Fish origins— fresh or salt water? 2 69 



deep water deposits, such as graptolitic shales, are due to necrocenosis, the floating 

 outward of cadavers or shed armour. But to Rueoemann and a minority of other 

 workers, the number of occurrences of eurypterids in lagoonai and near-shore 

 deposits suggests that the older Palaeozoic eurypterids were euryhaline, capable of 

 living in sublittoral and neritic marine environments. 



" No decision ? " 



The work on kidney structure and function makes it reasonably certain that fresh 

 waters were the original fish environment. However, it is obvious from the discussion 

 above, that it is difficult to reach a positive confirmation of this conclusion from 

 palaeontological studies, no matter how carefully conducted. Additional points are 

 sometimes advanced which tend towards the discouraging conclusion that attainment 

 of palaeontological proof is not only difficult but perhaps impossible. 



WEST 



DEVONIAN 



EAST 



Fig. 1. Diagrammatic east-west section through the Silurian of Pennsylvania, showing the transition 

 from sediments of continental type at the east to an essentially marine section at the west. Known 

 Silurian vertebrate occurrences are all in the eastern continental region, and although not confined to 

 a single area, are here shown in approximately their vertical and horizontal position. Conglomerates 

 and sandstones stippled; redbeds (essentially continental) hatched: limestone and other typical 

 marine beds unshaded. (Modified from Moore; data from C. K. and F. M. Swartz.) 



It may well be, for example, that the available material of early fishes is too late 

 in date to be of great value. We have little evidence to go on before very late Silurian 

 days, while the Ordovician scrap material shows that ostracoderms were well developed 

 by the middle of that period and probably originated at a much earlier time. There 

 may, therefore, have been considerable ecological shifting between streams and sea 

 by Silurian times, so that evidence from that period may have little meaning in terms 

 of truly primitive vertebrate environments. Again, it is possible that Silurian fishes 

 may have been to a considerable degree euryhaline in nature, with an ecologically 

 cosmopolitan range ; if such were the case, a distributional study would be meaningless. 

 Still further, ontogenetic development must be considered. As Westoll (1945) has 

 pointed out, it is probable that young ostracoderms had little or no armour, and 

 hence cannot be found under normal conditions as fossils. In consequence we cannot 



