| Summary: | International audience External test characters of Upper Cretaceous planktonic foraminifera are commonly com-pared to those of extant species. Similarities are used to interpret their life position: globu-lar pustulose forms (Whiteinella and Muricohedbergella) are supposed to have inhabited theupper part of the water column, whereas heavier keeled species (Rotalipora and Thalmanin-nella) would have evolved below the thermocline. However, this model cannot be applied toall time intervals. Indeed, during, Paleogene forms with a well-developed keel (Morozovel-lids) were common in the upper part of the water column.With the development of non-destructive X-Ray tomography, it is now possible to qualita-tively and quantitatively characterize all the ontogenetic stages of the planktonic foraminifera.Indeed, morphological test changes and internal volumes can be precisely constrained for eachchamber. CT techniques were therefore used to study, for the rst time, the growth patternsof three well-preserved Cenomanian species (Whiteinella aprica, Rotalipora cushmani andThalmaninnella greenhornensis) from the Greenhorn Formation of the Great Plains (SouthDakota, US). The ontogenetic succession of their chambers has been compared to those ofrecent fossil species (Miocene up to Holocene) still inhabiting oceans. Surprisingly, the re-sults would show that the globular W. aprica and the keeled R. cushmani have the samegrowth pattern which is similar to the thermocline dweller Globorotalia menardii. Th. green-hornensis growth pattern is clearly dierent and belongs to the deep dwellers group.Those results are important as they will allow a better understanding of the extinction steepsof rotaliporids (Rotalipora and Thalmaninnella) during the onset of the Oceanic Anoxic Event2.
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