Page:Eurypterids of the Devonian Holland Quarry Shale of Ohio.djvu/6

 remains are present. It is difficult to explain the intimate association of fishes and eurypterids not only in this black shale but also throughout the Silurian and Devonian sections other than by the conclusion that one served as prey for the other. In the Ohio Devonian, the pterygotids surpassed five feet in length (from the anterior of the carapace to the end of the telson) and were armed with stout and formidable chelicerae which, with their strong teeth, served as excellent grasping organs. Also, the pterygotids were active swimmers, as suggested by the flattened body and great telsonic flipper. In contrast to these active swimmers, the associated eurypterids, namely, Dolichopterus asperatus and Syntomopterus richardsoni, had simple, small chelicerae not serviceable as weapons of either defense or predation; both were probably benthonic forms, crawling on the mud but perhaps capable of swimming to a limited extent. These could well have been prey to the fishes and, indeed, to the large pterygotids. However, in this shale the fishes are the only animals with a covering rigid and hard enough to abrade the tough pincer-teeth of the pterygotids.

The numerous and persistent instances of the intimate association of pterygotids with primitive fishes throughout Silurian and Devonian time, as well as the presence of the greatly abraded teeth in the pterygotids herein described, point to the tentative conclusion that the fishes were a source of food to the great pterygotids.

The four forms from the Ohio Lower Devonian are different from any of the described North American species. In particular, there is no basis for comparison with the eurypterids of the Lower Devonian Beartooth Butte (listed above as "Wyoming"), as each species is quite different. The same is true of a comparison with the Lower Devonian of the Rhineland, Germany (Størmer, 1936). The Ohio Lower Devonian eurypterid fauna retains such Silurian elements as Erettopterus and Dolichopterus, two genera previously unknown above the Silurian. Pterygotus (Pterygotus) carmani likewise shows closer relationship to Silurian than to Devonian forms. The Downtonian eurypterids of the Welsh borderland seem to bear closest resemblance to the Ohio forms. The Downtonian is considered Devonian by some authors, although it has been recognized for many decades as a transition zone between Silurian and Devonian. Our knowledge of Devonian and even Downtonian eurypterids is still far from complete, and correlations on the basis of these fossils are not desirable at the present time. Undoubtedly, the highly specialized eurypterids will constitute good index fossils when more is known of their