Diagnosis: Shallow micro-etching with elliptical to biconvex outline. The morphology ranges from faint lateral grooves, to distinct ring-shaped grooves of varying depth, to elliptical or mandorla-shaped depressions which may show a faint central groove and deepest relief at the margin.

Description: This shallow attachment scar has an elliptical (Figs. 1, 2, and 3A–C) to mandorla-shaped (biconvex) (Figs. 3D–F) outline. The longitudinal axis of the bilateral symmetrical trace is 8 to 40µm long at a width of 5 to 25µm. The deepest relief (max. ca. 5µm is developed at the potentially overhanging margin of the very faint to shallow depressions. The central area may bear xenoglyphic patterns derived from the producing diatoms, for example, in form of afaint longitudinal groove (Fig. 4A). The traces are often in contact to but rarely superimpose each other, and often occur in clusters with a density of up to 6,000 traces per square millimeter (Figs. 1 and 2).