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.
TPO4托福综合写作听力原文文本：
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.
TPO4托福综合写作满分范文：
The professor actually contradicts the statements made in the passage. She is of the view that dinosaurs are not endotherms i.e. they were not able to keep their body temperature at a constant rate.
The professor contradicts the issue of dinosaurs being endothers based on the availability of fossils being available in the polar regions, she say that the polar regions in those days were not as cold as they are today i.e at least warm enough for dinosaurs to live. Durin harsh winters she says that there is a possibility of the dinosaurs actually migrating to warmer regions.
The issue of leg position and movement being used as a reason to clasify the dinosaurs as endotherms does not please the professor either. She says that dinosaurs had legs under their bodies to support their huge bodies i.e the legs under the body of the dinosaur were actually to support the huge weight of the dinosaur and not to provide it with a body structure like endotherms(which is actually suited for running).
The professor acknowledges the presence of haversian canals but also points out that that the fossils show the presence of growth rings. These rings occur due to the thickening of the bone. The thickening indicates that the dinosaurs were’nt actually growing continuously but were experiencing periods of rapid growth and periods of no growth in succesion. This pattern. she says is characteristic of non endothermic aanimals.Thus it can be inferred that the professor challenges the passage by giving reasons as to why she thinks that the dinosaur is not an endotherm.
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