Evolution of fish

Cartilaginous fish and bony fish

Let’s add a rough timeline to get an overview of the evolutionary history of fish. The most primitive vertebrates that exist today are the lamprey and hagfish. Their bone is soft cartilage, and they are characterized by having no jaw. The jawless fish are believed to have flourished in the sea approximately 400 million years ago (the Silurian period to the early Devonian period). Fish with jaws similar to modern chimaera were born in the middle Devonian period. Then, they evolved while the bones in their bodies remained soft in the Carboniferous period approximately 350 million years ago, and the cartilaginous fish, including sharks and rays, prospered. At the same time when the sharks prospered, bony fish evolved by calcifying bones to acquire a hard skeleton.

Lobe-finned fish and ray-finned fish

The diversification of the bony fish occurred in the Permian period (approximately 299 million to 251 million years ago). Although bony fish are described as “fish” with hard bones, they include four-legged animals (tetrapods). Because the four legs were evolved from fish fins, there are animals with fleshy fins that are called lobe-finned fish. Coelacanths and lungfish are primitive lobe-finned fish that exist today. These primitive lobe-finned fish prospered in the late Permian period (260 million years ago). Some lobe-finned fish that survived the mass extinction event at the boundary between the Permian period and Triassic period (PT boundary) differentiated into reptiles and mammals. Reptiles prospered on land in the Mesozoic era. Among the bony fish that survived the PT boundary, there are also the ray-finned fish with ray-shaped fins, which are the typical “fish” we think of today.

Ancient fish Sturgeon