Abstract

Extraordinarily rare phosphatized embryos liberated from fossiliferous limestone of the Middle Cambrian (about 500 Myr old) Gaotai Formation of Duyun, southern China, are assigned to Markuelia qianensis n. sp. Several specimens with increasing numbers of blastomeres may represent four successive cleavage stages, seemingly exhibiting a radial holoblastic cleavage pattern. No subsequent stages showing gastrulation were observed. However, several specimens are late, pre-hatching stages, each with a vermiform shape coiled in either left- or right-handed directions within the fertilization envelope. These specimens indicate that the intuitively assumed difference in preservation potential between early cleavage and late pre-hatching stages is probably not valid. This new material clarifies the affinity of the first fossilized invertebrate embryos ever described, from the same rocks, which were originally attributed to arthropods, presumably trilobites. Instead, they belong to scalidophorans, and this finding infers likely diversified cleavage patterns for stem members of this group, and yields fresh insight into the embryogenesis of early metazoans as a whole.

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