4. Dinosaurs were endotherms

리딩 지문 읽기 (3분)

Endotherms are animals such as modern birds and mammals that keep their body temperatures constant. For instance, humans are endotherms and maintain an internal temperature of 37°C, no matter whether the environment is warm or cold. Because dinosaurs were reptiles, and modern reptiles are not endotherms, it was long assumed that dinosaurs were not endotherms. However, dinosaurs differ in many ways from modem reptiles, and there is now considerable evidence that dinosaurs were, in fact, endotherms.
Polar dinosaurs
One reason for believing that dinosaurs were endotherms is that dinosaur fossils have been discovered in Polar Regions. Only animals that can maintain a temperature well above that of the surrounding environment could be active in such cold climates.
Leg position and movement
There is a connection between endothermy and the position and movement of the legs. The physiology of endothermy allows sustained physical activity, such as running. But running is efficient only if an animal’s legs are positioned underneath its body, not at the body’s side, as they are for crocodiles and many lizards. The legs of all modern endotherms are underneath the body, and so were the legs of dinosaurs. This strongly suggests that dinosaurs were endotherms.
Haversian canals
There is also a connection between endothermy and bone structure. The bones of endotherms usually include structures called Haversian canals. These canals house nerves and blood vessels that allow the living animal to grow quickly, and rapid body growth is in fact a characteristic of endothermy. The presence of Haversian canals in bone is a strong indicator that the animal is an endotherm, and fossilized bones of dinosaurs are usually dense with Haversian canals.


리스닝 듣기



리스닝 스크립트

Many scientists have problems with the arguments you read in the passage. They don’t think those arguments prove that dinosaurs were endotherms.
Take the polar dinosaur argument. When dinosaurs lived, even the Polar Regions, where dinosaur fossils have been found, were much warmer than today, warm enough during part of the year for animals that were not endotherms to live. And during the months when the Polar Regions were cold, the so-called polar dinosaurs could have migrated to warmer areas or hibernated like many modern reptiles do. So the presence of dinosaur fossils in Polar Regions doesn’t prove the dinosaurs were endotherms.
Well, what about the fact that dinosaurs have their legs placed under their bodies, not out to the side like crocodiles. That doesn’t necessarily mean dinosaurs were high-energy endotherms built for running. There is another explanation for having legs under the body. This body structure supports more weight, so with the legs under their bodies, dinosaurs can grow to a very large size. Being large had advantages for dinosaurs, so we don’t need the idea of endothermy and running to explain why dinosaurs evolved to have their legs under their bodies.
Ok, so how about bone structure? Many dinosaur bones do have Haversian canals, that’s true. The dinosaur bones also have growth rings. Growth rings are thickening of the bone that indicates periods of time when the dinosaurs weren’t rapidly growing. These growth rings are evidence that dinosaurs stopped growing or grew more slowly during cooler periods. This pattern of periodic growth, you know, rapid growth followed by no growth or slow growth, and then rapid growth again, is characteristic of animals that are not endotherms. Animals that maintain a constant body temperature year-round as true endotherms do grow rapidly even when the environment becomes cool.



30점을 위한 해설

리딩 주장
Dinosaurs were endotherms that maintained an internal temperature constantly. (공룡은 체온을 일정하게 유지했던 온혈 동물이었다)

첫번째 근거
The discovery of dinosaur fossils in Polar Regions indicates that dinosaurs were endotherms because only endotherms could survive in such cold climates. (극 지방에서 발견된 공룡 화석은 공룡이 온혈 동물이라는 것을 알려준다. 왜냐하면 오직 온혈 동물만이 그러한 추운 기후에서 생존할 수 있기 때문이다)

두번째 근거
The position and the movement of the legs of dinosaurs is evidence that dinosaurs were endotherms because their legs are positioned underneath their body. This is similar to the way the legs are positioned on all modern endotherms in order to sustain physical activity. (공룡 다리의 위치와 움직임은 공룡이 온혈 동물이라는 증거다. 왜냐하면 공룡의 다리는 몸의 밑에 위치되어 있는데 육체적인 활동을 지속하기 위해 현대 온혈 동물의 다리가 위치되어 있는 방식과 같은 것이다)

세번째 근거
Fossilized bones of dinosaurs have Haversian canals that contain nerves and blood vessels, which help animals to grow quickly. Haversian canals are usually a characteristic of endotherms. (공룡의 화석화된 뼈는 혈관과 신경을 포함하고 있는 하버스 관을 가지고 있는데 이 하버스 관은 동물이 빠르게 성장하는 것을 도와주는 것으로 온혈 동물의 특징이다)

리스닝 반박
The arguments mentioned in the reading passage are not convincing. (리딩에서 언급된 주장은 설득력이 없다)

첫번째 근거
Animals that were not endotherms could live in the Polar Regions because the regions were much warmer back then than today. Also, when the temperature of the Polar Regions was cold, dinosaurs could have moved to warmer places or hibernated elsewhere. Therefore, the discovery of dinosaur fossils in the Polar Regions does not necessarily mean the dinosaurs were endotherms. (온혈 동물이 아닌 동물도 극 지방에서 살 수 있었다. 왜냐하면 극 지방은 오늘날보다 더 따뜻했기 때문이며 극 지방의 온도가 추울때는 공룡은 더 따뜻한 지역으로 이동하거나 겨울 잠을 잤기 때문이다. 그러므로 극 지방에서의 공룡 화석 발견이 반드시 공룡이 온혈 동물이라는 것을 의미하는 것은 아니다)

두번째 근거
Considering the size of the dinosaurs, the position and the movement of the legs of dinosaurs were reasonable. The dinosaurs had this body structure because it could support the heavy weight of the dinosaurs and this had nothing to do with whether they are endotherms or not. (공룡의 몸집을 고려하면, 공룡 다리의 위치와 움직임은 논리적이다. 왜냐하면 공룡의 이런 몸 구조는 공룡의 엄청난 몸무게를 지탱하기 위한 것이기 때문이지 공룡이 온혈 동물인지와는 관계가 없다)

세번째 근거
Even though the dinosaurs have Haversian canals, the dinosaur bones have growth rings as well, meaning that dinosaurs stopped growing or grew slowly in cooler climates and they grew rapidly again in warmer climates. Endotherms do not have these characteristics. (비록 공룡이 하버스 관을 가지고 있을지라도 공룡의 뼈는 성장 나이테를 가지고 있다. 즉, 공룡은 더 추운 기후에서는 성장을 멈추거나 느리게 했고 더 따뜻한 기후에서 다시 빠르게 성장했다. 온혈 동물은 이러한 특성을 가지고 있지 않다)