Were A Worldwide Success Today We Find Their Fossils On Every Continent

behavior and ecology—how dinosaurs acted, what animals they lived with, and in what sort of environment. I can do this for duckbilled dinosaurs because I've found thousands of them, from every stage of development, including embryos still in the eggs and newborns in their nests. I can study their bones and see how fast they grew, study their nests and figure out how they raised their young, study their huge bone beds and figure out how they died. I can even compare their changing kinds over millions of years in one part of western Montana and figure out how changing environments influenced their evolution (some of that research is described in Digging Dinosaurs, a book I wrote with James Gorman).

But no one has ever found more than one reasonably complete I! rex at a time. No one has ever found the skeleton of a young T. rex, or a definite T. rex footprint, T. rex nest, or T. rex egg. And we're lucky to have the eleven T. rexes we've got. I and my crew wouldn't have had the chance to dig up the best T. rex then known if a rancher hadn't chanced to find it.

Dinosaurs ruled the earth, the whole earth, for 160 million years. Which brings up the question a lot of people, especially aggravated big city reporters, ask: "Why do we have to come to godforsaken places like eastern Montana to find T. rex?"

Well, I like eastern Montana. And it is true dinosaurs lived everywhere. We've found enough of them in places like the Arctic and Antarctic and many places in between, from New Jersey to Switzerland to Laos, to know that's so.

Rarely do we find the same kinds of dinosaurs in rock formations from two different times. That's because in the intervening millions of years between the originations of these rock deposits, new dinosaur species evolved and old ones died off. We do know more than three hundred kinds OAenera) of dinosaurs now, nearly half discovered in the last twenty years. And the fifty of us who look for dinosaurs—that's all the dinosaur scientists doing fieldwork in the world—find new kinds all the time. There's a new species described every seven weeks on average.

Species of dinosaurs were going extinct all the time over 160 million years. But until the time of T. rex,, 67 to 65 million years ago, new kinds of dinosaurs were always coming along to replace them. T. rex and its contemporaries, duckbills and horned dinosaurs like Trtceratops, went extinct, too. But after T. rex and its neighbors, no dinosaurs came along to replace them.

Another reason it's hard to find dinosaurs, even though they lived everywhere, is that there aren't dinosaurs in all the rocks all over the world. The earth is at least 4.5 billion years old. Dinosaurs may be among the most long-lived and successful animals ever to walk the earth. But they didn't come along until 235 million years ago. To find dinosaurs you first have to find the clay or sand or mud—the sediments long since turned to rock—in which dinosaurs died. In many places, the earth was eroding where and when the dinosaurs died. That ground and the dinosaurs in it long ago turned to dust and blew away. For instance, any dinosaurs that lived in highands will probably never be found, because highland terrain erodes.

Dinosaur fossils were made only in environments where sand or silt built up—floodplains, stream beds, river valleys, and sand dunes. To make a fossil out of a carcass or a single bone, streams and rivers have to be clogged with dirt and debris and flowing slowly enough to be settling out along the way. A good example today is the Mississippi Delta, where the river has slowed down and is dumping out all the sediment that it's carried downriver.

Eastern Montana at T. rex's time, 67 million to 65 million years ago, was on the eastern slope of the Rocky Mountains, which were then building up to the west. The slope was gradual, and there was enough sediment washing and blowing down from the mountains that layers of what would become rock were slowly building up in the lower elevations. Back then, the sediment covered over the bones of dinosaurs that died near the

A RECIPE FOR MAKI NG A FOSSIL. TOP TO BOTTOM:

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