For years scientists have argued about what exactly caused the extinct

For years scientists have argued about what exactly caused the extinction of the dinosaurs. A key to this question is dinosaur physiology. One topic of hot debate among paleontologists is thermoregulation or control of body temperature. All animals exhibit some form of thermoregulation through migration, perspiration, shivering, or hibernation; yet they fall into two distinct groups. Ectothermy is a form of thermoregulation, which requires that the animal regulate its temperature through behavior and autonomic, external environment. Reptiles and amphibians are ectotherms, which mean they gain heat from an outside energy source. They require little food, as their metabolic rate is low, however, ectotherms cannot withstand extreme cold. On the other hand, mammals and birds, are endothermic. They have a tachymetabolic (high-speed) metabolism, which produces body heat internally.

Endothermy is a highly effective but expensive state, as the elevated metabolic rate requires a high caloric intake each day. Because dinosaurs are extinct, their physiology cannot be measured by usual scientific method (temperature measurement, record of food consumption, and output of carbon dioxide and solid waste). Fossils initially indicated to early scientists that dinosaurs were ectotherms, because of their external physical similarity to modern-day reptiles (jawbones, scales, etc.). However, in recent years, scientists have reversed their preliminary findings as modern indirect evidence seems to suggest that dinosaurs were endotherms.

The debate is still open. Two dinosaur clans were herbivores with such developed an intricate dental equipment that scientists assume they must have been tachymetabolic, or endothermic. However, other dinosaurs lack those evolutions. Bipedal dinosaurs are thought to have been very fleet, suggesting endothermic behavior. Also leading scientists to that conclusion were the talons on their feet; balancing must have required a high degree of agility and activity consistent with endothermic behavior. Another compelling form of evidence is the fact that most dinosaurs stood upright, which means that their hearts had to be sophisticated and powerful enough to pump blood to their elevated brains. Scientists believe that dinosaurs had to possess the double-pump heart found in other endotherms. However, the fossil record is incomplete; scientists are basing their assumptions on partial evidence.

Scientists can draw no definitive conclusions. Just because dinosaurs had the ability to be endothermic, doesn't mean they were, at least, not exclusively. Modern technology has made possible the operation of cars, houses, appliances, etc. by solar energy; still, society has not given up its use of electricity, gas and oil. Current speculation among scientists places dinosaurs somewhere in between both extremes, a hypothesis which could lead to a new chapter in the story of evolutionary theory.

1. This passage was most likely excerpted from:

A. a paleontologist's science journal
B. an article on the methods involved in the study of fossils
C. a researcher's guide to the importance of not drawing conclusions from insufficient evidence
D. a summary of the debate surrounding dinosaurs' thermoregulation system
E. an explanation of the new advances in the theory of evolution


2. The main idea of the passage is:

A. to open a new direction for evolutionary study
B. to demonstrate that there is no clear answer to the question of dinosaur physiology
C. to suggest that there are large gaps in the fossil record
D. to examine the reasons that dinosaurs are extinct
E. to prove that dinosaurs are endothermic


3. According to the information in the passage, scientists base their theories on the thermoregulation of dinosaurs on:

A. conventional methods of inquiry into physiology
B. detailed investigations of dinosaur genetics
C. examination of records of consumption, waste and temperature fluctuation
D. advances in the area of physiological research
E. investigation of fossilized remains