Many millions of years before the first modern humans, the homo sapiens, appeared on earth, dinosaurs were the all-powerful rulers of the Earth. Their emperor was the tyrannosaurus rex, which means ‘king of the tyrant lizards’ and the titan was brontosaurus – the ‘thunder lizard’. They roamed the earth’s surface unchallenged during the Mesozoic Era – an interval of geological time roughly 230 million to 65 million years ago.
However, around 65 million years ago, at the end of the Cretaceous period – the last and the longest segment of the Mesozoic era, something nightmarish happened. The dinosaurs, after surviving for nearly 160 million years, became victims of the greatest mass extinction in the history of our planet. In an instant on the geological scale, their entire population was wiped out from the surface of the earth. Along with the dinosaurs, many other species of mammals, amphibians and plants died at the same time. This was the last of the five mass extinctions of life on our planet.
The question naturally arises: what killed the dinosaurs en masse? Controversy has surrounded this question for decades. Many hypotheses had been advanced to explain how the curtain fell on the dinosaurs. Devastating plagues, earth’s magnetic field reversals, increased geological activity, severe climate changes and supernova explosions hade all been proposed. While advocates of these hypotheses had convincing reasons in support of their ideas, for many years, climate change was the most credible explanation for the dinosaurs’ demise.
Dinosaurs were cold-blooded reptiles that thrived in the planet’s consistently humid, tropical climate. But before their extinction, evidence showed that the planet slowly became cooler. Lower temperatures caused polar region and the oceans to become extremely cold. Consequently, creatures like dinosaurs that obtained body heat from the sun and the air would not have been able to survive in significantly colder climates. Yet, some species of cold-blooded animals, such as crocodiles, did manage to survive. Moreover, climate change would have taken tens of thousands of years, giving the dinosaurs sufficient time to adapt. Hence, climate change theory doesn’t seem plausible.
In 1980, a team of researchers, including the father-son duo of Luis and Walter Alvarez, reported discovering lots of iridium in a thin worldwide layer of clay from the end of the Cretaceous period. Iridium is rare in Earth’s crust, but abundant in asteroids and other space rocks. This led to the suggestion that a huge extra-terrestrial object, perhaps an asteroid, collided with the Earth 65 million years ago. This now is arguably the leading explanation for the extinction of the dinosaurs, although it is by no means a universally accepted theory.
According to the impact theory, a large asteroid roughly 10 km across struck the earth, releasing as much energy as 10 million hydrogen bombs. The impact itself wasn’t the big killer. It was the after-effects of the impact – worldwide climate change and geologic disturbances, such as earthquakes, landslides and tsunamis – that might have killed the dinosaurs.
The impact set off major earthquakes around the world. In addition, huge quantities of dust, including the pulverized remnants of the killer asteroid, soared high into the atmosphere. The dust might have shrouded the planet for years, virtually blocking the sun’s rays. As a result, global temperatures plummeted and arctic ice spread southward. On the frigid, darkened surface, plants could not survive, thereby causing disruption of the entire food chain that sustained the dinosaurs.Thus, the after effects of the impact chocked the dinosaurs and eventually pushed them toward extinction.
The theory of the catastrophic asteroid impact got a boost in the 1990s when scientists found the tell-tale fingerprints of the fateful event. They discovered a heavily eroded but not completely obliterated crater of just the right size and age below the turquoise blue waters near Chicxulub in the Yucatan peninsula in Mexico.
Opponents of the asteroid impact theory argue that the iridium discovered by the researchers was most likely produced by volcanoes, not by the asteroid impact. Indeed, research shows that when the asteroid struck the earth, volcanoes in India’s Deccan Traps were wreaking havoc to the planet’s ecosystems. They believe that the lavas may have spewed poisonous levels of sulphur and carbon dioxide into the atmosphere and initiated the mass extinction of the dinosaurs through the resulting global warming and ocean acidification.
Proponents of the asteroid theory contend that while volcanic activities, probably set off by the shockwaves of the impact, and other factors might have contributed to the rapidly deteriorating environmental conditions, the crater near the Yucatan peninsula and the abundance of iridium in the layer of clay clearly indicate that the asteroid was the final coup-de-grace for the dinosaurs. Their claim was further bolstered by the impact of the comet Shoemaker-Levy 9 on Jupiter in 1994. Much of the debate now revolves around the question of whether the Chicxulub event was the smoking gun or merely accelerated the extinction process of the dinosaurs that was already under way seemingly due to climate change.
Finally, one might ask the question: why are we spending so much money and time to pin down the cause of an event that happened in an era when we were not even around? The answer is simple. If we want to survive on this planet, at least as long as if not longer than the dinosaurs, then it is important for us to understand how ecosystems respond to cataclysmic events. Otherwise, like the dominant species on earth, we are the ones who stand to face the same fate as dinosaurs.
The writer, Quamrul Haider, is a Professor of Physics at Fordham University, New York.