The earliest known fish fossils were found in the Late Cambrian strata about 500 million years ago, but only scattered scales failed to outline the physical state of fish for us. It was not until the late Silurian and Devonian 400 million to 350 million years ago that a large number of fish fossils were discovered. Some of these fish fossils are very different from each other in structural characteristics, indicating that there were many kinds of fish at that time. Probably before the fossil record, they had gone their separate ways and had gone a long way on their respective evolutionary paths.

The earliest fish were jawless fish. As the name implies, they have no upper or lower jaws, and only a funnel-shaped mouth is located at the front end of their bodies. This kind of mouth can't take the initiative to eat, and it can only bring tiny creatures into the mouth by electric current. In addition, they don't have ventral fins, but they have a membranous exoskeleton around their bodies. Therefore, jawless fish is also called turtle. Because of the existence of this exoskeleton, some disputes have arisen among scholars: which comes first, cartilage or hard bone? In the process of vertebrate embryogenesis, cartilage always appears first, and then hard bone is formed from cartilage. It is generally believed that individual development reflects systematic development. Accordingly, in the process of biological evolution, cartilage should be ranked first, and hard bone should be ranked last, but the earliest vertebrates first appeared hard bone. How do you explain this? Some people say that cartilage comes first, but cartilage cannot be preserved as a fossil. Finally, there is no conclusion.

Jaw-less fishes include two distinct classes: Cephalopods and Fin Whales, each of which has its own branch and various representatives of different types, which once flourished. But it didn't last long. By the middle Devonian (about 350 million years ago), most of them were extinct. Because some characteristics of alive lamprey and blind eel are consistent with cephalopods, scholars speculate that the former may be the living representative of the latter. Accordingly, cephalopods should not eventually become extinct. However, during the more than 300 million years from Devonian to modern times, no intermediate relationship among cephalopods, lampreys and blind eels was found (Figure 14). How these parasitic modern jawless fishes evolved from their ancestors is still an unsolved problem. Pinctada has no living representative and is considered an extinct category. However, because some characteristics of Allosaurus are similar to those of later jaw fish, some people say that Allosaurus may be the distant ancestor of jaw fish. Whether this is the case, more arguments are needed.

The earliest jaw fish is the shield fish, which has not only upper and lower jaws, but also lateral fins. So it is possible to take the initiative to feed. Shield fish are usually divided into Class A and Class B, both of which are covered with armor and most prosperous in the late Devonian. The former can be represented by tail fish, and the latter can be represented by channel scale fish. Some people think that the shield fish may be related to modern sharks, but others think that it may be closer to bony fish.

Claudiobranchia, also known as cartilaginous fish, includes sharks and cephalopods. Sharks are usually considered primitive fish because they have cartilaginous bones. Cartilage in front, hard bone in the back. However, some people think that shark cartilage is secondary, and the hard bone is "degraded", with the hard bone first and the cartilage behind. Don't turtles and shield fish have hard bones? The earliest hard bones are really primitive.

The earliest cartilaginous fish appeared in the early Devonian (380 million years ago), and the split shark is usually considered as one of the most primitive representatives and may be the ancestor of all sharks. It is a shark with a length of nearly 1 m, with a typical shark body shape-spindle shape, large eyes and close to the snout. Two dorsal fins. There is a stout backstab in front of the first dorsal fin. The pectoral fin is very large and the ventral fin is very small. The upper and lower leaves of the caudal fin are symmetrical in appearance, but the upper spine in the internal structure extends all the way to the end of the upper leaf of the caudal fin, so it is still a crooked tail. The lateral fin is a primitive fin with a wide base and a sharp tip. The teeth are in the shape of a pen, with the center of the tip high and the sides low (Figure 15). Starting from the split shark with the central spine similar to cartilaginous fish, various sharks have evolved in the later period, including typical sharks and rays with flat bodies. These sharks have lived in the ocean since Mesozoic, and they have not flourished particularly, but they have not been eliminated.

Bone fish is the most progressive fish, and it is also the "master" of the world waters today. It is generally believed that bony fish evolved from spiny fish. Spinosaurus is an early jawed fish, which appeared in Early Silurian (400 million years ago) and lasted until Permian (250 million years ago). This is a small fish, which was once thought to be related to shield fish and cartilaginous fish. In recent years, through the study of new materials, it is determined that they are related to teleost fish.

Bones are divided into two branches, one is called radial fin fish and the other is called fleshy fin fish. The former first appeared in the middle Devonian about 380 million years ago, and evolved into modern fish through three stages of evolution: chondroscleroderma (part cartilage, scalene muscle and obviously curved tail), whole bone fish (part cartilage, scalene muscle and slightly curved tail) and true bone fish (hard bone, round scale and straight tail). Carnivorous fish include stickleback and lungfish, and are divided into stickleback and fanfin fish. Latimai fish is the only living representative of empty stickleback fish, and all finfish are fossil species. The latter was once considered as the ancestor of terrestrial quadrupeds, but it was denied by China scholars in recent years. Lungfish began to appear in Devonian (360 million years ago), until now there are Australian lungfish, African lungfish and South American lungfish as representatives. As the name implies, lungfish can breathe with their lungs, which is the basic requirement of terrestrial vertebrates. Coupled with other characteristics, lungfish was once considered as the ancestor of terrestrial quadrupeds. Later, this "ancestor" status was replaced by a fan-fin fish with internal nostrils. In 1980s, with the denial of the internal nostril of fanfin fish, the ancestor theory of fanfin fish was shaken. So the scholars concerned went back to lungfish to look for the ancestors of terrestrial quadrupeds.