Why are there no Post-Paleogene Dinoflagellate cysts in the Southern Ocean

Dinoflagellates are an important component of Antarctic coastal and sea ice communities but comprise only a relatively minor component of Southern Ocean oceanic phytoplankton assemblages. However, living species capable of producing geologically-preservable cysts have been reported only rarely from Antarctic waters and no Quaternary cysts have ever been recovered from Southern Ocean surface sediments. The youngest fossil dinoflagellate cysts to occur anywhere in the Antarctic - Southern Ocean region are Oligocene. Geographic and thermal isolation has prevented the poleward migration of cyst-producing dinoflagellates, which require a continental shelf or slope pathway to migrate. The loss of shallow water shelves from the Antarctic continent must have contributed to the local extinction of the Paleogene cyst-forming groups. -from Author