As a result of a study based on three years of biological sampling in the east Mississippi Delta region, eight macro-invertebrate assemblages are recognized, each characteristic of a specific sedimentary environment ranging from the Mississippi Delta marshes to the edge of the continental shelf northeast of the Delta proper. The eight assemblages and their corresponding environments are: (I) the delta marshes, (II) delta front and lower distributaries, (III) lower Breton Sound and lower pro-delta clayey slopes, (IV) upper Breton Sound, (V) inlets, or areas of strong currents, (VI) the shallow continental shelf of the Gulf of Mexico from 0 to 12 fathoms, (VII) the deeper part of the continental shelf from approximately 13 fathoms to 60 fathoms, and (VIII) the living oyster reefs of the shallow protected bays of the Delta region. The boundaries of these environments were established by plotting the distributions of both living and dead representatives of species of invertebrates furnishing hard parts plus the distributions of living soft-bodied animals which were so abundant as to characterize regions where animals with hard parts were scarce though present.
Comparison of the distribution of the hydrographic factors with the physiography of the landmasses in this area with the macro-organism distributions made it possible to formulate criteria for the interpretation of ancient environments as far back as the Miocene on the Gulf and Atlantic coasts. Paleontologic literature shows that most of the present-day delta species have been found in the Pliocene, and most of the diagnostic forms have existed since the lower Miocene. The primary factors influencing distributions of these organisms are bottom type, salinity and temperature (especially the degree of variability), turbidity of the water, and currents. It was also found that comparative rates of deposition could be estimated by the ratio of the number of living to the number of dead, in equal-size samples. A series of marine bottom communities based on the community concept of European marine ecologists is recognized on the basis of the most abundant and widespread animals.