Did the silicoflagellate genus Octactis evolve from Bachmannocena?

摘要

It has long been assumed that Octactis and Stephanocha (previously known as Distephanus) are closely related. Both have apical rings, but with Octactis possessing an apical ring of larger diameter relative to the basal ring and made of thinner elements. However, closer examination of the basal rings and double skeletons (doublets) shows other differences that have received little attention. The location of the struts and pikes on each side of the Neogene Stephanocha basal ring are consistently rotated towards the basal corners. This establishes a distinct basal element between the struts and pikes that is tilted to form a zigzag basal design that creates an interlocking of the doublet skeletons. The pikes project into the region of the paired skeleton and the apical ring is also rotated with respect to the basal ring. In contrast, Octactis has a flatter basal ring without pikes, and possesses strut attachments located midway between the corners with the doublet held together only by organic material.