The Hunger Game, 65 Million Years Ago

During my weekly volunteering in the Past Worlds Gallery, I'm often asked what happened to the dinosaurs. Of course, I give the answer that most paleontologists and geologist agree upon today, that a comet or meteor impact killed them off. As I grew up with this hypothesis and enthusiastically watched it mature into a substantiated scientific theory, I have a personal interest in explaining it.

Visitors clearly are not satisfied with this pat answer, however, because they can't grasp the mechanics of it. I think most people know there are always survivors from every catastrophe, and that the survivors recover their numbers and restore what was lost. The Impact Theory does not explain why some species that survived the impact made it through the hard times thereafter, and other survivors of the initial disaster eventually perished. That puzzle makes the Impact Theory an incomplete answer, unless it is elaborated.

People also have a hard time grasping how a local impact can have global consequences, even when the explosion is huge. We all know from news headlines and history books that a volcanic mountain can blow its top, but if we are not near that mountain, we do not fear the consequences. So how did an impact on the Yucatan affect life in all the oceans, and life on land areas far away?

What follows is the response I generally give, phrased here as exposition rather than the Q/A style I like to use in conversation.

To begin: 

How much energy was available for the impact event that caused the Cretaceous-Paleogene extinction? 

A consensus ballpark estimate, based on the object mass and speed required to make the Chicxulub crater, is that the kinetic energy available before impact was 130,000,000 Megatons, using the unit for quantifying nuclear weapon energy yield.

A Wikipedia article on the history of nuclear weapons estimates that, in round numbers, about 130,000 nuclear weapons have been produced by all the world's nuclear powers over time. The yield of these weapons was quite varied, as they were designed for delivery by different systems to different targets. A reasonable guess for the average yield of all the nuclear weapons ever produced would be somewhere between 1 Megaton and 10 Megatons per weapon. A ten Megaton weapon would vaporize an entire metropolitan area.

For comparison purposes, the high side estimate is most useful, ten Megatons. Thus, the high side estimate of total Megaton yield capacity, cumulative for all nuclear weapons, is 1,300,000 Megatons.

This would put the impact energy available before atmospheric entry at 100 to 1000 times the explosive power of all the nuclear weapons ever produced. Upon entry and impact, this energy was converted to blast shock waves traveling through the atmosphere, the ocean and the earth's crust, a fireball of vaporized rock and superheated steam, ejected volumes of rock and water, an immense tidal wave, radiant heat, and seismic waves traveling through the earth.

The most significant contributor to extinction was the amount of small particle debris thrown into the air that stayed there for months, perhaps years, and was distributed around the globe. The impact cloud chilled the global climate by reflecting more sunlight back into space, and it reduced the sunlight available to plants, the base of food chains for land and ocean life. These are the effects of "Impact Winter", a term borrowed from studies of the global effect of total nuclear war, "Nuclear Winter". Most of the dinosaurs that survived the impact explosion starved to death or died of exposure soon after, even if they were far away from the impact site. In the chilled environment beneath darkened skies, there was little food to fuel them against the killing cold.

At this point in the narrative, kids raise their hands. Why didn't the bird branch of dinosaurs also go extinct? Why didn't the mammals? Why didn't all the world's oceanic fish die off for lack of sufficient phytoplankton to support the food chain? Let's rephrase the questions into one central question. Why did some species squeak through and most species did not?

It is probable that there were individual survivors of the impact across all the species on earth 65 million years ago, including the dinosaurs that today are extinct. But a new world had been created in the aftermath of the impact devastation, and the animals that were most adaptive to the drastically changed circumstances were favored. 

An animal that cannot compete with other animals for the same food resources will rapidly dwindle in number until no more are reproduced. An animal that adapts to new opportunities will survive and evolve. This is the way nature heals itself from global extinction events; the fossil record bridging the five major global extinction events provides testimony.

Flowering plants, such as the grasses, had evolved and proliferated late in the dinosaur age; classic dinosaurs ate them as part of their diet but had not evolved to exploit them fully. By contrast, insects, mammals and the avian dinosaurs had co-evolved with flowering plants. As flowering plants recolonized the land after the impact, the animals that could best utilize them were favored, and consequently they out competed the few remaining classic dinosaurs, who were ill suited to an environment in recovery. 

Here's the coda to the narrative, in scientific terms: The classic dinosaurs that survived the impact and its aftermath, eventually died off anyway because their long established success deprived their species of the genetic variation necessary to exploit drastic environmental change through adaptation by natural selection. One hundred, seventy million years of fairly stable ecological relationships did not prepare their genomes for a global catastrophe that gave the world a fresh start.

Of all the dinosaur families, only some species of birds made it through. Along with the mammals, they evolved to fill niches left empty by the absence of classic dinosaurs. Their genomes had the breadth of variability to diversify across generations of natural selection.

That's one story, consistent with what we know today. The job of science is to examine such stories, and find new facts that either support or refute them.