2. "How lively and energetic the movements are, jumping in the waves and floating in the sea", which makes people try their best to write about the vigorous vitality and lively movements of fish. What is the purpose?
3. "To live is to struggle, to advance in the struggle, and to use energy cleanly before death comes." What are the characteristics of poets expressing their feelings with the help of fish fossils?
1. The author of Yin thinks of Archean creatures, and the author of Fish Fossil thinks of energetic, lively and jumping fish and their disasters.
2. The intensity of the tragedy (setting off death with life).
3. With the help of the complete fossilization of fish fossils, express the poet's resistance and contempt for political doom.
As the oldest vertebrate on the earth, fish's long evolutionary history has always been an issue of interest to many biologists. The appearance of fish marks a qualitative leap from lower primitive invertebrates to vertebrates. The development and evolution of fish also put forward an obvious evolutionary pedigree of vertebrates. All higher animals, amphibians, reptiles, birds, mammals and even ourselves have developed on this basis.
Paleontological research is usually based on fossil materials. Scientists determine the absolute age of rocks by radioisotopes and divide them into different geological ages. The paleontology preserved in these geological times recorded the environmental conditions and biological information at that time, and formed fossils after thousands of years of deposition, which became the basis for studying geological history and biological evolution history.
Fish fossils are not very rich, but they can still show the development process of all kinds of fish in ancient and modern times. The earliest fish fossils were deposited in Cambrian and Ordovician rocks, about 400 million years ago. Through the study of rocks, people know that the earliest fish lived in salt water environment, or in the ocean, and their bodies were covered with an exoskeleton as hard as armor. These primitive fish are covered with hard armor, and the front and back armor are flat. Because they have no jaws, they are called jawless. They are arguably the oldest fish. Because they wear armor and can't swim, they can only live in underwater sediments. It should be said that they are a group of fish that can't swim. The endoskeleton of jaws has not been preserved, so scientists speculate that they have soft bones, just like cartilaginous fish, sharks and gill fish we see now.
A large number of complete jawless fossils were found in Devonian, which can be regarded as the infancy of fish. The Jurassic and Cretaceous of Mesozoic (about 65.438+0.3 billion years ago ~ 65.438+0.6 billion years ago) were the times of fish revival. In the Cenozoic, all kinds of ancient and modern fish * * * existed in the ocean and other waters on the earth, and the fish family reached its peak.
On the basis of jawless fish, the earliest jawed fish was also developed. The original jaw was transformed from several bony branchial arches. The branchial arch was originally buried in the muscle. In the process of evolution, the carapace of the jaw and head merged into one, thus forming a stronger and more efficient feeding organ-chewing device.
Primitive jaw fish, also known as shield fish, flourished in Devonian, but most of them became extinct at the end of Devonian. It is generally believed that both cartilaginous fish and bony fish evolved from shield fish, and they developed in different directions, but no clear evidence was found to prove this inference. Some shield fish still have flat bodies, just like their ancestors; But most of them have become streamlined and their armor has been reduced, which gives them a strong swimming ability. Chondroid fish also shed their heavy armor (but there are still traces of the backboard) and developed stronger muscle tissue suitable for swimming. Some scientists think that cartilaginous fish are "primitive" fish, but whether they are really more primitive than bony fish remains to be confirmed.
The research on the genesis and evolution of vertebrate jaws began with embryology research in19th century, which revealed an important process in evolution. The appearance of jaw shows that the appearance of a new important feature of animals can expand the life field of a group to the field that could not be lived in the past. Since then, fish have expanded rapidly and become the most common swimming organism group today.
Skeletal fish first lived in fresh water, then gradually extended to the ocean, and finally became the dominant group of marine fish. In the process of evolution, they produced internal hard bones, which turned stiff armor into thin scales, thus making their movements agile and flexible and improving their moving speed.
There are two kinds of bony fish, among which the number and species of radial fin fish greatly exceed that of another kind of fish-internal nostril fish. Internal nostril fish include some primitive species with special morphology and structure. They have internal nostril structure, which can close their mouths without affecting their breathing. Today, fish with internal nostrils can only see lungfish and marlin. Barracuda belongs to echinoderma. Known as a living fossil, it was considered an extinct species by scientists before 1938. 1938 A fisherman on the southeast coast of Africa caught the first pike tail, which caused a sensation in the world. Later, they were caught one after another, confirming that this ancient fish still lives in the modern ocean. The important feature of coelacanth is that its fins are leaflike, with muscles and connected spines, so that some fish can crawl on land. They are closely related to amphibians. People think that amphibians evolved from them.