Archaean stromatolites/microbial mats. (a) Domical, pseudocolumnar and branching columnar stromatolites, overlain by rippled sediments and (b) domical stromatolite, from the ca 2723 Myr old Tumbiana Formation, Western Australia (Walter 1972, 1983; Grey 1979); photo in (a) courtesy of I. R. Williams; scale, 10 cm. (c–e) Stratiform and conical stromatolites from the ca 2985 Myr old Insuzi Group, South Africa (Mason & von Brunn 1977; von Brunn & Mason 1977; Walter 1983; Beukes & Lowe 1989); photo in (e) courtesy of N. J. Beukes. (f) Laterally linked, low relief stratiform to domical stromatolitic mats from the ca 3245 Myr old Fig Tree Group of South Africa (Byerly et al. 1986; Byerly & Palmer 1991); photo courtesy of D. R. Lowe. (g) Stratiform microbial mats from the ca 3320 Myr old Kromberg Formation of South Africa (Walsh & Lowe 1985; Walsh 1992). (h–l) Laterallly linked (j) conical stromatolites from the ca 3388 Myr old Strelley Pool Chert of Western Australia (Hofmann et al. 1999; Hofmann 2000; Van Kranendonk et al. 2003; Allwood et al. 2004); (i) and (k) scale, 20 cm; (l) scale, 10 cm. (m) Domical and (n) stratiform stromatolites from the 3496 Myr old Dresser Formation, Western Australia (Walter et al. 1980; Buick et al. 1981; Groves et al. 1981; Walter 1983; Schopf & Walter 1983; Buick et al. 1995; Buick 2001).

ca 2500–2700 Myr old Archaean microfossils photographed in petrographic thin sections. (a–e) Broad prokaryotic (oscillatoriacean cyanobacterium-like) tubular sheaths (Siphonophycus transvaalense) from the ca 2516 Myr old Gamohaan Formation of South Africa (Klein et al. 1987; Buick 2001); scale for parts (a) and (b) shown in (b); scale for parts (c) and (d) shown in (c). (f–k) Solitary and paired (denoted by arrows) prokaryotic (bacterial or cyanobacterial) coccoidal unicells and (l–q) solitary and paired (denoted by arrows) bacterium-like rod-shaped unicells from the ca 2600 Myr old Monte Cristo Formation of South Africa (Lanier 1986; Buick 2001); scale for parts (f–q) shown in (f) (modified after Lanier 1986). (r) Narrow prokaryotic (bacterial or cyanobacterial) septate filament and (s) broad sheath-enclosed prokaryotic (oscillatoriacean cyanobacterium-like) septate filament from the ca 2723 Myr old Tumbiana Formation of Western Australia (Schopf & Walter 1983).

More than 3450 Myr old Archaean putative microfossils and ca 2000 Myr old Proterozoic unquestioned microfossils photographed in petrographic thin sections. (a–d) Unbranched carbonaceous filaments (Primaevifilum amoenum) described as degraded cellular fossil prokaryotes Incertae Sedis and interpreted as exhibiting oscillatoriacean cyanobacterium-like morphology (Schopf 1992, 1993), but regarded by Brasier et al. (2002, 2004) as being of questionable biogenicity, from the ca 3465 Myr old Apex chert of Western Australia; the faintness of the image of the filament illustrated in part (d) is due to its depth (range ca 65–90 μm) within the thin section studied; scale for parts (a–d) shown in (d). (e–i) Narrow carbonaceous filamentous bacterium-like putative fossils from the ca 3496 Myr old Dresser Formation of Western Australia (Ueno et al. 2001a); parts (e), (f), (h) and (i) show photomontages necessitated by the three-dimensional preservation of the sinuous filaments; part (g) shows an interpretive drawing of the specimen illustrated in (f); scale for parts (e–i) shown in (i) (modified after Ueno et al. 2001a). (j) Narrow prokaryotic (bacterium-like) carbonaceous filament (Gunflintia minuta) and (k, l) broader degraded septate prokaryotic (oscillatoriacean cyanobacterium-like) filaments (Gunflintia grandis) from the ca 1900 Myr old Gunflint Formation of southern Ontario, Canada (Hofmann 1971; Hofmann & Schopf 1983); scale for parts (j–l) shown in (j).

Temporal distribution of the reported occurrences of stromatolites/microbial mats listed in table 1.

More than 3200 Myr old Archaean microfossils photographed in petrographic thin sections. (a–f) Solitary and paired prokaryotic (bacterial or cyanobacterial) coccoidal unicells from the ca 3260 Myr old Swartkoppie Formation of South Africa, in (b–f) ordered in a sequence inferred to represent stages of cell division (Knoll & Barghoorn 1977); arrows point to dark organic contents within cells; scale for parts (a–f) shown in (b) (modified after Knoll & Barghoorn 1977). (g) Solitary prokaryotic (chroococcacean cyanobacterium-like) coccoidal unicells (Archaeosphaeroides barbertonensis) from the ca 3245 Myr old Sheba Formation of South Africa (Schopf & Barghoorn 1967); photo courtesy of A. H. Knoll. (h–j) Solitary, paired (denoted by arrows) and clustered prokaryotic (bacterial or cyanobacterial) coccoidal unicells from the ca 3320 Kromberg Formation of South Africa (Muir & Grant 1976; Schopf & Walter 1983); scale for parts (h–j) shown in (h). (k) Narrow prokaryotic (bacterium-like) filament and (l) broader prokaryotic (bacterial or cyanobacterial) filament from the ca 3320 Kromberg Formation of South Africa (Walsh & Lowe 1985; Walsh 1992; Schopf et al. 2002). (m, n) Colonial ensheathed prokaryotic (chroococcacean cyanobacterium-like) coccoidal unicells from the ca 3388 Myr old Strelley Pool Chert of Western Australia (Schopf & Packer 1987; Schopf 1992); scale for parts (m) and (n) shown in (n). (o) Broad septate prokaryotic (oscillatoriacean cyanobacterium-like) filament (Primaevifilum septatum) and (p) narrow prokaryotic (bacterium-like) filament (Archaeotrichion contortum), reported by Awramik et al. (1983) from stromatolitic cherts collected at a locality that maps within the ca 3470 Myr old Mount Ada Basalt (Van Kranendonk et al. 2003). Of unconfirmed stratigraphic provenance (Schopf 1999, pp. 83–84), these fossiliferous samples may be from a chert unit of the overlying ca 2700 Myr old Fortescue Group.

Carbon isotopic values of carbonate and organic carbon measured in bulk samples of the nine Palaeoarchean units reported to contain putative microfossils (Strauss & Moore 1992; Ueno et al. 2001a,b; Brasier et al. 2002).