Figure 4: EBSD analysis of Auca Mahuevo eggshells.

Images are representative of multiple specimens examined. (a,d,g) Grayscale diffraction maps in which lighter regions represent areas where greater diffraction is observed. (b,e,h) Colours represent different crystallographic planes of calcite, as shown in the colour-coded legend given in the bottom of the figure. (c,f,i) The corresponding pole figures of the planes projected on the {0001} plane of calcite. (a–c) Analysis of the prismatic layer (exterior portion, or palisade layer) of a titanosaur eggshell from Auca Mahuevo layer 4. This section of the eggshell exhibits excellent preservation on EBSD analysis, with the alternation {0–110} and {1–100} planes of calcite in the cross-section and the c axis parallel to the elongation of calcite crystals and perpendicular to the shell exterior, and seen in some modern avian eggshells⁴² and consistent with previous studies on titanosaur eggshells⁴³. (d–f) Mammilla (inner) layers of the same eggshell, also exhibiting excellent preservation in the regions where calcite crystals fan out from the cones and the c axis parallel to their elongation, but have a less constrained distribution of poles as shown in the pole figure (f). Areas between the cones were likely originally organic rich, but are now partially replaced by secondary calcite crystals (isolated orange–yellow–red poles in f). A similar feature can often be observed in both layer-2 and layer-4 Auca Mahuevo eggshells by cathodoluminescence, for example Fig. 3e. As described in the Methods, this area of secondary calcite on the interior surface is likely removed by drilling during the preparation stage. (g–i) Auca Maheuvo layer-2 eggshell that is heavily recrystallized. Each calcite crystal has a different orientation and without a preferred orientation of the c axis of calcite, indicative of recrystallization. The diffraction map shows regular edges of calcite crystals, defining prisms, which would suggest that the recrystallization is due to a closed-system replacement rather than open-system recrystallization involving dissolution and precipitation of secondary calcite.