Jul 28 2014
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 When I was in grade school, the textbooks still contained an outdated (and ridiculous) answer – they ran out of food.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.
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