Fish origins — fresh or salt water? 267 



crustaceans and very little, if anything, else (plant remains of probable Huvialile or 

 terrestrial nature may be present as well). There are various Silurian deposits with a 

 similar assemblage, and in default of evidence to the contrary, it is reasonable to 

 consider them likewise continental in nature. 



Need for detailed stratigraphic study 



If a geologic formation or group is dominantly marine (or continental) in nature, 

 it is frequently claimed that the fossil forms contained in it at every horizon must be 

 entirely marine (or continental) in origin. Such statements should not be made or 

 accepted without careful consideration of stratigraphic detail and possible facies 

 differences. For example, the early Permian Redbeds of north central Texas in which 

 I have worked extensively, are predominantly continental, with a wealth of terrestrial 

 reptiles and fresh water amphibians and fishes. But it would be absurd for me to make 

 a sweeping claim that all the fossil content of these beds (which includes, for example, 

 a number of cephalopods) is of a continental nature. The deposit is a deltaic one, and 

 from time to time the delta was invaded for a brief period by waters from a sea which 

 lay not far away. When in doubt, it is thus important to know, in the case of such a 

 continental formation, the exact horizon and locality from which a fossil has come 

 before attempting to reach conclusions as to its ecologic position. 



The same caution should be observed in the case of fossils contained in a dominantly 

 marine formation or zone- — particularly so for fossils of the time under special 

 consideration, the late Silurian, when over much of Europe there was occurring a 

 transition from marine conditions to the continental " Old Red " of Devonian times. 

 We are deahng with " Passage Beds ", tending toward and reaching deltaic conditions, 

 where fluctuations of a very minor nature could readily cause alternations of salt and 

 fresh water environments. Close examination of stratigraphic detail is requisite if the 

 truth is to be sought for. 



Fresh and brackish water invertebrates 



Most invertebrate palaeontologists working in the Palaeozoic are dealing with 

 marine faunas, and hence generally tend to assume, unthinkingly, that all Palaeozoic 

 invertebrates are unquestionably marine. This was obviously not the case; there is 

 an acknowledged, if small, fresh water fauna of invertebrates in the Carboniferous, 

 and there is every reason to believe, on theoretical grounds, that at whatever earlier 

 date a basal plant food supply had been established, an invertebrate fauna would have 

 soon evolved to occupy this ecological niche. Before the close of the Devonian a 

 continental flora of rather advanced and diversified nature was present. This must 

 have had an antecedent history of some length. It is not necessary, however, to wait 

 for the development of land floras to establish a fresh water biocenosis, and it may well 

 be that fresh waters may have contained, as far back as the Cambrian, at least, a 

 basal element in a potential food chain in the form of simple algae. 



Two problems, among many, which must be *' solved " by any type of animal 

 which attempts to leave the ocean and avail itself of the opportunities ofl'ered by a life 

 in fresh waters, are: (1) physiological adaptations to counteract dilution o\' body 

 liquids (with resulting death) in non-saline waters, and (2) some method — preferably 

 active locomotion — for maintaining the population in an inland environment and 

 preventing adults or young from being carried downstream, back the ocean. 



