Fish origins — fresh or salt water? 277 



Quite in contrast is Ko, the Padel Zone. Here there is an abundant marine inverte- 

 brate fauna, mainly of brachiopods and stromatoporites; forty-five species are listed. 

 It is not surprising to find that eurypterids are unknown and that of fishes there are 

 only isolated and apparently very rare scales of one species of Coe/o/cpis. 



K3, the Kaugatoma Zone, is, again, typically marine, with a fauna of forty-one 

 invertebrate species. Eurypterids are absent; fishes are found somewhat more 

 commonly than in K2, but only in the form of isolated scales of Gomphoclus and a few 

 spines of the Onchus type. 



In the K4 or Ohesaare Zone, exposed only in a very restricted area of the island, we 

 leave the limestone-dolomite formations typical of the lower zones, and find ourselves 

 in a deposit characterized for most of its extent by a rapid alternation between thin 

 limestones, typically crystalline and red in colour, and equally thin clay conglomerates; 

 in a profile little more than 3 metres in thickness Hoppe distinguishes no less than 70 

 such alternating layers. The situation, as Hoppe notes, indicates that we are very close 

 to the shore-line. There is a fairly good marine fauna in K4 as a whole, including 34 

 species of invertebrates. There are no eurypterids. There are, on the other hand, 

 numerous remains of fishes. But nowhere are there articulated specimens, the whole 

 material consisting of isolated scales, spines and fragments of various types, and 

 Hoppe notes that the fish remains are not normally associated with the marine 

 invertebrates, but occur only in bonebeds at three specific horizons. 



In attempting to review this interesting Oesel series, the most striking general feature 

 of the story is the fact that the fishes and marine invertebrates are (so to speak) 

 " allergic " to one another. The beds here follow the " rule of thumb " which we have 

 seen to apply in the Silurian in general : the more abundant the fishes, the fewer the 

 marine invertebrates, and vice versa. Zones K, and K3 are typically marine, and fish 

 are found rarely and only in fragmentary form; in K4 invertebrates and fish are both 

 abundant but are not found in the same layers; in Ki fish (and eurypterids) are abund- 

 ant and well preserved, while marine invertebrates are rare. 



With regard to the scale and spine findings of Zones K2-K4, we have a situation 

 comparable to that of the roughly contemporaneous Beyrichienkalk and the Scanian 

 scale-bearing beds farther west in the Baltic. The fragmentary nature of the remains 

 might be attributed to the work of scavengers if these fishes are considered marine or 

 to disintegration during transportation from stream mouths if they are believed to be 

 of fresh water origin. On the whole, however, consideration of the Oesel situation 

 in its entirety seems to me to throw the balance rather strongly towards fresh water 

 origin for these scales and spines. 



The critical problem is that of Zone K,, with its abundant and well-preserved 

 fauna. The assemblage here of fishes, eurypterids and ceratiocarids is one which 

 we have seen repeatedly in Silurian and early Devonian localities which are clearly 

 of fresh water nature, and O'Connell would have it that we are dealing here with a 

 fluviatile deposit. I fail to be convinced that this is the case. The conditions of 

 deposition suggest quiet waters, and the presence of at least a few definitely marine 

 invertebrates strongly indicates that, for the beds as a whole, the region certainly lay 

 no farther inland than could be reached by salt tidal waters. 



On the other hand, claims that the eurypterid-fish beds are typically marine seem 

 no better founded. The marine invertebrates reported are limited in variety and \cr\ 

 limited in numbers and there is no published evidence, as far as I am aware, that they 



