Trace fossils and other sedimentologic evidence of bioturbation in latest Cretaceous and earliest Tertiary marine deposits in Denmark provide important clues to understanding the ecology of the sea bottom immediately before, during, and after the terminal Cretaceous boundary event. Trace-fossil associations at all the onshore Danish boundary sites indicate a general upward shoaling of the late Maastrichtian chalk sea, which was accompanied by an abrupt shift from a carbonate ooze sea floor to a marly, clay-rich, unstable substrate at the boundary. Although the substrate condition may have become more stable in the early Danian, water depths never did return to their former deep level. There is evidence of localized anoxia during the deposition of some boundary sediments (in the Fish Clay) in shallow parts of the Danish Maastrichtian sea, but no clear evidence of anaerobic conditions occurring at the era boundary was observed in boundary strata deposited in deeper water near the center of the basin. In fact, earliest Danian marl there is totally bioturbated. A pulse of calcite dissolution in shallow water coincided precisely with the era boundary, and this event played a major role in the formation of the Fish Clay in eastern Denmark, which is a condensed series of smectitic clay-rich layers from which much calcite has dissolved. The effects of this dissolution episode, however, were not so extreme in the marl that was deposited at the same time in slightly deeper water in northwestern Denmark. Evidence from Cretaceous-Tertiary boundary sequences in Denmark and elsewhere suggests that no single catastrophe can account for the major biotic extinctions that occurred at the end of the Cretaceous Period. The primary causal factors of the terminal Cretaceous extinction event appear to be drastic global sea-level regression occurring simultaneously with extensive volcanism on land and a strong pulse of calcite dissolution in ocean surface waters.