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  3. Dung Fossils Suggest Dinosaurs Ate Grass Ancient pieces of plant minerals have offered up the first evidence that dinosaurs

Dung Fossils Suggest Dinosaurs Ate Grass Ancient pieces of plant minerals have offered up the first evidence that dinosaurs

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问题                         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 Evolution
    Along with the grass minerals, the coprolites were also found to contain evidence of other plants, including broad-leaved flowering plants, palms, and conifers (针叶树), etc. , which all showed the diversity of plants at that time.
    However, the grasses only formed a relatively low proportion of the total plant material that was found in the coprolites, which was an indication that they did not form the major part of the titanosaurs’ diet.
    Still, the inclusion of grass in the dinosaurs’ diets might mean that widespread occurrence of grasses had contributed to the titanosaurs’ success of existence, said Kenneth Carpenter, a curator at the Denver Museum of Nature and Science.
    "Cropping low vegetation, such as grass, may explain why some of these titanosaurs have very wide mouths reminiscent of the wide mouth of the grass-feeding hippo and white rhino," he said.
    Mammals called gondwanatherians also lived during the late Cretaceous and might have been more dependent on grasses for food than any other animals.
    The early mammals had high-crowned teeth, which puzzled paleontologists, because although high-crowned teeth would facilitate the eating of grass, it was thought queer for grasses were still quite rare at this time.
    Some researchers felt the teeth must have evolved for digging or gnawing on wood instead of chewing grasses.
    "Our study shows that grasses existed in India simultaneously with the.., gondwanatherians" Stromberg said.
    "These remarkable results will force reconsideration of many long-standing assumptions about grass evolution, dinosaurian ecology, and early plant-herbivore interactions," Piperno and Sues wrote in their review.
    But, she added, "I think it is fair to say that the classic hypothesis is still that hoofed mammals evolved high-crowned teeth during the Tertiary(第三纪) ,primarily in response to the spread of grass lands."
The paleontologists were puzzled by the fact that the early mammals had high-crowned teeth be cause the grasses at that time were ______.
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答案rare
解析 阅读题干可以发现这是有关牙齿的内容，所以定位在第二个小标题下。通过阅读发现这一标题下的第六段有it was thought queer for grasses were still quite rare at this time.。
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