Changes in bryozoan assemblages and bathymetric variations. Examples from the Messinian of northwest Algeria

Studies of Recent bryozoan faunas have shown that there is a correlation between colonial growth forms and various physical factors of the environment, especially bathymetry. Each given assemblage of zoarial forms can thus be related to a certain depth interval. In marine Neogene deposits numerous extant bryozoan species occur and some of them have more or less restricted depth ranges: they are used here to complement the indications given by the analysis of growth forms. In the Messinian (uppermost Miocene) of northwestern Algeria a general feature is the occurrence of well-developed diatomaceous deposits interbedded with marl beds where benthic invertebrate remains are often abundant. In the five sections under study bryozoans are represented by 87 species belonging to nine different colonial growth forms. Four different assemblages of these zoarial forms, characterised by presence/absence and number of fragments, are distinguished, corresponding to four depth intervals. The first and deepest assemblage (upper bathyal to lower circalittoral) occurs in the bottom part of the sections where colonies of a conescharelliniform species strongly predominate. The second assemblage (lower circalittoral) is represented by several species belonging to various zoarial forms: conescharelliniform, cellariiform, vinculariiform, adeoniform, and reteporiform. The third assemblage (upper circalittoral) is characterised by the absence of conescharelliniforms, but membraniporiforms and celleporiforms are abundantly represented, together with numerous erect colony fragments. In the fourth assemblage (infralittoral) membraniporiforms and celleporiforms predominate, although a number of erect zoarial forms are sometimes associated in various proportions. The presence of several extant bryozoan species associated with each of these four assemblages is used as a confirmation for the suggested depth intervals. A gradual shallowing is thus indicated by the study of the bryozoan assemblages through the five Messinian sections. The discussion shows that modern data on zoarial forms and species bathymetric distributions are clearly applicable to Cenozoic faunas but that caution must be exercised when using results obtained from different oceanographic regions.