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updated: 10/25/2011 7:49 AM

The story about dinosaurs is in fossils, rocks

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  • The Tyrannosaurus rex "Sue."

    The Tyrannosaurus rex "Sue."
    Daily Herald file photo, 2004


You wanted to know

"Why did dinosaurs become extinct?" asked students in Katherine Crawford's fifth-grade at West Oak Middle School in Mundelein.

If you had to answer a tough question, you might not rush out and grab a rock. But when trying to solve the puzzle of how dinosaurs became extinct, rocks hold the clues that experts use to decode the mysteries of the very distant past.

The story of dinosaurs goes way back. Fossils dating to 230 million years ago hold information about the very first dinosaurs; older dinosaur remains might exist but they haven't yet been discovered.

Fossils with dinosaur bones, eggs and footprints show these colossal beings existed during the Triassic, Jurassic and Cretaceous time periods.

Giant meat- and plant-eaters ruled the land and the seas for about 160 million years. Then, they didn't. While almost all dinosaurs failed to survive the catastrophic event that wiped them out, one dinosaur relative still exists -- birds.

"All birds that have ever lived were and are dinosaurs," said Bill Simpson, Field Museum collections manager, fossil vertebrates. "There are still about three times as many dinosaurs alive as mammals."

How could the majority of those gigantic reptilelike creatures have been wiped off the face of the Earth? Many types of scientists are working on this puzzle, among them are paleontologists, archeologists, astrophysicists, astronomers, geologists, ecologists and geochemists.

"Extinction is very hard to analyze," Simpson said. "The extinction event in this case is the Cretaceous-Tertiary extinction. The prevailing theory is that a large extraterrestrial body, like an asteroid, struck Earth, perhaps in the area of the Yucatán Peninsula, and threw up so much dust that a nuclear winter scenario began.

"The dust shut out sunlight, causing a broad extinction of plants followed by herbivores, and carnivores. Thus, the food chain collapsed. "

Other theories about the die-off include the possibility there was of a huge volcanic explosion, as well as a hypothesis that shifting tectonic plates could have triggered an avalanche of cataclysmic changes that resulted in a mass dinosaur and plant extinction.

Scientists have noted other mass extinctions. A catastrophic event occurred about 550 million to 250 million years ago that obliterated as much as 90 percent of the Earth's flora and fauna.

Learn more about dinosaurs and evolution at the Chicago Field Museum's Evolving Planet exhibit. See the Field Museum website for exhibit and special event information at

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