Ontogenetic growth and variation in the skeletal structure of two Late Neogene Sphaeropyle species (Polycystina radiolarians)

Two four-shelled species characterized by a pylome-bearing outermost shell - Sphaeropyle robusta and its descendant, Sphaeropyle langii - are selected for morphological study. Both species are morphologically identical to three-shelled Actinomma morphotypes, morphotypes A and B, respectively. Furthermore, the number of pores on half a circumference of the primary cortical shell, the distinguishing feature between Sphaeropyle robusta and Sphaeropyle langii, accounts for the same between Sphaeropyle robusta and morphotype A, and between Sphaeropyle langii and morphotype B. It suggests that the three-shelled morphotypes A and B are juvenile forms of the four-shelled Sphaeropyle robusta and Sphaeropyle langii, respectively. This is strongly supported by the same stratigraphic distributions of each pair. The quantitative analysis of morphological variations in both species also revealed that they have nearly identical morphology, except for the number of pores on half a circumference of the primary cortical shell. In addition, the size range of primary and secondary cortical shells and the range of the number of radial spines increase in Sphaeropyle langii, while the length range of radial spines decrease.