Abstract

Siliceous plankton have been an important component of the oceanic silica cycle since the evolution of Radiolaria during late Precambrian or early Paleozoic time. Diatoms did not enter the cycle until Jurassic time, but now account for as much as 90 percent of the suspended silica in the oceans. It is postulated that the tremendous evolutionary success of the diatoms is responsible for evolutionary trends observable in the Cenozoic fossil record of the Radiolaria. Such trends include decrease in test weight, as well as structural changes such as increased pore size, decreased bar width, reduction or loss of test processes, and more regular alignment of pores–all changes that result in less silica uptake per test. Natural selection, mediated by the role of the diatoms in the silica cycle, apparently favors radiolarian phenotypes which use less silica in test construction.

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