MIGRATION AND DISTRIBUTION OF THE EURYPTERIDS
Theory of Early Marine Habitat and Routes of Migration. As I stated above, the anomalies in the distribution of the eurypterids have not usually been given much consideration, though they are of the utmost importance. There is a current opinion that has somehow been formed about the bionomy of the eurypterid faunas and no one thinks of challenging it. When a eurypterid fauna has been found in a place where a marine fauna was not expected, it has had to be made to fit in with the preconceived opinion about the bionomic facies in which eurypterids are supposed to occur. It has been spoken of as a "most unusual occurrence;" "one which is most interesting because found in beds formerly supposed to be devoid of marine fossils," and so on. Again we read of the clear evidence of a marine passage between the Buffalo region and the Baltic area, because two almost identical species of eurypterids are found in these localities. Formations are declared to be marine because they contain eurypterids, and eurypterids are held to be marine, because they occur in formations considered on a priori grounds to be marine. Every writer seems to feel it necessary to fit the eurypterids into a marine or estuarine habitat; where the facts refuse to fall into line, they are cited as ineresting because they fail to, or else they are consciously suppressed or carelessly overlooked. The prevailing opinion as to the bionomy of the successive eurypterid faunas is as follows: Until well on in the Siluric the eurypterids were purely marine forms living in the seas and, inferentially, associated with the marine organisms therein. Toward the middle of the Siluric, the eurypterids all over the world left the seas and migrated into the various brackish water bodies then existing, seeking the mouths of rivers, the bays, lagoons and interior cut-off arms of the sea. From that time until the end of the Palaeozoic, they are supposed to have sought water of ever-decreasing salinity until they became entirely freshwater denizens. Their geographical distribution is accounted for by an assumed migration from one estuary or lagoon to another along the shores of various Palaeozoic continents.
Objections to Marine Habitat Theory. If this succession of events is the correct one, then the following question arises in connection with the distribution: If the eurypterids lived in pools or in marginal lagoons on the seashore, in estuaries, bays or cut-offs how did they get there to begin with?
