Nimbia occlusa is a form of Ediacaran fossil shaped like a circular or oval disk, with a thick rim around the margin. Within the rim the fossil is usually flat, but may have a central nipple or dimple. These fossils were generally believed to be those of cnidarians, but they have since been reinterpreted as structures made by microbial colonies (Grazhdankin, see Ediacaran biota for references and discussion). They can reach up to 6 cm in diameter, with a centimeter-thick rim. Some fossils are distorted.