This article was downloaded by:[University Of Oregon] On: 3 September 2007 Access Details: [subscription number 770397526] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Alcheringa: An Australasian Journal of Palaeontology Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t770322720 Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil Online Publication Date: 01 September 2007 To cite this Article: Retallack, Gregory J. (2007) 'Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil', Alcheringa: An Australasian Journal of Palaeontology, 31:3, 215 - 240 To link to this article: DOI: 10.1080/03115510701484705 URL: http://dx.doi.org/10.1080/03115510701484705 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. © Taylor and Francis 2007 Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil GREGORY J. RETALLACK RETALLACK, G.J., September, 2007. Growth, decay and burial compaction of Dickinsonia, an iconic Ediacaran fossil. Alcheringa 31, 215-240. ISSN 0311-5518. Dickinsonia is a Neoproterozoic, Ediacaran fossil, variously considered a polychaete, turbellarian or annelid worm, jellyfish, polyp, xenophyophoran protist, lichen or mushroom. Its preservation as unskeletonized impressions in quartz sandstones has been attributed to a Neoproterozoic regime of aerobic decay less effective than today, microbial pyritization much nearer the surface than today, or agglutinate-mineralization as in xenophyophorans. However, the great variation in thickness independent of width or length of South Australian Dickinsonia is evidence of decay like the wilting of a fossil leaf, lichen or mushroom, but unlike clotting and distortion during decay, wilting or osmotic shrinkage of modern and fossil worms and jellyfish. Decayed specimens of Dickinsonia arrayed in arcs have been interpreted as slime trails or tumble tracks, but can also be interpreted as rhizinous bases of decayed crustose lichens or mushrooms arranged in fairy rings. Dickinsonia is interpreted to be sessile because adjacent specimens show reaction rims indicative of competitive interaction, and because no overlapping well-preserved specimens have ever been found. Folded and bent Dickinsonia reveal firm attachment and limited flexibility, but no brittle deformation indicative of pyritic, sideritic or calcitic ‘death masks’ or xenophyophoran agglutinate skeletons. Dickinsonia was resistant to compaction by overburden, like fossil lichens such as Spongiophyton and Thucomyces, Downloaded By: [University Of Oregon] At: 23:32 3 September 2007 and more compaction-resistant than fossil logs, jellyfish or worms. Dickinsonia also shows indeterminate growth like lichens, fungi, plants, xenophyophorans and colonial animals. Growth, decay and burial compaction of Dickinsonia were more like those of plants, lichens and fungi, than of worms, jellyfishes or anemones. G.J. Retallack [[email protected]], Department of Geological Sciences, University of Oregon. Eugene, OR 97403-1272, USA; received 18.11.05, revised 23.3.06. Key words: Dickinsonia, Ediacaran, Precambrian, taphonomy, growth. DICKINSONIA is an iconic Ediacaran width, length and depth of Dickinsonia fossil, best known from Neoproterozoic from the Ediacara Hills and Brachina Gorge, sandstones of South Australia (Jenkins South Australia as tests of three fundamental et al. 1983, Jenkins 1992), and Russia questions about these enigmatic fossils. What (Fedonkin 1992, Grazhdankin 2004), and kind of organism were they? Did they decay? perhaps China (Niu & Shu 2000). It was the How were they preserved? giant of its age, reaching 1.4 m in length Biological affinities of Dickinsonia re- (Jenkins 1996). Dickinsonia has a striking main problematic. At first Dickinsonia was regularity of symmetrically disposed seg- considered a jellyfish (Sprigg 1947), and ments (Fig. 1A), unlike other unskeletonized, assigned to an order Dipleurozoa distin- irregularly circular and elongate Ediacaran guished by bilateral symmetry found in no fossils (Glaessner 1984). A variety of other other jellyfish (Harrington & Moore 1956). fossils such as Andiva and Vendia show simi- Comparison with the tiny parasitic poly- larities with Dickinsonia (Fedonkin 2002), chaete worm Spinther (Wade 1972), and but only Dickinsonia is considered here. interpretations as an extinct annelid worm This paper presents new measurements of (Conway Morris 1979) or turbellarian flat- worm (Termier & Termier 1968, Fedonkin ISSN 0311-5518 (print)/ISSN 1752-0754 (online) Ó 2007 Association of Australasian Palaeontologists 1981) have had the greatest sway (Runnegar DOI: 10.1080/03115510701484705 1982, Gehling 1991, Jenkins 1996), despite 216 GREGORY J. RETALLACK ALCHERINGA Downloaded By: [University Of Oregon] At: 23:32 3 September 2007 Fig. 1. Dickinsonia costata (A upper, B–E) and D. tenuis (A lower only) from the late Precambrian Ediacara Member of the Rawnsley Quartzite in the Ediacara Hills (A–B, D–E) and Brachina Gorge (C), showing marginal haloes (A), likely competitive interaction (B), undeformed overlying ripple mark (C), wrinkling and folding (D), and different degrees of decay (E). Scale bars are all 1 cm; specimens are located in the South Australian Museum (A ¼ F17462, B ¼ F13760, D ¼ F13977, E ¼ F14359) and Condon Museum, University of Oregon (C ¼ F34285). ALCHERINGA DECAY OF DICKINSONIA 217 Dickinsonia’s mattress-like construction hypothesis of decay, which should be re- (Seilacher 1989). Dickinsonia also has simi- vealed by varying depth and clarity of larities with xenophyophoran protists of the impressions, vs decay retardation, which deep sea (Zhuravlev 1993, Seilacher et al. should reveal consistent depth and clarity of 2005), although 1.4 m is unusually large for impressions. a single cell of this kind, and there is no The preserved resistance to burial com- physicochemical trace of the mineralized- paction of Ediacaran fossils has long been agglutinate skeleton found in such giant perceived as anomalous: see, for example, protists (Tendal 1972). The scleractinian Wade (1968), who postulated cementation coral Fungia, so-named for its resemblance before or shortly after burial, comparable to mushrooms, also has superficial similar- with iron oxide encrustation of leaves in ity to Dickinsonia (Valentine 1992). There is ponds (Spicer 1977). Another possibility is no trace of calcareous coralline skeletoniza- preservation as ‘death masks’ mineralized tion in Dickinsonia, and if it were a com- by pyrite within shallowly buried (1 – 2 cm) parable polyp or anemone, its mesenteric microbial mats (Gehling 1999). Yet another structure is bilaterally symmetrical rather view is that Dickinsonia had some kind of than the usual cnidarian radial symmetry. tough biopolymer, comparable with lignin Dickinsonia has also been interpreted as a of wood or chitin of lichens (Retallack mushroom or lichenized fungus or actino- 1994), perhaps a coherent biopolymer car- Downloaded By: [University Of Oregon] At: 23:32 3 September 2007 bacterium (Retallack 1994, 1995), but is apace (Fedonkin 2002). Cements or mineral strikingly regular compared, for example, skeletons are little compacted by burial with ridged crustose lichens, which are often compared with wood (Retallack 1994), but necrosed or grazed in older parts of the now Dickinsonia can be compared for the thallus (Brodo et al. 2001). There has been first time with burial compaction of newly gathering evidence for generally microbial recognized fossil fungi and lichens (Jurina & (Steiner & Reitner 2001, Grazhdankin & Krassilov 2002), including Devonian Spon- Seilacher 2002, Seilacher et al. 2003), and giophyton (Stein et al.1993, Retallack 1994, for specifically fungal (Peterson et al. 2003) Jahren et al. 2003), Siluro-Devonian Pro- or lichen (Yuan et al. 2005) affinities of totaxites (Hueber 2001), and Archaean Ediacaran fossils. Dickinsonia also has been Thucomyces (Hallbauer & Van Warmelo placed in the extinct phyla Vendozoa or 1974, Hallbauer et al. 1977, MacRae 1999). Vendobionta (Seilacher 1992, Seilacher Although Cloud (1976) considered Thuco- et al. 2003) or extinct triploblastic-metazoan myces to be an artefact of acid maceration, phylum Proarticulata (Fedonkin 2003). My Thucomyces is known within thin-sections measurements of size variation in Dickinso- and slabs, where it is cut by veins of meta- nia test these various hypotheses by evalu- morphic chlorite and quartz (MacRae ating modes of growth. 1999). The range of carbon isotopic com- The issue of decay in Ediacaran