Sediments from several Deep Sea Drilling Project sites and numerous surface samples were studied to determine the characteristics of radiolarian populations diagnostic of Campanian biogeographic zones and representative of different depths of deposition. Faunal composition showed three basic types of variation in response to the environment: (1) morphotypes present, (2) diversity, and (3) abundance. Few species of Radiolaria were ubiquitous. Of the 146 morphotypes studied, only members of the Dictyomitra multicostata and Protostichocapsa stocki groups occurred in all samples. Most forms were restricted to low and intermediate latitudes, but species of Prunobrachium were present only at high latitudes. Shallow and deep-water deposits contained a high percentage of nassellarians, while assemblages of intermediate depth were characterized by a predominance of spumellarians. Diversity varied directly with depth and inversely with latitude. Neritic assemblages were commonly dominated by only two or three morphotypes, which comprised from 60 to 90% of the population. Diversity indices for neritic populations determined on the Fisher scale (alpha = 1-20) showed values of alpha less than 3. In contrast, alpha values characteristic of deeper water assemblages ranged from 9 to 20. Although abundance is influenced by localized productivity and sedimentation rates, numbers of Radiolaria tended to increase with depth in any given depositional basin.

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