Gnathostomes

Abstract

The earliest known jawed fish-like vertebrates were the Placodermi. They arose in the Silurian period when the agnathans were at their peak. These first gnathostomes were marine fishes with the head and thorax generally covered with bony plates. They flourished in the Devonian and became extinct in the Carboniferous. The Gnathostomata thus includes almost all the living vertebrates which are divided into the six classes: Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammalia. These classes can be grouped in various ways according to the criteria used. The birds and mammals are warm-blooded (homiotherms) whereas the remainder are cold-blooded (poikilotherms). Alternatively, in these two classes and the reptiles, the developing embryo is protected by a membrane called the amnion. For this reason, they are grouped as Amniota. Fishes and amphibians, on the other hand, typically lay their eggs in water, and are called Anamniota because the embryo is not so protected. Finally, a distinction can be made between the fishes and the tetrapods, the former with fins and the latter with pentadactyl limbs. Distinctions based on the amnion or the limbs emphasize the essentially aquatic habit of the fishes compared with higher forms.