Figure 1: Fracture (SEM) and section (TEM) views of the AHFM of the Cavolinioidea.

(a) Radial section of the shell of Cuvierina columnella. Horizontal broken lines are spaced at about half a turn. In this specimen, the helices complete some three turns. Note how the spacing between half turns (and, thus, the lead angle) increases towards the shell interior. The inset is a sketch of the ideal morphology of one such helical fibres. (b) Top view of a fracture through the shell of Cuvierina columnella, approximately parallel to the shell surface. The observed changes in orientation of fibres are caused by the unevenness of the fracture surface. (c) Fracture perpendicular to the surface of the shell of Cuvierina columnella, showing the interpenetration between fibres (black arrows). Colour arrows show the paths of three individual fibres (in different colours). (d) Etched fracture through the shell of Cavolinia longirostris. Etching permits a view of the complete process of crossing between two fibres; the lower fibre (arrow) crosses the upper fibre completely from right to left. (e,f) Transversely fractured fibres of Clio pyramidata (e) and Cuvierina urceolaris (f), showing the complex outlines caused by interlocking. (g) Transversal section of the fibres of Cuvierina columnella, exposing their interlocking cross-sections. The vertical thin lines within the fibres are twinning planes. (h) Detail of an area similar to (g). (i) Transversal section of the fibres of Cuvierina columnella. Fibres consistently show an external band (~150–200 nm thick) with an unusual density of white spots. The inset is a close-up of two such fibres. Arrows point to some examples. The external shell surface is to the top in (a) and (c) to (i).