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    Last American Dinosaurs

Last American Dinosaurs

One of the largest and best exhibits on the second floor is a collection of dinosaur fossils from the Hell Creek Formation in the United States’ upper Midwest and Rocky Mountains. Here you’ll find a 30-foot, 3-horned Triceratops skeleton, and a 40-foot Tyrannosaurus Rex. The great thing about these dinosaurs is that they’re standing at ground level, easy for kids to see, and without any plastic or glass case around them. You can’t touch them, but you’ll be close enough to appreciate the size of these animals. (There are dinosaur fossils you can touch nearby, including an edmontosaurus annectens, a 30-foot long, duck-billed plant-eater.) Besides these headliner dinosaurs, fossils from many others are shown around the gallery, including raptors and fish.

The most important part of the Dinosaurs gallery is the explanation of how these animals went extinct. Until 1980, most scientists thought that a long period of worldwide volcanic activity around 65 million years ago, threw enough ash and lethal gasses into the atmosphere to kill off most plants and, eventually, the animals that fed on them.

In 1980, the father-and-son team of Luis (a physicist) and Walter Alvarez (a geologist) published a paper observing that rock formations around the world all include a thin layer of clay containing ash, shocked quartz crystals, tiny diamonds, and unusual quantities of minerals such as iridium, typically found on asteroids. The museum has on display several core samples, obtained by drilling deep into the earth, clearly showing this thin, colored layer of clay between two ordinary-looking layers of rock.

The Alvarez’s theory was that a huge asteroid crashed into earth 66 million years ago, sending up dust and debris around the globe, blocking sunlight, and then killing most plants and their consumers up the food chain.

It was a controversial theory that was not initially accepted by most geologists, who pointed out that an asteroid big enough to do that kind of damage would have left an enormous impact crater, which the Alvarez team had not found. However, two geologists using radar to look for oil deposits in Mexico later realized that such a crater did exist, in the ocean off the coast of the Yucatan peninsula. That crater, more than 110 miles wide and 12 feet deep, has been dated to exactly the same age as the worldwide clay ring with the iridium deposits. While some scientists today believe that volcanos and climate change helped kill off the dinosaurs, the “impact hypothesis” is the generally accepted cause of the dinosaur’s extinction.

The asteroid impact killed off most of the species alive at the time. When the proverbial dust settled, new life forms began to emerge and fill in the habitable parts of the earth. Among these were mammals and birds, which make up a large part of the earth’s ecosystems today.

At the end of the Last American Dinosaurs section is a small hallway with an exhibit titled Backyard Dinosaurs. It features a map showing where in the United States you’d find rocks of the right age to contain dinosaur fossils. There’s also a diorama showing what the area around Washington D.C. might have looked like during the Cretaceous period, with fossil evidence of ferns, plants, snails, turtles, and Strom Thurmond living nearby.

Other Lands at Smithsonian National Museum of Natural History