Description: Climate change is predicted to alter temperature, carbonate chemistry, and oxygen availability in the oceans, which will affect individuals, populations and ecosystems. We use the fossil record of benthic foraminifers to assess developmental impacts in response to environmental changes during the Paleocene Eocene Thermal Maximum (PETM). Using an unprecedented number of µ-Computer Tomography scans, we determine size of the proloculus (first chamber), number of chambers, and final size of two benthic foraminiferal species which survived the extinction at Sites 690 (Atlantic sector, Southern Ocean, paleodepth 1900m), 1210 (central equatorial Pacific, paleodepth 2100m), and 1135 (Indian Ocean sector, Southern Ocean, 600-1000m). The population at shallowest Site 1135 does not show a clear response to the PETM, whereas those at the other sites record reductions in diameter or proloculus size. Temperature was similar at all sites, thus not likely the reason for differences between sites. At Site 1210, small size coincided with higher chamber numbers during the peak event, and may have been caused by a combination of low carbonate ion concentrations and low food supply. Dwarfing at Site 690 occurred at lower chamber numbers, and may have been caused by decreasing carbonate saturation at sufficient food levels to reproduce. Proloculus size varied strongly between sites and through time, suggesting a large influence of environment on both microspheric and megalospheric forms without clear bimodality. The effect of the environmental changes during the PETM was more pronounced at deeper sites, possibly implicating carbonate saturation.