s full-on anti-GMO rhetoric with lots of Gish Galloping and JAQing, and you’ll be inundated with citations that are questionable at best, but weak sauce in the end. I just don’t have it in me to go hundreds of comments deep with someone who is so ideologically dogmatic.

    And before I get accused of being dogmatic myself, I think most of us can agree that a skeptic who has been presented with an argument in the past innumerable times and has already rejected it, may appear dismissive, but in reality they have already considered with the arguer thinks is new and compelling. As a skeptic, I’m concerned not only with having evidence, but the quality of evidence. Having standards of evidence is obviously a big part of being a skeptic. But, I think you guys have a pretty good idea now who you’re dealing with, an anti-GMO activist who wants them banned, and have Certified Organic that is pesticide free be the primary form of agriculture. Let’s see how well they spin their wheels trying to explain how that’ll feed billions or how indigent populations can afford boutique heirloom fruits and veggies.

  15. rezistnzisfutl says:

    Oops, this was meant to be posted on the Mike Adams thread. Not sure how exactly I ended up here… Maybe I need a good night’s sleep!

  16. ginckgo says:

    Worth noting are several other patterns related to the K/Pg extinction event (yes, I and a pedantic palaeontologist!):
    There is now decent evidence of several non-avian dinosaur species surviving well past (100,000s years) the impact event (see http://link.springer.com/chapter/10.1007/978-90-481-3428-1_26#page-1). This study also includes several ammonite species, the other major clade that defined the Mesozoic faunas, but died out around this time.
    Also, what about the patterns of other animals, especially in the oceans? Many of which when across the K/Pg boundary with nary a hiccup, yet many went into decline by the end of the Danian (the following Stage). Most of these clades have fossil records that are orders of magnitude more extensive and continuous through geological time, thus giving a clearer picture of patterns, without having to fall back on statistical methods to extrapolate.
    There are also some serious stress markers before the impact horizon. For example bryozoans (a group of marine animals I work on) indicate massive spikes in marine water temperatures in the million years preceding the impact (they are colonial, and the skeletal openings that the animals live in change size depending on water temperature, and the size halved for at least one period: http://www.stri.si.edu/sites/publications/PDFs/STRI-W_ODea_et_al_Palaeo3_KT_bryozoa.pdf)
    I agree that the asteroid seriously f*#ked up the place, but it is far from the whole story. Interestingly, there appear to have been numerous other asteroid impacts throughout the Phanerozoic that are on par with the K/Pg one in terms of size, yet none of them are correlated with any significant extinction event.

    Another thing to remember is what evolutionary characteristic caused the ‘extinction’ pattern; was it mass extinction (species extinction rates significantly elevated above background rates) or mass depletion (speciation rates significantly dropped below background rates). These are two fundamentally different dynamics, but can superficially give the same pattern in the fossil record. The possible decline pre-K/Pg could be a drawn out mass depletion.

  17. LDoBe says:

    @steven johnson

    Interesting point. I think the reason why diversity is so often used is because nobody gets published for digging up five score specimens from a well-known species, all of which are of average dimensions, condition and age.

    Everyone wants to get a shot at naming a new species, or finding the biggest or the smallest, or the best-preserved, etc. So there’s a smaller incentive for identifying cataloging and compiling a pile of iguanodons, for instance (let’s just say there’s tons of iguanodon fossils). So there’s a lot more reliable data in the frequency of new species because that’s what everyone’s looking for.

    On the other hand, fossilization is a pretty rare phenomenon anyway. And what we dig up isn’t necessarily representative of population size for the total area a species might have lived, since fossilization requires specific geological and climate conditions to be met.

    So it could possibly be in the long run, that diversity is a more reliable metric for determining how successful dinosaurs as a group were at any given time. If the environment is supportive enough to allow for the dinosaurs to evolve in tons of directions and a lot of species flourish, then we can say that they’ve been successful. If the environment has gotten too harsh, and selective pressure is too great, then most new species won’t be around for very long, but the stable and good solutions to living in the rough environment will stay for quite a while.

  18. Bruce says:

    “since fossilization requires specific geological and climate conditions to be met.”

    So, my layman’s understanding of this would then jump to saying that perhaps the meteor hit changed the climate in such a way that fossilisation occured more rarely, and that the actual number of those that survived might be under-estimated?

  19. Sylak says:

    You are right, even I f*****g love sciences reports it as a Bad luck. Well, Unfortunately I find that since it is now big, ifls is as bad as other media with the main titles of articles.

    The dinosaurs were a cool bunch, they went away with a bang :).
    I was reading about the spark plugs in my car last year, honda uses iridium tipped Plugs and on the denzo web site, they explain what iridium is. The great part is that they explain how the iridium used is from the t-k line and that it is one of the evidences for that big meteor. I didn’t know that before. I learned all this Just because I wanted to know more about my spark plugs lol.

    Am I the only one who thinks that traveling back in time to see that bolide hit earth (well, maybe looking at it from space) would be fascinating?

  20. BillyJoe7 says:

    Is Sylak short for Skylark? (:

  21. Stormbringer says:

    I wonder what amount of environmental specialization dinosaurs had. Think if they had diets like koalas or silkworms then even a robust population could be completely wiped out with an event that made a change to their environment.

  22. Sylak says:

    @Billeyjoe7 LOL no, it is the name of one of the character I had for a while back in time i was playing West end Game Starwars RPG. This is my Internet Nick ( online and gaming) for over a decade now hehe.
    But I did Owned a 1986 buick Skylark in between 1998 and 2000 lol.

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