The Shubuta Clay at the type locality in southeastern Mississippi consists of about 75 ft of gray, marine calcilutite. It yields a diverse and well-preserved invertebrate fauna of ostracods, foraminifers, and nannofossils. Study of more than 8,000 specimens of ostracods, differentiated into 36 species, provided a basis for morphologic, evolutionary, and paleoecologic interpretation of this rich mid-Tertiary fauna.

Comparisons of the Shubuta ostracods with those of the subjacent Pachuta Marl and the superjacent Red Bluff Clay indicate that the Shubuta fauna is clearly intermediate in character; however, a marked faunal change is found about 45 ft above the base of the Shubuta at the type locality. The lower assemblage is characterized by Bairdia woodwardensis Howe and Law, Cytheropteron montgomeryensis Howe and Chambers, Digmocythere russelli (Howe and Lea), and Hermanites? dohmi (Howe and Chambers) whereas the less diverse upper fauna is dominated by Haplocytheridea montgomeryensis (Howe and Chambers) and lacks many forms present in the lower part. The vertical change from high diversity to low diversity may reflect a shift to deeper water deposition during the Shubuta interval. This interpreta ion is corroborated by a comparison of the Shubuta ostracods with the depth distribution of related living forms.

The paleontologic evidence suggests correlation of the lower part of the Shubuta with Jacksonian (Upper Eocene) strata of the Gulf Coast. In contrast, the ostracods of the upper part show a closer affinity to Vicksburgian forms.

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