While fishes were numerous and large in the Devonian, throughout the Carboniferous they began to decline. By this time the land area had much increased, land plants became very abundant, there were immense forests of tropical vegetation, great swamps and peat bogs — all of which later sank below sea level — became covered up and changed into coal. Immense lizards lived in these forests and along the sea shores; these were the first land animals. At the close of the Carboniferous great changes took place; greater changes than at any time since the close of the Archaean. So marked were the changes at this time that it marks a new era in the geological history of the earth. All preceding the close of the Carboniferous is regarded as ancient geology; all since then as modern geology. It was at this time that plants and animals were represented by new forms more like those now living. The geological age following the Carboniferous is the Triassic. With this age began our modern sharks and fishes. They did not become abundant until the Jurassic and Cretaceous. All of the earlier sharks had strong spines in front of each dorsal fin and broad teeth made for crushing. One form of these known as Cestracionts were very abundant till the end of the Cretaceous. In the early Triassic they began to decline and the sharks, with pointed teeth, increased. These sharks, with pointed teeth, but rounded on the edges, commenced back in the Carboniferous. During the Triassic the sharks, with lancet shaped teeth, such as are now possessed by nearly all our sharks, commenced in small numbers. One of the important differences between the ganoids and the teleosts or true fishes is in the tail vertebrae. In the ganoids the tail vertebrae decrease gradually in size and curve upwards in the upper lobe of the tail. In the teleosts the tail vertebrae ends a short distance in front of ends of the middle fin rays of the tail fin. In the ganoids the upper lobe of the tail fin is the largest. In the teleosts both lobes are nearly the same size.

The tail of the ganoid fish is called heterocercal, that of our modern or teleost fishes is homocercal. The tail of all early ganoids was strongly heterocercal. In the Triassic and Jurassic its lobes in many cases became nearly equal, approaching the homocercal tail. The tails of all sharks are heterocercal, of all modern fishes it is homocercal except in a few families, as the cod and related fishes, it is Isocercal; that is, the vertebrae decrease in size, but do not form an upward curve. So far as we know the Shad family is the first of our teleosts or true fishes to appear, and these were quite abundant in the early part of the Triassic.

The rays, fish-like animals much like Sharks, but with the body and fins flattened or spread out in a broad flat disc, appeared in the Jurassic, The Chimeras, so abundant in the Devonian and which died out apparently at the close of the Devonian, also reappeared at the beginning of the Jurassic. These did not belong to the same families as did the more ancient Chimeras. The Chimeras no doubt flourished in the Carboniferous and Triassic, but migrated to some portion of the sea where now perhaps their remains lie buried in rocks below the bottom of the sea. Their survivors, which were able to modify their structure and habits to become suited to new conditions, returned in modified forms in the Jurassic, where in time their remains come to the surface as fossils.

At the end of the Cretaceous or beginning of the Tertiary we find all of our modern types of sharks and all of the important orders of teleosts. The sturgeons and ganoids decreased .throughout the Tertiary or Quaternary until at present we have but few living species. The sturgeons are the more abundant. Of the large group of Ganoids so abundant during all these geological ages but few forms are living today. These are the Ceratodus, lung fish of Australia; the Polypterus of the Nile, the Protopterus of Western Africa, the Dogfish and the three Garpike of North America. These few species are but the remnants of a once large and extensive group of fishes.