Adaptative strategies of the Toarcian benthic foraminiferal assemblages from the Middle Atlas (Morocco): Palaeoecological implications

• Benthic foraminifera reflect environmental changes during the Toarcian.
• Several foraminifera adapted to oxygen restricted conditions changing shape and size.
• Proliferation of opportunists under stressful conditions in the bottom during Early Toarcian.
• Then foraminifera adapted to shallowing trend during Mid-Late Toarcian oxygenated bottom.
• Foraminiferal morphogroups show great potential for reconstructing bottom conditions.

In the Toarcian deposits of the Middle Atlas (Morocco), the analysis of foraminiferal assemblages shows that benthic foraminifera reflect the changes in environmental conditions that influenced the diversity, composition of assemblages and morphology of the test (shape and size). Several species—belonging primarily to orders Lagenina, Robertinina and Textulariina—adopted particular shapes and sizes to survive in adverse conditions and sometimes to proliferate in niches subjected to stressing conditions (e.g. poor-oxygenation or muddy bottom). Lenticulina is the most frequent and diversified, due to the opportunist behaviour and low ecological requirements. Under oxygen-restricted conditions, Lenticulina exhibit adaptation mechanisms for survival such as decreasing the test size. Another means of adaptation to oxygen-restricted conditions is the flat compressed test (e.g. Planularia), where the high surface/volume ratio favoured gas interchange. The foraminiferal morphogroups characterized by elongated shells (Dentalina, Citharina, Ichtyolaria, Lingulina, Marginulina, Nodosaria, Vaginulina), colonise the low energy and deep environments occupying infaunal microhabitats in muddy soft grounds. Textularids are more frequent in relatively shallow environments, with tests showing agglutination of coarse siliceous grains and they are adapted to high energy conditions. The porcelaneous forms represented by Ophthalmidium survived under restricted oxygen conditions during the Early Toarcian with a reproductive strategy involving early maturation, thereby resulting in an abundance of small specimens of Ophthalmidium. Finally, the opportunist genus Reinholdella proliferated in oxygen restricted biofacies and dominated the foraminiferal assemblages of some sections (85%) in clayey confined environments during Early Toarcian.