With the end of Silurian, the evolutionary history of the earth entered the second half of Paleozoic, that is, Quaternary-Devonian. Just as Ordovician continued the Cambrian explosion, Devonian also inherited the vitality of Silurian, and the species and quantity of creatures reached an unprecedented scale. Lush ferns and rising insects and amphibians have grown to unprecedented scale. In particular, vertebrates, represented by fish, entered a period of rapid development, so Devonian is often called the "Fish Age".

In fact, the age of "fish in the north" is also the result of the evolution of paleogeography. Under the influence of Caledonian movement in early Paleozoic, Hercynian movement began in Devonian, and many areas rose, exposing the sea surface and becoming land, covering 85% of the earth's seawater, including the vast ancient Pacific Ocean in the northern hemisphere. Compared with the early Paleozoic, the paleogeographic characteristics have changed greatly: Gondwana, the largest and most complete ancient land, is distributed around the Antarctic region; Lauren and Boluo, two ancient lands, constitute the unified continent of Europe and America, and gradually approach Gondwana, an ancient land; In the late Devonian, the two ancient lands began to converge near the equator, which laid the foundation for the formation of Pangu supercontinent in the future.

Just like the representative of Ordovician evolutionary creatures, the 6-meter-long straight-shelled nautilus gave way to a new generation of marine cancer horseshoe crabs in Ordovician, the Devonian has also undergone a great change in sovereignty: vertebrates with jaws finally emerged and became the most dazzling stars. The dominance of arthropods and mollusks on the earth has become a thing of the past. Vertebrates firmly occupy the throne of the largest, strongest and fiercest creatures on earth.

In Devonian rivers and seas, vertebrates are mainly fish, such as jawless bony fish, soft-shelled turtle, scaly fish and miscellaneous fish (collectively referred to as "soft-shelled turtle"); Jaw fish has four branches: shield fish (the fish in the dungeon), spiny fish, cartilaginous fish (the shark) and bony fish. You sing too loudly. Among them, the jawless class flourished in the early Devonian, and with its strong armor, it can also occupy a place in the water where Silurian flounder is rampant. But in the middle Devonian, when fierce and strong jawless fish appeared, powerful jaws and sharp teeth not only completely shattered the rule of horseshoe crab, but also killed warm and heavy jawless relatives. Only a few specialized jawless animals survived and became the ancestors of modern round-billed animals.