Fish origins — fresh or salt water? 



271 



deposits dominantly continental in nature to western marine shales and limestones is 

 well illustrated in the east-to-west section of deposits in Pennsylvania shown in Fig. 1, 

 and the palaeogeography of the Silurian as a whole is well exemplified by the map 

 for the early part of the period given by Amsden (1955) and reproduced in Fig. 2. 

 The known vertebrates are present only in the most eastern and more dcfmiicly 

 continental formations; there is no trace of any vertebrate in the marine beds to the 

 west. This situation is in perfect agreement with the theory that the centre of distribu- 

 tion of the vertebrates of that day lay in fresh waters. 



In Europe, the Silurian geologic story is in strong contrast with that of North 

 America. For most of the period much of Europe was occupied by a widespread 

 ocean, in which were perhaps a few small islands; the shores of this ocean lay far 

 to the north and east (Fig. 3). Toward the end of the Silurian, however, the oncoming 

 of the Caledonian Revolution brought about an advance of the shore-line to the south, 



Fig 3 Map of Europe to show changes in land-sea relations in late Silurian times At the end ol the 

 Middle Silurian much of Europe was covered by a sea. Its northern shoreline (fulllme) is believed to 

 have run Xng the northern part of the British Isles to central Scandinavia and then south-eastward 

 through Finland and central Russia: islands are believed to have existed in Engand, in the rey.on 

 of the oresent Belgian coast, and in Saxony and Bohemia. By the end of the Dou^tonian. continental 

 ?oiSS had eSed southward over much of northern Europe (cross-hatched area). The Silurian 

 verteb a"e loclS^^^^^^^^^ all lie within the area of advance of continental sediments or close 



to the island areas. (Modihed from Born.) 



over the areas indicated by cross-hatching on the map. This brought about the 

 development in Downtonian times of beds of nearshore, estuarme and deltaic nature, 

 leading to the characteristic Devonian continental deposits ot " Old Red Sandstone 

 type It is in this area of advance of the shore-lines and oncoming ot continental 

 conditions that nearly all of the European Silurian tish-bearing deposits, indicated by 

 figures on the map, are to be found. The late appearance of fishes in the European 

 sflurian is thus closely correlated with the late appearance of " transitional or truly 

 continental beds in which remains of fresh-water dwellers would be expected. 



Of the deposits numbered on the map, numbers 1 and 2 are recogni/ed by Gross 

 as continental. The remainder are considered by him as dehmtely marine. Let us 

 bnefly review the stratigraphic evidence concerning these depc.its to test the validity 

 of such a conclusion. In doing so, 1 will restrict discussion to the materials considered 



