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What Wiped Out the Dinosaurs?

May 11, 2016

By Ronak Gupta

In the brimming potpourri of factors that encouraged the evolution of Homo sapiens, the extinction of dinosaurs also plays an important role.

In 1980, Luis Alvarez, a Nobel Prize winning physicist at the University of California, Berkeley, and Walter Alvarez, his geologist son, published a paper in the journal Science entitled Extraterrestrial Cause for Cretaceous-Tertiary (K-T) Extinction. In it, the duo postulated a theory since then called the impact hypothesis, which stated that an asteroid almost 10-km-wide collided in catastrophic fashion with Earth approximately 66 million years ago, leading to a mass extinction event that wiped out 70% of animal species including the dinosaurs. It was a radical proposition that challenging the old guard of uniformitarianism, forwarded by Charles Darwin and Charles Lyell, among other luminaries, and providing an answer for one of science’s most vexing questions: How did the dinosaurs die?

Until the 1980s, the prevailing idea was that gradual, uniform forces of nature like erosion were responsible for the ever-changing face of Earth’s geology and geography. This idea was dubbed uniformitarianism, standing as it did in contrast to catastrophism, which said Earth’s history was punctuated by certain catastrophic events that were responsible for the planet’s extant features.

Fossil records provide evidence that, across the K-T boundary, the geologic signature of rocks dating back to around 66 million years ago, there is an abrupt disappearance of fossils of new dinosaur species. This is indicative of the fact that somewhere between the end of the Cretaceous period and the beginning of the more recent Tertiary period, the dinosaurs became extinct along with a host of other organisms. However, extinction is far from a simple process to study. A sudden catastrophic event could be responsible for the demise of a group of animals – or a species could simply have failed to evolve, leading to its extinction. What happened in the case of the dinosaurs? There is no consensus yet.

Many in the field believe that the cause for the extinction of the dinosaurs was related to an sudden external event: involving the asteroid as hypothesised by the Alvarezs. When their seminal paper was published, it was greeted with much incredulity. While there was evidence to suggest the presence of an asteroid impact, such as high levels of iridium at the K-T boundary and certain zones of shocked quartz rock, a major piece of the puzzle was missing. Where was the impact crater?

The discovery of the Chicxulub crater in the Yucatan Peninsula, Mexico, in 1991 fit the bill perfectly and convinced most doubters. As geophysicist Sean Gulick told Space.com, “Once the crater at Chicxulub was recognised for what it was, it became the smoking gun for the idea that an extraterrestrial impact caused the mass extinction.” So, the dinosaurs were simply unlucky, then? As far as theories of this kind go, they evolve from the yoke of sensationalism and gain validation only after vetted evidence.

At the same time, the lack of testable evidence however hasn’t prevented physicists from entering into the debate. In 2014, Harvard physicists Lisa Randall and Matthew Reece published a study in Physical Review Letters claiming that dark matter could in theory be responsible for the death of the dinosaurs. The same dark matter as that near-invisible, almost non-interacting, elusive substance that has escaped detection since its existence was hypothesised in the 1970s.

According to Randall and Reece, it’s possible that a thin disk of dark matter, by virtue of its gravitational pull, disturbed the path of the comets and diverted one of them in the direction of Earth. If this hypothesis holds out, then it’s also possible that there have been multiple periodic impacts on our planet stemming from common origins.

Currently most of the geologists and palaeontologists lean towards the external catastrophe theory, even as there are loud voices of opposition. One of them belongs to Gerta Keller, a palaeontologist at Princeton University, New Jersey. Keller accords the demise of the dinosaurs to an intrinsic activity on Earth: massive volcanism. She points to a region called the Deccan Traps, a plateau to the west of central India, where remains of one of the most intense periods of volcanic activity on Earth have been preserved.

The alternative hypothesis is bolstered by the fact that volcanism has been linked to other mass extinction events. The Siberian Traps are believed to have been the cause of the ‘Great Dying’, technically called the Permian-Triassic event. Lava flows from volcanoes can inject copious amounts of harmful chemical substances in the air, including carbon dioxide. Global warming and ocean acidification are natural consequences. Keller and her colleagues were able to measure precisely when the volcanism event could have begun and ended, and herein lies the clincher. The major activity of the volcanic events continued for 750,000 years, overlapping the timestamp of when the dinosaurs were wiped out.

More precise measurements since either hypothesis was first injected into mainstream discourse have since mitigated the debate’s polarisation. Many eminent researchers agree that extraterrestrial impact and volcanism both are likely to have played a role in the K-T extinction. Walter Alvarez himself conceded in 2013 that “volcanism may have something to do with it as well”. A 2015 study published in Science concluded that the shockwaves produced by the asteroid impact may have lead to the formation of splits and chambers underground that, in turn, further intensified the volcanic activity in the Deccan Traps. Sam Bowring, a geologist at the Massachusetts Institute of Technology, Boston, summed it up well: “Perhaps the end of the dinosaurs was caused by a one-two punch.”

More recently, another question has found its way into this debate: Were the dinosaurs thriving until they fell prey to a cosmic hammer and a geologic anvil or were they on their way out anyway? A study, out of the University of Bristol, published earlier this year took a shot at answering this. By conducting an extensive statistical analysis of three major dinosaur groups (Ornithschia, herbivores with hips like birds’; Sauropods, long-necked herbivores; and Theropoda, lizard-like carnivores), it concluded that the dinosaur’s rate of species generation had begun to lag behind the rate of species extinction around 40 million years before the K-T extinction event. This made the dinosaurs easy pickings for a wipe-out once the asteroid hit: they couldn’t evolve fast enough in the aftermath to occupy what new ecological niches were available. At the same time, the reason for the gradual decline is not clear.

In the brimming potpourri of factors that encouraged the evolution of Homo sapiens, the K-T extinction event also plays an important role. While the Bristol study seems to suggest that the dinosaurs would have eventually become extinct – asteroid or not – according to some scientists that is not something set in stone. Duck-billed and horn-faced dinosaurs formed an exceptional group within the study, actually prospering before the asteroid hit. Paleontologist Stephen Brusatte, from the University of Edinburgh, told The Guardian, “I think if there was no asteroid you would still have dinosaurs around today.”



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