INTERLUDE 14-1 What Killed the Dinosaurs?
The name dinosaur derives from the Greek words, deinos (terrible) and sauros (lizard). Dinosaurs were no ordinary reptiles. In their prime, roughly 100 million years ago, the dinosaurs were the all-powerful rulers of Earth. Their fossilized remains have been uncovered on all the world's continents. Despite their dominance, according to the fossil record, these creatures vanished from Earth quite suddenly about 65 million years ago. What happened to them?

Until fairly recently, the prevailing view among paleontologists—scientists who study prehistoric life—was that dinosaurs were rather small-brained, cold-blooded creatures. In chilly climates, or even at night, the metabolisms of these huge reptiles would have become sluggish, making it difficult for them to move around and secure food. The suggestion was that they were poorly equipped to adapt to sudden changes in Earth's climate, so that they eventually died out. However, a competing, and still controversial, view of dinosaurs has emerged. Recent fossil evidence suggests that many of these monsters may in fact have been warm-blooded and relatively fast moving creatures—not at all the dull-witted, slow-moving giants of earlier conception. In any case, no species able to dominate Earth for more than 100 million years could have been too poorly equipped for survival. For comparison, humans have thus far dominated for a little over 2 million years.

If the dinosaurs didn't die out simply because of stupidity and inflexibility, then what happened to cause their sudden and complete disappearance? Many explanations have been offered for the extinction of the dinosaurs. Devastating plagues, magnetic field reversals, increased tectonic activity, severe climate changes, and supernova explosions have all been proposed. In the 1980s it was suggested that a huge extraterrestrial object collided with Earth 65 million years ago, and this is now (arguably) the leading explanation for the demise of the dinosaurs, although it is by no means universally accepted. According to this idea, a 10- to 15-km-wide asteroid or comet struck Earth, releasing as much energy as 10 million or more of the largest hydrogen bombs humans have ever constructed and kicking huge quantities of dust (including the pulverized remnants of the asteroid itself) high into the atmosphere (see the first figure below). The dust may have shrouded our planet for many years, virtually extinguishing the Sun's rays during this time. On the darkened surface, plants could not survive. The entire food chain was disrupted, and the dinosaurs, at the top of that chain, eventually became extinct.

Although we have no direct astronomical evidence to confirm or refute this idea, we can estimate the chances that a large asteroid or comet will strike Earth today, on the basis of observations of the number of objects presently on Earth-crossing orbits. The second figure shows the likelihood of an impact as a function of the size of the impacting body. The horizontal scale indicates the energy released by the collision, measured in megatons
of TNT. (The megaton—4.2 1016 joules, the

explosive yield of a large nuclear warhead—is the only common terrestrial measure of energy adequate to describe the violence of these occurrences.) We see that 100-million-megaton events, like the planetwide catastrophe that supposedly wiped out the dinosaurs, are very rare, occurring only once every 10 million years or so. However, smaller impacts, equivalent to "only" a few tens of kilotons of TNT (about one MX missile warhead from the U.S. nuclear arsenal), could happen every few years—we may be long overdue for one. The most recent large impact was the Tunguska explosion in Siberia, in 1908, which packed a roughly 1-megaton punch (see Figure 14.18).

The main geological evidence supporting this theory is a layer of clay enriched with the element iridium. This layer is found in 65-million-year-old rocky sediments all around our planet. Iridium on Earth's surface is rare because most of it sank into our planet's interior long ago. The abundance of iridium in this one layer of clay is about 10 times greater than in other terrestrial rocks, but it matches closely the abundance of iridium found in meteorites (and, we assume, in asteroids and comets too).

This theory has its problems, however. The amount of iridium in this clay layer varies greatly from place to place across the globe, and there is no complete explanation of why that should be so. And if this body was so massive as to kick up enough dust to darken the entire planet, then where is its impact crater— One leading candidate has recently been identified in the Yucatan Peninsula in Mexico, where possible evidence for a heavily eroded, but not completely obliterated, crater has been found. However, its identity as the site of the dinosaur-killing collision has not yet been conclusively proven. Perhaps, some scientists argue, the iridium layer was laid down by volcanoes and had nothing to do with an extraterrestrial impact at all.

Another potential difficulty concerns the speed at which the dinosaurs disappeared. The fossil and geologic records are rather imprecise in the ages they yield, and this translates into uncertainties in just how long the extinction process took. It seems to be no more than a million or so years, but that is still a very long time. If the dinosaurs vanished as a direct result of a collision and subsequent explosion, we might expect their disappearance to have been complete in a matter of decades, or perhaps centuries. It is hard to see how the process could have spanned tens or hundreds of millennia. Greatly improved geologic age—measurement techniques will be needed to settle this issue decisively.

Whatever killed the dinosaurs, dramatic environmental change of some sort was almost surely responsible. It is important that we continue the search for the cause of their extinction, for there's no telling if and when that sudden change may strike again. As the dominant species on Earth, we are the ones who now stand to lose the most.