A paleoecological study was made of a phosphatic nodule zone at the base of the Robbins shale (lower Virgilian, Upper Pennsylvanian) in Douglas County, Kansas. The nodules formed in shallow marine basins or bays which are inferred to have been characterized by foul bottom conditions. Fossils in some of the nodules include ganoid fish skulls with brain casts, nautiloids, ammonites, and orbiculoid brachiopods. The pelagic species presumably lived in the bays. The former regional picture is reconstructed as a warm, humid, semitropical area, with low-lying land areas nearby. The sea floor was nearly flat and only shallowly covered with sea water that lacked strong currents or overturning at the time the nodules formed.

The stratigraphic section studied has a total thickness of one foot and consists (in upward order) of a lower marine shale with phosphatic nodules at the base, a goethite bed, and an upper shale bed. The lower marine shale contains typical corals, brachiopods, and bryozoans and was deposited in an oxidizing environment. Seemingly the lower marine shale marks the beginning of a new cycle of deposition, with the upper shale bed of the one-foot section deposited in brackish water. The short-lived environment of the lower shale did not produce a typical marine assemblage of clay minerals.

The ganoid fish remains are described because of their unusual occurrence and preservation.