FOSSIL EGGS FROM ZHEJIANG PROVINCE, CHINA: EVIDENCE OF A REDUCING ENVIRONMENT FACILITATED BY ORGANIC DECOMPOSITION

Fossil eggs from the Mid Cretaceous, Tiantai basin in Zhejiang Province, China occur in fine-grained quartz-rich red sandstone, of alluvial fan origin. These rounded to subrounded eggs are referable to the oogenus Spheroolithus, within the oofamily Spheroolithidae. Nearly every egg of the 196 specimens examined at the Zhejiang Natural History Museum, exhibits a white halo (0.2-17.8mm in width) immediately adjacent to the eggshell on the interior and exterior surfaces. Similar halos in fine-grained deposits surround Sinornitho​mimus skeletons from an Inner Mongolia bonebed and fossil eggs at the Auca Mahuevo (Argentina) suggesting a common origin.

To investigate these halos, we characterized the mineralogical composition of the halo and surrounding sandstone using light microscopy and X-ray diffraction. Petrographic thin sections show white halos that closely outline the eggshell, and their composition and texture are identical to that of the surrounding red sandstone matrix. Six thin sections of eggshell reveal round dark rimmed spherical structures (160µm in diameter) that occur exclusively in the eggshell. These structures display multiple generations of sparry calcite fill and are interpreted as pedotubules. X-ray diffraction analyses of sediment samples show no quantifiable difference in the overall sedimentology and clay mineralogy between the halos and red sandstone. In contrast, iron staining of a Zhejiang thin section revealed the presence of Fe²⁺ and Fe³⁺ associated with the white halo and red sandstone, respectively. The greater abundance of Fe²⁺ suggests limited dissolved oxygen immediately surrounding the eggshell, therefore, indicating a reduced state immediately adjacent to fossil specimens. Decay of organics associated with the fossils may have decreased the amount of oxygen in sediments resulting in iron reduction. The halos provide potential evidence for interactions between organisms and eggs during early diagenesis. However, halos also occur in secondary rock fractures, therefore alternative explanations may include microbial activity and the capacity of fossils to attract or repel ions as mechanisms for the displacement of Fe²⁺ and Fe³⁺.