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A reconsideration of the biogenicity of Arumberia banksi Glaessner & Walter a
Duncan McIlroy & Malcolm R. Walter
a
a
School of Earth Sciences, Macquire University, NSW, 2109 Email: Published online: 27 Nov 2008.
To cite this article: Duncan McIlroy & Malcolm R. Walter (1997): A reconsideration of the biogenicity of Arumberia banksi Glaessner & Walter, Alcheringa: An Australasian Journal of Palaeontology, 21:1, 79-80 To link to this article: http://dx.doi.org/10.1080/03115519708619187
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Palaeontological Note A r e c o n s i d e r a t i o n of the b i o g e n i c i t y of Arumberia banksi Glaessner & Walter.
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DUNCAN MCILROY AND MALCOLM R. WALTER The Ediacarian body fossil Arumberia banksi (Fig. 1) was originally described by Glaessner & Walter (1975) from the Neoproterozoic part of the Arumbera Sandstone in central Australia. Glaessner & Walter (1975) interpreted abundant radiating casts on the soles of current-bedded sandstones as fossils of erect cup-like organisms. The structures are often superimposed upon sedimentary structures, especially flute marks on which the organisms were inferred to have grown. Arumberia has subsequently been recorded from several parts of the world and has been proposed as a marker for the latest Neoproterozoic (for a review ofoccarrences see Bland 1984; Wangetal. 1984;Rahaetal. 1991; Liu 1981). It is particularly abundant in siliciclastic units of the Nama Group in southern Namibia (J.G. Gehling pe rs comm.). In the original investigation of Arumberia the biogenic interpretation was favoured because the authors were unaware of any physical mechanism that could explain the radial arrangement of grooves and ridges that characterise the structure. Somewhat problematic to the interpretation of Arumberia was the paucity of distinctive margins to individuals. Subsequent to the original documentation of Arumberia, Alien (1982), and previously Dzulynski & Walton (1965), presented the results of experiments on flute cast formation; the experimentally produced flutes have striking resemblances to A. banksi. Vortices generated by fluid flow around objects on
Fig. 1. "Arumberia banksi" Glaessner & Walter,paratype CPC 14955 from the Neoproterozoic Arumbera Sandstone, Amadeus Basin, Northern Territory. Scale bar 10era. 0311/5518/97/01079-02
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the sediment surface were shown to have radiating flow patterns (Fig. 2A) and were produced experimentally by the action of a unidirectional current on a firm substrate such as plaster of paris or cohesive clay (Figs 2B, C). These are similar to the impressions of "ribs and grooves" on Arumberia. This abiogenic interpretation therefore goes some way to explaining the paucity of edges toAruraberia. 'A. banksi' from the Longmyndian of Shropshire, U.K., examined by the first author, is associated with small circular structures, commonly superimposed on the ribs of Arumberia. Such circular impressions were inferred by Bland (1984) to be resting cysts ofAruraberia. A petrographic thin section of one such slructure was illustrated by Bland (1984). We suggest that they may result from the weathering of volcanic iapilli which are relatively common in the Longmyndian (Greig et al. 1968). Similar structures were also generated in the experiments summarised by Allen (1982)(Fig. 2D). These may likewise be formed by interference between the scouring current and small objects exhumed by it. Both Glaessner & Walter (1975) and Bland (1984) noWxlthe alignment of Arumberia relative to the regional palaeocurrent and related it to taphonomic alignment of the original organisms. These data are also consistent with our reconsideration of these forms as sedimentary structures generated by unidirectional currents. In response to the abiogenic mode of formation of Arumberia favoured by Brasier (1979; based on observations by Dzulynski & Walton i 965), Bland (1984) listed three lines of evidence against a sedimenlary interpretation. We disagree with all of these and address them in order below. 1) Fine grooves on natural casts of Arumberia were considered to be junctions between ribs, but not the result of flow structures. The relative thickness of the ribs and furrows is, in a sedimentological interpretation, governed by the width of the radial grooves (erosional furrows) which could be so broad to leave only a fine radial ridge as observed on some specimens of Arumberia. 2) The lateral extent of the radial grooves and examples of cross cu~ing without distortion were considered by Bland not to be reconcilable with a sedimentary origin because current vortices could not overlap. This however may be aocounted for by the short-lived nature of vortices and superimposition of radial structures generated by a series of vortices, or"through printing" from contiguous laminae. 3) The presence of double ridges was thought by Bland not to be producible by a vortex. However, Allen (1982) illustrates several experiments which resulted in double ridges, generated by erosion around two adjacent objects on the sediment surface (Fig. 2E). The rectangular nature of many specimens of Arumberia was also considered to be problematical for its generation by sedimentary means. However, many of the flutes produced in the experiments of Allen (1982) have a rectangular cross section (Fig. 2B). A problem with our abiogenic interpretation ofArumberia i~ its apparent time-restriction within the latest Neoproterozoic. Such structures are not yet recorded from the pre-Ediacafian Proterozoic record, despite the conspicuousness of such traces and intensive searching of these stratigraphic levels for body fossils by many researchers, over many years. The high cohesiveness of sediment that would be needed for the preservation of such sedimentary structures may have been generated by microbial binding of sediment as suggested by Gehling (1991) to account for the preservation of the Ediac,ara fauna. If this were the case, the rise of bioturbating metazoa
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may account for the loss of such firm muddy substrates by bioturbation and microbial cropping (Walter & Heys 1985 and references therein). This could account for the lack of reports of Phanerozoic Arumberia, though very similar structures of Tertiary age are described as "frondescent markings" by Dzulynski & Walton (1965, p. 133, fig. 92). We conclude that them is no need to maintain the original interpretation ofArumberia banksi, and the most parsimonious interpretation is that it formed by the action of currents on cohesive muddy substrates which may have been microbially bound. Further work is needed to explain the temporal restriction of conditions suitable for the formation and preservation of such sedimentary structures. REFERENCES ALLEN,J.R.L., 1982. Sedimentary structures, their character and physical basis. Part IL Volume30B of Developments in Sedimentology Series.Elsevier, Amsterdam, 1-676. BLAND,B.H., 1984. Arumberia Glaessner & Walter, a review of its potential for correlation in the region of the Precambrian-Cambrian boundary. Geological Magazine
121,625-633. BrtAsma, M.D., 1979. The Cambrian radiation event. In, The origin of the major invertebrate groups M.R. House, ed. Systematics Association Special Volume 12. Academic Press, London, New York, 103-159. DZULVNSrd,S. & WALTON,E.K., 1965. Sedimentary features of flysch and greywackes. Developments in Sedimentology 7, Elsevier, Amsterdam, 1-274. GEHLING,J.G., 1991. The case for Ediacaran fossil roots to the
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metazoan tree. Memoirs of the Geological Society of India 20, 181-223, GLAESS~a,M. & WALTER,M.R., 1975. New Precambrian fossils from the Arumbera Sandstone, Northern Territory, Australia. Alcheringa 1, 59-69. Cmn~, M.A., Wmcm',J.E.,HA~s, B.A. & Mrrcrmt.LG.H., 1968 Geology of the country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee. Memoir of one inch sheet 166. H.M.S.O., London, 66-72. LIu, X., 1981. Metazoa fossils from the Mashan Group near Jixi, Heilongjiang. Bulletin of the Chinese Academy of
Geological Sciences 3, 71-83. RAHA,P.K., Morrax, A.K., SaAMA,D.C., KUMAR,A. E, RAMA, R.M., 1991. Search for microfossils in the Bhima and Kaladgi-Badami sequence of South India.Records of the Geological Survey of lndia 124, 10. WALar_a,M.R. & I-hvs, G., 1985. Links between the rise of the Metazoa and the decline of the stromatolites.Precambrian Research 29, 149-174. W A N G HONGZHEN, LIN BAOYU & LIo XIAOLIANG, 1984. Cnidarian fossilsfrom the Sinian System of China and their stratigraphic significance. In, Recent advances in the palaeobiology and geology of the Cnidaria. W.A. Oliver, W.J. Sando, S.D. Cairns, G. Coates, I.G. Macintyre, EM. Bayer, & J.E. Sorauf, eds.Paleontographica Americana 54, 136-140.
D. Mcllroy & M.R. Walter, School of Earth Sciences, Macquarie University, NSW, 2109. Enquiries to D. Mcllroy at l
[email protected].
Fig. 2. "Arumberia"-Iike structures produced experimentally by Allen (1982), reproduced with permission. A, Radial fluid flow patterns in a flute; B-D, experimentally generated flute-like structures showing abundant pimple-like markings and radial ridges; E, experimentally produced structure with a bilobed appearance generated by two adjacent surface inhomogeneities and their resultant vortices.
