Early Cambrian animal diapause embryos revealed by X-ray tomography

Discoveries of animal embryos have profoundly improved our understanding of the early evolution of animal development. However, the fossil record of early animal embryos is extremely sparse. Here we present some three-dimensionally (3-D) phosphatized Archaeooides from the basal Cambrian in southern Shaanxi Province, China. The 3-D reconstructions of a number of specimens, aided by high-resolution X-ray tomography, demonstrate that these soft-bodied fossil organisms have a thick cyst characterized by pustule-like ornaments and vesicular structures. Furthermore, a multicellular inner body undergoing palintomic cell division is enclosed by the cyst. The suite of characters, including submillimeter to millimeter scale, a palintomic pattern of cell division, and a complex cyst wall microstructure, corroborate the hypothesis that Archaeooides fossils represent the embryonic remains of animals. More specifically, the structure of the cyst wall bears close comparison to the resting cysts of living invertebrates, allowing us to interpret Archaeooides as a diapause embryonic stage adapted to the temporally and spatially heterogeneous redox conditions that extended from the Ediacaran to the early Cambrian. The global distribution of Archaeooides indicates that these conditions were geographically widespread. Ultimately, Archaeooides provides evidence of the early evolution of this metazoan life history strategy as an adaptation to adverse environmental conditions. Its widespread occurrence in both conventional and exceptional taphonomic windows provides the potential for reconstructing its embryology and, by inference, the developmental evolution of early animals and their body plans.