The oldest known dinoflagellates: Morphological and ...

ARTICLES Chinese Science Bulletin 2005 Vol. 50 No. 12 1230—1234

Abstract Abundant and well-preserved organic-walled microfossils including acanthomorphic acritarchs have been found in Mesoproterozoic Beidajian Formation in the Yongji area of Shanxi Province, North China. The morphological and ultrastructural features of these acanthomorphic acritarchs resemble living dinoflagellates (e.g. double-walled and polygonal structures), which leads to the interpretation of these fossils as probably the oldest dinoflagellates. The detection of dinosterane, a dinoflagellate biomarker, from pyrolytic product of these fossils further supports the morphological inference. This finding is consistent with molecular clock estimate that dinoflagellates may have diverged 700 to 900 million years (Ma) before previously known fossil record.

cystment structures are different from archeopyles of dinoflagellates, and the reflected tabulation in dinoflagellates is absent in acritarchs. However, the living dinoflagellates of the order Gymnodiniales do not produce clearly defined cysts or reflected tabulation[11]. 50% of living dinoflagellate orders do not produce fossilizable cysts, thus the fossil record of dinoflagellates is biased[12]. An alternative approach to test the ancestry of dinoflagellates is to study their biomarkers[13] . Triaromatic dinosteroids and dinosterane are regarded as diagnostic dinoflagellate biomarkers[6], because the precursors of triaromatic dinosteroids and dinosterane (dinosterols) have been found in most of dinoflagellates at rather high concentrations and are hardly present in other organisms[14,15]. Triaromatic dinosteroids have been detected in Precambrian to Permian rocks[13] . The occurrence curve of triaromatic dinosteroids through the Phanerozoic time is similar to the species diversity curves of Paleozoic acritarchs and Mesozoic dinoflagellates. The similarity implies that many Paleozoic acritachs are probably dinoflagellates lacking diagnostic features[13], and may also account for the coincidence between the decline of acritarchs after Paleozoic and the diversification of dinoflagellates in Mesozoic[16]. The occurrence frequency of triaromatic dinosteroids detected in Precambrian does not accord well with the species diversity curve of acritarchs, which may indicate that many Precambrian dinoflagellates cannot be preserved as fossilized cysts, but only as biomarkers [13]. The presence of dinosterane in pyrolytic product of isolated kerogen from Lower Cambrian acanthomorphic acritarchs further indicates that acanthomorphic acritarchs have close affinity with dinoflagellates [17].

Keywords: Mesoproterozoic, Beidajian Formation, acanthomorphic acritarch, dinoflagellate, dinosterane.

1 Fossil horizon and morphological features

The oldest known dinoflagellates: Morphological and molecular evidence from Mesoproterozoic rocks at Yongji, Shanxi Province MENG Fanwei1 , ZHOU Chuanming1, YIN Leiming1, CHEN Zhilin2 & YUAN Xunlai1 1. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China; 2. Organic Geochemistry Laboratory of Geological Academy, Shengli Oil Field, Dongying 257000, China Correspondence should be addressed to Yuan Xunlai (email: xlyuan@ nigpas.ac.cn)

DOI: 10.1360/982004-543

Dinoflagellates are primitive planktonic eukaryotic algae. Known as “mesokaryotes”[1], dinoflagellates may have a very early origin[2,3]. Studies of RNA molecular sequence and mitochondrial cristae of living dinoflagellates indicate that dinoflagellates emerged earlier than the Foraminifera and Radiolaria which have a fossil record in the Cambrian[4]. Indeed, several lines of evidence suggest that dinoflagellates originate in the Neoproterozoic[5]. The earliest and undisputed fossil dinoflagellate cysts, however, were found in the Upper Triassic[6]. “Dinoflagellates” older than Middle Triassic are not widely accepted due to the absence of diagnostic features[7,8]. There are 22 species of thermally altered organic-walled microfossils similar to dinoflagellate cysts found in the Silurian[3,9]. Some organic-walled microfossils with excystment structures discovered in Neoproterozoic rocks have also been regarded as possible dinoflagellate[10]. It has been suggested that there are primordial dinoflagellates in acritarchs in Precambrian and Paleozoic, although the acritarch excystment structures are different from archeopyles of 1230

In Yongji of Shanxi Province, North China, Mesoproterozoic and Neoproterozoic strata unconformably overlie the Archean gneiss of Songshui Group, and unconformably underlie the Sinian Luoquan diamictite. In ascending order, the Mesoproterozoic and Neoproterozoic strata consist of the Ruyang Group, Luoyu Group, and Sinian[18]. Abundant well-preserved acritarchs were found in the Beidajian Formation of Mesoproterozoic Ruyang Group. These acritarchs are transparent, and yellow to light brown in color (Fig. 1). The diversity of Ruyang acritarchs is so low that among them Dictyosphaera and Shuiyousphaeridium account for more than 90% of the total specimens[19]. The acritarch assemblage from Beidajian Formation can be correlated well with the acritarch assemblage in the Roper Group of Australia, which suggests that the age of Beidajian Formation is probably between 1200―1500 Ma[19,20]. Carbon isotope chemostratigraphic correlation also suggests a late Mesoproterozoic age for the Beidajian Formation[21]. Chinese Science Bulletin Vol. 50 No. 12

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ARTICLES TEM studies on Dictyosphaera and Shuiyousphaeridium show that an organic inner membrane layer joins closely with outer walls bearing polygonal structures. This typical double-wall structure (Fig. 2) is quite similar to that of living dinoflagellates [22]. Carbon isotope analysis of individual acritarchs of the genus Dictyosphaera suggests

that it is a photosynthetic eukaryote [23]. The polygonal structure of Dictyosphaera and Shuiyousphaeridium resembles reflected tabulation of dinoflagellate, although the polygons are much smaller but much more numerous than those in dinoflagellates (Fig. 3).

Fig. 1. (a) Well-preserved Dictyosphaera in Beidajian Formation; (b) well-preserved Shuiyousphaeridium in Beidajian Formation.

Fig. 2. TEM images showing double-wall structure (arrow) of Shuiyousphaeridium.

Fig. 3. (a) Dictyosphaera showing outer wall bearing polygonal structure under transmitted light microscope; (b) Shuiyousphaeridium showing outer wall bearing polygonal structure under scanning electron microscope .

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ARTICLES There are two hypotheses about the evolution of reflected tabulation of dinoflagellates. One states that the number of reflected tabulation increases[24], while the other states that it decreases with time[25]. Thus Dictyosphaera and Shuiyousphaeridium with smaller but more numerous plates would support the second hypothesis[25]. However, this inference needs more evidences to confirm the dinoflagellate affinity of Dictyosphaera and Shuiyousphaeridium. To this end, we present the biomarker evidence in support of the dinoflagellate affinity of the Ruyang acritarchs. 2 Biomarker investigation and discussion In order to understand the phylogenetic affinity of Ruyang acanthomorphic acritarchs, we did a biomarker analysis on both whole rock extracts and the pyrolytic product of kerogens. Whole rock sample’s preparation and analysis followed procedures described by Shi et al.[26]. To eliminate free-lipid contamination from surface and cracks, we removed the outer part of rock samples, washed the samples with distilled water, soaked samples in absolute alcohol for 24 h and then in distilled CHCl3 for another 24 h. The samples were allowed to dry and were then finely ground and exhaustively extracted. The biomarker analysis was performed on the HP 6890 GC/5973N MSD gas chromatography-mass spectrometry-mass spectrometry (GC-MS-MS) system in the Shengli Oil Field Organic Geochemistry Laboratory. The results indicate that triaromatic dinosteroid (a dinoflagellate biomarker) is present in whole rock samples (Fig. 4), whereas biomarkers derived from higher plants are absent, which suggests that the extracts are not affected by recent contamination[27]. Triaromatic dinosteroids detected from whole rock extracts partly came from free-lipids derived from unfossilizable dinoflagellate cysts, and partly from free-lipids derived from the degradation of organic-walled microfossils (Table 1). In order to explore the relationship between biomarkers

Fig. 4.

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Table 1

Biomarker ratios in extracts of whole rock samples of Beidajian Formation Biomarker ratios Distributions 0.3719 C27/(C27+C28+C29)ααα20R sters 0.2807 C28/(C27+C28+C29)ααα20R sters 0.3474 C29/(C27+C28+C29)ααα20R sters

from whole rock extracts and organic-walled microfossils in the Beidajian Formation, we analyzed pyrolytic product of kerogens[17,28]. The first two steps of sample’s preparation are the same as procedures described above. After these procedures, we crushed the sample exhaustively, and then extracted the sample in distilled CHCl3 for another 72 h. Next we put these extracted powder into 15% HCl for 72 h, and then neutralized them in distilled water. Then we put the remains in 50% HF for 72 h to eliminate the free-lipids from linking cracks and fissures. At last, we washed the kerogen and dried them at room temperature. Dried kerogen was placed in a 0.5-cm-in-diameter new steel tube which was previously heated at 400℃ for 72 h and then, after cooling, soaked in pure CHCl3 for 72 h to eliminate organic contamination. Steel tube with dried kerogen was evacuated to 10−2 Pa. Samples were heated at 350 ℃ for 72 h. After the steel tube cooled to room temperature, the pyrolytic product of kerogen was taken away and put in pure CHCl3 for 144 h. Later we evaporated the CHCl3 and did the biomarker analysis using a metastable reaction monitoring (MRM)-GC-MS system. Experiment was performed on the Agilgent 6890GC/QUATTROⅡMS-MSD in the Shengli Oil Field Organic Geochemistry Laboratory. Blank experiment was done routinely to ensure credible results[25]. Dinoflagellate biomarkers were detected from the extracts of pyrolytic product of Beidajian Formation kerogen (Fig. 5). The relative abundances of C27-C28-C29 steranes between pyrolytic product of Beidajian kerogen and whole rocks extracts are different (Tables 1, 2; Fig. 6), and the predominance of C27 steranes is higher in pyrolytic product

GC-MS-MS traces showing triaromatic dinosteroids in whole rock extracts of Beidajian Formation (m/z→245).

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Fig. 5. MRM-GC-MS traces showing dinosteranes in Beidajian kerogen pyrolysate (for small samples MRM-GC-MS is used because it gives higher sensitivity than GC-MS-MS). Table 2

Biomarker ratios in pyrolytic product of kerogens of Beidajian Formation Biomarker ratios Distributions 0.4467 C27/(C27+C28+C29)ααα20R sters 0.2846 C28/(C27+C28+C29)ααα20R sters 0.2687 C29/(C27+C28+C29)ααα20R sters

Fig. 6. The C27-C28-C29 sterane distributions in the rock extract and microfossil pyrolysis experiment of Beidajian Formation. 1, The C27-C28-C29 sterane distributions in the rock extract of Beidajian Formation; 2, the C27-C28-C29 sterane distributions in the microfossils pyrolytic product of Beidajian Formation.


of Beidajian kerogen than whole rocks extracts of Beidajian Formation, which suggests that the pyrolytic products of kerogen are independent of free lipids of the rock, and pyrolytic analysis reflects original biochemical characteristics of kerogen fractions[17]. Relative abundance of C27-C28-C29 steranes shows the predominance of C27 steranes, which is similar to the relative abundance of C27C28-C29 steroles (sterane precursors) of living dinoflagellates. The results are also similar to biomarker analysis of kerogen pyrolytic product of the Cambrian acanthomorphic acritarchs[17]. In conclusion, both morphological features and biomarker analysis of kerogen pyrolytic products indicate that Dictyosphaera and Shuiyousphaeridium, especially Shuiyousphaeridium, from the Beidajian Formation probably represent primitive dinoflagellates. The small size but large number of polygons on their walls can be compared with the reflected tabulations of dinoflagellates, and suggest that primitive dinoflagellates have more reflectes tabulations and the number of reflected tabulations reduced gradually during evolution. This 1233

ARTICLES finding is broadly consistent with molecular clock estimate of dinoflagellate divergence[4,5]. The research implies that dinoflagellates may have appeared 700 to 900 Ma before previously known dinoflagellate fossils[17], and it pushes back the antiquity of dinoflagellate biomarkers by 100 to 400 Ma[29]. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 40472003), the Ministry of Science and Technology of China (Grant Nos. G2000077701 and 2003CB716805), and the Chinese Academy of Sciences (Grant No. KZCX3-SW-141). We thank He Chengquan and Xu Jinli for constructive discussion, Huang Fengbao and Wang Jinlong for technical help.

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