Studies of Neogene ostracods from the western Amazonia region, began in 1979, mainly dealing with taxonomic identification, recording a vast, diverse, and endemic fauna, predominantly represented by the Cyprideis genus. This genus is euryhaline and adaptable to the most diverse environmental conditions, such as water salinity level, temperature, and oxygenation, presenting high ecophenotypic plasticity. Thus, the rapid adaptive response of the genus to bioevents, during the deposition of the Pebas System, in the Neogene, in the western region of the Amazonia, led to the development of peculiar morphological characteristics and high intraspecific variation, making it difficult to differentiate species of the genus within this system. Although several authors have studied the Cyprideis genus from the Neogene of Western Amazonia, the number of species is still underestimated. In that context, the present study aimed to survey and taxonomic refinement of the species of this genus, from different boreholes (1AS-1-AM, 1AS-4-AM, 1AS-8-AM, 1AS-32- AM, 1AS-33-AM and 1AS-36-AM) drilled in the western Amazon, west of the state of Amazonas. A total of 9353 valves were
counted, in which 30 species and 8 genera of ostracods were identified. Out of these, 22 species belong to the Cyprideis and 8 belong to other genera (Cypria aqualica, Cytheridella danielopoli, ?Paracypris sp., Pellucistoma curupira, Penthesilenula olivencai, Perissocytheridea acuminata, Perissocytheridea ornellasae and Rhadinocytherura amazonensis). The species Cyprideis goeldiensis and Cyprideis javariensis are new and
Cyprideis sp. 1 remains in open nomenclature. A new variation of the species Cyprideis matorae was identified and the species Cyprideis santaelenae, originally described for the Pebas Formation, is recorded here for the first time for the strata of the Solimões Formation. The presence of index species allowed the identification of four ostracod zones in core 1AS-32-AM (Cyprideis caraionae Zone, Cyprideis minipunctata Zone, Cyprideis cyrtoma Zone and Cyprideis paralela Zone), compatible with the Grimsdalea palynozone, from the middle to
upper Miocene. A new Ostracod Association Zone named Cyprideis cyrtoma Association Zone is here proposed, which has an inferred age in the Tortonian and records the peaks of abundance and diversity of species, especially of the genus Cyprideis, throughout the cores studied herein and in those from previous studies. The great predominance of species of the Cyprideis genus in the studied material, in addition to the associated occurrence of rare freshwater and marine genera, points to a lacustrine, semi-confined, brackish and mesohaline environment.
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