November 14, 2007 > TechKnow Talk
What Killed the Dinosaurs?
There has been a tremendous amount of research on this question since 1980. Many unknowns remain, but answers are beginning to emerge. In general, scientists fall into two camps. Some believe the dinosaurs died over a lengthy period - perhaps as long as a million years - due to environmental changes originating from within the earth. Others believe they died very suddenly, in a few thousand years or possibly much faster, from a dramatically altered climate traceable to a collision with an asteroid or comet. Not surprisingly, many geologists and paleontologists favor the former hypothesis, while astronomers and physicists are more likely to subscribe to the latter.
Dinosaurs were the dominant life form on land for some 140 million years, relegating all other creatures to secondary roles. But about 65 million years ago (MYA), they vanished. The impact of this mass extinction event extended far beyond dinosaurs; 70% of all species in existence at that time were killed, including much of the marine life. Even mammals were severely affected, though they subsequently rebounded in the absence of the dinosaurs. Only ferns seemed to emerge from this period in better evolutionary health than before, widespread and genetically diverse.
What happened 65 MYA? Geologists have long been aware of a rock stratum that is found throughout the world known as the K-T boundary. This thin deposit marks the transition between two geologic era; the end of the Mesozoic, the era of the dinosaurs, and the beginning of the Cenozoic, in which we currently live. The K-T boundary has been dated to about 65.5 MYA, tying very nicely to a mass extinction event.
In 1980, it was discovered that the K-T boundary layer contained significant amounts of Iridium, an element not found naturally in the earth's crust. There are two possible sources of Iridium: an extraterrestrial object such as an asteroid or material ejected from the mantle, deep within the earth. Thus the two leading theories for the mass extinction are the impact of a large asteroid and massive volcanism.
The asteroid impact theory was bolstered by the identification in 1990 of a feature near the tip of the Yucatan peninsula as an enormous impact crater. The dimensions of the Chicxulub Crater (about 100 miles across) indicate a meteorite of about six miles in diameter, which closely matches the calculations of scientists based on the extent and composition of the K-T layer. The age of the crater is roughly 65-66 MYA, adding further credence to the impact theory, though there is still some disagreement as to whether the impact occurred at precisely the proper time to trigger the mass extinction.
A body six miles across would have impacted the earth with a force millions of times greater than an atomic bomb, ejecting a massive cloud of dust and debris into the atmosphere, blanketing the earth, blocking sunlight, and reducing temperature for several years. This would have had a disastrous effect on plant life and disrupted the entire food chain.
In addition, as hot debris rained down, firestorms would have raged across the planet. Smoke would have combined with the dust cloud magnifying its effect and duration. As if that weren't bad enough, when the dust finally settled, the atmosphere would have been oxygen-depleted and rich in carbon dioxide, leading to a period of greatly increased temperatures caused by the greenhouse effect.
The theory of extensive volcanic activity posits a similar scenario. Geologic evidence does confirm a period of intense volcanism leading up to 65 MYA, particularly from a vast area in present-day west-central India. Huge volcanoes spewed massive amounts of material from deep within the earth into the upper atmosphere. This material would have covered the earth, blocked sunlight, started firestorms, and resulted in greenhouse heating.
One important difference is that mass extinction tied to volcanism alone would have been a more gradual process, occurring over thousands or possibly even millions of years. The geologic and fossil dating methods for material that ancient is not accurate enough, and the fossil record not complete enough, to determine if the mass extinction occurred suddenly or more gradually.
Several other theories have been put forward to explain the extinctions but most are not sufficient in themselves to explain such a profound global event and need to be considered possible contributing causes, coupled with impact and/or volcanism. These theories include multiple collisions with comet debris, tectonic activity, and the failure of dinosaurs to evolve quickly enough to survive natural climatic cycles or compete with emerging mammalian species.
One contributing theory worth discussing is ocean regression. During the reign of the dinosaurs, most land on earth was combined into one huge continent, called Pangaea. At the time of the mass extinction, Pangaea was still in the final stages of breaking apart, and new oceans were being formed. North America, Europe, and Asia were still one contiguous landmass at this time.
During the period preceding the extinctions, oceans receded dramatically, sea level dropping perhaps as much as several hundred feet. This was probably due to the movement of the new continents and uplift of the land in combination with a subsidence of the ocean floors. These changes may have been an important contributing factor, especially in the extinction of marine life.
The explanation for the mass extinction event, including the demise of the dinosaurs, is almost certainly climate change. Most scientists agree on this, though they may differ on the root cause(s) of that change or its precise nature. There is little doubt that a huge asteroid impacted the earth about the same time, and the majority of scientists now lean toward this event as the primary, if not sole cause. The presence of large-scale volcanism prior to the mass extinction is also well-established and was very possibly a significant contributing factor.
By the way, there is a popular perception of dinosaurs as slow-moving, stupid, and unable to adapt to environmental changes. Granted they were probably not very bright, but we should nonetheless give credit where due. They were the pre-eminent animals on our planet for more than 140 million years and able to survive, adapt, and thrive during tremendous challenges from a changing environment and competing species.
By comparison, humans and their human-like predecessors have managed to survive for only about three million years so far. In fact, had the path of an asteroid not intersected that of earth 65 MYA, humans may never have evolved at all, and mammals might still be nothing more than handy snacks for carnivorous dinosaurs. Opportunities to test our adaptability to severe climatic change will arrive sooner or later. Will we be as successful as the dinosaurs?