Modern day radiolarian distributions can be grossly divided into warm and cold water spheres separated by the polar convergences and the associated pycnocline. During the Paleogene both warm and cold water sphere radiolarian diversities appear to have been lower than during the Neogene.
One major reason for this difference appears to be the difference in the number of “packages” of water (water masses essentially) for radiolarians to inhabit during these times. The number of “packages” or provinces increased in the Neogene due to the development of
polar convergences and their associated polar, shallow subpolar and intermediate waters, the initiation of the formation of Antarctic (Polar) Bottom Water and associated Circumpolar Water, and the development of the surface Eastern Tropical Pacific as a specific “package” of water.
Specific examples of radiolarians evolving into these packages support these contentions. Another Neogene development was the creation of a new “package” that apparently resulted in the evolution, or expansion, of a new niche, that of the symbiont bearing radiolarians by a variety
of taxa. This apparently was preceded by changes in Antarctic geographies (and perhaps some biological influences) that resulted in Antarctic glaciation and the development of this niche.
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