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4. Ultrastructure

Heterococcoliths and nannoliths are composed of complex shaped calcite crystals. These terms provide the vocabulary to describe this ultrastructure. Crystallographic orientation terminology is discussed separately.

4.1 Types of ultrastructural component

(this terminology is largely based on Young and Bown, 1991).
Element Apparently discrete component of a coccolith. This is an observational term, several elements may unite to form a crystal-unit.
Crystal unit A group of elements from different cycles in crystallographic continuity. These are the fundamental components of coccoliths and their identification is a key objective of ultrastructural research.
Segment one symmetrically repeated part of the coccolith, including elements from each cycle, consisting of one or more crystal-units.
Lamina platy sub-structure within a crystal-unit (e.g. Braarudosphaera).
Contact-surface plane of contact between two elements. (alternative term attachment surface, see appendix).
Suture trace of contact-surface on surface of coccolith.
Cycle ring of elements or crystal-units.
Tier one of a set of vertically superposed cycles (e.g. Arkhangelskiella, Lapideacassis).

components

4.2 Element shapes

(N.B. a,b,c three orthogonal axes, with any orientation)
Block nearly equidimensional element (a≈b≈c).
Tile broad and thin element (a≈b>c) N.B. Plate has been used in this sense but we prefer to use it for larger structures, not for single elements).
Lath elongate and wide element(a>b>c).
Rod elongate and narrow element (a>b≈c)
Wedge tapering nearly equidimensional element.
Petal/petaloid element tapering broad and thin element.
Ray tapering elongate and wide element.
Spine tapering elongate and narrow element.
Granule small and irregular or variable-shaped element (e.g. blanket elements of Helicosphaera, spine-forming elements of Cretarhabdus). N.B. Crystallite has been used in this sense but we prefer to only use it for holococcolith elements.

element shapes

4.3 Element modifications

Curvature curving of elements. Laevogyre - elements curve to the left when traced radially outward. Dextrogyre - elements curve to the right when traced radially outward. Straight - elements not curved.
Node block-shaped projection from element.
Keel lath-shaped projection from element.
Ridge rod-shaped projection running along element.
Tooth rod or wedge-shaped projection from element.
Kink angular bend in element.
Offset displacement of an element from radial growth due to a double kink.

element curvature

4.4 Special structures

Scissor-structure crystal-unit structure formed of two elements growing at only slightly different angles, and forming a two-layered shield (e.g. Coccolithus upper and lower proximal shield elements, Fig. 6) or tube (e.g. Toweius, Emiliania inner and outer tube elements, Fig. 6). {Young 1992b}
Cross-over zone belt around which two cycles of crystal-units cross (this usually corresponds to the proto-coccolith ring, e.g. Coccolithus, Fig. 6). {Young 1992b}

4.5 Openings

Canal narrow elongate opening within a coccolith or nannolith (Figs. 10, 14).
Cavity broad opening within a coccolith or nannolith (Figs. 10, 14).
Common opening opening formed by several individuals; i.e. the space within a coccosphere or group of associated nannoliths.
Depression pit on the surface of a coccolith or nannolith (e.g Fasciculithus).
Hole opening running through one element (e.g. Pemma basquensis). {Farinacci et al. 1971}
Opening general term for any space not filled by elements.
Perforation small opening between two or more elements. {Farinacci et al. 1971}
Slit elongate perforation (e.g. Emiliania).
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