The First Occurrence in the Fossil Record of an Aquatic Avian Twig-Nest with Phoenicopteriformes Eggs: Evolutionary Implications
Figure 4
(a) SEM radial sections reveal a three laminated eggshell structure, indicative of an avian eggshell, with an average total thickness of 470 µm, consistent with Kohring [20] observations on modern flamingos. The 163 µm thick layer 1 consists of elongated calcite crystals that radiates mostly outward from nuclei at the eggshell base. The 262 µm thick layer 2 is characterized by a crystallographic arrangement only easily described with EBSD microcharacterization (Figures 5 a and b). The 63 µm thick layer 3 displays long rectangular crystals with a spongy appearance due to multiple voids in the crystalline structure. (b) Backscattered electron microscopy (BSEM) of the same eggshell fragment. BSEM allows a better observation of the outer most eggshell surface covered with a thin calcium phosphate granular layer that does not appear to be a diagenetic artifact. Note the well-defined basal crystals and obvious large pore canals, which by themselves indicate a high moisture level in the nest, here congruent with the floating nest. (c) SEM of the basal crystals shows clearly well-defined spherulitic crystals that surround the nuclei. (d) BSEM of the same specimen upper eggshell section reveals the spongy appearance of this eggshell, the high concentration of micro size pore canals (black arrows), most of which seem to abut below the outer-most thin covering to indirectly connect with the eggshell surface. The sum of these microscopic features support a flamingo type eggshell and egg. The white arrows point to the granular relief of the surficial layer.