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DISTIBUTION IN TIME & SPACE -pg 4.

The rise of the dominant diatoms and the sharing of niches during the Cenozoic had a major impact on radiolarian diversity. Extensive interspecific competition between diatoms and radiolarians for dissolved silica is suspected to be the cause of an observed decrease in the average weight of radiolarian skeletons during the Cenozoic. However, cold/deep water radiolarians did not experience a weight loss, therefore, only groups in ecological contact with diatoms were affected.

The final extensive reoganization of radiolaria came in the Paleogene-Neogene transition. Several groups evolved and/or diversified at this transition, including the Tholonids, Antarctisins, Collosphaerids, Artiscins, Spongasterins, and Pyloni. The circumpolar water masses formed in the Neogene are thought to have been a factor in the evolution of the Tholonoids and the Antarctisins, since both are warm-water sphere forms. The development of nutrient-poor subtropical water masses provided a niche for the Collosphaerids, Artiscins, Spongasterins, and Pylonids, which are all warm-water sphere forms.

Image from F. Cordey's The Rad Page (see Links)

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