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The analysis of the ostracods from three boreholes (1AS-7D-AM, 1AS-8-AM, and 1AS-31-AM) drilled in Atalaia do Norte town, Amazonas State, Brazil, allowed to recognize 30 species and nine genera. Cyprideis is the most abundant and diverse genus and is represented by 19 species already identified for the Solimões Formation, besides two new species described herein: Cyprideis atalaiensis sp. nov. and Cyprideis dictyon sp. nov. Intraspecific morphological variability, as well as the presence of nodes and tubercles, are observed in some species of Cyprideis and interpreted as a response to the paleoenvironmental changes during the Neogene in western Amazon region. Keywords: Cyprideis, morphological variability, paleoenvironmental changes, western Amazonia.

IDENTIFICATION AND PALEOENVIRONMENTAL SIGNIFICANCE OF THE OSTRACOD FAUNA FROM CODO FORMATION (NEOAPTIAN), EAST OF THE GRAJAU BASIN, MA, BRAZIL. The study of ostracods from sedimentary successions exposed in the east portion of the Grajau Basin, Maranhao State, related to Codo Formation, revealed a Neoaptian lacustrine fauna characterized exclusively by Harbinia and Candona represented by the species Candona sp., Harbinia micropapillosa (Bate), Harbinia angulata (Krommelbein & Weber), Harbinia sinuata (Krommelbein & Weber), Harbinia sp. aff. H. symmetrica (Krommelbein & Weber), Harbinia salitrensis (Krommelbein & Weber) and Harbinia sp. The stratigraphical distribution patterns of the ostracod palaeocommunities in the studied sections confirmed the occurrence of a cyclic depositional lacustrine environment, as assumed by previous faciological studies. These cycles are associated with events of expansion and contraction of the lake, characterized mainly as hipersaline, with shallow-water and very low oxygen levels. The sudden environmental changes caused by these events produced a monospecific occurrence of some resilient species, followed by a mass death.