268 Alfred Sherwood Romer 



The modern fresh water invertebrate fauna is relatively sparse as compared with 

 that of the ocean, but contains a considerable variety of types. As would be expected, 

 sessile forms, and those with relatively poor locomotor ability, such as sponges, 

 coelenterates, bryozoans and brachiopods are rare or absent; the molluscs, however, 

 have achieved success by means of highly specialized modifications of developmental 

 processes. More common are active-swimming bilaterally symmetrical forms 

 including (in addition to fishes) a variety of members of the worm phyla and, most 

 especially, diversified crustaceans, mainly phyllopods, cladocerans, copepods and 

 ostracods. In addition to truly fresh-water assemblages, there are, of course, a variety 

 of other invertebrates derived from both marine and inland faunas which are eury- 

 haline, able to live in brackish water deposits of the sort which may be encountered 

 in estuarine and deltaic regions (cf. O'Connell, 1916, 70-76). 



Although it seems certain that a fresh-water invertebrate fauna containing elements 

 analogous to those present today was developed at an early stage, little attention has 

 been paid to this subject by invertebrate palaeontologists. As in recent times, however, 

 most of the forms present would probably have been soft-bodied, and hence not 

 likely to be fossilized, or of small size and hence liable to escape observation. In the 

 Devonian and Silurian fish-bearing beds which are reasonably suspected of being of 

 fresh-water origin recorded invertebrates are few in number. There are occasional 

 reports of lingulids, Leperditia and a few other forms which may indicate a brackish 

 layer or a brief incursion of salt waters. Two invertebrate types, however, occur time 

 after time with fishes — ceratiocarids and eurypterids. It may be reasonably claimed 

 that they are fresh water in habitat or are euryhaline forms which were able to inhabit, 

 with fishes, fresh water streams into which typical marine life could not extend. 



The ceratiocarids are small crustaceans, apparently of phyllopod affinities, which 

 have attracted little scientific attention. Study of them from an ecological point of 

 view should prove interesting. Dictyocaris is another phyllopod which has a distribu- 

 tion likewise suggestive of fresh waters (Stormer, 1934). 



The eurypterids are the one group of Palaeozoic invertebrates which exhibit such 

 distinctive features in their apparent ecological surroundings that students of inverte- 

 brate palaeontology have had to hesitate, at least, in their habitual tendency to claim 

 a marine environment for all fossils. The eurypterid environmental picture is compar- 

 able in many ways to that for the older fishes, but with somewhat less evidence for fresh 

 water habitats. We can here but briefly note the main points in the discussion; 

 among the major papers concerned are those of O'Connell (1916), Pompeckj, 

 Versluys and other discussants in a 1923 symposium, and Ruedemann (1934, etc.); 

 a recent resume is that of Prantl and Pribyl (1948, 108-111). Eurypterids are most 

 abundant in the Silurian; they are rare earlier, and although surviving until the 

 Permian become increasingly uncommon from the Middle Devonian onward. The 

 post-Silurian forms are definitely fresh water, and these, together with certain of the 

 Silurian finds, are often associated with fish faunas which likewise appear to be fresh 

 water in nature. In the Silurian, however, the eurypterids occur in a number of 

 situations in which the evidence for a fresh-water habitat is far from certain (as the 

 " water limes ") and, further, are present to some degree in beds of definitely marine 

 nature. As Prantl and Pribyl note, the " almost universally accepted opinion " 

 today is that the eurypterids were fresh water organisms throughout their history. 

 It appears to be rather generally accepted that occurrences of eurypterid remains in 



