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  3. Look at the Size of Those Things! Sauropods(蜥脚龙)unprecedented bulk has long posed a thorny problem for biologists. How did t

Look at the Size of Those Things! Sauropods(蜥脚龙)unprecedented bulk has long posed a thorny problem for biologists. How did t

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问题                         Look at the Size of Those Things!
    Sauropods(蜥脚龙)unprecedented bulk has long posed a thorny problem for biologists. How did they get to be so big? Why have no other land animals reached such massive proportions before or since? There have not been convincing answers to these questions. Until now.
    "We now have a coherent theory on how dinosaur gigantism evolved," says Martin Sander, a paleontologist at the University of Bonn in Germany. For six years, Sander has headed an international team of scientists put together to tackle the gigantism mystery. It turns out that sauropods had a unique set of biological features that combined to propel them to unrivalled sizes.
Bigger is better
    Sander’s starting point was observations made by the 19th-century paleontologist(古生物学家)Edward Drinker Cope, who noticed that animal lineages(血统)tend to get bigger over evolutionary time, starting out small and leaving ever bigger descendants. This process came to be known as Cope’s rule.
    Getting bigger has evolutionary advantages, explains David Hone, an expert on Cope’s rule at the Institute of Vertebrate Paleontology and Paleoanthropology. " You are harder to predict and it is easier for you to fight off competitors for food or for mates. But we also know that big animals are generally more vulnerable to extinction," he says. Larger animals eat more and breed more slowly than smaller ones, so their problems are greater when times are tough and food is scarce. So on one hand natural selection encourages animals to grow larger, but on the other it eventually punishes them for doing so.
    This balance between opposing forces has prevented most land animals from exceeding about 10 tonnes.
Small fry
    What struck Sander was the size imbalance between adult sauropods and their small eggs and clutch sizes. The nest sites also reveal no sign of parental care, further increasing the adults’ ability to produce lots of offspring.
    Egg laying and a lack of parental care, however, cannot be the whole story as all dinosaurs laid eggs and few cared for their young. So Sander looked elsewhere in search of further pieces of the puzzle.
    To understand dinosaur growth rates, thin sections of their bones are examined under microscopes. Most dinosaurs have growth lines in their bones, indicating the fitful growth typical of animals with a slow metabolism(新陈代谢). Sauropod bones, in contrast, have a pattern of continuous growth similar to that seen in mammals and birds. Sander concludes that sauropods had a fast metabolism, which enabled them to attain immense sizes relatively quickly. Research by his team on a 30-tonne Asian sauropod called mamenchisaurus(马门溪龙)shows how this rapid growth translated into astonishing weight gains. At its peak, it grew up to 2 tonnes a year. In comparison, an African elephant gains at most 200 kilograms in a year.
    Fast growth is all well and good, but once an animal reaches an immense size, how does it deal with the demands of its body and its lifestyle? Sauropods all conformed to the same basic body plan: a long neck terminating with a small head, a huge barrel-like body and, inevitably, thick sturdy legs. Sander and others now argue that the creatures’ unique structural combination—inside and out—was key to its sizeable success.
    In the 1980s, Jyrki Hokkanen of the University of Helsinki in Finland tackled one part of this problem—how to support and move a massive body. By analysing bone and muscle strength in large animals, he concluded that even the largest sauropods were nowhere near the theoretical upper limit for body size. "Brachiosaurus could have been at least a couple of times bigger and still have walked on land," he concluded. So, while a large sauropod would have been heavy, that in itself did not inhibit its size.
Bird-like lungs
    A related problem is how to get enough oxygen. In 2003, Mathew Wedel of the Sam Noble Oklahoma Museum of Natural History solved this by showing that sauropods had bird-like lungs.
    Birds breathe in a far more efficient way than mammals. When they inhale, air fills their lungs and also air sacs(气囊)further inside their body. Upon exhaling, fresh air from the air sacs flows out and replaces the air that was in the lungs. This means that the lungs contain a constant stream of fresh air and can extract up to two-and-a-half times as much oxygen per breath as a mammal.
    Bird-like breathing would have helped to support a large size in a variety of ways. First, it solves the problem of getting enough oxygen. Secondly, the air sacs were located in, greatly reducing their weight. Finally, breathing like a bird would solve another problem; how the sauropods stopped themselves from overheating. A high metabolism coupled with a huge body, with its low surface area-to-volume ratio, would normally spell trouble. "Big sauropods could probably breathe to cool themselves off," says Wedel.
Anatomy(解剖学)
    Anatomy also explains how an 80-tonne animal could obtain enough to eat. The largest land animals today are all vegetarians that survive by eating huge amounts of plant material of poor nutritional quality. This is because there is not enough higher-quality food such as fruits and seeds to sustain a large animal, but grasses, leaves and branches are much more abundant. It is assumed that this is true for the extinct giants, too.
    Large sauropods probably needed to eat a tonne of vegetation a day, so how did they manage it? Sander sees the crane-like neck and small head as being the key.
    The lightweight vertebrae(脊椎)allowed their necks to grow longer, which would have increased their feeding range, both side to side and up and down. This would have allowed them to stand still while their necks did all the work, helping to conserve energy.
    What is more, instead of chewing their food, sauropods used their simple peg-like teeth to pluck leaves and branches from plants before swallowing them whole. This allowed them to cram in much more food per day than if they had spent time chewing. It also meant they had no need of heavy grinding teeth and the elaborate musculature(肌肉组织)that goes with them, reducing the mass of their heads and allowing their necks to grow even longer.
    The nutrients from this huge unchewed meal would have been extracted by lengthy microbial fermentation inside their huge bodies. That, however, posed yet another problem. As flowering plants did not evolve until late in the sauropods’ reign, their diet was limited to plants. According to animal nutritionist Jurgen Hummel at the University of Bonn, it is commonly believed that such food is of exceptionally low nutritional quality. How did the sauropods manage to survive on this restricted diet?
    Hummel set about trying to find out. In 2008, he simulated dinosaur digestion by placing samples of these primitive plants among the gut microbes of sheep. It turns out that many of the plants were more nutritious than they had been given credit for. " When you give the ancient plants enough time, they are digested quite reasonably. A long retention time in the digestive tract of a sauropod would have been the solution," he says.
    With their unique combination of reproduction, growth and anatomy, sauropods were able to overcome the limits on body size that have constrained all other land animals, and it was a hugely successful design. The giant sauropods were a fixture of the dinosaur age, persisting for 145 million years.
According to Hummel, ancient plants were digested quite reasonably with______ in the digestive tract of a sauropod.
选项
答案a long retention time
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