Nannotax

Description: 6-rayed discoaster with birfucate tips; central-area with proximal and distal ridges, but weak bosses. Bifurcations well developed. Slightly concavo-convex and proximal and distal sides well-differentiated.

Remarks: D. variabilis differs from D. exilis primarily in having stronger differentiation of the proximal and distal surfaces, and stronger bifurcations. On the distal side there is normally a distinct central area, with scalloped depressions on the beginning of each ray, and often a central boss, mid-Miocene discoasters rarely have well-developed distal areas of this type.

According to Young (1998) D. variabilis evolved from D. exilis during the early Late Miocene (NN9-10), but the first occurrence is gradational, and of no use for high resolution biostratigraphy. By contrast, Prins (1971) and Theodoridis (1984), derive D. variabilis directly from the D. deflandrei group, and record it throughout the Middle Miocene.

D. variabilis varies considerably (hence the name) in form, in particular the bifurcations vary considerably in length, from less than the width of the ray to two or three times it. Several variants have been described as species or subspecies, as listed below.

Range: NN10 to 16.

Variants:

• Discoaster decorus (Bukry, 1971) Bukry, 1973 [D. variabilis decorus]- small bifurcations
• Discoaster icarus Stradner, 1973 - bifurcations broad and space between often partly filled by thin “webbing”
• Discoaster mirabilis Duan, 1985 - asymmetrical 5-rayed variety
• Discoaster pansus (Bukry and Percival, 1971) Bukry, 1973 [D. variabilis pansus] - large, wide, bifurcations