the standing crop of the living animals increases considerably from 

 north to south, possibly suggesting that the resources available within 

 each niche increase at lower latitudes. Within the tidal creek environ- 

 ment, the number of types of niches occupied by the mollusc fauna does 

 not appear to change with latitude, although the specific types do 

 (infaunal and epifaunal species present, feeding types change with 

 latitude) . 



The identity, relative abundance and size-frequency distributions 

 of the more common species within the living fauna are adequately 

 represented in their death assemblages. The assemblages differ significantly 

 from the living fauna in both the total number of species and the 

 absolute density. Thirty of the 62 species recorded from the death 

 assemblages appear to represent introductions from other environments; 

 all but two of these "accidental" species were represented by only a 

 few (i.e., less than 10) individuals. 



Although a wide variety of environmental parameters appear to affect 

 the accumulation and distribution of dead material, many of the differences 

 seen between the living and dead fauna can be explained by the local 

 redistribution of empty shells by fresh water runoff and tidal activity. 



Published evidence indicates that both salt marshes and the tidal 

 creeks that dissect them represent distinct sedimentary environments. 

 The results of this investigation therefore, in confirming that the 

 community structure within these habitats is similar in the types of 

 niches occupied, can help to explain the fact that comparable lithofacies 

 of different ages often yield fossil assemblages containing closely 

 related and ecologically similar species. 



Of the models currently available in the ecological literature, only 

 the logarithmic curve gives an adequate representation of the individuals- 

 species relationships observed in the living fauna. The fit to 

 observation is far poorer in the case of the death assemblages, but 

 becomes adequate when all the species represented by single occurrences 

 are eliminated. 



The numbers of mollusc species found in the fauna of successively 

 larger salt marshes or tidal creek systems are not significantly different. 

 This suggests that increases in species diversity noted from successively 

 larger regional samples may reflect the presence of a greater variety 

 of habitats within the sample rather than a real enrichment of the 

 fauna. 



The results of this investigation suggest that quantitative studies 

 of fossil assemblages probably can yield useful information on the 

 evolution of the "structure" seen in present day communities of plants 

 and animals. Since salt marshes and tidal creeks represent rather 

 "specialized" habitats, it is essential that studies similar to that 

 described here be conducted in other environments so that adequate 



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