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micrometer to atomic scale and with these observations have established the precise
relationships between element morphology and crystal lattice (Fig. 5; Henriksen et al. in
press a). High resolution AFM of the flat surfaces of the elements (Fig. 5B and D) reveal
an atomic pattern identical to that known from rhombic {1014} faces of inorganic calcite
(Stipp et al. 1994; Stipp 1999). It shows the characteristic surface unit cell, atomic rows
defining the orientation of the
a-
axis vector, and a pairing of these rows that constrains
the crystallographic orientation fully (Henriksen et al. in press a).
C. pelagicus
has
{1014} faces on the whole distal shield surface and has element edges defined by
rhombic cleavage (Fig. 5E), with individual elements stuck together by imbrication. By
contrast, in
O. fragilis
the crystal lattice is rotated so that acute crystallographic corners
make the complex element edges. This enables elements to interlock without overlap,
allowing for very thin and delicate coccoliths (Fig. 5F). The outer part of the distal shield
consists of rhombic faces that are broken by a series of steps forming the slope towards
the middle, probably induced by constraints on growth space by the vesicle. Thus, in both
species the rhombic calcite motif is much in evidence, but detailed variations in
orientation allow them to construct coccoliths with very different properties.
An example of the importance of the interplay of rhombic growth directions with
vesicle geometry is provided by comparing coccoliths of the closely related species
Calcidiscus leptoporus
,
Umbilicosphaera foliosa
and
Umbilicosphaera sibogae
(Fig. 8).
These belong to the family Calcidiscaceae and have their distal shields entirely formed
from V-units (Young 1998; Young et al. in press).
Calcidiscus leptoporus
(Fig. 8A)
shows flat, overlapping elements with curved radial edges. The surfaces are almost
certainly rhombic calcite faces, as can be seen from overgrown specimens, from
examination of face relationships in the central area, and by analogy with the related
species
Coccolithus pelagicus
and
Oolithotus fragilis
.
Umbilicosphaera sibogae
(Fig.
8B) is similar, but has a wider central opening and radial edges showing a pronounced
kink.
Umbilicosphaera foliosa
(Fig. 8C) also has flat, probably rhombic faces, but has
two cycles in the shield. The inner cycle is imbricated clockwise, the outer is imbricated
counter-clockwise, with both cycles showing crystallographic edges. Superficially, this is
a very complex crystal structure. However, when a large number of specimens is
investigated at high resolution, paying close attention to overgrown and malformed
examples, it becomes evident that both shield cycles are made up of the same crystal
units. This is illustrated in Figure 9. The crystal units actually have a rather simple basic
shape, as indicated by the dotted lines on Figure 9B, which also reflect the shape of the
elements on the proximal surface. This simplicity is in marked contrast to the apparent
complexity on the distal surface, caused by counter-clockwise growth on the surface of
the crystal in the outer part of the coccolith and clockwise growth in the inner part. The
difference in growth direction is in fact a predictable result of the plane of the crystal
surface intersecting obliquely with a conical surface (Fig. 9C). This is not seen in
C.
leptoporus
since the elements are less obliquely oriented, and there are fewer of them.
Figure 8D shows a specimen of
U. foliosa
with rather irregular growth that has the
complete range of element types, from simple elements like those typical of
C.
leptoporus,
through kinked elements like those of
U. sibogae,
to the offset bicyclic
elements typical of
U. foliosa
. The fact that all three can occur on one coccolith as a
result of malformation proves that no major changes in mechanism are necessary to form
one rather than the other. Further, this suggests that the range of distal shield element
morphologies seen in the three species arise as a product of interaction of crystal
orientation and coccolith shape rather than through any direct control of crystal growth.
This example is thought-provoking because it shows that in biomineralization, a trivial
change in process can result in a large morphological effect.
