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People used to think that there is no grass before the extinction of dinosaurs.
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听力原文：M: I wish I knew how to swim．
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Animals without high-crowned teeth cannot chew grasses.
Dung Fossils Suggest Dinosaurs Ate Grass
Ancient pieces of plant minerals have offered up the first evidence that dinosaurs ate grass and grasses actually exist earlier than people have imagined, a new study says.
The proof was found in what might seem to be an unlikely location: fossilized dung left by titanosaurs (无法龙).
The coprolites -- the technical, and polite, term for dung fossils -- were found in India and this could date to about 65 million years ago.
Evidence of ancient plants is often found in fossils that contain outlines of easily visible leaves and stems. Such fossils of grasses have been dated to about 55 million years ago but no older. This gives people an impression that there is no grass in existence 55 million years ago.
The plant evidence found in coprolites, however, is based on microscopic bits of minerals that form. in plants. When plants are eaten or decay, the mineral bits are released and pass through an animal's digestive system. And this has become the very precious evidence for the study nowadays. By carefully examining the contents in the dung, the scientists or researcher can have a general idea of what these animals ate millions of years ago and also possibly their living habits.
Researchers were able to examine and date minerals from ancient grasses found in the fossilized dinosaur dung. The scientists will describe the fossils in tomorrow's issue of the journal Science.
Their work "is the first unambiguous evidence that grasses originated and had already diversified during the Cretaceous (白垩纪时代) ," Dolores Piperno and Hans-Dieter Sues of the Smithsonian Institution's National Museum of Natural History wrote in a review accompanying the journal paper.
And we must keep in mind that the Cretaceous period extends from 145.5 to 65.5 million years ago, a period much earlier than 55 million years ago.
Grassy Dino Diets？
The fossils containing the plant minerals were found close to Pisdura in central India and date within the late Cretaceous.
Coprolites are very common in the area and are often found in rocks that have been worn down by weather. Based on their common association with titanosaur bones, many of the dung fossils probably come from the massive plant-eating reptiles(爬虫类).
The finding is the first indication that grasses evolved before the dinosaurs went extinct. This finding is contradictory to the previous thinking that there is no grass existing at the time when dinosaurs lived. Just as what Caroline A. E. Str0mberg, a co-author of the paper and a researcher at the Swedish Museum of Natural History in Stockholm, said that until now "it has been assumed that dinosaurs lived in virtually grass-free ecosystems."
Grasses exist today on every continent except Antarctica. It has become very important in today's life in that not only many animal, but also human beings themselves depend on them for food to a great extend.
Scientists have long believed that the now ubiquitous plants first began to spread and diversify some 70 to 60 million years ago.
However, fossil evidence had suggested that grasses evolved along with early plant-eating mammals. For example, the Hoofed animals with high-crowned teeth suitable for chewing grass first began to appear about 25 million years ago.
But the grass minerals in the Indian coprolites were much older than the hoofed mammals and were at that time already diversified. According to the latest findings by the researchers, five different species were evident, which means that grasses likely diversified substantially before the end of the late Cretaceous.
The researchers believe that various species of grass had spread before India became geographically isolated from other continents about 125 million years ago.
Tooth Ev
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