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Journal of Paleontology 91(3):369–392
mostly crop out from dolostones of the third sedimentary member. SHRIMP U-Pb zircon ages of 1625.9±8.9 Ma and 1559±12 Ma have been obtained from the second sedimentary member of the Dahongyu Formation and the Gaoyuzhuang Formation, respectively (Gao et al., 2008; Li et al., 2010), demonstrating the age of the third sedimentary member of the Dahongyu Formation to be ca. 1600 Ma.
Materials and methods
All fossiliferous samples in this research are silicified peloidal arenite sediments that were systematically collected from the dolostone-dominated strata of the third sedimentary member. The sampling point is located ~15km north of Jixian County, Tianjin, China (Fig. 1, coordinate: 40.16594°N, 117.47364°E). At the outcrop scale, silicified peloidal sediments are mottled black to brownish gray in color. Uncompacted peloidal grains suggest that these materials had undergone early diagenetic cementation. These peloidal deposits are heterogenous in texture with organic-rich peloids surrounded by organic-poor silicious cements. Microfossils are abundant inside peloidal grains. Thirty-two chert samples were collected and two petro- graphic thin sections were made for each of the samples. Seventeen samples contain well-preserved microfossils. Sub- sequently, fifty thin sections were prepared for each sample and abundant microfossils were recovered, measured, and photo- graphed using biological microscope Leica DM5500 and Leica Application Suite 4.0.0 software.
Repository and institutional abbreviation.—All illustrated specimens are in petrographic thin sections of chert. Locations and position coordinates for each specimen in this study are stored with the thin sections in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China.
General characteristics of the newly studied Dahongyu microbiota
This is the first time that well-preserved silicified microfossils have been recovered from the Dahongyu Formation of the Jix- ian section. In total, 19 species were recognized, six species have been identified informally and seven species have been identified in open nomenclature (Table 1). Simple cyano- bacterial filaments and coccoids are dominant elements, but most diversity is contributed by rare taxa. By observing their preserved state and living habits, colonially preserved micro- fossils and isolated individuals were recovered separately.
Colonies.—Colonially preserved microfossils in the new microbiota are mostly simple cyanobacteria, including hormo- gonian and chroococcacean. Based on their state of preservation and living habit, they are mainly recognized as builders that contributed to mat-building.
Filament colonies.—Filament colonies, including filamentous trichomes and sheaths, are abundant. These filaments exhibit differing living habits; some growing upwards, some were
randomly scattered in the matrix, and others grew parallel to the bedding plane. Based on their morphology and size, two genera and nine species are recognized: Oscillatoriopsis (O. obtusa, O. amadeus, O. longa, and O. princeps) and Siphonophycus (S. thulenema, S. septatum, S. robustum, S. typicum, and S. kestron).
Oscillatoriopsis.—Filaments of Oscillatoriopsis form macro- scale aggregates as ubiquitous elements (Fig. 3.1–3.3). Based on their distributional pattern in the matrix, three different aggre- gates have been recovered (Figs. 3.1–3.3, 4.2). These three aggregates exhibited very different features. The first Oscillatoriopsis aggregate is composed of
O. obtusa and O. amadeus (Figs. 3.1, 4.2, aggregate 1; specimens could only be assigned to particular species based on measurement because their diameters are distributed in a continuous zone). These trichomes, which are arranged closely in twos and threes, have been observed to be parallel and bend together in the same region. Their arrangement suggests that they may have been bounded together by a common envelope during the process of bending. The tentative multi-trichome nature is comparable with Eoschizothrix composite, which has been reported from the overlying Gaoyuzhuang Formation (Lee and Golubic, 1998). The second Oscillatoriopsis aggregate is formed by morphologically uniform filaments (Figs. 3.2, 4.2, aggregate 2),
which are O. amadeus, O. longa,and O.
princeps.These filaments are short, flexible, and do not possess tapering ends. Their diameters vary from 9.8 μmto 27.3μm and form a single main diameter-frequency distributional peak, which is 13–19μm
(Fig. 4.2, aggregate 2). This characteristic indicates that these filaments may have belonged to the same biological taxon. Filaments of the third Oscillatoriopsis aggregates are
variable in size but uniform in morphology (Fig. 3.3). Oscillatoriopsis longa and O. princeps are the key elements of this aggregate. SomeO. amadeus filaments also emerged (Fig. 4). Their diameters varied from 8.2 μmto35.8μm and formed several distributional peaks of diameter-frequency (Fig. 4.2, aggregate 3). This nature suggests that they could belong to different biological taxa.
Siphonophycus.—Five morphologic species of Siphonophycus (S. thulenema, S. septatum, S. robustum, S. typicum, and S. kestron) have been identified in the studied assemblage (Figs. 5, 6). As the most abundant filaments, Siphonophycus mostly appears in dense or loose aggregates and only a few emerge as individuals. Three different types of aggregates have been recovered according to their distributional pattern in the matrix (Figs. 5.2, 6). The first Siphonophycus aggregate is dominated by
S. thulenema, which usually arrange into small bundles of aggregates (Fig. 5.2, aggregate 1; Fig. 6.1, 6.2). Siphonophycus septatum is occasionally present within these small aggregates as a rare member (Fig. 6.2 arrow). Most S. thulenema filaments extended parallel to the bedding plane or along with the outer edge of a pelletoid (Fig. 6.1, 6.2). They do not exhibit an upward trend in growth direction, which may indicate that they were not photosynthetic organisms, or that their environment was not affected by sunlight. These aggregates are distributed in
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