Three typical Neogene discoasters. Proximal surface is coloured green distal surface red. Note that alignment of the ridges in the central area is inter-radial on the proximal surface (green) and radial on the distal surface (red).
Left D. bollii, centre D. quinqueramus, right D. pentaradiatus.
Discoasters are an extinct group of nannoliths they are common throughout the Tertiary (mid Palaeocene to end Pliocene) and very important for nannofossil biostratigraphy. There is a gradual trend toward reduction in numbers of rays in the group and typical Palaeogene discoasters are rossette shaped with 10-40 rays in contrast to the star-shaped Neogene discoasters as illustrated here.