Page:The Habitat of the Eurypterida.djvu/105

 of the sea. It is often argued that such alternations of sandstones and shales as we see in this series indicate near-shore oscillatory conditions, the shales marking a slight advance of the sea and of fine deposition, the sands marking a retreat and the seaward advance of continental clastics. In the present instance it is difficult to explain the presence of eurypterids as marine organisms if we account for the lithological variation in the customary manner, for it is in the sandstones which mark the dominance of terrigenous sedimentation that the eurypterids are more abundant, while they are scarce in the shales which accompany the advance of the sea. If they were living along shore they should be abundant in the shales. The reply may be made that the eurypterids at that time preferred the sandy facies only and that the occurrence of dead individuals or shed exoskeletons in the muds was fortuitous. A phenomenon can hardly be called fortuitous which occurs again and again in response to a given set of conditions. Furthermore, if the eurypterids did live in the sandy facies then there is no reason why their remains should not have been preserved, for it is a mistake to believe that such exoskeletons would be destroyed by the waves, except perhaps on a shingly beach, a facies with which we are not here concerned. A short distance out to sea eurypterid remains would quickly be buried, the hollow case being soon filled by infiltrating sand. Anyone familiar with the occurrence of Limulus exoskeletons on sandy shores knows that they are easily filled by and buried in the sand and that they are preserved in toto, not broken to pieces. Thus we cannot account for the occurrences of the eurypterids on the assumption that they are marine organisms. In the Normanskill beds not a single entire specimen has been found, the whole fauna being made up mostly of carapaces with some separated abdominal segments. In the Schenectady sandstones the conditions are the same, but in the shales the preservation is better.

The evidence clearly militates against a marine habitat for the eurypterids in these two regions and the hypothesis of a fluviatile origin while not yet very strongly supported at least accounts for the observed facts. If the eurypterids were living in the rivers in Middle and Upper Ordovicic time, then it is to be expected that their remains would be carried out to sea. In rivers of moderate gradient it is not so likely that an abundance of remains of fluviatile organisms will be washed seaward, for they may be entirely broken up during transportation, or they may be caught in hollows along the banks, or even