Fossilized metazoan embryos from the earliest Cambrian

Small globular fossils known as Olivooides and Markuelia from basal Cambrian rocks in China and Siberia, respectively, contain directly developing embryos of metazoans. Fossilization is due to early diagenetic phosphatization. A nearly full developmental sequence of Olivooides can be observed, from late embryonic stages still within an egg membrane, to hatched specimens belonging to several ontogenetic stages. Earlier cleavage stages also occur, but cannot be identified to taxon. Olivooides shows similarities to coronate scyphozoans and to their probable Paleozoic representatives, the conulariids. Markuelia eggs contain looped embryos of a segmented worm with short. conical processes covering the body.