I really like the Paleozoic. Many important events in evolution occurred during this era, and the spread of land vertebrates is by far the most important of them. Now I want to share some thoughts about what was happening in the late Paleozoic. As you all know, from the Cambrian to the beginning of the Middle Devonian, mollusks and arthropods were the dominant animals in ecosystems (mollusks most likely until the end of the Ludlow epoch of the Silurian). But then noticeable changes occur: vertebrates become the dominant group, first in the seas, and then on land.
The number of trilobites steadily decreased from the Silurian to the Permian, chasmataspidids became extinct in the Devonian, eurypterids moved from the ocean to rivers and lakes from the mid-Devonian, large myriapods and insects became extinct by the middle of the Permian period, and large arachnids even earlier. Why did this happen? I haven't seen any reputable research on this topic, but it seems to me that the whole point is that vertebrates beat the competition with arthropods thanks to their jaws. The jaws of vertebrates turned out to be the best weapon in the fight for survival, since they made it possible to effectively hunt arthropods that were vulnerable, especially during molting.
If we consider this issue in more detail, then... In the Silurian and Early Devonian, many eurypterids lived in the seas. But after the number of jawed fishes increased and large fishes appeared, the giant eurypterids disappeared, and their relatives began to seek salvation in fresh waters. Scorpions, their relatives, have long occupied the safe land and have been apex predators there for at least 30 million years. But after the first amphibians began to walk on land, large scorpions also began to decrease in number. The last giant scorpions are known from Mississippian (Early Carboniferous) deposits. It can be assumed that some of them also lived in the Pennsylvanian period ("late Carboniferous), hiding in deserts from vertebrates that could not yet move far from water. As far as I know, there were no longer any large species among the Mississippian marine arthropods, since cartilaginous shark-like fishes became widespread during that time, and in the Pennsylvanian eugeneodontiforms appeared with teeth ideally suited for chewing shells and carapaces. Large myriapods and insects lived in swampy forests, but there were no aquatic forms among them. The only possible exception is griffinflies. They probably spent the first part of their lives in water, where they risked becoming prey to numerous amphibians. The end of the Carboniferous and beginning of the Permian is characterized not only by the drying of the climate, but also by the appearance of large terrestrial vertebrates, each of which had muscular jaws. And soon after that, large land arthropods also disappeared. In my opinion, the conclusion suggests itself that large arthropods appeared not because of the high concentration of oxygen in the atmosphere, but due to the absence of terrestrial predators. When these predators appeared, large arthropods disappeared. And mostly small species remained, unable to compete with large vertebrates. My conclusion is that our Paleozoic ancestors, beginning in the Devonian period, became dangerous animals. More dangerous than arthropods. And they became like that thanks to their jaws. What do you think about this?