Extreme Morphological Plasticity Within Orbulina-“Praeorbulina-Like” Assemblages Related to Environmental Stress

International audiencePlanktic foraminifera, unicellular microzooplankton with a calcitic shell, have produced an exceptional fossil record, revealing an invaluable archive of biodiversity, morphological and evolutionary changes. The evolutionary lineage starting from Trilobatus Spezzaferri 2015 (= “Globigerinoides”) culminating in Orbulina universa d’Orbigny 1839 is a fascinating example of peramorphic spherisation lineage (increasing involution, coupled with increasing shell curvature). This chapter focuses on the extreme morphological variability observed in the Orbulina group in some horizons from Chélif Basin in Algeria, just preceding the well-known Messinian (Late Miocene) salinity crisis in the Mediterranean basin. Surprisingly, in such horizons, spherical Orbulina universa lineage end-member specimens coexist with ancestor-like morphotypes, such as Orbulina suturalis Brönnimann 1951 and the supposed extinct Praeorbulina Olsson 1964, as well as with malformed specimens. Many authors considered in fact that Praeorbulina last occurred within the Langhian stage in the Middle Miocene. A similar recovery of individuals which show an intergradation between a typical Orbulina morphology and morphologies close to the ancestors Orbulina suturalis and Praeorbulina was also reported in Last Glacial Maximum sediments from the northern Arabian Sea. In this Late Pleistocene case, AMS 14C data showed clearly unreworked character of this “Praeorbulina-like” populations. We discuss the possible link between this extreme morphological plasticity of Orbulina group in specific time horizons and possible stress conditions of the water column