The Paleocene-Eocene Thermal Maximum (PETM) event, which represented a sudden and abnormal rise in temperature during the early Cenozoic Era, is regarded as one of the most important global geologic phenomena. Two important index microfossils (nannoplankton and Ostracoda) were utilised to understand and predict the paleoenvironment and describe the changes during this period. The basis of the study was 12 cutting samples taken from Aaliji and the lower part of Jaddala formations of a subsurface section of (Ba-8) borehole in central Iraq. Some geophysical data were used to determine the upper and lower contacts of the Aaliji Formation and define the shale rate in the studied formations. The micropaleontologic investigation reveals twenty-four nannoplankton species and twenty species belonging to seven genera of Ostracoda. The use of Nannoplankton fossils led to the identification of two types of biozones based on two species belonging to the genus Discoaster, which are ordered from bottom to top as follows; 1- Discoaster nobilis Interval Biozone (CP7) and 2- Discoaster multiraditus Interval Biozone (CP8). The biozones were compared locally and regionally with their equivalent biozones, which deduced the age of the Aaliji Formation as (Late Paleocene-Lower Early Eocene) whereas (Early Eocene) for the studied part of the Jaddala Formation. The determination of the upper and lower boundaries was determined by interpreting the geophysical logs. Ostracoda fossils were used to predict paleoecology and its changes in the area during the PETM episode. The transmutation of nanoplankton fossils from the Paleocene to the Eocene indicates an abnormal rise in global temperatures, flourishing and high diversity of some nanoplankton, such as some species belonging to Discoster, especially those in the CP8 zone.
