Fish (scientific name: Piscium), is the body covered with bony scales, gill breathing, through the tail and trunk of the swing and fins of the coordinated role of swimming and feeding with the upper and lower jaws of the variable-temperature aquatic vertebrates collectively,
belongs to the chordata of the vertebrate subphylum, is the oldest vertebrates. [1] Fish is rich in animal protein and phosphorus, among other things, and plays a major role in the development of human physical and intellectual capacity.
Fish are the lowest of the five major classes of vertebrates and have appeared on Earth for the earliest time. We are all familiar with present-day fish, but we are more rusty on early fish in geological history and how they evolved into present-day fish. Let's go forward in time.
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Fish
The earliest known fish fossils, found in the late Cambrian strata about 500 million years ago, are just a few scattered scales, failing to give us an outline of the fish's body shape.
It was not until the late Silurian and Devonian periods, between 400 million and 350 million years ago, that large numbers of fish fossils were discovered. These fish fossils, some of which are very different from each other in terms of structural features, indicate that multiple types of fish existed at that time. In all likelihood, they had already gone their separate ways and traveled a considerable distance on their own evolutionary paths before the fossil record was available.
The first fish to appear were jawless. As the name suggests, they did not yet have upper and lower jaws, but only a funnel-shaped mouth located at the front of the body. This mouth, which could not actively feed, only relied on water currents to bring tiny organisms into the mouth. Further they have no ventral fins, but have a membranous exoskeleton,
wrapped around the outside of the body. This is why jawless fish are also known as armored fish. The existence of this exoskeleton has led to a debate among scholars about whether cartilage or hard bone comes first. In the process of vertebrate embryogenesis, always first appear cartilage, and then from cartilage to form a hard bone. It is generally believed that individual occurrence reflects phylogeny.
Accordingly, in the process of biological evolution, cartilage should come first and hard bone later, but the earliest vertebrates appeared first but hard bone, how can this be explained? Some people say, or cartilage in the first, just cartilage can not be preserved as fossilized. The answer to this question is not conclusive.
The jawless fishes include two distinct groups: cephalopods and fin-nailed fishes, each of which has its own branch, with a variety of representatives of the different types, and which flourished for a while. But the good times did not last long, to the middle of the Devonian period (about 350 million years ago), most of them extinct. Only because of the living seven-gill eel and blind eel of certain characteristics and cephalopods consistent, scholars speculate that the former may be the living representatives of the latter. Accordingly, the cephalopods are not yet extinct.
But there is no intermediate link between the cephalopods and the seven-gill and blind eels from the Devonian period to more than 300 million years in the modern era (Figure 14). Exactly how these modern, parasitic, jawless fish evolved from their armor-clad ancestors is uncertain.
Finiformes have no extant representatives and are considered an extinct group. However, because of the close similarity of some features of the isopods of the fin-nailed group to those of the later jawed fishes, it has been suggested that the isopods may have been the distant ancestors of the jawed fishes. Whether this is the case or not, more arguments are needed.
The earliest jawed fish was the peltate, which not only had upper and lower jaws, but also had even fins. This made active feeding possible. Shield-skinned fishes are usually divided into segmented armor and carapace armor, both of which were clad with armor and flourished most in the late Devonian period. The former can be represented by the caudal bony fishes and the latter by the furrow-scaled fishes.
Some have suggested that shield-skinned fishes may be related to modern sharks, but others think they may be more closely related to bryozoans.