Why were dinosaurs so huge?

 　　When it comes to dinosaurs, the first thing that comes to mind for most people is "giant". So why did dinosaurs have such a huge size? Why have land mammals never reached such a scale? What limits the limitless growth of animals? How did dinosaurs push the limits of their size? How did dinosaurs support their huge bodies?

what limits the limitless growth of animals

　　Back in the 19th century, British paleontologist Edward Kopp noticed a phenomenon: animals of a species tend to get bigger and bigger as they evolve. This phenomenon came to be known as Kopp's Law. Evolutionarily speaking, the larger the individual, the more difficult it is to be invaded by predators, and the easier it is to defeat opponents in the search for food and mates. But it also brings some disadvantages: large animals are more prone to extinction, because large animals have a larger appetite and slower reproduction than small animals, so in an era of food shortages, the probability of extinction is higher. In addition, large animals create other problems. For example, how to support a huge weight? How to get enough food and oxygen? How to prevent the body from overheating? On the one hand, natural selection encourages animals to grow bigger, but on the other hand, they eventually pay for it, and these two forces mutually restrain each other, so that most terrestrial animals generally weigh no more than 10 tons. But it is incredible that the challenges posed by the above disadvantages were all overcome by sauropods.

How dinosaurs pushed the limits of size

　　Environmental factors

　　In fact , the birth of giant creatures such as dinosaurs is the result of a combination of factors. Of course, the environmental factors of the earth account for a large part of the reason. First, during most of the age of the dinosaur family, the oxygen content of Earth's atmosphere was relatively high. Scientists believe that the oxygen content in the Earth's atmosphere during the Cretaceous period should have exceeded 30%, which is 1.5 times the oxygen content in the atmosphere today. In such an oxygen-rich environment, animals and plants grow faster and grow very large. This is more obvious in small animals such as mosquitoes, flies, dragonflies, moths, butterflies, etc., which used to be 2 to 3 times the size of the same species now, and some are even more than 10 times larger than today. Secondly, the climate and ecological environment at that time were better, and the living habitats of animals on the earth were much larger than now. At that time, the climate on earth was relatively warm and humid, and the plants grew lush and lush, and the abundant rainfall and light conditions allowed the plants to grow very large. Because of the abundance of food, animals are not at risk of nutritional deficiencies, so they can also grow large.

　　Dinosaurs Had Efficient Respiratory

　　Systems Sauropods had a respiratory system similar to that of birds. Birds breathe more efficiently than mammals because they not only have lungs, but also air sacs. When they inhale, air fills both their lungs and the air sacs in their bodies. During exhalation, although the air in the lungs flows out, fresh air from the air sacs enters the lungs at the same time (humans have no fresh air in their lungs when they exhale), so their lungs are always full of fresh air. It is calculated that in one breath, the oxygen that the capillaries in the lungs of birds can inhale is 2.5 times that of mammals with the same lung capacity. There is evidence that sauropods also had air sacs like those of birds. There are also many ways in which this style of breathing can help support a large body. First, it solves the problem of how to get enough oxygen; second, the air sacs are located near the vertebrae of the body, like a soft sponge, which greatly reduces the pressure on the vertebrae; third, in order to adapt to this breathing style, sauropods Dinosaur-like vertebrae also have many hollow air cavities, which reduce the weight of the vertebrae; finally, this breathing method can also facilitate heat dissipation through panting, thus solving the problem of heat dissipation.

　　Dinosaurs have unique ways of digesting

　　There is a hidden "ceiling" of size that limits the growth of the animal, which is how digestion works. The ability to eat enough in a limited time is an important factor in determining the size of an animal. An elephant needs to spend 18 hours a day eating non-stop, and can eat about 200 kilograms of plants a day. How did large sauropods eat about a ton of plants a day? Sauropod dinosaurs can be said to have pushed the limits of chewing, because they eat without chewing at all. Sauropods did not have a developed chewing system, nor a stomach grinding (gastric stone plus peristalsis) system, and they could even pile food from the end of the digestive tract all the way to the neck. The vertebrae of sauropods were hollow and light-weight, which allowed them to grow longer necks. As a result, they have a wider range of food up and down, left and right. They can stand still and eat only by twisting their necks, which also reduces energy loss. And when they eat, they don't chew, but use their teeth to collect leaves and branches, fill their mouths, and then swallow them together. Doing so allows them to pack more food into their stomachs each day than they would spend time chewing. Sauropod dinosaurs had a "fermenter" in their digestive system, which gave bacteria plenty of time to fully break down the nutrients of the plants they tucked in.

　　Dinosaurs reproduce differently from mammals

　　Another "ceiling" that limits the limitless growth of animals is the viviparous system of mammals. Giving birth in large mammals takes a lot of time and effort. In general, the bigger the animal, the bigger the fetus; the longer the fetus is bred in the mother's body, the longer the entire reproductive process will be, and the longer the reproductive cycle will lead to their rapid extinction. Dinosaurs, however, avoided this problem entirely. Large dinosaurs did not produce large cubs, but relatively small eggs. This method of producing offspring has an advantage: the fetus develops outside the mother, unburdening the pregnancy and thus removing the growth restriction. We know that the larger a mammal is, the fewer pups it produces and the longer the interval between pregnancies. However, because of oviparity, large dinosaurs can continue to lay broods of eggs, so they have a very large number of offspring. Elephants only give birth every 4 years, and a sauropod dinosaur can probably lay hundreds of eggs in the same amount of time. When sauropods increased in size, the number of their offspring did not decrease. In this way, sauropods avoided the risk of extinction of ordinary large animals due to slow reproduction. In case of danger, sauropod populations also recovered faster than larger mammals.

　　All in all, sauropod dinosaurs are an anomaly in the evolutionary history of vertebrates, and they themselves possess a series of structural and physiological characteristics that are conducive to large-scale large-scale formation. Sauropod dinosaurs have a very powerful respiratory system, they can get more oxygen; at the same time, the special way of eating allows them to get more food without consuming too much energy. Therefore, the inhaled oxygen is sufficient, and the body does not consume too much. As the generations evolve, the body will naturally become stronger and stronger, and the body will become larger and larger. Through their unique reproductive, physical and lifestyle advantages, sauropods finally overcame the disadvantages that limit the growth of other terrestrial animals and grew into unprecedented giants.

How Dinosaurs Supported Their Huge Bodies

　　After knowing the origin of the "big body" of dinosaurs, some people may ask: As the largest animal in the history of the earth, how did sauropod dinosaurs support their huge bodies? Sauropods were able to support their massive bodies because their joints were more flexible than modern land mammals. Scientists believe that sauropods had more layers of cartilage connecting their bones than modern mammals, allowing their bones to hold more weight and grow huge bodies. Why were sauropods so flexible? This is because of the presence of "trabecular bone". So, what is trabecular bone? It is an extension of cortical bone in cancellous bone, and it is an irregular three-dimensional network structure in the bone marrow cavity, such as loofah-like or spongy, and plays a role in supporting hematopoietic tissue. Studies have found that with the increase of age, the trabecular bone will be continuously absorbed, and at this time, the bones will become heavier and heavier. There is a folk saying that "the older you are, the heavier your bones are", which is actually the relationship between trabecular bone and age. As vertebrates, sauropods naturally have trabecular bone. However, the trabecular bone of sauropod dinosaurs is not absorbed with age. Their trabecular bone structure is very special. The - belongs to a very light spongy substance. This particularity also causes the bone weight of sauropods to not change with age. Otherwise, if the sauropods are older, their bones will become heavier. Soon there will be no way to move and walk, and naturally they will not be able to survive on Earth for hundreds of millions of years.