1963; Shechter et al., 2008a), in the later deposited external layers
of the gastrolith these spheres are more loosely packed and less
fused (Fig. 3A) than in earlier deposited inner parts (Fig. 3B), with
a sphere-size ranging from 150 nm (Fig. 3A) to 190 nm (Fig. 3B) in
most regions. Here, what appears to be loose chitin fibers with a
diameter of about 5 nm can be observed around the spheres and
sticking out of cracks (Fig. 3C). Also around the cracks in the middle
of the gastrolith (Fig. 4), the spheres show an exceptional loose
packing and particle sizes ranging from slightly smaller than average
(region a: 140 nm) to exceptionally large spherical particles
(b: 265 nm, c: 440 nm). The lower amount of interconnections
between the large separate spheres could explain why the gastrolith
is broken at this position. Additionally, between regions a and
b we see a high amount of organic material, containing chitin
fibers, which seem to separate both sizes of spheres. Up to this
point, comparing the SEM data with the transmission light microscopy
images, we can deduce that the light–dark layering is caused
by the alternation of loosely to more densely packed or fused submicron
sized spheres. Proceeding toward the innermost part of the
gastrolith in Fig. 3D we observe multiple, large layers (right) separated
from the main body of the gastrolith (left). On a higher scale
of hierarchy (Fig. 3E), these separated large layers consist of a finer
layering (every 0.5 lm) of mostly 10–20 nm-sized chitin fibers
and protein sheets in between a granular amorphous mineral, with
average particle size of approximately 100 nm (see arrows Fig. 3E).
The increase in size of the chitin fibers with respect to the later
deposited external parts possibly indicates that they are covered
by mineral or protein.