These Wildebeest And Zebra Do


But I do most of my looking in western Montana, the land I grew up in and know best. If I dig a dinosaur someplace else, like the T. rex in eastern Montana, it's because someone else found it and asked me to dig it up. That's just how it was with the T. rex we dug up.

The next question people ask me is usually, "How come T. rex is found only around Montana and South Dakota?" Most, but not all, of the few T. rexes we know do come from eastern Montana and western South Dakota. The answer to that one has to do with both the location and the age of the rocks. Dinosaurs lived all over the world, but not the same dinosaurs. T. rex seems to have made its home in western North America—we've found its remains in Alberta, Canada, as well as Montana, Wyoming, Colorado, and South Dakota. So far, we haven't found the same animal on any other continents. But there is a nearly identical creature, named Tarbosaurus, which lived near to T. rex's time in Mongolia. That's not so surprising because close Asian cousins to a lot of the dinosaurs of western North America lived at the end of dinosaur days. The two continents were joined from time to time in this period, and populations of dinosaurs could spread across the Arctic to new territories.

The other reason we can find T. rex in eastern Montana has to do with time. Each rock formation we explore for dinosaurs is a window into one specific relatively brief period of time. In the Hell Creek Formation that time is a few million years, ending 65 million years ago, the time of T. rex.

We know the age of those rocks better than almost any others from dinosaur time. I can't figure the age of rocks precisely. I'm a paleontologist—I study fossils. But some geologists and physicists have been able to date those rocks. To get a good "absolute" date for the age of rock you need a certain kind of rock. The kind that is most likely to contain dinosaurs is sedimentary rock. However, for geochemical dating you need rock formed suddenly by volcanoes, at one single time. That's igneous rock.

In the Hell Creek Formation we're lucky to have not just sedimentary rock but igneous rock, in the form of ash beds from volcanic eruptions. In that ash are tiny, but detectable, radioactive isotopes of elements. Over millions of years some of those elements decay into other elements, at a constant rate. For instance, radioactive potassium eventually breaks down into argon, an inert gas. Geochemists know the rate of that change. If they measure the amount of potassium in the volcanic ash versus the amount of argon, they can tell, give or take a hundred thousand years, how much the composition of isotopes has changed. When we know how much change has occurred in how long a time, we can figure when that rock and the animals in it were made. We call this radiometric dating.

Scientists know the age of many forms of extinct life precisely, no matter what some creationists say. The idea that scientific dating methods could be off by billions of years, and that humans ran around with dinosaurs, is not true. But that hasn't stopped many


people from believing those things. Just the same, it's important to keep our information straight and not confuse the fact of evolution with ideas from biblical authors and interpreters.

Though it isn't full of errors, radiometric dating of rocks is a difficult and time-consuming business. You don't just run around and say, "OK, I've found some really nice ash beds here. I'm gonna take home a piece of rock and date it."

Usually, good chemical dating isn't available to us, and we rely on other less precise, but still pretty useful indications for the age of fossils. Paleontologists have compiled a pretty thorough collection of small mammals, pollen spores, and tiny marine creatures from various dinosaur times. Find any of these near a layer of rock with dinosaur bones in it, and you can get a pretty good fix on the age of the rock. And dinosaurs themselves can help date the rocks. We know something of which dinosaurs lived at which time. So if you find T. rex and Triceratops bones, it's a pretty strong indication the rock they are in is 65 to 67 million years old.

If I wanted to find a dinosaur bone of a certain age, I would first look on a geological map for sediments of that age, then go to that place and hope it is all busted up into badlands that make it easy to follow the age of one layer of sediment. I'm relying on data from geologists, who have mapped this land pretty thoroughly. And geologists know about changes in the terrain, like the upthrust of mountains, and when they happened. I've learned a bit of geology myself, enough to figure out what the local environment looked like in dinosaur times (see Chapter 4).

For instance, the yellowish rock is sandstone left by streams, and the grayish rock is mudstone or siltstone made on the floodplain when the streams flooded. That is simple enough, and you find fossils in both stream channel and floodplain environments. What is harder is reading the breaks in the stripes of sediment where the ancient stream channels ran, and finding one layer of time across the crumbled and jumbled layercake of the parched badlands of eastern Montana.

But none of this information can give you a good idea of what time you are in when you walk through the badlands. Channel and floodplain deposits like those you walk in here look pretty much the same, no matter how old they are.

So, knowing all this, how come we have only eleven incomplete skeletons to show for a century of digging? With good people looking and lots of information about the time and place of T. rex., you would think we might find more of it. This was, after all, a huge animal, and its kind seems to have survived for several million years across the West.

Truth is, dinosaur skeletons are pretty scarce, period. We've got only about 2,100 of them in all the world's museums. A lot of dinosaurs we know only from a single tooth or chip of bone. It takes special conditions to make a fossil, andalotoflucktofindit. Evenifyou've found the rock of the right age, with a T. rex fossil in it, you've got to come on it at the right time. Rocks, and the fossils in them, erode fast in the badlands. One season a bone is freshly exposed. The next it may be dust, "exploded" as we say. The best fossil specimens, like our T. rex, are those where just a tip of bone is exposed. Then you can dig down a few feet and get out the rest of it before the elements get at it. And it was lucky that our T. rex was in sandstone, not mudstone. Mudstone has a lot of volcanic ash in it, stuff called bentonite. Bentonite is used to plug up dams, because it swells up in water. In the badlands, when bentonite gets wet it swells up just the same way. Then it shrinks

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