Small elliptical mounds and depressions (about 1 m long) occur on and within a bed of fine-grained Middle Ordovician limestone in the Ottawa Valley, Canada. Both the mounds and the bottoms of the depressions contain abundant fossils in contrast to the surrounding bed, which is essentially barren. The mounds and depressions formed in the outer and deepest part of a quiet tropical marine lagoon, behind a carbonate bank, where bottom conditions were normally anoxic. An unusually diverse and abundant fauna lived on the bottom just prior to deposition of the bed containing the mounds and depressions, indicating an abnormally high oxygen concentration at the sediment/water interface at the time. As bottom conditions deteriorated, many of the species survived only on the mounds, which had formed by the trapping and holding of sediment by large branching bryozoans. The suspension-feeding fauna that flourished on the mounds, slightly above the barren sea floor, implies a location at the interface between the oxygenated normal sea water and a bottom lens of anoxic water of somewhat higher salinity. The depressions are washout features that formed when sediment was eroded from the mounds.