The PETM is accompanied by a mass

The PETM is accompanied by a mass extinction of 35-50% of benthic foraminifera (especially in deeper waters) over the course of ~1,000 years - the group suffering more than during the dinosaur-slaying K-T extinction. Contrarily, planktonic foraminifera diversified, and dinoflagellates bloomed. Success was also enjoyed by the mammals, who radiated profusely around this time.

The deep-sea extinctions are difficult to explain, as many were regional in extent (mainly affecting the north Atlantic). General hypotheses such as a temperature-related reduction in oxygen availability, or increased corrosiveness due to carbonate-undersaturated deep waters, are insufficient as explanations. The only factor which was global in extent was an increase in temperature, and it appears that the majority of the blame must rest upon its shoulders. Regional extinctions in the North Atlantic can be attributed to increased deep-sea anoxia, which could be due to the slowdown of overturning ocean currents,[12] or the release and rapid oxidation of large amounts of methane.[20][verification needed]

In shallower waters, it's undeniable that increased CO2 levels result in a decreased oceanic pH, which has a profound negative effect on corals.[21] Experiments suggest it is also very harmful to calcifying plankton.[22] However, the strong acids used to simulate the natural increase in acidity which would result from elevated CO2 concentrations may have given misleading results, and the most recent evidence is that coccolithophores (E. huxleyi at least) become more, not less, calcified and abundant in acidic waters.[23] Interestingly, no change in the distribution of calcareous nanoplankton such as the coccolithophores can be attributed to acidification during the PETM.[23] Acidification did lead to an abundance of heavily calcified algae[24] and weakly calcified forams.[25]

The increase in mammalian abundance is intriguing. There is no evidence of any increased extinction rate among the terrestrial biota. Increased CO2 levels may have promoted dwarfing[26] – which may (perhaps?) have encouraged speciation. Many major mammalian orders – including the Artiodactyla, horses, and primates – appeared and spread across the globe 13,000 to 22,000 years after the initiation of the PETM.[26]