The evolution of the most primitive placoderm fish

The most primitive gnathostomes discovered so far are placoderms (class). They first appeared in the late Silurian Period and flourished for a while in the Devonian Period.

Placoderms also have bony armor to protect their bodies, usually wrapped around the front of their bodies. The bone armor of armored fish is an undivided, immovable tube that contains the entire body; while the bone armor of placoderm fish is divided into several pieces and can move with each other, so that the shield Skin fish are much more flexible in movement than armored fish.

ichthyosaur

These advantages of placoderm fishes allowed them to overwhelm armored fishes in the competition for survival, and by the Devonian period they had developed into a wide variety of groups. They include the following orders: Arthroplastids, Flatfishes, Turtlefishes, Sharians, Leaf-Lepid Fishes, Frill-toothed Fishes, and Paleospondyllales. Among these groups, the most prosperous are the joint-necked fishes and the turtle-shelled fishes.

The head and trunk of joint-necked fish are wrapped by strong bony plates. The two parts of the bony plates form a self-contained system and are connected by only a pair of joints. The structure of the upper and lower jaws is very special. When eating, contrary to ordinary vertebrates, the lower jaw does not move, while the upper jaw lifts upward and then cuts downward, like a guillotine. Some of these fish developed huge forms in the middle Devonian period, such as the dinofish, which could reach a length of 10 meters! The skull is huge, the jaws are strong, and the front end is equipped with large and sharp bony plate-like teeth. Such bone plates form a perfect scissor-like sharp edge, which is a very effective prey-catching device. Dinosaurs could prey on any fish at that time and were the overlords of the primitive oceans. Fossils of placoderm skulls similar to those of dinosaurs have been discovered in Jiangyou County, Sichuan Province, my country. This fish is called Jiangyou fish.

Grooved scale fish

Turtle-shelled turtles are smaller primitive jawed species that lived in the Middle and Late Devonian, with body length generally only about 30 centimeters. Their heads, trunks and pectoral fins are covered with bone armor composed of multiple pieces of armor. The armor on the trunk is particularly developed, as if a box made of bone plates is set on the outside of the fish body. There are quite a lot of turtle fossils discovered in my country, especially in Yunnan Province. In addition to the most common groove-scale fish, there are also Wuding fish, Yunnan fish, Yunnan fish, etc.

The rear part of the trunk of the fish is exposed, and the front part of the body is wrapped in bony armor; the head armor is hexagonal and has a "V" shaped sensory groove. Some fossil fishes still retain impressions of the soft body. Through these impressions, scientists found that there was a pair of air sacs on both sides of the esophagus that communicated with the throat, which were probably rudimentary lungs with respiratory function. Such a structure has also been found in some early bony fish, so scientists speculate that the lungs existed at the origin of vertebrates, but secondary degeneration occurred in some later fish-like vertebrate animals.

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