Close up of the phosphate rich (I) and phosphate poor (II) regions in the middle of the gastrolith by EDS and high-resolution SEM. (I) and (II) indicate regions left and right of the crack. Size of the spheres in sub-regions a–d: (a) 137 ± 17 nm, (b) 265 ± 49 nm, (c) 437 ± 68 nm and (d) 243 ± 31 nm. Sphere size is presented as mean diameter ± standard deviation. Scale bar in I and II is 1 lm.