Geometrical transformations of selected Mesozoic radiolarians

Radiolarians are Cambrian to recent holoplanktonic marine protists with morphologically very diverse siliceous skeletons. In the Mesozoic, two main groups are differentiated: nassellarians (mostly conical, composed of one or more consecutive segments) and spumellarians s.l. (generally spherical, composed of one or more concentrical shells). Following the extensive radiolarian research over the past decades, it is now possible to trace the development of some Mesozoic radiolarians through time and to reliably reconstruct several phyletic lineages. In this study, we analyse some lineages with well-marked trends in skeletal development and compare these trends with those observed in other marine organisms. The most usual geometrical transformations occurring in radiolarians are characterized by an increase of the surface of the shell. In several nassellarians, we observe a progressive inflation of the test, leading to a spherization. In some groups, such a trend leads to cryptocephalization, a phenomenon analogous to the orbulinization of some Tertiary foraminifers. The development of a terminal tube or of apertural arches is also frequent in the nassellarians. In many respects, the transformations observed in the spumellarians are related to the same kind of geometrical modifications. For example, the addition of an arch to a polar spine in Baumgartneria, of a ring in the Saturnalids, of a button or spine also in the ring of Aurisaturnalis, are clear examples of an increase of the shell surface through time. Proterogenesis is also frequent in nassellarians and spumellarians s.l., especially in the Pyloniaceae and Centrocubidae where new structures first develop in the earliest ontogenetic stage. The initial antapical spicule frequently evolves by doubling or quadrupling of the antapical spine, leading to entirely new modes of growth in subsequent descendants and to forms which are cryptogenic with regard to their ancestors. During extinction crises corresponding to highly stressful periods, radiolarians tend to lose the structures which were developed during their preceding evolution: loss of the ring in the Axoprunids, loss of the twisting of the spines in Tipperella, disappearance of inflated nassellarians in the Early Jurassic, etc. Such losses lead to the reappearance of primitive characters.