Evolution of the Calcareous Nannofossil Genus Micula in the Late Cretaceous

Abstract

The genus Micula appeared in the Turonian and disappeared at the end of the Maestrichtian. In early specimens of Micula staurophora, from which all other species evolved, the morphology is cubical and compact, commonly with secondary overgrowth (morphotype of Tetralithus pyramidus). In the Coniacian and Santonian the elements of Micula staurophora formed extensions leading to Micula concava. Micula staurophora coexisted with the generally less abundant M. concava. In the upper Maestrichtian Micula murus evolved from M. staurophora by reduction of the upper cycle of elements and extension of the elements of the lower cycle. All 3 species of Micula seem to have disappeared simultaneously at the end of the Cretaceous. Quadrum gartneri is considered a synonym of Micula staurophora (Tetralithus pyramidus morphotype) and thus the genus Quadrum is synonymous with and not ancestral to Micula. Micula praemura is just an isolated cycle of Micula staurophora. Evolution of the calcareous nannofossil genus Micula in the Late Cretaceous