Shi et al.—Silicified Paleoproterozoic microbiota from China
water flow. According to the sequence stratigraphic information and stromatolites, the fossiliferous beds are located in the upper part of a transgressive sequence. They were deposited in an intertidal environment and are dominated by shallow-water carbonates. Fossil-bearing rocks containing a large amount of broken fragments of microbial mats, pelletoids, and concentrical ooids indicated a turbulent water environment, which allowed plankton habitation. Combined with environmental and tapho- nomic information, these broken isolated individuals could be considered as potential plankton. In summary, this new microbiota, which contains endo-
benthic, epibenthic, and allochthonous elements, shows a vertical distributional view of an intertidal microbiota in the transitional time from Paleoproterozoic to Mesoproterozoic.
Regional disparity of the Dahongyu microbiota
All of the previously reported microfossils from the Dahongyu Formation were concentrated at the Pangjiapu section, which is located in the west of the Yanshan basin and is 185km away from the Jixian section. The Dahongyu microbiotas from two localities (the Pangjiapu and the Jixian sections) both have high abundance, but also possess some obvious differences. To date, several studies have been conducted in the Pangjiapu section, with only nine taxa being recovered: Gunflintia, Huroniospora, Gloeotheceopsis aggregate, Gloeodiniopsis mattiformatum, Glenobotridion granulosum, Siphonophycus inornatum, Eohyella campbellii, Coniunctiophycus conglobatum, and Sphaerophycus sp. (Zhang, 1984, 1988; Zhang and Golubic, 1987; Zhang et al., 1995). These nine taxa of the Pangjiapu section are colonial microfossils with simple morphology. Compared with the Pangjiapu section, the newly studied
Jixian section contains more diversified microfossils. Besides colonial microfossils, the Jixian section also contains abundant allochthonous microfossils, which were not present in the
Pangjiapu section. As in the Pangjiapu section, in many silici- fied microbial mat biotas in the Paleoproterozoic and Meso- proterozoic (e.g., microbiotas from the Gaoyuzhuang Formation, Wumishan Formation, Salkhan limestone, Yus- mastakhan Formation, and Dismal Lakes Group), isolated individuals usually were rare elements and only account for ~2% of microbiota (Horodyski and Donaldson, 1980; Zhang, 1981, 1985; Sergeev et al., 1995; Sharma, 2006). In the new Dahongyu microbiota, the content of large isolated individuals is up to 20%. The high content of isolated individuals made the Jixian section standout compared to other silicified microbiotas in Paleoproterozoic and Mesoproterozoic strata. Gunflintia and Huroniospora have been reported from the
Dahongyu Formation of the Pangjiapu section (Zhang, 1984), but no typical Gunflint elements have been discovered in the same formation of the Jixian section. Only some S. thulenema share a similar preservable pattern with the interstitial filaments,
which have been assigned to Gunflintia minuta from the Gun- flint Formation (Lanier, 1989). Gunflint microbiotas are con- sidered a result of a special environment (Zhang, 1984). They are comprised of special components restricted to an exclusive time interval (2.1–1.7 Ga) and mostly associated with banded iron formations (Licari and Cloud, 1968; Knoll and Barghoorn, 1976; Awramik and Barghoorn, 1977; Tobin, 1990).
381 Disparity of the sedimentary environment could be the
cause of biological disparity. By comparing both microbiotas of the Jixian section and the Pangjiapu section, it could be inferred that there was significant environmental disparity between the two localities. The Pangjiapu section, which is 185km from the Jixian section, is located in the same Yanshan basin with the Jixian section. The whole Dahongyu Formation of the Pangjiapu section, which contains well-preserved microfossils, is dominated by carbonates like the third member of the Dahongyu Formation in the Jixian section. In both localities, microfossils are recovered from cherts and silicified carbonates. Besides these similarities, absence of volcanic materials and detrital rocks in the Pangjiapu section is the main difference between the two localities. As mentioned above, widespread and long-standing volcanic activities occurred and induced significant regional disparity of volcanic sediments and terres- trial inputs during deposition of the Dahongyu Formation (Ren, 1987). Compared to the Pangjiapu section, the Jixian section is closer to the provenances and more nutrition provided by volcanic materials could have influenced the microbiota to flourish. Although there are numerous environ- mental possibilities that could influence size and diversity in ancient basins, nutrition is an important factor that could limit or promote the growth of microfossils in a carbonate tidal-flat environment.
Comparison with the Chuanlinggou microbiota
The ca. 1650 Ma Chuanlinggou microbiota, recovered from the underlying Changcheng Group, is well known for bearing the earliest ornamented acritarch Valeria lophostriata (Yan and Liu, 1993; Peng et al., 2009). The Chuanlinggou microbiota was recovered from shales, which were considered to have been deposited in a barrier-island lagoon with hydrostatic environ- ment and sufficient terrestrial inputs (Xu et al., 2002b). Abun- dant microfossils have been reported from the Chuanlinggou Formation (Yan, 1982, 1985, 1991; Yan and Liu, 1993; Sun, 2006), and most of these microfossils are smooth-surfaced (Peng et al., 2007; Peng et al., 2009). The Dahongyu microbiota was recovered from cherts and silicified carbonates, which are chemical sediments representing littoral-neritic sea environ- ments. Microbiotas from totally different environments, such as the Chuanlinggou and Dahongyu microbiotas, usually are composed of different microfossils. However, the presence of plentiful isolated individuals made the Dahongyu microbiota comparable with the Chuanlinggou microbiota. These two microbiotas share main microfossil components:
(1) the Chuanlinggou microbiota and the Dahongyu microbiota are both dominated by filaments and small coccoids (Xing and Liu, 1973; Yan and Liu, 1993); (2) large spherical microfossils are abundant elements in both microbiotas (Yan and Liu, 1993); and (3) large fusiform (or ovoidal-shaped) microfossils, which are half roll-ups of a regular ruptured envelope, are frequent microfossils in the Chuanlinggou Formation (Peng et al., 2007; Peng et al., 2009). The same fusiform envelopes are also common microfossils in the Dahongyu microbiota (Fig 11.6–11.7). In addition to similarities, rare microfossils differ between these two
microbiotas.Anew type of small fusiform microfossil
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