Silicoflagellate double skeletons in the geologic record

Modern silicoflagellates are known to produce double skeletons, which are considered as pre-division stages; members of modern doublets are aligned at basal corners, using either pikes or organic material between the abbasal surfaces of the basal rings to hold together. In contrast, fossil doublets from the Cretaceous and Paleogene often have the corners of each skeleton oriented between those of the paired member in what is known as the Star-of-David configuration. Until recently, however, virtually nothing was known about the fine structure of fossil double skeletons. As a follow-up to our recent study on extant silicoflagellate doublets, we present a compilation of data on the fossil record of silicoflagellate double skeletons, with particular focus on the Cretaceous and Paleogene. Fossil silicoflagellate double skeleton specimens are extremely rare, with the oldest being from the late Santonian–early Campanian. In order to test whether silicoflagellates may have produced doublets prior to that time, we have constructed three-dimensional computer models of silicoflagellate double skeleton morphologies. Our findings indicate that doublet formation was part of the silicoflagellate reproductive cycle already since early in the evolutionary history of the group, before the development of the basal ring. Two distinct double skeleton configurations developed at some point in this evolution, with the Star-of-David group likely becoming extinct in the Oligocene, and the corner-aligned group represented by the Neogene and modern silicoflagellates. At present, the doublet configuration in fossil silicoflagellates can only be determined confidently with paired skeletons, but the occurrence, position and orientation of pikes may offer useful clues to interpret the doublet configuration from single skeletons. Unusual pike morphologies of some silicoflagellates from the Eocene strongly suggest a Star-of-David configuration. Silicoflagellate skeletal morphologies with pikes appear to be less abundant in the fossil record as double skeletons, and may disassociate more readily than morphologies that lack pikes and are instead held together by organic material between the basal rings.