Locality map and relative generalized stratigraphy of the Doushantuo Fm. (A) Location of the Weng'an phosphorite deposits in Guizhou Province, China. Outcrops of the Doushantuo Fm are shown by the black areas. (B) Relative stratigraphy of the Terminal Proterozoic and Cambrian. The Doushantuo Fm is older than the late Neoproterozoic paleocommunity Zone III, which bears fossil evidence of cnidarian as well as bilaterian forms. Sponges are clearly present in Precambrian deposits (41, 42), including the Doushantuo (26). Beneath the Doushantuo Fm is a layer of tillite, deposited during the global glaciation, which is believed to have terminated about 590 Ma ago. On the left are interpolated absolute dates; the solid horizontal lines represent divisions dated by U-Pb geochronology. Also indicated are representative Cambrian lagerstätten. The dates shown for the Cambrian are from Landing et al. (43, 44) and Davidek et al. (45); the Precambrian/Cambrian boundary date is from Bowring et al. (30) and Grotzinger et al. (5). See text for discussion and references. The fossils shown are, from top to bottom: Opabinia, a middle Cambrian Burgess Shale arthropod; a Lower Cambrian olenellid trilobite (left) and Haikouella lanceolata (46), an early Cambrian chordate (right); a small shelly fossil of the Lower Cambrian; Arkarua, a possible Precambrian echinoderm (left) and a sea pen, Charniodiscus, from the later Precambrian (right); a possible sponge embryo from the Precambrian Doushantuo Fm (left), and a fossil embryo resembling a deuterostome gastrula (right; for more details see Fig. 3A).

Putative cnidarian embryos and larvae. (A) Oblique section of a possible fossil anthozoan planula. (B) Schematic view of a transverse section of the late planula of the anthozoan Euphyllia rugosa. The larval stage represented in A and B is constituted of an outer monocellular layer, the ectoderm, within which is an inner endodermal layer with various mesenteric folds and immature septa. This complicated bilayered structure is typical of anthozoan late planula larvae. Note the individual cells visible in the ectodermal layer at lower left in A, where it has separated from the endodermal layer. (Scale bar, 100 μm.) (C and D) Putative fossil gastrula of hydrozoan medusa; (C) Bright field; (D) Polarized light. Under polarized light (D), both layers show the same crystal orientation at arrows, as indicated by the same colors. The modern hydrozoan embryo shown in E is Liriope mucronata. B is from Chevalier (47); E from Campbell (48). (Scale bar in C is 50 μm.)

Putative fossil embryos that resemble bilaterian gastrulae. (A–G) Fossils resembling deuterostome embryos; (H) Modern example (gastrulae of the sea urchin Mespilia globulus, ref. 49) In A, C, and E, the archenteron is bent to one side, and in A and C displays bilobed outpocketings; (A) The nearer ectodermal layer is thicker compared with the opposite one (possible oral and aboral ectoderms, respectively; compare H). (C) A section in the plane indicated by the small arrowheads in A. (B and D) Polarized light microscope images, showing that the cells comprising the outpocketings are differently oriented, as they appear in different colors from those constituting the walls of the gut. In A, part of the outer wall is deformed (arrow) by a crystal grain visible in B (light pink). (G), Another specimen displaying invaginating archenteron at early midgastrula stage. (H) Modern sea urchin gastrulae (49). (I and J), Fossils resembling modern spiralian gastrulae; (K) Modern polychaete embryos in which the dashed lines indicate yolky endoderm cells and dots represent mesoderm cells (Eupomatus, left; Scoloplos, right, redrawn from Anderson, ref. 50). In the fossils I and J, the archenteron is thick-walled (cf. cross section in C), and in J all of the cells in the embryo, including the ectodermal wall, are conspicuously larger relative to the size of the embryo. Note also the column of cells along the archenteron in J. (Scale bars represent 50 μm.)

Oncoids and an undefined embryo, larva, or adult form. Bright-field (A) and polarized (B) light images of an oncoid found in one of the thin sections. The inner mass consists of a unique homogeneous crystal, as shown by its uniform color in polarized light, surrounded by a laminar structure. Note the crack in the center. The two outside thin layers are uniform, and they do not delimit any cavity. (C) A smaller oncoid; note the uniform circumferential wall, which in polarized light is the same color throughout. (D) Unidentified biological form. There is a large blastocoel and a gut of some kind. The top of the fossil embryo is slightly deformed, suggesting that this structure was soft. (Scale bars are 50 μm.)