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The Importance of Nama Group Sediments and Fossils to the Debate About Animal Origins Introduction

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The Importance of Nama Group Sediments and Fossils to the Debate About Animal Origins Introduction Palaeont. afr., 34, 1- 13 ( 1997) THE IMPORTANCE OF NAMA GROUP SEDIMENTS AND FOSSILS TO THE DEBATE ABOUT ANIMAL ORIGINS by C. K. Brain Transvaal Museum, P. 0. Box 413, Pretoria 0001 , South Africa ABSTRACT The purpose of this review is to draw attention to the contribution that Nama sediments and fo ssils have made, and potentially can make, to the ongoing debate about metazoan origins. T wo important features of this debate concern the nature and systematic position ofthe late Proterozoic "Ediacaran" fauna as well as the reasons for the sudden appearance in the fossil record of representatives of almost all known animal phyla, during the Early-Middle Cambrian radiation. An additional vexing question is the reason for the apparent absence of preserved representatives of ancestral metazoan lineages in Proterozoic sediments, despite the fact that molecular evidence shows that such lineages had a long history, prior to Cambrian times. ama fo ssil s and their enclosing sediments have made crucial contributions to this debate and will surely continue to do so in the future. KEYWORDS: Proterozoic, Metazoan origins, Nama Group, Ediacaran fauna. INTRODUCTION information central to the interpretation of animal Between 1908 and 1914, in the early days of origins. German South West Africa as it was then called, the In the interim, during 1946, ao Australian geologists, P . Range, H. Schneiderhohn and geologist, R. C. Sprigg, was examining an old lead H. Yon Staff found impressions of soft-bodied mine in the Ediacara Hills north of Adelaide, South organisms in Nama quartzites on the farms Plateau, Australia, when he came upon impressions of soft­ Aar and Kuibis, east of Aus, in southern Namibia. bodied organisms in the Late Proterozoic Pound T hese came to the attention of the German Quartzite there . The following year he described palaeontologist G. Gi.irich, who discussed them at some of these impress ions (Sprigg 1947) as the 15th International Geological Congress, held in " among the oldest direct records of animal life in Pretoria in 1929 (Gi.irich 1930). Subsequently he the world", observing that "they all appear to lack published descriptions of the following five fossil hard parts and to represent animals of very varied organisms new to science at that time (Gi.irich affinities". Since then, a wide variety of taxa based 1933): Rangea schneiderhohni, Rangea (?) on body-fossil impressions, has been described brevior , Pteridinium simplex, Orthogonium from the Flinde rs Ranges of the Ade laide parallelum and a m edusoid-like impression, geosyncline (eg. Gehling 1991 , Runnegar & Paramedusium africanum. The last specimen came Fedonkin 1992) and the term "Ediacaran" was from a locality much further to the east, near proposed by Jenkins (1981 ) as the terminal Ariamsvlei, whe re S.H. Haughton had found , Precambrian subdivision. Subsequently, Cloud and during geological mapping of the area in 1927, G laessner (1982) introduced an enlarged concept tubular impressions in quartzite that he interpreted of "Ediacarian", to embrace the time period from as being archaeocyathids (Haughton 1962). Apart the end of the Upper Proterozoic glacials till the end from an extensive review of South West African of the time period of the soft-bodied fossil fossils by Richter (1955), little further attention was assemblages. The Ediacaran stratotype occurs in given to Nama palaeontology until Hans Pflug the South Australian Flinders Ranges and the (1 966, 1970a, b, 1972a, b) collected and described Ediacaran time perim:l corresponds to the proposed large numbe rs of soft-bodied fossil impressions Vendian Period (Sokolov & Fedonkin 1984) based from the original localities of Plateau and Aar. on sequences on the Russian Platform. Ge r ard J . B. Germs also undertook a major A ssemblages of impressions of soft-bodied stratigraphic and palaeontological investigation of organisms, similar to those from Namibia and the Lower Nama Group in southern Namibia, with South Australia have since been found in various fieldwork being done between 1967 and 1970, as a parts of the world, though it seems somewhat ironic doctoral project at the University of Capetown that these should be referred to as the Ediacaran (Germs 1972c). Results of this investigation, which fauna in view of the fact that the first examples were will be discussed shortly, did a great deal to re­ discovered in Namibia, rather than South Australia. kindle interest in the Nama basin as a source of But as m y Australian fri e nd and colleague , 2 Malcolm Walter, remarked to me "What matters is reassigned the Vendozoa to a new Kingdom, the not who finds something for the first time, but Vendobionta, which he defined as follows (p. 607): rather who does something with it! ". It is true that "Immobile foliate organisms of diverse geo­ the Aus tralians have made more of their metries that were only a few millimetres thick, but Proterozoic fossils than have the southern Africans, reached severa l d ecimetres in size. A shared though this situation is now being corrected. characteristic is the serial or fractal quilting of the flexible body wall, which stabilized shape, maxi­ FEATURES OF THE CURRENT DEBATE ON mized external surface and compartmentalized the METAZOAN ORIGINS living content. Since no organs can be recognised, The abrupt appearance of fossilised representa­ this content is thought to have been a plasmodial tives of virtually all extant animal phyla in fluid rather than multicellular tissue. Included are sediments of early Mid-Cambrian times has the Petalonamae (Pflug 1972b) and a variety of commonly been called the Cambrian radiation of forms previous ly interpreted as soft-bodied animal life. But this event was preceeded by an ancestors of metazoan phyla. Range: Vendian. earlier radiation (Conway Morris 1993), in Late C la imed Cambrian survivors seem to show Proterozoic times, of soft-bodied organisms different preservational properties." comprising the Ediacaran fauna. As mentioned In a later development of this concept, Buss and earlier, the first examples of this fauna came from Seilacher (1994) put up the hypothesis that the Namibia, followed by s pecimens from So uth Vendobionta should be regarded as a phy lum, Australia. Since then, Ediacaran fossils have come constituting a monophyletic sister group of the to light in various parts of the world: Morocco in the Eumetazoa. They speculate that the Vendobionta Adoudou Series in Antia tlas (Houzay 1979); are cnidarian-like organisms that lacked stinging Charnwood Forest, near Leister, England (eg. Ford cells or cnidae and that cnidarians arose later 1958); South Wales (Cope 1977); County Wexford, through acquistion of cnidae by symbiosis with Ireland (eg. Palmer 1996); Norway (Kulling 1972); microsporidians. In this view, Ediacaran fossils are Sardinia (D e brenne & Naud 1981 ); Finn mark not thought to have been ancestors of living (Farmer et al 1991 ); the White Sea region of Russia cnidarians. (Keller & Fedonkin 1977, Fedonkin 1992); Siberia The resistant quilted construc tion of many (eg. Sokolov 1975, Fedonkin 1990); China (eg Ediacaran organisms has also recently impressed Xing & Liu 1979); Northwestern Canada Retallack (1994). He suggests that the resistance to (Hofmann, et al. 1990); Newfoundland (Anderson compaction shown by these quilted organisms is & Misra 1968; Anderson & Conway Morris 1982) similar to that of woody fossils preserved in and Sonora, Mexico (McMenamin 1996). sandstones and has made a case that the Ediacaran organisms were, in fact, lichens. He concludes that Ideas as to the affiliation of Ediacaran " the diversity of Ediacaran body plans can be organisms compared with the variety of form in fungi, algae There is probably no other group of fossil and lichens". As might have been suspected, the organisms that has generated such a diversity of pro posal has elicited a good dea l of critical opinion, concerning their affinities, as has the comment (eg. Waggoner 1995). Ediacaran fauna. There has been reasonable In the course of his studies of quilted organisms agreement as to the coelenterate affinities of the from the Nama sandstones in Southern Namibia, medusoid organisms that left impressions in the late Pf lug (1972a) concluded that most Edlacaran Proterozoic sediments, although Seilacher (1992) fossils should be accommodated in a new phylum, expressed the opinion that these organisms were the Petalonamae, "distingui shed by a characteristic very different from the contemporary jellyfish that feathered surface structure" (p. 134). they superficially resemble. H e coined the term As early as 1971, G laessner argued for the PsammocaraJlia, or sand-corals and wrote (p.611): incorporation of the various components of the " In the new inte rpretation, these fossils are Ediacaran fauna in existing phyla. This proposal regarded as internal sand skeletons of coelenterates has been accepted by various subsequent workers, comparable to actinians. They were built of sand with some modifications. For instance, in their grains that e ntered the gastric cavity and the n review of Proterozoic metazoan body fossils, became phagocytized and deposited in place of a Runnegar and Fedonkin (1 992) assign all known mesogloea between ecto and endoderm". fo rms to the following phyla: Pe talonamae, In an earlier paper (Seilacher 1989) proposed the Trilobozoa (as proposed by Fedonkin, 1985), informal ta xon Ve ndozoa to accommodate Cnidaria, " Ve rmes" ( inc luding molluscs a nd remaining elements of the Ediacaran fauna. He extinct phyla), Arthropoda, Echinodermata and believed that all these showed "a unique, quilted itncertae sedis. To this li st , a new Phy lum, type of biological construction that had no Tribrachidia, has been proposed by Jenkins (1992) counterpart in the modern, or even the Phanerozoic to accommodate Tribrachidium from the Flinders biosphere". In 1992 he went one step further, and Ranges. 3 As an example of the extreme divergence of ordinal-level standing diversity, which they list as views as to the affiliations of Ediacaran organisms, follows (p.
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