On the problem of the antiquity of the deep-sea fauna

The colonization of the abyssal zone by animals has extended over a very long geological time. The recent deep-sea fauna contains species which immigrated into the abyssal zone at different periods.

It is impossible to agree with Bruun that owing to a sharp decrease of the temperature of bottom waters during the late Tertiary the ancient deep-water fauna died out and that the abyssal zone was repopulated by young Quaternary forms. Since the determinations of paleotemperatures on which this concept is based were obtained by using shallow-water rather than deep-water Foraminifera such a conclusion seems questionable. It is further contradicted by the existence in the abyssal zone of many undoubtedly ancient elements (2 species of Neopilina, Spirula, Pogonophora etc.) and by the absence of a true deep-water fauna in deep basins, which were formed during the Quaternary (Japan, Mediterranean and Red seas).

The computations of Menzies and Imbrie, which led these authors to the concept of a relatively young deep-water fauna are likewise far from convincing. The groups selected for analysis are not characteristic of the deep-sea fauna; they include only about 11 per cent of the species recorded from depths exceeding 3000 m. The dominant groups of the deep-water fauna, rich both in number of species and in biomass, are not preserved in fossil condition and were not taken into account by Menzies and Imbrie.

An analysis of the systematic position and pattern of vertical distribution of many important groups of deep sea animals permits us to distinguish among them ancient and young settlers of great depths. Approximate counts show that the percentage of primitive archaic forms in the abyssal fauna is far higher than in the fauna of the shelf, thus providing evidence of the greater antiquity of the abyssal fauna.