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Now listen to part of a lecture on the topic you just read about

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Sauropods were plant-eating dinosaurs from the Jurassic and Cretaceous periods notable for their enormous size, the largest species may have reached 50 meters in length and weighed up to 100 tons. It has been suggested that the evolution of gigantism (enormous size) in sauropods was caused by environmental factors. Scientists have proposed several theories identifying environmental factors that may have been responsible.
High Levels of Atmospheric oxygen
One theory suggests that sauropods reached gigantic sizes thanks to very high levels of oxygen in the atmosphere. According to this theory, the atmospheric oxygen content during the Jurassic and Cretaceous periods was significantly higher than it is today. The high oxygen levels led to improved respiration (breathing) in sauropods. Improved respiration allowed sauropods to grow extremely large
Plentiful Food Sources
A second theory holds that the large size of sauropods was the result of the availability of plentiful food sources. During the Jurassic and Cretaceous periods, the climate favored the rapid growth of lush forests. Sauropods had virtually unlimited supplies of plants to feed on. So the theory goes that without the limitations of food resources that normally control body size, sauropods evolved larger and larger bodies to become giants.
Warm Climate
According to a third theory, the main factor that allowed sauropods to evolve large bodies was the very warm climate of the Jurassic and Cretaceous periods. Animals living in warm climates do not need to generate as mush body heat as animals living in cold climates do. Sauropods could therefore use most of the energy they obtained from food to grow and maintain body tissues rather than to generate body heat. This allowed sauropods to grow larger than animals in other periods.

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Actually, the environmental factors mentioned in the passage were probably not responsible for the large size of sorapods.

First, the oxygen theory. This theory assumes that the oxygen levels in the atmosphere during the Jurassic and the cretaceous periods reached as much as 33. 0%, which is much higher than today's levels of about 21%. However, new research shows that this Assumption is not accurate. Specialists have analyzed samples from the Jurassic and the cretaceous periods and concluded that oxygen levels for much of the duration of those periods were actually in the 12 to 20% range. In other words, the Jurassic and cretaceous oxygen levels did not even reach today's 21% levels. Most of the time, we cannot really say those were high levels of oxygen.

Second, it's true that the Jurassic and cretaceous conditions encouraged rapid plant growth, but this faster growth resulted in the plants being less nutritious. In fact, the Jurassic and cretaceous plants were probably less nutritious than plants from other periods that grew at a slower rate. So although there was a greater quantity of plants, this would not have mattered because the plants were of such poor nutritional quality. So in the end, there wasn't a significant improvement in terms of food resources during the Jurassic and cretaceous periods.

Third, the warm climate. Actually, animals living in warm climates face a distinct challenge. They must avoid overheating. This means they need to find ways to lose heat and cool off. Now, losing heat and cooling off is much more difficult for large animals than it is for small animals. So having a large body in a warm climate is actually a disadvantage. So you really could say that sorapods evolve their gigantic bodies despite the warm climate in which they lived, not because of it.
问题

Summarize the points made in the lecture, being sure to explain how they challenge the specific theories presented in the reading passage.

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  • The reading passage discusses three environmental factors that may have contributed to sauropod gigantism: high oxygen levels, plentiful food, and a warm climate. However, the listening passage disputes each theory with contradictory evidence.
    First, while the reading claims that high oxygen levels during the Jurassic and Cretaceous facilitated sauropod respiration and growth, the listening argues that oxygen levels at the time ranged between 12% and 20%, similar to or lower than today’s 21%. This suggests oxygen levels were not a key factor in their gigantism.
    Second, the reading suggests abundant vegetation supported larger sauropod bodies. The listening counters that rapid-growing plants during this era had low nutritional value, making the food supply less advantageous than the reading implies.
    Third, the reading states that the warm climate helped sauropods grow by reducing the need for energy to generate body heat. The listening disputes this, noting that large animals in warm climates often struggle to cool down, making the warm climate more of a challenge than a benefit.

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