Micula murus is one of the main calcareous nannofossil biostratigrapic markers of Tethyan and Intermediate provinces in the upper Maastrichtian (uppermost Cretaceous). A review of its first occurrence at 14 deep-sea sites and sections shows that it is time transgressive from the Tropical Realm of the Atlantic and Pacific Oceans to the intermediate latitudes of the North Atlantic, South Atlantic, Indian Ocean, and the northern Tethys. M. murus remained confined to the Tropical Realm for ∼1.2 m.y. in the early–late Maastrichtian, thus supporting high-latitudinal thermal gradients. It subsequently spread out in the late Maastrichtian to temperate latitudes and to the Tethys in coincidence with the onset of a thermohaline circulation change ca. 67.5 Ma, suggesting a major change in surface-water circulation and interocean communications.