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. 2009 May 12;106(19):7729-34.
doi: 10.1073/pnas.0812460106. Epub 2009 Apr 24.

The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older

Stefan Bengtson  1 , Veneta Belivanova, Birger Rasmussen, Martin Whitehouse
Affiliations

The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older

Stefan Bengtson et al. Proc Natl Acad Sci U S A. .

Abstract

The age of the Vindhyan sedimentary basin in central India is controversial, because geochronology indicating early Proterozoic ages clashes with reports of Cambrian fossils. We present here an integrated paleontologic-geochronologic investigation to resolve this conundrum. New sampling of Lower Vindhyan phosphoritic stromatolitic dolomites from the northern flank of the Vindhyans confirms the presence of fossils most closely resembling those found elsewhere in Cambrian deposits: annulated tubes, embryo-like globules with polygonal surface pattern, and filamentous and coccoidal microbial fabrics similar to Girvanella and Renalcis. None of the fossils, however, can be ascribed to uniquely Cambrian or Ediacaran taxa. Indeed, the embryo-like globules are not interpreted as fossils at all but as former gas bubbles trapped in mucus-rich cyanobacterial mats. Direct dating of the same fossiliferous phosphorite yielded a Pb-Pb isochron of 1,650 +/- 89 (2sigma) million years ago, confirming the Paleoproterozoic age of the fossils. New U-Pb geochronology of zircons from tuffaceous mudrocks in the Lower Vindhyan Porcellanite Formation on the southern flank of the Vindhyans give comparable ages. The Vindhyan phosphorites provide a window of 3-dimensionally preserved Paleoproterozoic fossils resembling filamentous and coccoidal cyanobacteria and filamentous eukaryotic algae, as well as problematic forms. Like Neoproterozoic phosphorites a billion years later, the Vindhyan deposits offer important new insights into the nature and diversity of life, and in particular, the early evolution of multicellular eukaryotes.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Geologic map of the Vindhyan basin, central India. After Azmi et al. (4), based on several sources. Localities mentioned in the present study are labeled in white letters on black.
Fig. 2.
Fig. 2.
Bundles of tubular filaments similar to calcifying cyanobacteria, in phosphorite clasts from the Jankikund section, sample Ind06110805. Thin section, S156413. (A and B) Transmitted light. (C) Back-scattered SEM image.
Fig. 3.
Fig. 3.
Globular structures, interpreted as formed by gas bubbles within microbial mats, Jankikund section, sample Ind06110804. SEM (A–D, F–H) and SRXTM (E) images. (A) Globules of different size within filamentous fabric. S156414. (B) Negative casts of globules with polygonal surface structure, apparently formed by packing of smaller globules in matrix. S156415. (C) Two flattened globular objects with wrinkled surface membrane. S156416. (D) Packed globules forming foamy structure. S156417. (E) Clast containing foamy assemblage of globules of different sizes, viewed as transparent surface rendering of interfaces constructed by selective thresholding of gray-level values in voxel stack. S156418. (F) Globule with apatite-overgrown interior filamentous network. S156419. (G) Globules with probable gas escape structures. S156420. (H) Small globule within a larger one. Note palisade-like apatite overgrowth with occasional spherulitic structure. S156421.
Fig. 4.
Fig. 4.
Annulated and segmented tubes, Jankikund section, sample Ind06110804. Light (A and C) and SEM (B, D, and E) images. (A and B) Phosphorite clast with cast of 2-mm-long tube with weakly expressed annulations (arrows in B). S156422. (C) Tube with external annulations expressing transverse septa. S156423. (D and E) Tube with external annulations and evidence of prediagenetic plastic deformation. E is a back-scattered electron image of a polished transverse section through the tube. Note diagenetic apatite with spherulitic structure. S156424.
Fig. 5.
Fig. 5.
Age regressions of Pb-isotope data from fossiliferous phosphorite from the Jankikund section, samples Ind 06110701 (circle) and Ind06110805 (squares). Analytic uncertainties are smaller than the symbol size. MSWD, mean square of weighted deviates.
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References

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