Dinosaur Extinction

echemists have spent a lot of time in eastern Montana in the last decade, because it is there that the best evidence of the iridium layer is found. That's the band of the element iridium, rare on earth and more common in rocks from outer space. The iridium-rich layer in rocks in many places around the world, all of them about 65 million years old, is a key piece of evidence for those who think an asteroid or meteor stuck the earth then and produced huge climatic changes that killed off the dinosaurs and many other life forms. Now that there's more evidence of a huge crater made in the Gulf of Mexico at the end of the dinosaur era, many people assume the case is settled—extraterrestial objects killed the dinosaurs.

I can't say I'm much interested in that theory. If an object from space did hit the earth then, there's no direct evidence to link it with the dying off of the dinosaurs at the "K-T boundary" (the end of the Cretaceous period and the beginning of the Tertiary period) 65 million years ago. We don't find any dinosaurs within 100,000 years of the iridium band. In eastern Montana there's no dinosaur skeleton closer than three meters below the K-T boundary line and none above. Maybe dinosaurs lived until then, maybe they lived later. But we don't have the skeletons to prove it either way.

Although therewasasudden pulse of extinctions for many life-forms, including dinosaurs, a lot of other animals, from turtles to mammals, survived. And the sudden extinction doesn't seem to have been worldwide. What evidence we have suggests the Southern Hemisphere might not have suffered from such a catastrophe.

Dinosaur paleontologists aren't expected to look above the K-T line into the Tertiary, though I've searched for fossils from Tertiary stream environments to compare them with the preservation of T. rex. The joke among our colleagues is that we'd get nosebleeds if we went up into the Tertiary. It isn't particularly funny or true,

but there aren't many comedians in paleontology.

As for what killed the dinosaurs, there were enough significant, if not sudden, changes in the environment at the time of T. rex to account for all dinosaurs' extinction. In the last half of the Cretaceous period (which lasted in all from 144 million to 65 million years ago), the water level rose drastically worldwide. A lot of seaways invaded the continents. In North America, for example, a seaway extended from the Gulf of Mexico clear to the Arctic Ocean until T. rex's time, when it began receding. It allowed for a relatively warm climate way up north. Storms generated in the Gulf of Mexico pumped up into what is now Canada. So we got some tropical climate right along the seaway clear up to Montana. We find crocodile fossils all the way to Canada from this time.

At the end of the Cretaceous those seaways were leaving the continent, and a land bridge had formed, hooking western North America to eastern North America. The air flow and water currents, the temperature regimes and climatic conditions changed, and the climate become more extreme. I'm sure that change had some effect on the dinosaurs. It does seem from some samples that dinosaurs were getting less and less common closer to the K-T boundary.

However, a recent and thorough study by Pete Sheehan of the Milwaukee Public Museum and Dave Fastovsky of the University of Rhode Island showed the opposite—that there was no big dropoff in the diversity of dinosaurs (at the broad level of evolutionary families, not genera or species) in one stretch of time from a few million years before their sudden disappearance to the boundary of their era. It's intriguing information, but I'm not convinced the data relate to mass extinctions. I just don't think we can say for sure how the dinosaurs died or when their decline began.

And the truth is, I don't really care how the dinosaurs died. I'm interested in how they lived. Of course, to figure that out now, I need the bones of dead dinosaurs, the more the better.

of the country into a badland desert. And the ice ages that preceded modern times by tens of thousands of years wiped away far more of the country, carving great valleys far wider than the small streams that now wind across their floors.

Today the badland ground is breaking up, eroding faster than plants can establish themselves. So you don't have any vegetation growing over the rocks and fossils. And not many buildings either, since practically nobody lives out here.

I used to live in New Jersey, where I worked cleaning fossils for Princeton University. There's a lot of dinosaurs there, including the first skeleton of a duckbill ever found in North America (one of my favorites, Hadrosaurus). But there you've got to dig under somebody's house, or under a parking lot or a mall, to find a dinosaur. In the badlands, digging for dinosaurs is a lot more convenient and appealing.

How do you actually find a dinosaur? In the past few years, scientists have begun experimenting with sound waves, infrared cameras, magnets, Geiger counters, and a lot of other gadgets to look for dinosaur fossils. So far, none of that high-tech stuff has been proven to work efficiendy. And it's too expensive for us, anyway.

To find a dinosaur, I just walk up and down the bare hills with my head down and stop when I come to something that looks like a dinosaur bone. Sometimes I crawl around on my hands and knees to find little bones. I know that sounds too simple, but it's pretty much the way dinosaur hunters have done it for 150 years. Dinosaurs could be anywhere in badlands like those in eastern Montana. So if you're looking for a fossil, you pretty much just have to go up and down through the hills. It takes a lot of persistence. You could look over the side of one hill and never find anything. And if you didn't go on the other side of the hill, you'd miss the find of your life. So I try to hit every spot.

If I have success where no one else has, maybe it's because I'm more persistent. Or maybe I'm just lucky. I've found fossils when I was just trying to dig a hole for




my tent stake. Nice accidents happen to other fossil hunters too. My friend Phil Currie, a dinosaur paleontologist in Canada and an expert on T. rex., once found the skull of a predatory dinosaur when his camera case rolled down a hill and came to rest on top of it.

None of us can get to every promising spot. And what's exposed changes quickly in the badlands, so today's promising fossil find will be dust in twenty-five years if it isn't excavated and preserved.

What you find depends also on what you are looking for. Someone else might be looking for big specimens and so might stand on a hill and look around to scan for big bones. I'm interested in every fossil, including the little ones. So I cover a lot of ground.

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