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Alfred Sherwood Romer 



tell where these " fry " lived; they may have stayed in the same environment through- 

 out their lives, or may have migrated from salt to fresh waters or vice versa at the end 

 of the larval period. 



These are discouraging thoughts. But despite them, I feel it is nevertheless profitable 

 to follow the known evidence as far as it can lead us. 



THE EUROPEAN SILURIAN RECORD 

 We now come to the specific problem under discussion — the fact that, while the 

 North American Silurian fish fauna, although limited in extent, includes a series of 

 finds extending over the greater part of the duration of the Silurian which is almost 

 purely fresh water in apparent origin, the European record of fishes is confined almost 

 entirely to the closing phases of the period and, according to Gross's interpretation, 

 is dominantly marine in origin. How can these contrasts be reconciled? 



Fig. 2. A map of the north-eastern United States area in Lower Silurian times to show the distribution 

 of some major faunal elements. The midwestern region was one of typical marine conditions, with 

 limestone deposition dominant. Farther east, toward the presumed land area, is a transitional zone 

 with few fossils except the problematical Arthrophycus. The most easterly zone in which Silurian 

 sediments are preserved is a seemingly continental deposit of conglomerates and sandstones with a 



eurypterid-fis i fauna. (From Amsden.) 



Part of the differences may be readily accounted for if we review the differences in 

 the Silurian history of the two areas. In North America, as may be seen from the 

 summaries in any standard text (as, for example, Moore, 1933, 159-177) the Mississippi 

 Basin region was occupied throughout the Silurian by a sea. Along the eastern border 

 of the present continent, however, there is believed to have been an area or areas of 

 high land — the land mass of " Appalachia ", or an island chain. Throughout nearly 

 the length of the Silurian period materials from this region were carried westward, 

 to be deposited along the sea margins as conglomerates, sandstones and — particularly 

 in late Silurian times — red shales. The gradual lateral change in sediments from eastern 



