In describing the geologic occurrence of the fish and eurypterids, Chamberlin lays special stress on the abrupt appearance of both groups of fossils in the Siluric, heralded by very scanty remains in the preceding formations, and gives the following account of the phenomena of their distribution in the rocks.
In the Ludlow "bone bed" of England, where they first make their appearance in abundance, the fish remains are associated with eurypterids, probably the most gigantic crustaceans that have ever lived, some of them attaining two meters in length. There is the same association on the continent, notably in the island of Oesel in the Russian Baltic and in Podolia and Galicia, and so again in the waterlime group of America in which the Pteraspis [Palaeaspis] americana of Claypole occurs. The physical conditions in all these cases seem to have been peculiar, and in the case of the waterlime group they were singularly so, for they permitted a host of these larger eurypterids and other crustaceans to flourish in seeming luxuriance, while only a meager and pauperate marine fauna found an occasional entrance into the series. The conditions seem to have been congenial to the fish and eurypterids but not to a typical marine fauna.
In the Old Red sandstone of the Devonian both in Europe and America a similar association obtained. A most extraordinary group of fishes and a family of most gigantic crustaceans flourished where marine life found only an occasional and meager presence. These few marine forms, here and there in a massive deposit, no more imply prevalent salt water than the present marine species in the bay of San Francisco imply that the gravels, sands and silts of the valley of California and of the Great Basin, which seem to be analogues of the Old Red sandstone, are prevailingly marine. The further association of the fishes and eurypterids with land plants and fresh-water mollusks, together with a total absence of marine relics from the same beds, leaves no solid ground for hesitating to accept the dominant view of English and other geologists that the typical Old Red sandstone and its homologues are the deposits of fresh waters and that both the fishes and the eurypterids found congenial conditions of life in them. As fishes and eurypterids were found both earlier and later in marine deposits the question arises: Were the fishes and eurypterids primarily marine and later became adapted to fresh water, or were they primarily fresh-water forms which were occasionally carried out to sea, and which later became adapted to salt water? The two cases do not necessarily
