Fish evolved 530 million years ago, with the oldest known being Pikaia gracilens. Jawed fish evolved 420 million years ago, with placoderms dominating until the Devonian period. Many modern and extinct forms evolved during this period, including sharks, rays, and lobe-finned fish, which eventually evolved into terrestrial vertebrates. Acanthodians became extinct, while lobe-finned fish became a minority after giving rise to tetrapods. The find of a living coelacanth off the coast of South Africa was considered one of the greatest zoological finds of the 20th century.
“Fish” is a word used to refer to any non-tetrapod vertebrate. The evolutionary history of fish began 530 million years ago, in the middle of the Cambrian period. Some of the oldest known animals in the evolutionary history of fish are Pikaia gracilens, which resembles the modern lancet, found in the famous Burgess shale assemblage, and Haikouichthys and Myllokunmingia from the Maotianshan shale of southern China. These simple jawless fish existed for about 100 million years until the first fish with jaws evolved. These are also the earliest known vertebrates.
Initially, fish were minority members in an ecosystem dominated by invertebrates, particularly brachiopods, sponges and arthropods such as trilobites. It would not have been until the Silurian period, about 420 million years ago, that jawed fish evolved and began to compete effectively with invertebrates. The first jawed fish were the placoderms, a family of fish with special armor for the head and chest to protect them from predators. These fish are the ancestors of all modern vertebrates, including humans and all of our pets and livestock.
The jawless fish were quickly defeated, leaving behind only a few small lineages that eventually gave rise to modern hagfish, lampreys, and lancets. Sometimes these animals are not considered true fish due to their radically different physiology. There is also some disagreement as to whether lampreys are vertebrates, as their cartilaginous “skeleton” is so primitive.
The evolutionary history of fish continued with the dominance of jawed fishes, especially placoderms, growing to 6 m (20 ft) in apex predators such as Dunkelosteus telleri. Large predators such as Dunkelosteus are considered the earliest vertebrate apex predators and firmly established the role of vertebrates as apex predators in the world’s ecosystems, a role that would continue for the rest of evolutionary history. Placoderms dominated throughout the Silurian period, for several tens of millions of years, until the Devonian, when fish began to diversify rapidly.
The Devonian was the most important period in the evolutionary history of fish, when many modern and extinct forms evolved from placoderm seed, including sharks and rays, acanthodians (“spiny sharks”, now extinct), ray-finned fishes (which dominate today’s seas) and lobe-finned fishes, which eventually evolved into terrestrial vertebrates. Fish have successfully overtaken many other marine organisms to become the dominant mobile marine animal, sharing the seas with small arthropods such as copepods and krill.
Although lobe-finned fishes and acanthodians demonstrated momentary success during the Devonian, acanthodians were practically extinct by the end of the following period, the Carboniferous, while lobe-finned fishes became a small minority after giving rise to the tetrapods during the same geological period. For many years they were thought to be represented only by lungfish, until a coelacanth was lured off the coast of South Africa. The find of a living coelacanth has been considered one of the greatest zoological finds of the 20th century.
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