A comparison of the Ostracoda from the Shubuta clay (Jacksonian Stage) and the overlying Red Bluff Clay (Vicksburgian Stage) of the east-central Gulf Coast region reveals an easy and reliable method of delineating the Jacksonian-Vicksburgian boundary which separates these Cenozoic shelf deposits.

Ten or more Shubuta ostracods species continue into the overlying Red Bluff, but another six span the Jacksonian-Vicksburgian boundary as evolutionary descendants. Argilloecia subovata Huff, Buntonia shubutaensis Howe, Eucythere shubutaensis Howe & Howe, Occultocythereis broussardi (Howe & Chambers), Trachyleberis? montgomeryensis (Howe and Chambers), and Trachyleberis? quadrata Howe & Howe are Shubuta Clay species which evolved into direct evolutionary descendants in the Red Bluff Clay. The descendants were respectively: Argilloecia sp. aff. A subovata Huff, Buntonia n. sp., Eucythere woodwardsensis Howe, Occultocythereis kempi (Howe & Law), Actinocythereis quadrataspinata (Howe & Law), and Actinocythereis thomsoni (Howe & Law).

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