This data indicates a periodic deposition of chitin and mineral, which is much more dense in the early deposited layers, as has been observed before (Travis, 1963). Furthermore, though only observed in embedded slices of the smaller gastrolith (in Fig. 1C), synchrotron small-angle X-ray scattering (SAXS) at the inner, dense, parts of the sample shows a slope of 2 up to q = 0.3 nm1 (Fig. 5). Despite the very close packing, a slope of 2 indicates a more plate-like structure (Glatter and Kratky, 1982) and would correspond to such an ordered distribution of thin chitin layers (fitted by plates with a thickness of 4.7 nm), inside a dense calcium carbonate (Fig. 5), under the presumption of a large enough density difference between the chitin and the mineral. Interestingly, the SAXS-pattern of the external layers shows a hump at around 0.9 nm1,