The Early Evolution of Fishes

Abstract

A review of the advances in this subject in the past three decades. On the basis of new discoveries of hundreds of fossil types and the study of these by improved techniques involving microdissec-tion and reconstructions after precision grinding, anatomy and structural details have been ascertained. It is found that endochondral bone existed in Ostracoderms much earlier than previously supposed and prior to the existence of the Chondrichthyes. The new concept of fish phylogeny is now as follows: Agnatha to Ostracoderms to Placodermi (having bone and primitive jaws). From here the route divides into two principal branches of which the one, and shorter, ends in the Chondrichthyes. The other, larger branch, proceeds from the Placodermi to primitive Osteichthyes, from which one branch proceeds and terminates in the teleosts via the Actinopterygii and ganoids. From the Osteichthyes another branch leads to the Crossopterygii, and terminates in the Amphibia. Dipnoi are a side branch of this route. The revolutionary changes of this concept of evolution are, Cyclo-stomata have been displaced from the basal position on the line of ascent to a terminal position on a side branch emanating from the primitive Agnatha. Ostracodermi are placed in the main line of ascent, as are the Placodermi. Chondrichthyes are terminal on a side branch of the main line from the Placodermi. Dipnoi are terminal on a side branch from Choanich-thyes, while Crossopterygii are on the main line preceding Amphibia.