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Coccoid microfossils from the Doushantuo Formation (late Sinian) of South China
Precambrian Research, 28 (1985) 163--173 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
163
COCCOID MICROFOSSILS FROM THE DOUSHANTUO FORMATION (LATE SINIAN) OF SOUTH CHINA
ZHANG ZHONGYING
Department of Geology, Nanfing University, Nanjing (People's Republic of China) (Received May 15, 1983; revision accepted October 12, 1984)
ABSTRACT Zhang, Z., 1985. Coccoid microfossils from the Doushantuo Formation (late Sinian) of South China. Precambrian Res., 28: 163--173. Eight categories of organic-walled coccoid microfossils in the c. 700-Ma-old Doushantuo Formation are described and named. Of these, one genus and species (Paratetraphycus giganteus) is new. Most of the microfossils are interpreted as being the remains of the Chroococcaceae and are morphologically comparable to those of the 650-Maold Yudoma microbiota and the 850-Ma-old Bitter Springs microbiota. These microfossils occur in non-stromatolitic cherts, adding preservational information to the study of Precambrian life.
INTRODUCTION Palaeontologists and biologists are probing the early history of life on the Earth. During the past decade, and especially within recent years, considerable progress has been made in China in t he study o f Precambrian life. In South China a large n u m b e r o f microfossils from Precambrian rocks, particularly f r o m t he Sinian, has been reported. These discoveries provide i m p o r t a n t insight into the evolutionary development and biostratigraphic usefulness of Precambrian microbiotas in China. I have r e p o r t e d some filamentous microfossils, including Eomycetopsis robusta Schopf, emend. Knoll and Golubi6, cf. Gunflintia minuta Barghoorn, Palaeolyngbya sp., Rhicnonema antiquum H ofm ann, Siphonophycus sp., Doushantuonema peatii Z. Zhang and ObrucheveUa minor Z. Zhang f r o m the Doushantuo F o r m a t i o n (late Sinian) of western Hubei Province, China (Zhang, 1981a, b, 1982, 1984). Recently during examination of about 400 petrographic thin sections, I have discovered m any t axa o f coccoid microfossils which have n o t been previously r e p o r t e d from the D oushant uo Formation. The purpose o f this paper is t o illustrate, identify, and describe these newly-discovered coccoid forms, thus f u r t h e r supplementing our understanding o f t he D ous ha nt uo microbiota.
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
164 REGIONAL
STRATIGRAPHY
The Sinian, occurring as the first covering strata on the Yangtze platform, crops out extensively in western Hubei Province, particularly in the eastern Yangtze Gorges district around the flanks of the Huangling anticline. Early in 1924, the late Chinese geologist Lee Siguang and his assistant (Lee and Chao, 1924) investigated the geology of this district and established the Sinian section there, thus providing a preliminary standard for Sinian study in South China. Since then, especially during the past decade, extensive studies on the Sinian biostratigraphy and geochemistry of western Hubei Province have been carried out and great advances have been made. In this district, an unmetamorphosed Sinian sequence up to 1000-m thick is well-exposed. The Sinian rests upon the erosion surface of the Huangling Granite dated as 819 + 54 Ma old (Rb--Sr isochron age) (Zhao et al., 1980), or the metamorphosed Kongling Group with a distinct angular unconformity, and is comformably overlain by lowest Cambrian strata containing small shell fossils, such as hyolithids, monoplacophorans, branchiopods, and gastropods. The latest radiometric data give a time span of 615 + 20 to 800 + 30 Ma ago for the Sinian (Zhao et al., 1980), which is thus considered as late Precambrian. There is a long established subdivision of the Sinian in western Hubei Province. In this area, in accordance with changes in lithology and sedimentary cycles as well as palaeontological assemblages, the Sinian is naturally divisible into four formations {Fig. 1). From the base upwards, these are: Lower
Cambrian
I[ II
II
Dengying
II
Fm.
II o
II
I1
a "~
t.
C
]1
II
248-671m a
II •
.m
"~
•~ E
m
I
II
•
Douehentuo
Fm.
II
II
8
It
|70-230m -2 . -
Nantuo Fro.
i
~':
50-180m
• ........
o ..J
• ..
L i a n t u o Fro. -260m
Huengling
Granite
÷ I-
+ .I-
+ ÷
~. +
F i g . 1. Stratigraphic sequence of the Sinian and adjacent rocks in western Hubei Province, C h i n a . Thickness data f r o m Z h a o et al. ( 1 9 8 0 ) .
165
(1) the Liantuo Formation, chiefly feldspar-quartz-sandstones with basal conglomerates; (2) the Nantuo Formation, tillites; (3) the Doushantuo Formation; and (4) the Dengying Formation, mainly dolomites with cherty beds and siliceous nodules. The Liantuo Formation and the Nantuo Formation comprise the Lower Sinian which is characterized by terrestrial and shoreline clastic sediments, including glacial sediments in its upper part which indicate a cold climate. The Doushantuo Formation and the Dengying Formation comprise the Upper Sinian which represents shallow water marine sediments found in an arid climate. The boundary between the Upper and the Lower Sinian is now set at 700 Ma (Zhao et al., 1980). The above-mentioned subdivision of the Sinian has been reiterated by the Meeting for the Stratigraphical Classification and Nomenclature of the Late Precambrian convened in Beijing in 1982. The Doushantuo Formation of western Hubei Province is 170--230-m thick, and mainly consists of microdolomites, intercalated with black carbonaceous shales, mudstones, siliceous limestones, cherty beds, and siliceous or phosphatic nodules. Evaporite relicts, such as pseudomorphs after gypsum, sometimes occur in its lower part. This formation yields diverse and abundant microfossils, which are mostly composed of organicwalled residuals three-dimensionally preserved in a structurally-intact condition. SYSTEMATIC PALAEONTOLOGY
Division: Cyanobacteria Class: Cocogoneae Order: Chroococcales Chroococcaceae-like coccoids Genus: Paratetraphycus gen. nov. Type species: Paratetraphycus giganteus sp. nov.
Diagnosis: Cell-like units spheroidal, ellipsoidal, hemispheroidal or slightly polygonal due to mutual compression, 3--21 ~m in diameter. Walls thin, distinct; surface texture psilate to finely granular. Spheroids commonly in planar tetrads and diads, occasionally in triads and cross tetrads. Tetrads commonly occur in planar clusters (one or two layers), without an encompassing amorphous organic matrix. Etymology: With reference to morphological similarity to the genus Tetraphycus D. Z. Oehler. Discussion: In general, the morphology of the cell-like units, and their common arrangement in planar tetrads and diads, in this taxon resemble Tetraphycus D. Z. Oehler, 1978. However, Doushantuo specimens of Paratetraphycus giganteus differ from members of the four species of Tetraphycus (Oehler, 1978) previously reported in their lack of adherent amor-
166
phous organic matrix, and their larger size of cell-like units. These important distinctions warrant the erection of a new genus. Tetraphycus conjunctum described by Lo (1980) from the Lower Yudoma Formation of eastern Siberia also show its lack of adherent amorphous matrix. Therefore, it seems reasonable to refer T. conjuncture to the present genus. The specimens here assigned to Paratetraphycus display a distinct cyanophytic habit of spheroids commonly in planar tetrads and diads. Based on size and colonial form, I prefer to assign Paratetraphycus to the Chroococcaceae-like blue-green algae. A closer modern analogue to Paratetraphycus seems to be the genus Gloeocapsa Elenkin.
Paratetraphycusgiganteus sp. nov. (Plate I, 1, 4, 6, 7; Plate II, 6) Diagnosis: Qualitative characters as for genus. Cell-like units 10.8--21.6 ~m in diameter, with an average of 15.5 #m (101 cell-like units measured in 6 clusters). Walls < 1-pm thick. Type specimens: The cluster shown in Plate I, 6 has been chosen as holotype (thin-section GS454-2, Palaeobot. Coll. Nanjing Univ. No. B8309, stage coordinates 81.6x/17.2y). All other specimens illustrated are paratypes. Etymology: After the extremely large size of the cell-like units. Discussion: In gross morphology, this species resembles Paratetraphycus conjunctum (Lo) from the Lower Yudoma Formation of eastern Siberia (Lo, 1980). However, the cell-like units of Paratetraphycus conjuncture (Lo) range in diameter from 3.1 to 6.2 gm, averaging 4.9 ~m, and the significantly larger and non-overlapping size range of P. giganteus cell-like units distinguishes the two species. Clusters of this species are abundant in the Doushantuo microbiota. Type locality and occurrence: Yinjiagou and Tiejiangya, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Genus Aphetospora Lo, 1980 Aphetospora euthenia Lo, 1980 (Plate 2, 7, 9) Plate I. Structurally-preserved coccoid microfossils in petrographic thin sections of black cherty beds and siliceous or phosphatic nodules in microdolomites of the Doushantuo Formation (late Sinian) (~700 Ma old) of western Hubei Province, China. Bar scale is 10-~m long. Locality, thin-section number and the Palaeobot. Coil. Nanjing Univ. No. are given for each specimen. 1, 4, 6, 7: Paratetraphycus giganteus gen. et sp. nov. 1, Yinjiagou, GS454-1, B8307. 4, Tiejiangya, GS 59, B8308. 6, holotype, Yinjiagou, GS454-2, B8309. 7, Tiejiangya, YK176, B8310 (the specimen is deposited in the Hubei Institute of Geology). 2: Huroniospora sp., Jiangjiapo, GS86-2, B8311. 3: unnamed larger spheroids, Jiangjiapo, GS86-2, B8312. 5: Myxococcoides sp., Yinjiagou, GS454-1, B8313. 8: Nanococcus vulgaris J.H. Oehler, Liantuo, LAR-8-1TS06, B8314.
-,1
,,.l
, L,
1
o~
169 Description: Cell-like units spheroidal, ellipsoidal or hemispheroidal, 2.4-20.4 # m in diameter, averaging 9.3 # m (126 cell-like units measured in three aggregates). Surface may be psilate, granular to muricate; walls distinct, solid and thick, ranging up to 1 p m in thickness, or not well
Genus Nanococcus J.H. Oehler, 1977 Nanococcus vulgaris J.H. Oehler, 1977 (Plate I, 8; Plate II, 8) Description: Cell-like units spheroidal to subspheroidal, 1.1--6.2 ~m in diameter, with an average of 2.9 ~m (27 cell-like units measured). Cell-like units less c o m m o n l y solitary, rarely in pairs, and generally occur as clusters composed of a few to several tens of cells loosely distributed within a common, amorphous organic matrix. Walls psilate to granular, or not welldefined, preserved as interrupted surface, Individual cellular sheaths lacking. Generally devoid of inner dark bodies. Discussion: The Doushantuo fossils assigned to this species are similar in general morphology to the specimens from the H.Y.C. Pyritic Shale Member of the Barney Creek Formation (McArthur Group) (Oehler, 1977) and the Balbirini Dolomite (McArthur Group) (Oehler, 1978) of Australia.
Plate II. Structurally-preserved coccoid microfossils in petrographic thin sections of black cherty beds and siliceous or phosphatic nodules in microdolomites of the Doushantuo Formation (late Sinian) (~ 700 Ma old) of western Hubei Province, China. Bar scale is 10-~m long. Locality, thin-section number and the Palaeobot. Coll. Nanjing Univ. No. are given for each specimen. 1: Myxococcoides inornata Schopf, Tiejiangya, GS69, B8315. 2, 3: unnamed double-walled spheroids. 2, Jiangjiapo, GS86-2, B8316. 3, Liantuo, LAR-8-1-TS06, B8317.4, 5: unnamed larger spheroids. 4, Jiangjiapo, GS86-2, B8318. 5, Jiangjiapo, YK237, B8319 (the specimen is deposited in the Hubei Institute of Geology). 6: Paratetraphycus giganteus gen. et sp. nov., Yinjiagou, YK451, B8320 (the specimen is deposited in the Hubei Institute of Geology). 7, 9: Aphetospora euthenia Lo. 7, Jiangjiapo, GS86-2, B8321. 9, Jiangjiapo, GS86-2, B8322.8: Nanococcus vulgaris J'H. Oehler, Liantuo, LAR-8-1-TS02, B8323.
170
A minor difference from the type is the slightly larger size and broader size range of the Nanococcus vulgaris in the described material. It seems to me that Muir's specimen of Huroniospora psilata (Muir, 1976, fig. 6A) can also be assigned to this species. Some of specimens of Sphaerophycus parvum from the Belcher Supergroup (Hofmann, 1976; plate 3, fig. 4, for example) and the Dismal Lakes Group (Horodyksi and Donaldson, 1980; fig. 5A, for example), however, are morphologically indistinguishable from Nanococcus vulgaris. Therefore they might be conspecific. Type locality and occurrence: Liantuo, Yichang County, western Hubei Province, China; i.e., the Doushantuo Formation, late Sinian. Genus Myxococcoides Schopf, 1968 Myxococcoides inornata Schopf, 1968 (Plate II, 1)
Description: Cell-like units spheroidal to subspheroidal, rarely eUipsoidal. Surface texture psilate to finely granular; walls thin, occasionally not welldefined, ~0.5-~m thick. Cell-like unit diameters vary between 12.0 and 25.2 ~m, with an average of 17.3 #m (28 cell-like units measured). Individuals commonly grouped in colonies of a few tens of cell-like units closely packed within a non-lameUated, somewhat granular amorphous organic matrix. Discussion: This description fits that given by Schopf (1968) for Myxococcoides inornata from the Bitter Springs Formation of central Australia. Although only one well-preserved specimen of M. inornata has been detected in the Doushantuo assemblage, it is more complete than the Bitter Springs (Schopf, 1968) and the early Proterozoic Belcher (Hofmann, 1976) specimen of the same species. Type locality and occurrence: Tiejiangya, Yichang County, western Hubei Province, China; the Doushantuo Forrhation, late Sinian. Myxococcoides sp. (Plate I, 5) Description: Cell-like units spheroidal, hemispheroidal or polygonal due to mutual compression, 19.2--45.6 p m in diameter, with an average of 28.9 ~m (30 cell-like units measured). Walls thin, distinct, < 1 ~m in thickness; surface texture psilate to finely granular. Cell-like units closely packed in dense aggregations. Amorphous organic matrix, if present, indistinct. Discussion: Among previously described material of Myxococcoides, the cell-like unit diameters of the present specimen are relatively large. Because only a single representative of this type has been found in the Doushantuo Formation, assignment to a particular species has not been made. Type locality and occurrence: Yinjiagou, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian.
171
Genus Huroniospora Barghoorn, 1965 Huroniospora spp. (Plate I, 2)
Description: Cell-like units solitary, spheroidal to subspheroidal, 5--15 ~m in diameter. Surface psilate, finely granular to muricate; walls thick, ranging up to 1 ~m in thickness. Organic sheath and preserved internal contents apparently lacking. Discussion: Huroniospora Barghoorn (Barghoorn and Tyler, 1965) is a wide-ranging genus, both geographically and geochronologically. The systematics and taxonomy of the genus is presently under revision, with emphasis being placed on cell-size rather than on wall-texture (Awramik, 1976; Awramik and Barghoorn, 1977; Mendelson and Schopf, 1982). Thus the DouslSantuo specimens described here are not referred to presently defined species. However, it is possible that these spheroids may represent isolated members of the Aphetospora euthenia specimens. Type locality and occurrence: Jiangjiapo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Unnamed double-walled spheroids (Plate II, 2, 3)
Description: Spheroids solitary, double-walled, ~15--25 #m in diameter. Dark organic body occasionally within inner layer. Surface psil~te, granular or scabrous. Discussion: Only a few specimens have been found in the Doushantuo assemblage. More specimens are necessary to interpret these spheroids satisfactorily. Type locality and occurrence: Jiangjiapo and Liantuo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Unnamed larger spheroids (Plate I, 3; Plate II, 4, 5)
Discussion: A few larger, isolated thick-walled spheroids, ranging from 21.0--33.6 p m in diameter, occur in the Doushantuo Formation. These spheroids are morphologically similar to the Aphetospora euthenia specimens described above, but they axe much larger than the mean for the A. euthenia spheroids. Thus, it is unreasonable to suppose that these larger spheroids represent isolated members of the A. euthenia specimens. The Doushantuo specimens are also much larger than those from the upper part of the Yudoma Formation, which are named Huroniospora spp. (5-10 pm) (Mendelson and Schopf, 1982). However, they appear to be indistinguishable from individual spheroids of Myxococcoides sp. described above, when detached from a colony. Type locality and occurrence: Jiangjiapo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian.
172 DISCUSSION AND SUMMARY
(1) The c. 700-Ma-old Doushantuo microbiota is one of the youngest Precambrian microbiotas known. The diverse microbiota is composed of two basic types of microfossils, namely the coccoids and the filaments. This recent discovery enlarges our knowledge of Precambrian life and provides an important record of a silicified microbiota of both evolutionary and biostratigraphic significance. (2) The algal fossils described here are all coccoid microorganisms, which are designated to eight categories, i.e., Paratetraphycus giganteus gen. et sp. nov., Nanococcus vulgaris J.H. Oehler, Aphetospora euthenia Lo, Myxococcoides inornata Schopf, Myxococcoides sp., Huroniospora spp., and two morphogroups. All these microfossils are possibly entirely of cyanophytic affinity. In this study, most of them are interpreted as having affinity to the Chroococcaceae. (3) Most Doushantuo coccoid microfossils are morphologically comparable to those of the 650-Ma-old Yudoma microbiota and the 850-Ma-old Bitter Springs microbiota. Among the micro fossils Paratetraphycus giganteus gen. et sp. nov. and Aphetospora euthenia Lo are most abundant and prominent. As mentioned above, the genus Paratetraphycus includes Tetraphycus conjunctum Lo from the Lower Yudoma Formation. Aphetospora euthenia Lo is also abundant in the Yudoma Formation. Their presence is one more suggestive point in favour of an Ediacarian age for the Doushantuo microbiota. (4) Almost 30 well-preserved Precambrian stromatolite-forming microbiotas are known from Australia, Canada, India, South Africa, Greenland, the United States and the Soviet Union (Schopf, 1977). Although the Doushantuo microbiota is comparable to other microbiotas reported from stromatolites, it occurs in non-stromatolitic cherts. In fact, few stromatolites from the Doushantuo Formation of western Hubei Province have been reported. This report on the Doushantuo non-stromatolitic microbiota adds some information about the preservation of Precambrian microbiota. ACKNOWLEDGEMENTS
I thank the Department of Geology, Nanjing University, China, and the Department of Geology, Royal School of Mines, Imperial College, London, U.K., for supporting this research. I am especially grateful to Professor S. Golubid and Dr. S.E. Cambell for valuable discussion on many aspects of Prec~nbrian microfossils and for critically reviewing this manuscript. Thanks are also due to the Hubei Institute of Geology for providing many rock samples and petrographic thin sections.
173 REFERENCES Awramik, S.M., 1976. Gunflint stromatolites: microfossil distribution in relation to stromatolite morphology. In: M.R. Walter (Editor), Stromatolites. Elsevier, Amsterdam, pp. 311--320. Awramik, S.M. and Barghoorn, E.S., 1977. The Gunflint microbiota. Precambrian Res., 5: 121--142. Barghoorn, E.S. and Tyler, S.A., 1965. Microorganisms from the Gunflint chert. Science, 147 : 563--577. Hofmann, H.J., 1976. Precambrian microflora, Belcher Islands, Canada: significance and systematics. J. Paleontol., 50: 1040--1073. Horodyski, R.J. and Donaldson, J.A., 1980. Microfossils from the Middle Proterozoic Dismal Lakes Group, Arctic Canada. Precambrian Res., 11: 125--159. Knoll, A.H. and Golubi~, S., 1979. Anatomy and taphonomy of a Precambrian algal stromatolite. P~ecambrian Res., 10: 115--151. Lee, J.S. and Chao, Y.Y., 1924. Geology of the Gorge district of the Yangtze (from Ichang to Tzekuei), with special reference to the development of the Gorges. Bull. Geol. Soc. China., 3: 351--391. Lo, S.-C.C., 1980. Microbial fossils from the lower Y u d o m a Suite, earliestPhanerozoic, eastern Siberia. Precambrian Res., 13: 109--166. Mendeison, C.V. and Schopf, J.W., 1982. Proterozoic microfossils from the Sukhaya Tunguska, Shorikha, and Y u d o m a formations of the Siberian Platform, USSR. J. Paleontol., 56: 42--83. Muir, M.D., 1976. Proterozoic microfossils from the Amelia Dolomite, McArthur Basin, Northern Territory. Alcheringa, 1: 143--158. Oehler, D.Z., 1978. Microflora of the middle Proterozoic Balbirini Dolomite (McArthur Group) of Australia. Alcheringa, 2: 269--309. Oehler, J.H., 1977. Microflora of the H.Y.C. Pyritic Shale M e m b e r of the Barney Creek Formation (McArthur Group), middle Proterozoic of northern Australia. Alcheringa, 1: 315--349. Schopf, J.W., 1968. Microflora of the Bitter Springs Formation, late Precambrian, central Australia. J. Paleontol., 42: 651--688. Schopf, J.W., 1977. Biostratigraphic usefulness of stromatolitic Precambrian microbiotas: a preliminary analysis.Precambrian Res., 5: 143--173. Zhang, Z., 1981a. Precambrian microfossils from the Sinian of South China. Nature (London), 289: 792--793. Zhang, Z., 1981b. A new Oscillatoriaceae-likefilamentous microfossil from the Sinian (late Precambrian) of western Hubei Province, China. Geol. Mag., 118: 201--206. Zhang, Z., 1982. Filamentous microfossils from the Doushantuo Formation (late Sinian) of South China. J. Paleontol., 56: 1251--1256. Zhang, Z., 1984. O n the occurrence of ObrucheveUa from the Doushantuo Formation (late Sinian) of western Hubei Province and its significance. Acta Palaeontol. Sin., 23: 447--451. Zhao, Z. et al.,1980. The Sinian System of eastern Yangtze Gorges, Hubei. In: Tianjin Institute of Geology and Mineral Resources, Chinese A c a d e m y of Geological Sciences (Editor), Research on Precambrian Geology: Sinian Suberathem. Tianjin Science and Technology Press, Tianjin, pp. 31--35.
163
COCCOID MICROFOSSILS FROM THE DOUSHANTUO FORMATION (LATE SINIAN) OF SOUTH CHINA
ZHANG ZHONGYING
Department of Geology, Nanfing University, Nanjing (People's Republic of China) (Received May 15, 1983; revision accepted October 12, 1984)
ABSTRACT Zhang, Z., 1985. Coccoid microfossils from the Doushantuo Formation (late Sinian) of South China. Precambrian Res., 28: 163--173. Eight categories of organic-walled coccoid microfossils in the c. 700-Ma-old Doushantuo Formation are described and named. Of these, one genus and species (Paratetraphycus giganteus) is new. Most of the microfossils are interpreted as being the remains of the Chroococcaceae and are morphologically comparable to those of the 650-Maold Yudoma microbiota and the 850-Ma-old Bitter Springs microbiota. These microfossils occur in non-stromatolitic cherts, adding preservational information to the study of Precambrian life.
INTRODUCTION Palaeontologists and biologists are probing the early history of life on the Earth. During the past decade, and especially within recent years, considerable progress has been made in China in t he study o f Precambrian life. In South China a large n u m b e r o f microfossils from Precambrian rocks, particularly f r o m t he Sinian, has been reported. These discoveries provide i m p o r t a n t insight into the evolutionary development and biostratigraphic usefulness of Precambrian microbiotas in China. I have r e p o r t e d some filamentous microfossils, including Eomycetopsis robusta Schopf, emend. Knoll and Golubi6, cf. Gunflintia minuta Barghoorn, Palaeolyngbya sp., Rhicnonema antiquum H ofm ann, Siphonophycus sp., Doushantuonema peatii Z. Zhang and ObrucheveUa minor Z. Zhang f r o m the Doushantuo F o r m a t i o n (late Sinian) of western Hubei Province, China (Zhang, 1981a, b, 1982, 1984). Recently during examination of about 400 petrographic thin sections, I have discovered m any t axa o f coccoid microfossils which have n o t been previously r e p o r t e d from the D oushant uo Formation. The purpose o f this paper is t o illustrate, identify, and describe these newly-discovered coccoid forms, thus f u r t h e r supplementing our understanding o f t he D ous ha nt uo microbiota.
0301-9268/85/$03.30
© 1985 Elsevier Science Publishers B.V.
164 REGIONAL
STRATIGRAPHY
The Sinian, occurring as the first covering strata on the Yangtze platform, crops out extensively in western Hubei Province, particularly in the eastern Yangtze Gorges district around the flanks of the Huangling anticline. Early in 1924, the late Chinese geologist Lee Siguang and his assistant (Lee and Chao, 1924) investigated the geology of this district and established the Sinian section there, thus providing a preliminary standard for Sinian study in South China. Since then, especially during the past decade, extensive studies on the Sinian biostratigraphy and geochemistry of western Hubei Province have been carried out and great advances have been made. In this district, an unmetamorphosed Sinian sequence up to 1000-m thick is well-exposed. The Sinian rests upon the erosion surface of the Huangling Granite dated as 819 + 54 Ma old (Rb--Sr isochron age) (Zhao et al., 1980), or the metamorphosed Kongling Group with a distinct angular unconformity, and is comformably overlain by lowest Cambrian strata containing small shell fossils, such as hyolithids, monoplacophorans, branchiopods, and gastropods. The latest radiometric data give a time span of 615 + 20 to 800 + 30 Ma ago for the Sinian (Zhao et al., 1980), which is thus considered as late Precambrian. There is a long established subdivision of the Sinian in western Hubei Province. In this area, in accordance with changes in lithology and sedimentary cycles as well as palaeontological assemblages, the Sinian is naturally divisible into four formations {Fig. 1). From the base upwards, these are: Lower
Cambrian
I[ II
II
Dengying
II
Fm.
II o
II
I1
a "~
t.
C
]1
II
248-671m a
II •
.m
"~
•~ E
m
I
II
•
Douehentuo
Fm.
II
II
8
It
|70-230m -2 . -
Nantuo Fro.
i
~':
50-180m
• ........
o ..J
• ..
L i a n t u o Fro. -260m
Huengling
Granite
÷ I-
+ .I-
+ ÷
~. +
F i g . 1. Stratigraphic sequence of the Sinian and adjacent rocks in western Hubei Province, C h i n a . Thickness data f r o m Z h a o et al. ( 1 9 8 0 ) .
165
(1) the Liantuo Formation, chiefly feldspar-quartz-sandstones with basal conglomerates; (2) the Nantuo Formation, tillites; (3) the Doushantuo Formation; and (4) the Dengying Formation, mainly dolomites with cherty beds and siliceous nodules. The Liantuo Formation and the Nantuo Formation comprise the Lower Sinian which is characterized by terrestrial and shoreline clastic sediments, including glacial sediments in its upper part which indicate a cold climate. The Doushantuo Formation and the Dengying Formation comprise the Upper Sinian which represents shallow water marine sediments found in an arid climate. The boundary between the Upper and the Lower Sinian is now set at 700 Ma (Zhao et al., 1980). The above-mentioned subdivision of the Sinian has been reiterated by the Meeting for the Stratigraphical Classification and Nomenclature of the Late Precambrian convened in Beijing in 1982. The Doushantuo Formation of western Hubei Province is 170--230-m thick, and mainly consists of microdolomites, intercalated with black carbonaceous shales, mudstones, siliceous limestones, cherty beds, and siliceous or phosphatic nodules. Evaporite relicts, such as pseudomorphs after gypsum, sometimes occur in its lower part. This formation yields diverse and abundant microfossils, which are mostly composed of organicwalled residuals three-dimensionally preserved in a structurally-intact condition. SYSTEMATIC PALAEONTOLOGY
Division: Cyanobacteria Class: Cocogoneae Order: Chroococcales Chroococcaceae-like coccoids Genus: Paratetraphycus gen. nov. Type species: Paratetraphycus giganteus sp. nov.
Diagnosis: Cell-like units spheroidal, ellipsoidal, hemispheroidal or slightly polygonal due to mutual compression, 3--21 ~m in diameter. Walls thin, distinct; surface texture psilate to finely granular. Spheroids commonly in planar tetrads and diads, occasionally in triads and cross tetrads. Tetrads commonly occur in planar clusters (one or two layers), without an encompassing amorphous organic matrix. Etymology: With reference to morphological similarity to the genus Tetraphycus D. Z. Oehler. Discussion: In general, the morphology of the cell-like units, and their common arrangement in planar tetrads and diads, in this taxon resemble Tetraphycus D. Z. Oehler, 1978. However, Doushantuo specimens of Paratetraphycus giganteus differ from members of the four species of Tetraphycus (Oehler, 1978) previously reported in their lack of adherent amor-
166
phous organic matrix, and their larger size of cell-like units. These important distinctions warrant the erection of a new genus. Tetraphycus conjunctum described by Lo (1980) from the Lower Yudoma Formation of eastern Siberia also show its lack of adherent amorphous matrix. Therefore, it seems reasonable to refer T. conjuncture to the present genus. The specimens here assigned to Paratetraphycus display a distinct cyanophytic habit of spheroids commonly in planar tetrads and diads. Based on size and colonial form, I prefer to assign Paratetraphycus to the Chroococcaceae-like blue-green algae. A closer modern analogue to Paratetraphycus seems to be the genus Gloeocapsa Elenkin.
Paratetraphycusgiganteus sp. nov. (Plate I, 1, 4, 6, 7; Plate II, 6) Diagnosis: Qualitative characters as for genus. Cell-like units 10.8--21.6 ~m in diameter, with an average of 15.5 #m (101 cell-like units measured in 6 clusters). Walls < 1-pm thick. Type specimens: The cluster shown in Plate I, 6 has been chosen as holotype (thin-section GS454-2, Palaeobot. Coll. Nanjing Univ. No. B8309, stage coordinates 81.6x/17.2y). All other specimens illustrated are paratypes. Etymology: After the extremely large size of the cell-like units. Discussion: In gross morphology, this species resembles Paratetraphycus conjunctum (Lo) from the Lower Yudoma Formation of eastern Siberia (Lo, 1980). However, the cell-like units of Paratetraphycus conjuncture (Lo) range in diameter from 3.1 to 6.2 gm, averaging 4.9 ~m, and the significantly larger and non-overlapping size range of P. giganteus cell-like units distinguishes the two species. Clusters of this species are abundant in the Doushantuo microbiota. Type locality and occurrence: Yinjiagou and Tiejiangya, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Genus Aphetospora Lo, 1980 Aphetospora euthenia Lo, 1980 (Plate 2, 7, 9) Plate I. Structurally-preserved coccoid microfossils in petrographic thin sections of black cherty beds and siliceous or phosphatic nodules in microdolomites of the Doushantuo Formation (late Sinian) (~700 Ma old) of western Hubei Province, China. Bar scale is 10-~m long. Locality, thin-section number and the Palaeobot. Coil. Nanjing Univ. No. are given for each specimen. 1, 4, 6, 7: Paratetraphycus giganteus gen. et sp. nov. 1, Yinjiagou, GS454-1, B8307. 4, Tiejiangya, GS 59, B8308. 6, holotype, Yinjiagou, GS454-2, B8309. 7, Tiejiangya, YK176, B8310 (the specimen is deposited in the Hubei Institute of Geology). 2: Huroniospora sp., Jiangjiapo, GS86-2, B8311. 3: unnamed larger spheroids, Jiangjiapo, GS86-2, B8312. 5: Myxococcoides sp., Yinjiagou, GS454-1, B8313. 8: Nanococcus vulgaris J.H. Oehler, Liantuo, LAR-8-1TS06, B8314.
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169 Description: Cell-like units spheroidal, ellipsoidal or hemispheroidal, 2.4-20.4 # m in diameter, averaging 9.3 # m (126 cell-like units measured in three aggregates). Surface may be psilate, granular to muricate; walls distinct, solid and thick, ranging up to 1 p m in thickness, or not well
Genus Nanococcus J.H. Oehler, 1977 Nanococcus vulgaris J.H. Oehler, 1977 (Plate I, 8; Plate II, 8) Description: Cell-like units spheroidal to subspheroidal, 1.1--6.2 ~m in diameter, with an average of 2.9 ~m (27 cell-like units measured). Cell-like units less c o m m o n l y solitary, rarely in pairs, and generally occur as clusters composed of a few to several tens of cells loosely distributed within a common, amorphous organic matrix. Walls psilate to granular, or not welldefined, preserved as interrupted surface, Individual cellular sheaths lacking. Generally devoid of inner dark bodies. Discussion: The Doushantuo fossils assigned to this species are similar in general morphology to the specimens from the H.Y.C. Pyritic Shale Member of the Barney Creek Formation (McArthur Group) (Oehler, 1977) and the Balbirini Dolomite (McArthur Group) (Oehler, 1978) of Australia.
Plate II. Structurally-preserved coccoid microfossils in petrographic thin sections of black cherty beds and siliceous or phosphatic nodules in microdolomites of the Doushantuo Formation (late Sinian) (~ 700 Ma old) of western Hubei Province, China. Bar scale is 10-~m long. Locality, thin-section number and the Palaeobot. Coll. Nanjing Univ. No. are given for each specimen. 1: Myxococcoides inornata Schopf, Tiejiangya, GS69, B8315. 2, 3: unnamed double-walled spheroids. 2, Jiangjiapo, GS86-2, B8316. 3, Liantuo, LAR-8-1-TS06, B8317.4, 5: unnamed larger spheroids. 4, Jiangjiapo, GS86-2, B8318. 5, Jiangjiapo, YK237, B8319 (the specimen is deposited in the Hubei Institute of Geology). 6: Paratetraphycus giganteus gen. et sp. nov., Yinjiagou, YK451, B8320 (the specimen is deposited in the Hubei Institute of Geology). 7, 9: Aphetospora euthenia Lo. 7, Jiangjiapo, GS86-2, B8321. 9, Jiangjiapo, GS86-2, B8322.8: Nanococcus vulgaris J'H. Oehler, Liantuo, LAR-8-1-TS02, B8323.
170
A minor difference from the type is the slightly larger size and broader size range of the Nanococcus vulgaris in the described material. It seems to me that Muir's specimen of Huroniospora psilata (Muir, 1976, fig. 6A) can also be assigned to this species. Some of specimens of Sphaerophycus parvum from the Belcher Supergroup (Hofmann, 1976; plate 3, fig. 4, for example) and the Dismal Lakes Group (Horodyksi and Donaldson, 1980; fig. 5A, for example), however, are morphologically indistinguishable from Nanococcus vulgaris. Therefore they might be conspecific. Type locality and occurrence: Liantuo, Yichang County, western Hubei Province, China; i.e., the Doushantuo Formation, late Sinian. Genus Myxococcoides Schopf, 1968 Myxococcoides inornata Schopf, 1968 (Plate II, 1)
Description: Cell-like units spheroidal to subspheroidal, rarely eUipsoidal. Surface texture psilate to finely granular; walls thin, occasionally not welldefined, ~0.5-~m thick. Cell-like unit diameters vary between 12.0 and 25.2 ~m, with an average of 17.3 #m (28 cell-like units measured). Individuals commonly grouped in colonies of a few tens of cell-like units closely packed within a non-lameUated, somewhat granular amorphous organic matrix. Discussion: This description fits that given by Schopf (1968) for Myxococcoides inornata from the Bitter Springs Formation of central Australia. Although only one well-preserved specimen of M. inornata has been detected in the Doushantuo assemblage, it is more complete than the Bitter Springs (Schopf, 1968) and the early Proterozoic Belcher (Hofmann, 1976) specimen of the same species. Type locality and occurrence: Tiejiangya, Yichang County, western Hubei Province, China; the Doushantuo Forrhation, late Sinian. Myxococcoides sp. (Plate I, 5) Description: Cell-like units spheroidal, hemispheroidal or polygonal due to mutual compression, 19.2--45.6 p m in diameter, with an average of 28.9 ~m (30 cell-like units measured). Walls thin, distinct, < 1 ~m in thickness; surface texture psilate to finely granular. Cell-like units closely packed in dense aggregations. Amorphous organic matrix, if present, indistinct. Discussion: Among previously described material of Myxococcoides, the cell-like unit diameters of the present specimen are relatively large. Because only a single representative of this type has been found in the Doushantuo Formation, assignment to a particular species has not been made. Type locality and occurrence: Yinjiagou, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian.
171
Genus Huroniospora Barghoorn, 1965 Huroniospora spp. (Plate I, 2)
Description: Cell-like units solitary, spheroidal to subspheroidal, 5--15 ~m in diameter. Surface psilate, finely granular to muricate; walls thick, ranging up to 1 ~m in thickness. Organic sheath and preserved internal contents apparently lacking. Discussion: Huroniospora Barghoorn (Barghoorn and Tyler, 1965) is a wide-ranging genus, both geographically and geochronologically. The systematics and taxonomy of the genus is presently under revision, with emphasis being placed on cell-size rather than on wall-texture (Awramik, 1976; Awramik and Barghoorn, 1977; Mendelson and Schopf, 1982). Thus the DouslSantuo specimens described here are not referred to presently defined species. However, it is possible that these spheroids may represent isolated members of the Aphetospora euthenia specimens. Type locality and occurrence: Jiangjiapo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Unnamed double-walled spheroids (Plate II, 2, 3)
Description: Spheroids solitary, double-walled, ~15--25 #m in diameter. Dark organic body occasionally within inner layer. Surface psil~te, granular or scabrous. Discussion: Only a few specimens have been found in the Doushantuo assemblage. More specimens are necessary to interpret these spheroids satisfactorily. Type locality and occurrence: Jiangjiapo and Liantuo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian. Unnamed larger spheroids (Plate I, 3; Plate II, 4, 5)
Discussion: A few larger, isolated thick-walled spheroids, ranging from 21.0--33.6 p m in diameter, occur in the Doushantuo Formation. These spheroids are morphologically similar to the Aphetospora euthenia specimens described above, but they axe much larger than the mean for the A. euthenia spheroids. Thus, it is unreasonable to suppose that these larger spheroids represent isolated members of the A. euthenia specimens. The Doushantuo specimens are also much larger than those from the upper part of the Yudoma Formation, which are named Huroniospora spp. (5-10 pm) (Mendelson and Schopf, 1982). However, they appear to be indistinguishable from individual spheroids of Myxococcoides sp. described above, when detached from a colony. Type locality and occurrence: Jiangjiapo, Yichang County, western Hubei Province, China; the Doushantuo Formation, late Sinian.
172 DISCUSSION AND SUMMARY
(1) The c. 700-Ma-old Doushantuo microbiota is one of the youngest Precambrian microbiotas known. The diverse microbiota is composed of two basic types of microfossils, namely the coccoids and the filaments. This recent discovery enlarges our knowledge of Precambrian life and provides an important record of a silicified microbiota of both evolutionary and biostratigraphic significance. (2) The algal fossils described here are all coccoid microorganisms, which are designated to eight categories, i.e., Paratetraphycus giganteus gen. et sp. nov., Nanococcus vulgaris J.H. Oehler, Aphetospora euthenia Lo, Myxococcoides inornata Schopf, Myxococcoides sp., Huroniospora spp., and two morphogroups. All these microfossils are possibly entirely of cyanophytic affinity. In this study, most of them are interpreted as having affinity to the Chroococcaceae. (3) Most Doushantuo coccoid microfossils are morphologically comparable to those of the 650-Ma-old Yudoma microbiota and the 850-Ma-old Bitter Springs microbiota. Among the micro fossils Paratetraphycus giganteus gen. et sp. nov. and Aphetospora euthenia Lo are most abundant and prominent. As mentioned above, the genus Paratetraphycus includes Tetraphycus conjunctum Lo from the Lower Yudoma Formation. Aphetospora euthenia Lo is also abundant in the Yudoma Formation. Their presence is one more suggestive point in favour of an Ediacarian age for the Doushantuo microbiota. (4) Almost 30 well-preserved Precambrian stromatolite-forming microbiotas are known from Australia, Canada, India, South Africa, Greenland, the United States and the Soviet Union (Schopf, 1977). Although the Doushantuo microbiota is comparable to other microbiotas reported from stromatolites, it occurs in non-stromatolitic cherts. In fact, few stromatolites from the Doushantuo Formation of western Hubei Province have been reported. This report on the Doushantuo non-stromatolitic microbiota adds some information about the preservation of Precambrian microbiota. ACKNOWLEDGEMENTS
I thank the Department of Geology, Nanjing University, China, and the Department of Geology, Royal School of Mines, Imperial College, London, U.K., for supporting this research. I am especially grateful to Professor S. Golubid and Dr. S.E. Cambell for valuable discussion on many aspects of Prec~nbrian microfossils and for critically reviewing this manuscript. Thanks are also due to the Hubei Institute of Geology for providing many rock samples and petrographic thin sections.
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