The prehistoric creatures
Intern reporter Dai Xiaopei
In the evolutionary history of dinosaurs, one of the biggest mysteries is: Are there aquatic dinosaurs? Although we know that ichthyosaurs, plesiosaurs and canglong all live in water, according to their position on the evolutionary tree, they are not strictly dinosaurs, but only close relatives of dinosaurs. So why are aquatic dinosaurs so rare?
Spinosaurus is an atypical aquatic dinosaur?
In fact, among the dinosaur fossils found, there are not dinosaurs that did not live in water at all.
In 1912, people discovered a kind of theropod dinosaur in Egypt — — Fossils of Spinosaurus, which lived in the Cretaceous period from about 110 million to 93.5 million years ago.
According to the early data released by scientists, Spinosaurus was 12.6 to 18 meters long and weighed 7 to 20.9 tons. In recent years, more complete specimens support early research, and scientists found that Spinosaurus could grow to a length of 15 to 15.6 meters. However, the latest data estimate that its weight is 6.4 tons to 7.2 tons. Unlike most dinosaurs, Spinosaurus had some unique spines on its back, forming a sail-like structure. According to public information, the spines on the back of Spinosaurus were at least 1.65 meters long. The spine of Spinosaurus is an extension of its vertebrae, which researchers believe can help Spinosaurus regulate its body temperature or show its unique charm.
"After comparing Spinosaurus with crocodiles, people think that Spinosaurus may be aquatic." Xu Xing, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology of China Academy of Sciences, told the Science and Technology Daily reporter that according to the skull shape of Spinosaurus, it was found that the skull of Spinosaurus was long and narrow, similar to that of modern crocodiles. Specifically, the mouth is slender, and the direction and size of teeth help it catch fish and fix its prey in the mouth.
In addition, by analyzing the fossils of individual Spinosaurus, some scientists found that there were fish scale residues in the stomach contents of Spinosaurus. "This shows that Spinosaurus ate creatures in the water, and it can be speculated that Spinosaurus may live on the shore or in the water." Xu Xing said that another evidence that Spinosaurus lived in water is that studies have found that the isotopic composition of Spinosaurus bones is close to that of aquatic animals.
However, Xu Xing stressed that there is no very conclusive evidence that Spinosaurus must have lived in water. "Because Spinosaurus didn’t have a fin-like structure like aquatic animals such as sea lions or seals. This shows that Spinosaurus cannot live completely in water and is not a typical aquatic animal. " He said that according to the existing findings, it is not easy to judge whether Spinosaurus lived in the water or on the shore for a long time.
"According to the teeth and skull of Spinosaurus, people think that Spinosaurus mainly lived by eating fish, but Spinosaurus is just a kind of dinosaur with a special feeding habit, and generally speaking, Spinosaurus just goes into the water to prey and then returns to land life. Therefore, we cannot put forward ‘ Aquatic dinosaur ’ The concept. " Xing Lida, an associate professor at China Geo University, said.
Dinosaurs were aquatic animals?
Some scientists believe that dinosaurs were originally aquatic animals. This statement stems from a puzzle: how did the slender legs of dinosaurs support its huge body? Brian Ford, a cell biologist at Cambridge University in England, tried to explain that dinosaurs mainly lived in water and supported their bodies by buoyancy. He said that dinosaurs swam in the lake most of the time, and their tails provided assistance, and their footprints were printed into the mud.
According to Ford, it can explain why archaeologists often dig dinosaur footprints, but there are few traces of dinosaur tails. Compared with modern large animals such as elephants and rhinoceroses, dinosaurs had small legs but fat tails.
"It makes sense to think that dinosaurs lived in water." Ford said, "dinosaurs used water to support their weight, float their tails, regulate their body temperature and get food."
However, Paul Barrett, a paleontologist at the Natural History Museum in London, believes that this statement makes no sense. He said that with the help of the principle of load-bearing structural engineering, researchers have proved that the strength of dinosaur leg muscles is enough to support its body to walk easily on land. Moreover, dinosaurs had difficulty breathing and walked slowly in the water. "They may live near water, and sometimes they go into the water to cool off, but I don’t accept the theory that they live in water." Paul said.
In 2007, scientists found traces of dinosaurs’ claws at the bottom of the water in the Carmelos basin in northeastern Spain, which became conclusive evidence that "dinosaurs could swim". In 2011, Chinese and American paleontologists found a group of strange footprints with only toe marks and no heels in Chicheng County, Hebei Province. Experts believe that this is the hard evidence of dinosaurs playing in the water — — Tracers leave scratches on the bottom of the lake after entering the water. As the depth of the lake increases, their feet can touch less and less water bottom, and their footprints gradually weaken, and their three toes can not be completely preserved.
Completely aquatic dinosaurs don’t exist?
Although there is evidence that some dinosaurs lived in water, Xing Lida still opposed the concept of "aquatic dinosaurs". “‘ Aquatic dinosaur ’ It is only a hypothesis put forward in recent years, but it is extremely easy to include Syngnathus, Canglong, plesiosaur, etc., confusing concepts. "
Xu Xing also told reporters: "According to the existing research, there are no pure water dinosaurs. Let’s just say that there is some evidence that dinosaurs lived in water for some time. The limbs of plesiosaurs, ichthyosaurs or sea lions and seals will undergo height changes to adapt to life completely in water, but there is no evidence that such dinosaurs exist at present. "
Strictly speaking, no real aquatic dinosaurs have been found. Xu Xing believes that there are two possibilities. First, dinosaurs did not give priority to occupy the water niche in the era when they lived. At that time, the water environment might be occupied by crocodiles and other aquatic animals. Second, there may be aquatic dinosaurs, but they have not become mainstream species, and the fossils of such dinosaurs have not been preserved or discovered.
"In recent years, with the deepening of field trips, people have discovered more and more strange dinosaurs. The current fossil record has not fully revealed the diversity of dinosaurs. " Xu Xing said that the original fossil record is deficient, some deficiencies may be remedied, and some deficiencies may exist forever.
What’s the point of studying dinosaurs and even arguing about whether dinosaurs were aquatic or terrestrial?
"First of all, it can prove some evolutionary phenomena. Species are always changing, such as from big to small, from land to air. Scientists hope to reveal the law of change. " Xu Xing said that the current ecosystem is called the world of birds and flowers, characterized by beautiful birds and flowering plants, and this ecosystem evolved from the Mesozoic ecosystem dominated by dinosaurs. We need a lot of data and fossil records to reveal this evolution process.
At the same time, scientists should also reveal which factors affect the evolution of life, such as genes or environment. "It is also important for human beings to understand their own lives to understand the laws of the evolution of all things." Xu Xing said.
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This dinosaur can also swim and hunt in the water.
At the end of 2017, research teams from Italy and other countries discovered a new dinosaur fossil in the late Cretaceous strata of Mongolia, which was roughly waterfowl-like. This new dinosaur species was named Eichzka, commonly known as "duck dinosaur". "Duck Dinosaur" has a long and curved neck, and its forelimbs are fin-shaped. It can walk on land like a duck and swim with its forelimbs. They also have many tiny teeth suitable for eating fish. Another feature of "Duck Dinosaur" is that its beak and upper jaw have many small holes, which were once filled with nerve endings. This kind of mouth has a similar structure to the beaks of some waterfowl. This shows that "duck dinosaurs" may have preyed in the water or at the bottom of rivers and lakes.