Fig 1.
Sketch map showing the location of the fossil site.
Map of the People’s Republic of China (No. GS(2008)1826) downloaded from the website of the National Administration of Surveying, Mapping and Geoinformation (http://219.238.166.215/mcp/Default.html).
Fig 2.
Sedimentary succession at the fossil site (Based on [35]).
The red arrow indicates the fossil horizon.
Fig 3.
Photographs of mollusk fossils found in the Nihewan Basin.
Fig 3a-1, 3a-2, 3a-3 and 3a-4 show the same specimen from different angles, as do Fig 3b-1, 3b-2 and 3b-3. Fig 3d-1 and 3e-1 display the morphology of the left valve and the right valve of different specimens, while Fig 3d-2 and 3e-2 are of their inner surface.
Fig 4.
Photographs of fossil pearls found in the Nihewan Basin.
The red arrow points to the biggest pearl we found.
Fig 5.
Photographs showing the pearls under micro-CT scan machine.
Fig 5a to 5d are scanning photos, while Fig 5e to 5g are shadow projections. Four pearls on one specimen were scanned, among which the pearl of No. 2 is the biggest. Fig 5b, 5d and 5f also show this pearl. The red arrows point to the nucleus.
Fig 6.
Changing curve showing the gray levels of each pixel along the red line on the left.
Labels a and b show the amorphous matrix layer.
Fig 7.
XRD patterns of one fossil pearl and the shell to which it was attached.
Table 1.
X-ray powder diffraction data of fossil pearl and shell and calcium carbonate minerals.
Fig 8.
SEM photographs showing the internal structure of the fossil shells (a-c) and pearls (d-o). (Fig 8a-8c) Nacreous layers of the fossil shell. (Fig 8d) The pearl surface. (Fig 8e) Nacreous layer of the fossil pearl. (Fig 8f) Growing surface of the aragonite crystals. (Fig 8g and 8h) Spherulitic structure formed by fibrous aragonite. (Fig 8i) Junction area of the spherulitic structures. (Fig 8j and 8k) Interlacing aragonitic prisms and tablets. (Fig 8l) Thickness variations of different layers among the tablets interlayer. (Fig 8m and 8o) Longitudinal and transverse sections of the fibrous aragonite crystals. (Fig 8n) Junction between two fibrous prisms. Labels 1–4 indicate the thicknesses of each layer, 1 = 0.34 μm; 2 = 0.37 μm; 3 = 3.23 μm; 4 = 2.3 μm; 5, the thickness of the 11 tablet layers is 7.32 μm (0.67 μm on average). P and T in Fig 8j indicate prism layers and tablet layers respectively.