The skeletal remains of radiolaria and planktonic foraminifera are among the most important microfossils in interpreting marine sediments. Radiolaria are of particular interest because of their wide geographic distribution and diverse skeletal morphologies. Much radiolarian micropaleontological data is based on analyses of mature skeletons. Likewise, reconstructions of radiolarian phylogenies have largely used mature skeletons (e.g., Knoll and Johnson 1975, Goll 1979, Goll and Björklund 1980, Riedel and Sanfilippo 1981, Sanfilippo and Riedel 1992). However, a more thorough knowledge of skeletal ontogeny may be a useful source of additional evidence to decipher phylogenetic lineages and better interpret paleoenvironments based on a wider range of skeletal forms.