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, 97 (25), 13684-9

Eumetazoan Fossils in Terminal Proterozoic Phosphorites?

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Eumetazoan Fossils in Terminal Proterozoic Phosphorites?

S Xiao et al. Proc Natl Acad Sci U S A.

Abstract

Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation, South China, contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg cases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stem-group eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean.

Figures

Figure 1
Figure 1
Diagenetic modifications of microfossils in the Doushantuo Formation. Specimens illustrated in A–J and N–Q are from phosphorites at Weng'an, South China; K-M occur in chert nodules. Phosphatic rims occur on algal thalli (A), cyanobacterial filaments (B), the inner surfaces and collapsed contents of acanthomorphic acritarch vesicles (C), and small coccoidal cells (D). Spheroidal fossil containing diagenetically formed inner and outer rims that display crystal zonation (E) and regular orientation (F, cross-polarization); note how the inner rim is templated on the surface of preserved organic contents. (G) Phosphatic spherulitic coating on the inner surface of a vesicle (arrow), forming bulbous microstructures similar to those interpreted by Chen et al. (7) as large ectodermal cells. (H) The spot marked by an arrow in G is magnified to show crystal forms and orientation. (I) Cross section of phosphatized filament, again illustrating crystal forms and orientation. (J) Bilayered spheroidal structure similar to that in E, showing clear evidence of zoned crystal growth. (K) Silicified organic-walled vesicle with invagination produced by postmortem deformation; if phosphatized, the specimen would resemble forms interpreted as bilaterian gastrulae by Chen et al. (7); viewed in thin section, the phosphatized fossil with postmortem infolding in P would also resemble proposed gastrulae. (L and M) Individual organic-walled algal vesicles with partially collapsed internal contents, drawn from a large population of 90- to 150-μm fossils in Doushantuo cherts; postmortem phosphatization would yield structures with a size range, internal morphology, and crystal orientation comparable to those used by Chen et al. (7) to infer eumetazoan origins. (N) Internally complex spheroid, similar in organization to specimens interpreted as anthozoan planulae by Chen et al. (7), but not interpretable in biologically meaningful terms; the SEM image in O shows diagenetically phosphatized filaments and other internal structures that, viewed in thin section, would resemble N. (Q) Multicellular algal thallus (the small dark structures are cell lumens) containing a decay feature comparable in organization to structures interpreted as anthozoan planulae by Chen et al. (7). (The scale bar in A represents 100 μm for A and B; 60 μm for C, N, and Q; 80 μm for D; 50 μm for E and F; 40 μm for G, K, and L; 4 μm for H and I; 20 μm for J; 30 μm for M; 200 μm for O; and 250 μm for P.)
Figure 2
Figure 2
Sinocyclocyclicus guizhouensis, tabulate fossils interpreted as possible stem cnidarians. (A) Twenty clustered tubes, seen in oblique section. (B) Detail of slightly curved cross-walls. (C) Tube showing expansion at top. (E) The same specimen at higher magnification, illustrating the thickening and curvature of cross-walls where they meet tube walls (arrow). (D and J) Tube with large chamber at upper end; cross-walls are incomplete and curve downward to make side walls of the chamber—cross-walls beneath the chamber are complete. (F) Tube with phosphatic rim along inner surface of tube walls and both complete and incomplete cross walls, preserved as boundaries between the phosphatic infillings of adjacent chambers. (G) Detail of complete and incomplete cross-walls (arrow in F). (H) SEM of tube with a bulbous structure at the end (arrow), as well as a laminated phosphatic rim on the tube wall. (I) Folded tube, demonstrating original flexibility of wall. (The scale bar in A represents 100 μm for A and F; 25 μm for B, D, and E; 60 μm for C; 20 μm for G and J; 200 μm for H; and 150 μm for I.)
Figure 3
Figure 3
Sinocyclocyclicus guizhouensis, tabulate fossils interpreted as possible stem cnidarians. (A) SEM of branched tube preserved as phosphatic internal molds of tube chambers; note branching pattern as well as wedge-shaped chamber formed where an incomplete and complete cross-wall meet (arrow). (B) SEM of four clustered tubes. (C) SEM of curved tube. (D and E) Cross and longitudinal sections through this specimen. (F) An enlarged SEM view of the surface, showing cross-walls, phosphatic laminae on the wall, and a longitudinal ridge on the concave side. (G) Saffordophyllum newcombae, an Ordovician tabulate showing bending and thickening of cross-walls where they meet side walls, as well as apical budding (reproduced with permission from Ref. 36); compare with Figs. 2E and 3A. (The scale bar in A represents 140 μm for A; 200 μm for B; 150 μm for C; 80 μm for D and E; 30 μm for F; and 1 mm for G.)

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