The Fortunian fossil embryos exhibit the embryonic development of ancient animals at the lowermost Cambrian, thus they have important significance in evolutionary developmental biology. They have radial symmetry as a ...
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The Fortunian fossil embryos exhibit the embryonic development of ancient animals at the lowermost Cambrian, thus they have important significance in evolutionary developmental biology. They have radial symmetry as a fundamental body plan, and may be early representatives of cnidarians. Here we report new material of three-dimensionally phosphatized fossil embryos from the Fortunian Kuanchuanpu Formation and coeval strata in northern Sichuan and southern Shaanxi Provinces, South China. The new material includes previously reported fossil embryos assignable to Pseudooides prima with biradial symmetry or pseudo-hexaradial symmetry, Quadrapyrgites quadratacris with tetraradial symmetry, and Olivooides multisulcatus with pentaradial symmetry. Stereo light microscope reveals that, during the embryonic development of Olivooides multisulcatus, the emergence of pentaradial oral lobes predates that of the stellate structures. More important is the recovery of two new types of fossil embryos, i.e., Embryo I with hexaradial symmetry and Embryo Ⅱ with octaradial symmetry, and they are suggested to represent two new types of cnidarians. In contrast to the diverse radial symmetry of the Fortunian cnidarians, modern cnidarians exhibit mainly tetraradial symmetry, with exceptions of biradial symmetry in anthozoans and bilateral symmetry in siphonophores(Hydrozoa). Therefore, the tetraradial symmetry that is common in modern cnidarians is just an evolutionary relic derived from their ancestors in the Fortunian Stage.
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