The ostracode assemblage of the Arkadelphia Formation, upper Maastrichtian, of southwestern Arkansas is characterized by a well-preserved, relatively diverse, and abundant fauna representing 36 podocopid species and an undetermined number of species of the platycopid genus Cytherella. Thirteen of these account for 85 percent of the fauna. The dominant forms are Cytherella spp., Brachycythere rhomboidalis (Berry, 1925), Haplocytheridea renfroensis Crane, 1965, Haplocytheridea bruceclarki (Israelsky, 1929), and Brachycythere ovata (Berry, 1925), which account for about 57 percent of the specimens found. Other species that are less common, but are characteristic of the Arkadelphia, are Antibythocypris macropora (Alexander, 1929), Ascetoleberis hazardi (Israelsky, 1929), Aversovalva fossata (Skinner, 1956), Brachycythere ledaforma (Israelsky, 1929), Curfsina communis (Israelsky, 1929), Cytheromorpha arbenzi (Skinner, 1956), Escharacytheridea micropunctata (Alexander, 1929), and Veenia arachoides (Berry, 1925). The ostracode assemblage indicates that the Arkadelphia was deposited in the inner sublittoral zone.
The species Veenia parallelopora (Alexander, 1929) and Brachycythere foraminosa Alexander, 1934, are restricted to the Arkadelphia Formation and allow correlation with other Gulf Coastal Plain Upper Cretaceous units. A new ostracode interval zone, the Veenia parallelopora Zone, is proposed. This zone divides the Platycosta lixula Zone.
Calcareous planktic microfossils reported from the Arkadelphia and its correlatives in the Veenia parallelopora Zone indicate that these deposits are late Maastrichtian in age, not middle Maastrichtian as some authors have thought.