The origination of six nannoplankton species in the late Paleocene to middle Eocene involves significant change in the entire nannofossil assemblage. The change began at the first occurrence and accelerates at the increase in abundance of the new species. The changes in the environment generally reflect the preferred ecology of the originating species. The species that originated in the late Paleocene to early Eocene (outside of the Paleocene-Eocene thermal maximum (PETM)) are generally thought to be oligotrophic taxa adapted to warm water conditions. During this long interval, temperatures were generally increasing and nutrient availability was decreasing; conditions that would have been advantageous for the originating taxa. Conversely, the species that originated in the middle Eocene are generally thought to be mesotrophs adapted to colder water conditions. These taxa were selected for during a time of generally increasing nutrient availability and decreasing temperature. Orbital stratigraphy indicates that the first occurrences of three Paleocene taxa are time transgressive by up to 400. kyr. The only event that appears to be synchronous is the abundance increase of Zygrhablithus bijugatus during the PETM. This species is thought to be mesotrophic and replaces the genus Fasciculithus at a time when nutrient availability was likely increasing rapidly.
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