Terminated Dinosaur Era Reading Passage
Terminated Dinosaur Era Reading Passage
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A huge meteor impact 65 million years ago may have been the end of the age of dinosaurs. But it may also have been the beginning. Researchers recently found what they think is the first direct geological evidence of a meteor impact 200 million years ago. This event happened at the same time as a mass extinction that killed off half of the major groups of life and opened the way for dinosaurs to evolve.
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Many people have different ideas about why and when dinosaurs became more common. We don't know much about the time between when dinosaurs first appeared and when they became the most common animals on Earth, so we can't say for sure what happened. Dr. Paul E. Olsen, a researcher of earth and environmental sciences at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., said, "There is a geochemical characteristic of something important happening, probably a strange impact from asteroids, quite earlier the time when familiar dinosaur-centric groups appear."
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Olsen and his colleagues looked at fossils of vertebrates from 80 sites in four different ancient rift basins. These basins are part of a chain of rifts that formed as North America began to break away from the supercontinent that existed between 230 and 190 million years ago. Scientists discovered many rare elements of iridium in the outer layer of rock. However, it was found around at the time of the extinction. Iridium is a precious metal that is in the same group as platinum. It is more common in meteorites than in rocks.
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At home, in the 1970s, a similar rise in iridium levels in rocks that were 65 million years old led to the idea that a meteor killed the dinosaurs. For years, people disagreed about that theory, even after it was backed up by other evidence and the impact site was found off the coast of the Yucatan Peninsula. Scientists will have to do the same thing with the new iridium anomaly. The levels are only about 10% as high as they were during the next extinction. That could mean that the meteor was smaller or had less iridium in it, or that it wasn't a meteor at all. Iridium can also come from inside the Earth, when volcanoes erupt. Dr. Michael J. Benton, a senior lecturer of vertebrate paleontology at the Bristol University in England, said that the data are "the first reasonably compelling proof of an iridium spike."
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In a database of 10,000 fossilized footprints found in old lake basins from Virginia to Nova Scotia, the scientists found more proof of a rapid extinction. The tracks of a house cat, for example, look like those of a baby tiger. However, footprints are much more common than fossil bones and can give a more complete picture of the types of animals walking around. Dr. Olsen said, "It makes it very easy for us to see the very clear signs of big changes in wildlife." Since sediment builds up quickly in lake basins, scientists were able to date each footprint by looking at the rock layer where it was found. They found that about 200 million years ago, the animals that walked along what is now the East Coast of North America changed all at once.
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Several large groups of reptiles left tracks that go almost all the way up to the layer of rock that marks the end of the Triassic geologic period, which ended 202 million years ago. After that, the tracks of these reptiles disappear in younger layers from the Jurassic period. Geology professor at the University of Washington, Dr. Peter D. Ward, said, "I think the footprint method is very new and very exciting." He said that the information was "very interesting," but that it needed more research. Scientists led by Dr. Ward reported last year that the types of carbon in rocks changed quickly at this time, which suggests that plants died all at once in less than 50,000 years. The footprint research backs up the idea that the extinction happened all at once.
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Several groups of dinosaurs lived on after the extinction, and their footprints show that new groups appeared soon after. Before the extinction, dinosaurs left about one-fifth of the footprints. After the extinction, they left more than half of the footprints. Researchers said that the changes happened in less than 30,000 years, which is a very short time in geological terms. Scientists think that the asteroid or comet impact and the deaths of Triassic competitors made it possible for a few groups of meat-eating dinosaurs to grow very quickly and take over the top of the food chain on land around the world.
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At the time of the extinction, rauisuchians, which were 15 feet long and had teeth like knives, and phytosaurs, which looked like big crocodiles, were the most dangerous predators. Dinosaurs first appeared about 230 million years ago, but they were small and had to compete in an ecological niche that was already full. Before the end of the dinosaurs 200 million years ago, the biggest ones that ate meat were about the size of large dogs. Not very interesting," Dr. Olsen said. The dinosaurs grew up very quickly. The length of a Jurassic meat eater's foot from toe to heel was on average 20% longer than that of its Triassic ancestor. Scientists think that the dinosaurs got twice as big because their feet got twice as big. This led to the terrifying Velociraptors, Tyrannosaurus rex, and other large meat-eating dinosaurs.
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The speed up in evolution is like how mammals became more common after the dinosaurs died out. During the time of the dinosaurs, mammals were no bigger than small dogs. After the dinosaurs died out, they changed into tigers, elephants, whales, and even humans. Dinosaurs may not have made it through the second extinction because they were so successful after the Triassic-Jurassic extinction. "When disasters happen, small animals always do better. Dr. Olsen said this is because they can live on less food. He also said that scientists now think that the small dinosaurs did make it. He said, "We call them birds."
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