ABSTRACT The occurrence of exclusively agglutinated assemblages in the fossil record has remained an enigma in terms of the environmental significance because the modern examples are confined either to the deep ocean or intertidal marshes. Two questions can be asked: are the fossil examples primary (and therefore not represented by modern analogues), or are they the dissolution residues of originally calcareous assemblages? In this paper we address the second question by making a comparison between original, mainly calcareous, modern assemblages and agglutinated assemblages experimentally produced from them. The samples studied were from the outer shelf, continental slope and abyssal plain. All were dominated by calcareous taxa. Following dissolution by acetic acid, each yielded a diverse assemblage of agglutinated forms. The agglutinated taxa show distribution patterns that can be broadly correlated with water mass and substrate tranquility or disturbance. We conclude that many fossil agglutinated assemblages are the result of partial or total loss of the calcareous element through dissolution. However, differential loss or preservation of organically-cemented taxa during early diagenesis may further alter the taxonomic composition of recent assemblages, leading to a further difference between the modern fauna and the “flysch-type” fauna of the Palaeogene North Atlantic.

ABSTRACT The occurrence of exclusively agglutinated assemblages in the fossil record has remained an enigma in terms of the environmental significance because the modern examples are confined either to the deep ocean or intertidal marshes. Two questions can be asked: are the fossil examples primary (and therefore not represented by modern analogues), or are they the dissolution residues of originally calcareous assemblages? In this paper we address the second question by making a comparison between original, mainly calcareous, modern assemblages and agglutinated assemblages experimentally produced from them. The samples studied were from the outer shelf, continental slope and abyssal plain. All were dominated by calcareous taxa. Following dissolution by acetic acid, each yielded a diverse assemblage of agglutinated forms. The agglutinated taxa show distribution patterns that can be broadly correlated with water mass and substrate tranquility or disturbance. We conclude that many fossil agglutinated assemblages are the result of partial or total loss of the calcareous element through dissolution. However, differential loss or preservation of organically-cemented taxa during early diagenesis may further alter the taxonomic composition of recent assemblages, leading to a further difference between the modern fauna and the “flysch-type” fauna of the Palaeogene North Atlantic.