The earliest known fish fossils were found in the late Cambrian strata about 500 million years ago. However, they are only some scattered scales and cannot give us a detailed picture of the fish's body shape. contour. It was not until the late Silurian and Devonian periods, 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 terms of structural characteristics, indicating that many types of fish existed at that time. It is likely that before there are fossil records, they have already gone their separate ways and traveled a considerable distance on their respective evolutionary paths.

The earliest fish to appear were jawless fish. As the name suggests, they have no upper or lower jaws, only a funnel-shaped mouth located at the front of the body. This kind of mouth cannot actively take food, and can only rely on water flow to bring tiny organisms into the mouth. Furthermore, they do not have pelvic fins, but have a membranous exoskeleton wrapped around the body. Therefore, jawless fish are also called armored fish. The existence of this exoskeleton has caused some debate among relevant scholars: Which comes first, cartilage or hard bone? During vertebrate embryogenesis, cartilage always appears first, and then hard bone is formed from cartilage. It is generally believed that ontogeny reflects phylogeny. According to this, in the process of biological evolution, cartilage should come first and hard bones should follow. However, the earliest vertebrates appeared first with hard bones. How to explain this? Some people say that cartilage came first, but cartilage cannot be preserved as fossils. How exactly that will happen has not yet been determined.

Jawless fishes include two completely different categories: cephalothorax and fin beetles. Each category has its own branches, with different types of representatives, and they also flourished for a while. But the good times did not last long, and by the middle of the Devonian Period (about 350 million years ago), most of them became extinct. Just because some of the characteristics of living lampreys and hagfish are consistent with those of cephalothorax, scholars speculate that the former may be the living representatives of the latter. According to this, the cephalothorax should not be finally extinct. However, no intermediate link has been found between the cephalopods, lampreys and hagfishes, from the Devonian to modern times for more than 300 million years (Figure 14). Exactly how these modern jawless fishes that live a parasitic life evolved from their armored ancestors is still an unsolved mystery. There are no living representatives of the fin beetles and they are considered an extinct species. However, because some of the characteristics of the heterochaetes among the fin beetles are similar to those of later jawed fishes, some people say that the heterochaeids may be the distant ancestors of the jawed fishes. Whether this is the case requires more proof.

The earliest jawed fish was the placoderm, which not only had upper and lower jaws, but also even fins. In this way, it is possible for it to take the initiative to eat. Placoderm fish are usually divided into segmented beetles and carapace beetles. They are all covered with armor and were most prosperous in the late Devonian period. The former can be represented by caudal fish, and the latter can be represented by groove-scaled fish. Some believe that placoderms may be related to modern sharks, but others believe that they may be more closely related to bony fishes.

Elasmobranchs are also called cartilaginous fishes, including sharks and holocephalians. Sharks are often considered primitive fish because of their cartilaginous skeletons. The cartilage comes first and the hard bones come last. But some people think that the cartilage of sharks is secondary and is "degenerated" from the hard bone. The hard bone comes first and the cartilage comes later. Don’t armored fish and placoderm fish both have hard bones? The earliest hard bones are truly primitive.

The earliest cartilaginous fish appeared in the early Devonian period (380 million years ago). The rip shark is often regarded as one of the most primitive representatives and is probably the oldest of all sharks. ancestor. It is a shark nearly 1 meter long, with a typical shark body shape - spindle shape, large eyes, close to the tip of the snout. There are two dorsal fins, and there is a thick dorsal spine in front of the first dorsal fin. The pectoral fins are particularly large and the pelvic fins are small. The upper and lower lobes of the caudal fin are symmetrical in appearance, but the upper spine of the internal structure extends to the end of the upper lobe of the caudal fin, so it is still a crooked tail. Even fins have a wide base and a pointed tip, which is a primitive type of fin. The teeth are "pencil" shaped, with the cusp in the center high and the cusps on the sides low (Figure 15). Starting from the central trunk of the rip shark, which is similar to a cartilaginous fish, various later sharks evolved, including typical sharks and rays with flat bodies. These sharks have been living in the ocean from the Mesozoic Era to the present. They have never been particularly prosperous, but they have not been eliminated either.

Bony fishes are the most progressive fish and are the "masters" of the world's waters today. It is generally believed that bony fishes evolved from sticklebacks. Sticklebacks are early jawed fishes that appeared in the Silurian Period (400 million years ago) and continued into the Permian Period (250 million years ago).

This is a small fish that was once thought to be related to placoderms and cartilaginous fishes. In recent years, through the study of new materials, it has been determined that it is related to bony fishes.

Bony fishes are divided into two major branches, one is called ray-finned fishes and the other is called lobe-finned fishes. The former first appeared in the middle Devonian period about 380 million years ago, and evolved through cartilaginous squamates (partial cartilage, trapezoidal scales, obviously crooked tail) and holoosteus (partial cartilage, trapezoidal scales, Modern fish come from three evolutionary stages: light crooked tail) and teleost fish (bony fish, round scales, straight tail). Lobe-finned fishes include lobe-finned fishes and lungfishes, which are further divided into coelacanths and fan-finned fishes. Latimer is the only living representative of coelacanths, while fan-finned fishes are all fossil species. The latter was once considered the ancestor of terrestrial quadrupeds, but has been denied by Chinese scholars in recent years. Lungfishes began to appear in the Devonian Period (360 million years ago), and are still represented by Australian barramundi, African barramundi and South American barramundi. As the name suggests, lungfish can breathe with lungs, which is a basic requirement for terrestrial vertebrates. Together with some other characteristics, lungfish was once thought to be the ancestor of terrestrial tetrapods. Later, this "ancestral" status was replaced by the fan-fin fish "with internal nostrils". In the 1980s, with the denial of the internal nostrils of fan-finned fish, the theory of fan-finned fish ancestry was shaken. So the relevant scholars returned to lungfish to look for the ancestors of terrestrial quadrupeds. Hope to adopt it!