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.
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.
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.
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.
Main points: Evidence that dinosaurs were endotherms.
Sub point 1: Polar dinosaurs found.
Sub point 2: Leg position suggests endotherms.
Sub point 3: Bone structure indicates endothermy.
Attitude: Arguments. (Can’t prove endotherms.)
Sub Point 1: Polar regions were warmer. (Migrate or hibernate.)
Sub Point 2: Legs just for support more weight.
Sub Point 3: Growth rings. (Periodic growth.)
The professor actually contradicts the statements made in the reading passage. She is of the view that dinosaurs were not endotherms, or that they were not able to keep their body temperatures at a constant rate.
The professor disputes the issue of dinosaurs being endotherms. Based on the limited availability of fossils from the polar regions, she asserts that these regions were not as cold as they are today, and could sustain dinosaur life during warmer seasons. During harsh winters she believes that there is a possibility that the dinosaurs actually migrated to warmer regions.
The issue of leg position and movement being used as a reason to classify the dinosaurs as endotherms does not satisfy the professor either. She says that dinosaurs had legs under their bodies to support their massive weights, and not to provide it with a body structure like other endothermic creatures, and onemore suited to 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 are due to the thickening of a developing bone. The thickening indicates that the dinosaurs weren’t actually growing continuously but were experiencing periods of both rapid growth and no growth in succession. She says this type of bone growth is characteristic of non-endothermic animals.
Thus, it can be inferred that the professor challenges the passage by giving reasons supporting her position that the dinosaur is not an endotherm.
1. The professor disputes the issue of…
2. Acknowledge： 承认
We acknowledge the need of revolution.