Dinosaurs refer to a large group of organisms belonging to the "general order of dinosaurs", the earliest dinosaurs appeared in the Triassic 230 million years ago, and almost disappeared 65 million years ago due to a huge catastrophe of an asteroid hitting the earth. Probably the most well-known dinosaur is the Tyrannosaurus rex that appeared in the movie Jurassic Park.
△ Dinosaurs in Jurassic Park
But a question for everyone: Were dinosaurs cold-blooded? I'm afraid that most people will say without thinking after hearing this question, of course! And the reason is also very simple, dinosaurs are a kind of lizard, since lizards are cold-blooded animals, then dinosaurs should be too.
Dinosaurs were originally thought to be very similar to lizards
The Latin meaning of the English word "dinosaur" is "scary lizard", which shows that scientists did not hesitate to think that this animal was a kind of lizard after discovering dinosaur fossils, so until the 1970s people believed that dinosaurs should be cold-blooded animals.
But dinosaurs were not a type of lizard, and in fact, dinosaurs may be more closely related to today's ostriches. So wishful thinking that dinosaurs were cold-blooded animals is too arbitrary, and now more and more evidence makes people have to believe that things are not so simple: dinosaurs are most likely not a cold-blooded animal.
In fact, the term "cold-blooded animals" is less accurate, and it is more accurate to say that it should be "warm animals". This animal exists in relation to thermostatic animal.
For example, we humans are a thermostatic animal, when the external environment is reduced, we will generate more heat by accelerating metabolism to help us maintain body temperature; and when the external environment is tall, we will dissipate heat and reduce body temperature by sweating - in short, our body temperature will be maintained at about 36.5 ° C.
The body of the warming animal does not have this temperature regulation mechanism, and their body temperature can only change according to changes in the environment. Therefore, the warm animals cannot move when the weather is too hot and too cold. For example, the lizard just mentioned is a very typical warming animal, which needs to hibernate to cope with cold weather, and at noon in the summer, they also need to hide in a cool place and stay motionless to prevent overheating. The picture below is a lizard with almost the same ambient temperature, and the human body temperature is different from the ambient temperature in comparison.
△ The lizard is almost the same as the ambient temperature△
So was the extinct dinosaur a thermostatic animal or a thermostatic animal? Dinosaurs are extinct, so it is impossible to observe dinosaurs directly, and we need to look for clues from fossils.
In the early part of the last century, there was almost no doubt that dinosaurs were a temperate animal – partly because the idea that dinosaurs were a lizard was deeply rooted, and partly because of the lack of archaeological discoveries. So for a long time, people thought that dinosaurs were slow-moving warming animals.
And some people have come up with evidence to prove that dinosaurs are a kind of variable temperature animals, for example, as early as the eighties of the last century, a Canadian scholar Paul Johnstown proposed after studying a large number of dinosaur skeleton fossils, dinosaur bones with obvious "annual ring" characteristics, which shows that dinosaurs are very sensitive to hot and cold in the process of growth, and there are such skeletal characteristics between spring and autumn, just like the annual rings of trees.
△ Dinosaur skeleton with annual rings △
Correspondingly, most of today's temperate animals also have the characteristics of this bone. For example, crocodile teeth will also have this "annual ring" after cutting, so people think that dinosaurs must be cold-blooded animals.
The first scholar to suggest that dinosaurs were thermostatic animals was paleontologist Buck, who first proposed this hypothesis in 1968 and caused an instant stir in the biological community, because people had never made such a hypothesis.
Buck's view began with questioning the idea that dinosaurs were a lizard (reptile). He believes that the limbs of dinosaurs are located directly below the torso, which is obviously very different from reptiles. Bodies like dinosaurs show that they have the ability to move faster than reptiles, and they don't need to cling to the ground and hide themselves like reptiles. So dinosaurs should have had a higher body metabolic rate, and they were most likely a thermostatic animal with a thermoregulatory mechanism.
△ The hip joint of the crocodile (left) and the dinosaur (right).
In addition to such rough "evidence", some scientists have studied the lines on dinosaur skeleton fossils, and found that these lines indicate that the microvessels of dinosaurs are very dense, which is obviously evidence of a relatively fast metabolic rate; there are also scientists who have studied the proportion of predators and predators in the dinosaur group, and found that the proportion of predators is much higher than that of predators, which shows that these predators need more prey to feed themselves, which is also a manifestation of high metabolism, and so on, and so on.
Even the rings on dinosaur bones that have been used as evidence to prove that dinosaurs were thermostatic animals have been used in turn to prove that dinosaurs are thermostatic animals. Because there is a large amount of evidence that this annual ring structure also appears in the bones of thermostatic animals, and compared with the bones of lizards and crocodiles, the annual rings of the thermostatic animals are more similar to the annual rings of dinosaurs, which indicates that dinosaurs are a thermostatic animal with a high metabolic rate.
△ Mammalian bones also have annual rings
Some scientists will even suggest that the sail-like structure behind the spinosaurus body is used to regulate the temperature of the body, as if carrying a huge heat sink.
△ The sail-like structure behind Apatosaurus
Although there is a lot of evidence that dinosaurs have a high metabolic rate that does not conform to thermostatic animals, compared with real thermostatic animals, dinosaurs do not seem to resemble thermostatic animals, for example, their growth rate is lower than that of thermostatic animals.
So another scientist put forward a view that dinosaurs are neither thermostatic animals nor thermostatic animals, but a magical "medium temperature animal". Although this animal can obtain heat through the metabolism in the body, their metabolic mechanism is not so perfect, resulting in their body temperature is not stable.
This animal has a lower body temperature in its infancy, and then increases its body temperature after maturity, such as tuna and leatherback turtles. This is because as animals grow up, the heat dissipation area on their body surface rapidly decreases in proportion to their own volume, which leads to this strange phenomenon.
As shown in the chart below, it is a paper published in the Journal of Science in 2014 in which the authors made this unique point by comparing the growth rates of different animals. In the picture, the blue animal is a thermotropic animal, the black animal is a medium temperature animal, and the red animal is a thermostatic animal.
△ Unique properties of different creatures △
Therefore, the question of what kind of creature the dinosaur was is not a very simple answer now. As new evidence arrives, perhaps there will be a new understanding of the temperature of dinosaur blood.
There is plenty of evidence that dinosaurs were not a cold-blooded animal (a thermostatic animal), but there are some differences between this creature compared to the thermostatic animals we see now. But with the extinction of this creature, the truth of the matter may require advances in technology and more fossil evidence to be gradually discovered.
In short, it's a magical world, and we probably wouldn't believe at first that this creature, known as the "scary lizard," wasn't actually a cold-blooded killer.