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.