HOW THE FISHES LEARNED TO SWIM ' 



By Anatol Heintz 

 Paleontological Museum, Oslo 



Through investigations during the last few years, an astonishing 

 amount of new information has been discovered concerning the earli- 

 est known vertebrates. Not only have a number of new orders, 

 families, genera, and a profusion of new species been described, but 

 what is more important, investigators have succeeded in classifying 

 all the new and old discoveries into a fairly reliable system and to 

 outline the general relationships between ail these various forms. 



In my first figure I have attempted to show the latest systematic 

 classification of both fishes and fishlike forms based chiefly upon the 

 investigations of Kiaer, Stensio, Broili, Gross, and others. We see 

 the first class formed by the ostracoderms, which are most nearly 

 related to the recent cyclostomes or lampreys. They reach their 

 greatest abundance in the upper Silurian and through the entire 

 Devonian but become extinct with the latter period. Characteristic 

 of almost all Paleozoic forms is the presence of a more or less 

 strongly developed dermal armature of mail which covered the head 

 and frequently also the anterior part of the body. 



The true fishes, which form a second class of craniate vertebrates, 

 are subdivided into three large divisions: the Placodernis, the Elas- 

 mobranchs or sharklike forms, and the Teleostomes or bony fishes. 

 Of greatest interest to us are the Placoderms, which are the best- 

 known group of the oldest true fishes. They began to appear in 

 the upper Silurian, but are chiefly characteristic of the Devonian 

 period. At the transition to the Carboniferous the Placoderms dis- 

 appeared. They, also, had a strong dermal armature covering both 

 the head and the anterior part of the body. 



The earliest remains of the forms belonging to the two last divi- 

 sions, the Elasmobranchs and Teleostomes, are also known from the 

 Devonian. But only a few groups of these divisions reached any 

 abundance as early as the Paleozoic era. 



1 Translation published by permission from Naturen, vol. 58, nog. 7 and 8, Bergen, 

 July and August 1934. 



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