Double skeletons of silicoflagellates: Their reciprocal position and taxonomical and paleobiological values

Abstract

The author addresses some problems concerning the double skeletons of silicoflagellates using examples from the Middle Miocene of Romania in conjunction with examples published in the recent literature. The main problem concerns the cause of the reciprocal position of the two individuals in double skeletons, which may occur either with the basal spines and corners of the basal rings of the two specimens aligned, or rotated from one another under different angles. Since the basal ring of the daughter skeleton must be in close proximity to that of the mother skeleton during its skeletogenesis, the main cause of rotation or non-rotation is attributed to the presence or absence of sustaining spines and their position on skeleton, namely beneath the struts or migrated onto the basal ring. A secondary cause is the position of basal spines relative to the plane of the basal ring. An approximate value of the angle of rotation of the basal rings is 180° per number of sustaining spines or struts. Species without sustaining spines, or with these spines migrated clockwise beneath the basal ring, are not rotated. In addition, species without sustaining spines are rotated when some basal spines or all are tilted under the plane of the basal ring. Cretaceous species without a basal ring are also rotated during skeletogenesis under angles depending on number of struts according to the same formula. The importance of this study is multiple: it indicates that silicoflagellates multiply asexually, it may solve some taxonomic problems, and it provides a mechanism in which new morphologies can appear suddenly, without intermediaries. From paleobiological point of view, this study provides an explanation of why in fossil materials, the position of the daughter skeleton with respect to the mother one was in many cases different from that of living species. A short taxonomic appendix is included with a brief discussion of the taxonomic problems of some genera and species mentioned in this article.