Intemal Morphology and Taphonomic History of the Neoproterozoic Vase-Shaped Microfossils from the Visingso Group, Sweden

Intemal morphology and taphonomic history of the Neoproterozoic vase-shaped microfossils from the Visingso Group, Sweden MONICA MARTf MUS & MALGORZATA MOCZYDLOWSKA Marti Mus, M. & Moczydlowska, M. Intemal morphology and taphonomic history of the Neoproterozoic vase-shaped microfossils from the Visingsii Group, Sweden. Norsk Geologisk Tidsskrift, Vol. 80, pp. 213-228. Oslo 2000. ISSN 0029- 196X. The morphology and mineral composition of the vase-shaped microfossils (VSMs) from phosphatic nodules of the upper Visingsii Group in the Lake Vattem area have been studied using confocal and transmitted light microscopy along with the EDX analytical technique. The taphonomic history of the microfossils and the internal anatomy of the original organism were reconstructed. An early diagenetic precipitation of phosphate accounts for the formation of phosphatic surfaces replicating the extemal and internal morphology of the vase-shaped organism. The vase-shaped vesicle had an internal compartrnent containing a reproductive cell or cyst. The presence of these structures indicates that the vesicle does not in itself represent an encystrnent stage of a protist as previously suggested by some authors. Hypotheses on the biological affinities of the VSMs are reviewed. The VSMs global record is considered in the palaeogeographic and palaeoenvironmetal contexts, suggesting that the vase-shaped microfossils were produced by stenothermal planktonic organisms, thriving in shallow marine seas in tropical to temperate climatic zones. M. Martf-Mus & M. Moczydlowska, Department of Earth Sciences, Historical Geology and Palaeontology, Uppsala University, Norbyviigen 22, SE-752 36, Uppsala, Sweden Introduction their world-wide distribution and their presence in a variety of lithologic facies supports the interpretation of Vase-shaped microfossils (VSMs) are a morphologically VSMs as representing marine planktonic organisms distinct group of microfossils known from Neoproterozoic (Bloeser et al. 1977; Knoll & Vidal 1980; Bloeser 1985; marine sediments world-wide. They have a characteristic Knoll et al. 1989; Fairchild et al. 1991). These intriguing vase shape, being rounded at the aboral pole and showing a microfossils thus constitute the earliest record of plank short, tapering, truncated neck with a narrow terminal tonic organisms showing morphological polarity (Bloeser opening at the oral pole. The vesicle has a smooth wall and 1985) which, together with the morphological complexity lacks processes. VSMs were first recorded by Ewetz of the oral pole and the herein described internat structures, (1933), though at the time un-named and simply called places them among the most complex Neoproterozoic 'single-celled organism with shell', from phosphatic microfossils recovered to date. nodules of the Visingso Group, southern Sweden. Since It has been observed that the mode of preservation of then, several occurrences of VSMs have been reported VSMs changes according to the lithology, but little is from Neoproterozoic (850-650 Ma) sediments of many known about the details of their preservation or the parts of the world (Table 1). The organically preserved taphonomic processes which they have undergone in specimens from the shales of the Kwagunt Formation different depositional environments. In only a few occur (Bloeser et al. 1977; Bloeser 1985) are the best-known rences the VSMs are preserved as three-dimensional morphologically and the only ones formally described organic-walled vesicles that can be extracted from the under the generic name Melanocyrillium Bloeser, 1985. sediment by standard palynological techniques (Bloeser et Differences in preservation between these exceptionally al. 1977; Vidal l979; Vidal & Ford 1985; Bloeser 1985). In preserved specimens and specimens from other localities most of the localities VSMs are preserved as moulds and have hindered detailed morphological comparisons and casts of entire, undeformed vesicles, and the organic have generally prevented the identification of the latter matter seems to be partially or comp1etely degraded. Knoll ones as Melanocyrillium. Even so, the different micro (1992, p. 57) pointed out that 'Marked taphonomic fossils summarized in Tab le l share distinct morphological differences between these fossils and acritarchs suggest similarities and have preservational features in common original differences in wall chernistry' (see below). (perhaps associated with the chemistry of the wall; see The VSMs from phosphatic nodules of the Visingso below) that suggest close biological affinities. Group were first described and figured by Ewetz (1933) VSMs occur in rocks of various lithologies: shales, and subsequently studied in more detail by Knoll & Vidal carbonates, cherts and phosphatic nodules, encompassing a (1980). Ewetz (1933) observed that the VSMs possessed wide array of shallow marine depositional settings. Both 'shells' with rough internat surfaces and that phosphate 214 M. Marti Mus & M. Moczydlowska NORSK GEOLOGISK TIDSSKRIFT 80 (2000) Table l. Global record of VSMs. Region Stratigraphic unit Age References North America KwaguntFm Late Proterozoic Bloeser et al. 1977 Bloeser 1985 Vida! &Ford 1985 Horodyski 1987, 1993 Porter & Knoll (in press) Pahrump Group Late Proterozoic Horodyski 1987, 1993 Greenland Limestone-Dolomite 'Series' Late Proterozoic Vida! 1979 (Eleonore Bay Group) Green et al. 1988 Sweden Visingso Group Late Riphean Ewetz 1933 Knoll & Vidal 1980 This study Saudi Arabia Jabal Rokharn Carbonates Late Precarnbrian Binda & Bokhari 1980 Svalbard RyssoFm Late Riphean Knoll l982 Knoll & Calder 1983 DrakenFm Late Riphean Knoll et al. 1991 Backlundtoppen Fm Late Riphean Knoll et al. 1989 Elbobreen Fm Late Proterozoic Knoll 1990, 1992 Tasmania Smithton Dolomite Late Precambrian Saito et al. 1988 had entered the fossil vesicles through their 'mouths'. He of alternating micaceous shale and siltstone, with coarse also observed an internal, round, brownish structure, which sandstone at the bottom and interbeds of dolomitic he interpreted as a resting spore, and concluded that the limestone with stromatolites at the top (Vidal 1972, Visingso VSMs could represent fossil rhizopods. Knoll & 1976, 1985). The total thickness of the unit was estimated Vidal (1980) described the morphology and dimensional as being more than 580 m (Collini 1951). The observed variation of a greater population of VSMs and compared sedimentary structures and the presence of abundant them structurally and ecologically with modem tintinnids. marine planktonic microfossils in the upper Visingso In this study, newly observed internal structures Group conform with deposition in a shallow marine preserved in the VSMs are reconstructed in the light of environment with oscillating water depth, including shelf the taphonomic history of the microfossils, and their and subtidal to intertidal carbonate mud flats (Vidal 1972, biological significance is assessed. 1976, 1985; Samuelsson & Strauss 1999). The cosmo politan acritarch species dominating the assemblage also attest to the existence of a connection between the Vattern Basin and the global ocean (Vidal 1985) maintaining the Geological setting and palaeoenvironments free dispersal of microplankton. The total organic carbon The Visingso Group is a sedimentary succes si on consisting (TOC) content in the whole-rock upper Visingso samples of terrigenous rocks with minor carbonates of early has an average value of 3.56 mg C/g for shales (Strauss & Neoproterozoic age that extend in a restricted area of Moore 1992; Samuelsson & Strauss 1999). This value is Lake Vattern in south-central Sweden (Fig. lA, B). The higher than the average value for late Proterozoic shales sediments are preserved in a graben within the crystalline (2.30 mg C/g; Strauss et al. 1992) suggesting that not only basement presently occupied by the lake. The successi on is high burial of organic matter, but also high organic unmetamorphosed and exposed in the coastal cliffs of the productivity must have prevailed in the Visingso Basin. lake and on the Vising Island (Visingso), and is known also This, together with the presence of phosphate nodules in from a number of boreholes in the vicinity of the lake the upper Visingso Group is compatible with the Visingso (Brotzen 1941; Collini 1951; Vidal 1974, 1976, 1982, basin being influenced by upwelling of nutrient-rich 1985; Fig. lB). Three informal lithostratigraphic units waters, as suggested by Knoll & Vidal (1980). (lower, middle and upper) which are in the rank of The climatic conditions prevailing during the deposition formations (Vidal 1985), are distinguished in the Visingso of the Visingso Group, related to the palaeogeographic Group, and together they exceed 1000 m in thickness position of the host craton, have been constrained by (Collini 1951). several independent lines of evidence; i.e. palaeobiology, The lower unit of the Visingso Group consists of cross diagenetic mineralogy and palaeomagnetic data. The bedded, fluvial quartz sandstone with minor conglomerate presence of stromatolites in the upper Visingso Group, and was deposited in a delta plain environment. The characteristic of tropical and warm temperature beits, is a middle unit is characterized by alternating feldspathic direct indication of the climatic conditions (Vidal 1972, arenite, conglomerate and siltstone and was accumulated 1976). The abundance of detrital biotite suggests dry in a shallow marine prodelta environment (Vidal 1974, climatic conditions (Vidal 1985). The early diagenetic

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