Fossocytheridea (Cretaceous) is the ancestor to the extant ostracode Cyprideis. Most fossil taxa assigned to these genera are invariably associated with marginal marine biota and sedimentary facies. Geochemical and morphometric techniques on the normal pores of the calcite carapace have been applied to Quaternary Cyprideis to develop high-resolution proxies for paleosalinity. Using these same principles, we evaluated Fossocytheridea for both d¹⁸O values and normal pore morphometric variability to develop a proxy for Cretaceous estuarine seawater.

Twenty samples of F. kirklandi and F. posterovata were selected for isotope analysis (4–10 valves per sample). Care was taken to analyze only disarticulated adult valves that revealed little evidence for diagenetic alteration (e.g., clear, pristine calcite). The d¹⁸O values ranged from -7.8‰ to -11.7‰ PDB (average -9.6‰ PDB). Comparison with published isotope values for Mesozoic normal marine (-0.5-to -2.9‰ PDB) and freshwater (-13.6-to -14.6‰ PDB) data sets indicates that the values obtained for Fossocytheridea fall in between marine and freshwater end members.

Using a Scanning Electron Microscope, high quality digital photomicrographs were obtained for Fossocytheridea and modern Cyprideis. Using the image analysis software ImageJ 1.31v (W. Rasband, National Institute of Health, USA), we calculated both the area and the circularity for each pore observed on the carapace. The pore areas were plotted against the circularity values. Our results demonstrate that in environments where salinity was inferred less than normal, the pores of the ostracode carapace develop irregular shapes with increasing size.  Pore variations in Fossocytheridea resemble those observed for Cyprideis obtained from modern marsh deposits. Fossocytheridea kirklandi occurs in an oligohaline assemblage with charophytes and brackish mollusks and it reveals the most irregular pore shapes and the least variability in its isotopic signature that trends toward freshwater.

Pore morphometric (size and shape) and d¹⁸O analyses both support the previously inferred brackish water paleoecological interpretation for the late Cretaceous Fossocytheridea and together, they provide a multidisciplinary proxy for brackish seawater in ancient marginal marine strata.