Multilayered Metazoan Eggshells from Cambrian Series 2 and Their Possible Affinities with Panarthropods

Cambrian eggs and embryos have been documented from multiple regions since the 1990s. In case of some fossil sites, mineral replacement has preserved fine-scale surface microstructure with high fidelity, as in certain Olivooides fossils. However, fossil eggs preserving nano-scale eggshell structures have not been described. We have found new silicified metazoan egg fossils from the Coonigan Formation (Cambrian Series 2, Stage 4) of New South Wales, Australia. Specimens occur as elongate three-dimensional bodies measuring 374–379 μm in length that represent the first Cambrian occurrence of eggs exhibiting clearly differentiated multiple chorionic layers. FE-SEM observations show a thick outer layer and a flexible inner layer connected by mushroom-shaped pillars. The outer layer comprises a granular external zone, possibly representing a taphonomic artifact or replaced organic material, and a thick porous internal zone. The inner layer bears regularly scattered micropores and stalk-like bases of the pillars. X-ray Micro CT analysis reveals possible remains of embryonic tissue or yolk, but no diagnostic details remain. Nevertheless, thepresence of a porous outer layer with pillar-like interlayer connections suggests affinity with Panarthropoda. The structural complexity in these Cambrian eggshells suggests specialization of chorion ultrastructure during initial metazoan diversification.