The first hantkenids (Hantkenina mexicana) lived in deep planktonic environments with minimum oxygen levels. However, isotopes suggests they migrated to fully oxygenated shallower waters at about 43.8 Ma.[2] During the Eocene the climate went through climate changes. The peak of the Cenozoic warmth occurred during the Early Eocene. Afterwards, global cooling began, persisting to Early Oligocene.[3] The first large ice caps appeared in Antarctica during this time.[4] This climate change also affected deep-water environments.
The cooling process caused gradual changes in the structure of the water column. As the water cooled, the rates of remineralization of organic matter at the near surface water altered. A consequence of this global cooling was that bacterial metabolic rates at the upper water column slowed down and allowed sinking organic matter to descend further to deeper environments in the ocean. This made the oxygen minimum zone, less intense and more spread out. New niches for deep-dwelling zooplankton that were able to tolerate low levels of oxygen opened up. Pearson and Coxall (2013) speculate that the evolution of Clavigerinella and Hantkenina was related to this global cooling,[3] as well as pulses of deep-water anoxia [5]
