Faunal composition and diversity of macroinvertebrate assemblages in the gray facies of nineteen "core" shales of Mid-continent Pennsylvanian cyclothems confirm the offshore marine origin previously established for those shales. Diverse assemblages of only calcitic fossils that show grain corrosion at northern Midcontinent localities (Nebraska/Iowa) indicate deposition under oxic to slightly dysoxic conditions within the calcite lysocline but below the aragonite compensation depth for this particular sea. Southward (Kansas), sparser calcitic faunas indicate that "core" shales there were deposited farther from the northern shoreline in water of greater depth and less bottom oxygen, closer to or locally below the calcite compensation depth. Farther southward near the Ouachita detrital source (Oklahoma), greater influx of sediment permitted rapid burial of shelled organisms to overprint the effects of lysoclines and compensation depths. Both the originally aragonitic molluscs as well as calcitic organisms are preserved in those shales. Fine detrital grain size, radiolarian-bearing nonskeletal phosphate, and pelagic fauna (ammonoids and conodonts) further substantiate an offshore marine origin for the black "core" shale facies that is surrounded by the gray facies, and which was deposited in a probably deeper, more continually anoxic environment where most benthos was excluded.