Fishes are largest group of vertebrates. These are represented by four separate major sub-divisions or classes :
The Agnatha or Cyclostomata
The first vertebrate known from fossil record were the Ostracoderm, a diverse group of jawless fish like vertebrates with bony armours and plates covering the head and body.
The Ostracoderm fossils first appeared in deposits of Ordovician age, approximately 450 million years ago. The lamprey and marine hag fish are the modern day survivor of this group.
The first jawed vertebrates were the placodermi or plate-skinned fishes, which in turn gave rise to the Osteichthyes or bony fishes and the Chondrichthyes or cartilaginous fishes (sharks, skates and rays).
All four groups were present in the Devonian period ( about 450 million years ago, which is referred to as the “age of fishes“.
The modern bony fishes are represented by two striking different groups or sub-classes, the Sarcopterygii ( flesh or lobe-finned fishes), and the Actinopterygii ( ray-finned fishes).
The latter comprise over 99% of the living bony fishes.
The Teleostei is the most diverse group of vertebrates living today and has been the dominent group of fishes for almost 100 million years.