First Data on Maastrichtian Ostracods of the Eastern Crimea (Mount Klementieva): Stratigraphic Potential in the Northern Peri-Tethys

Maastrichtian deposits in Eastern Crimea are represented by isolated outcrops of clayey-carbonate rocks of the Klementyevo and Beloyarsk formations. Their ostracod assemblages remain unstudied, and many aspects of the depositional environment are still debated. This study provides the first detailed investigation of ostracods from the Maastrichtian of Eastern Crimea and aims to identify stratigraphically significant levels, and to correlate them with zonal schemes based on planktonic and benthic foraminifera and calcareous nannoplankton. A rich ostracod assemblage has been identified in the Klementyevo section, represented by both isolated valves and complete carapaces belonging to 67 species, along with numerous indeterminate fragments. The distribution of ostracods is highly uneven: some intervals yield rare or no specimens, whereas others are characterized by abundant and diverse assemblages. Despite generally good to moderate preservation, most taxa remain in open nomenclature, reflecting both endemism and the fact that many taxa in the literature are identified only to the genus level (Andreu, 2001; Andreu and Odin, 2001, 2008; Damotte and Odin, 2001, a. o.). Four distinct ostracod assemblages (O1–O4) are recognized within the section. Assemblage O1 corresponds to the lower part of the upper Maastrichtian planktonic foraminiferal Abathomphalus mayaroensis Zone and the lower part of nannoplankton Zone CC26a. It is characterized by highly variable diversity (2–17 species per sample) and dominance of large eurytopic ostracods such as Cytherella obovata Jones et Hinde, C. ovata (Roemer), C. contracta (Van Veen), Xestoleberis ovata Bonnema, Krithe cushmani Alexander, K. kritheformis (Bonnema), Argilloecia communis Bonnema, Bairdia denticulataMarsson and Bairdoppilata pseudoseptentrionalis Mertens. Small-sized taxa occur sporadically. Assemblage O2 corresponds to the upper part of the A. mayaroensis PF Zone and the lowermost part of Pseudoguembelina hariaensis Zone, as well as to the upper part CC26a Zone and the lowermost of CC26b. The species Pontocyprella sp. first appears in this interval and becomes a persistent component of the assemblage. Additionally, rare but taxonomically diverse small ornamented ostracods occur, including Cytherelloidea hindei Kaye, different CytherelloideaAnnosocythere, Eucytherura. Assemblage O3 is characterized by relatively high diversity (12–14 species per sample) and the appearance of new small ornamented taxa, notably Saida sp. 1, Saida sp. 2, Phacorhabdotus lonsdaleianus (Jones), different Annosacythere, Eucytherura, and Neocythere. This assemblage corresponds to the middle part of the Ps. hariaensis and CC26b Zones. Assemblage O4 is marked by a significant decline in diversity, down to approximately five species per sample. Small representatives of the family Cytheruridae disappear completely, and only rare specimens of Cytherella, Krithe, and Argilloecia persist. This assemblage corresponds to the terminal part of the Ps. hariaensis and CC26b Zones. Thus, the recognition of ostracod assemblages has allowed a more detailed subdivision of the section, distinguishing four assemblages compared to only two based on planktonic foraminifera and two based on calcareous nannoplankton. The study was supported by the Russian Science Foundation Grant No. 25-77-10106, https://rscf.ru/project/25-77-10106/.