The First Occurrence in the Fossil Record of an Aquatic Avian Twig-Nest with Phoenicopteriformes Eggs: Evolutionary Implications
Figure 5
(a) An EBSD orientation map of a cross section through the Vs-1 eggshell, with the outer surface of the eggshell at the top of the image and non-indexed regions marked in black. The colour scheme relates to the orientation of the calcite crystal lattice, with red colours indicating an alignment of the c-axis with the eggshell surface normal. High angle grain boundaries (>10° misorientation) are marked by black lines and low angle boundaries (2–10° misorientation) by grey lines. The grain structure can be subdivided into 3 distinct layers, with layer boundary positions highlighted by the black arrows. Numerous low angle boundaries are present in the outermost layer, some of which are marked by blue arrows. (b) A comparable EBSD orientation map of a cross section through P. ruber eggshell, using the same colour scheme as in (a). This shows many of the same features visible in (a), including the presence of 3 distinct layers (marked by black arrows) and an abundance of low angle boundaries in the outermost layer (blue arrows). The boundaries between EBSD grain structure seem to mirror the eggshell structural layer boundaries. Some subtle differences in the crystallographic structure between these extinct and modern eggshell are visible such as the less serrated nature of the grain boundaries in the middle layer and the greater spacing of the individual shell units in the modern specimen. (c) and (d) SEM photomicrographs of fractured cross sections through through the Vs-1 and P. ruber eggshells respectively. These images complement the EBSD orientation maps, highlighting the 3-layered structure of both eggshells but lacking the detailed grain structure information visible in (a) and (b). The scale bars in a, b, and c images are 200 µm.