The developmental cycles of early Cambrian Olivooidae fam. nov. (?Cycloneuralia) from the Yangtze Platform (China)

•Olivooids represent some of oldest metazoans with informations on embryology.•Life cycles of three species of olivooids are reconstructed.•Affinities and taxonomy of Quadrapyrgites and Olivooides are re-discussed.•Olivooids probably represent stemgroup Cycloneuralia and not Cnidaria.

Eggs, embryos, and hatchlings of olivooids are common and distinctive components of the earliest Cambrian small shelly fossil assemblage (Anabarites trisulcatus–Protohertzina anabarica Zone) at the northern and northwestern edge of the Yangtze Platform. Embryos are preserved in 3D as phosphatized permineralizations with superficial cellular structures. Nanofocus X-ray computed tomography reveals that diagenetic structures characterize the interior of most phosphatized eggs and embryos. The Olivooidae fam. nov. is herein distinguished from the possibly related hexaconulariids. Statistical size investigations and morphological studies indicate that at least three, but possibly five or more species of olivooids occur in the early Cambrian of South China. Fossilized smooth egg stages reflect a high biodiversity, encompassing 8–10 or more biological species. Eggs and embryos of Olivooides multisulcatus, Olivooides mirabilis and Quadrapyrgites quadratacris are represented by distinct and characteristic size clusters. The size ranges of embryonic parts of hatchlings roughly correspond to the size clusters of the embryos. From reconstruction of the developmental series of Olivooides multisulcatus it appears that the cleavage mode of olivooids was transitional between superficial and total cleavage. Late embryos and hatchlings of Olivooides multisulcatus and Olivooides mirabilis are characterized by pentaradial symmetry, whereas tubes of Quadrapyrgites quadratacris exhibit tetraradial symmetry. Tubular hatchlings of olivooids may represent larval or adult stages, or perhaps, nonfeeding larvae. Although the biological affinity of olivooids is weakely constrained, we hypothesize that they belong within an extinct clade of the Cycloneuralia. Their assignment to the Cnidaria is considered unlikely.

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