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 atte ntion to the contribution that Nama sediments and foss ils 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 his tory, prior to Cambrian times. ama fossils and their enclosing sediments have made c rucial contributions to this debate and will surely continue to do so in the future. KEYWORDS: Proterozoic, Metazoan origins, Nama Group, Ediacaran fauna. INTRODUCTION Between 1908 and 1914, in the early days of German South West Africa as it was then called, the geologists , P . Range , H. Schneiderhohn and H. Yon Staff found impressions of soft-bodied organisms in Nama quartzites on the farms Plateau, Aar and Kuibis, east of Aus, in southern Namibia. T hese came to the attention of the German palaeontologist G. Gi.irich, who discussed them at the 15th International Geological Congress, he ld in Pretoria in 1929 (Gi.irich 1930). Subsequently he published descriptions of the following five fossil organisms new to sc ience at that time (Gi.irich 1933): Rangea schneiderhohni , Rangea (?) brevior , Pteridinium simplex, Orthogonium parall elum and a m edusoid-like impression , Paramedusium africanum. The last specimen came from a locality much further to the east, near Ariamsvlei, whe re S.H. Haughton had found , during geological mapping of the area in 1927, tubular impress ions in quartzite that he interpreted as be ing archaeocyathids (Haughton 1962). Apart from an extensive review of South West African fossils by Richter (1955), little further attention was given to Nama palaeontology until Hans Pflug (1966, 1970a, b, 1972a, b) collected and described large numbe rs of soft-bodied fossil impressions from the original localities of Plateau and Aar. Ge r ard J . B. Germs also undertook a major stratigraphic and palaeontological investigation of the Lower Nama Group in southern Namibia, with fieldwork being done between 1967 and 1970, as a doctoral project a t the University of Capetown (Germs 1972c). Results of this investigation , which will be discussed shortly , did a great deal to re­ kindle interest in the Nama basin as a source of information central to the interpretation of animal origins. In the interim, during 1946, ao Australian geologist, R. C. Sprigg, was examining an old lead mine in the Ediacara Hills north of Adelaide, South Australia, when he came upon impressions of soft­ bodied organisms in the Late Proterozoic Pound Quartzite there . The following year he described some of these impress ions (Sprigg 1947) as " among the oldest direct records of animal life in the world", observing that " they all appear to lack hard parts and to represent animals of very varied affinities". Since then, a wide variety of taxa based on body-fossil impress ions, has been described from the Flinde rs Ranges of the Ade laide geosy ncline (eg. Gehling 1991 , Runnegar & Fedonkin 1992) and the term "Ediacaran" was proposed by Jenkins (1981 ) as the terminal Precambrian subdivision. Subsequently , Cloud and G laessner (1982) introduced an enlarged concept of "Ediacarian", to embrace the time period from the end of the Upper Proterozoic glacials till the end of the time period of the soft -bodie d fossil assemblages. The Ediacaran stratotype occurs in the South Aus tralian Flinders Ranges and the Ediacaran time perim:l corresponds to the proposed Vendian Period (Sokolov & Fedonkin 1984) based on sequences on the Russian Platform. A ssemblages of impress ions of soft-bodied organisms, similar to those from Namibia and South Australia have since been found in various parts of the world, though it seems somewhat ironic that these should be referred to as the Ediacaran fauna in view of the fact that the first examples were discovered in Namibia, rather than South Australia. But a s m y Australian fri e nd and colleague , 2 Malcolm Walter, remarked to me "What matters is not who finds something for the first time, but rather who does something with it! " . It is true that the Aus tralians have made more of their Proterozoic fossils than have the southern Africans, though thi s s ituation is now being corrected. FEATURES OF THE CURRENT DEBATE ON METAZOAN ORIGINS The abrupt appearance of foss ilised representa­ ti ves of virtually all extant animal phyla in sediments of early Mid-Cambrian times has commonly been called the Cambrian radiation of animal life. But this event was preceeded by an earlier radiation (Conway Morris 1993), in Late Proterozoic times, of soft -bodied organi sms comprising the Ediacaran fauna. As mentioned earlier, the first examples of this fauna came from Namibia, followed by s pec imen s from So uth Australia. Since then, Ediacaran fossils have come to light in various parts of the world: Morocco in the Adoudou Series in Antia tlas (Houzay 1979); Charnwood Forest, near Leister, England (eg. Ford 1958); South Wales (Cope 1977); County Wexford , Ireland (eg. Palmer 1996); Norway (Kulling 1972); Sardinia (D e brenne & Naud 1981 ); Finn mark (Farmer et al 1991 ); the White Sea region of Russia (Keller & Fedonkin 1977, Fedonkin 1992); Siberia (eg. Sokolov 1975, Fedonkin 1990); China (eg Xing & Liu 1979); Northwestern Canada (Hofmann, et al. 1990); Newfoundland (Ande rson & Misra 1968; Anderson & Conway Morris 1982) and Sonora, Mexico (McMenamin 1996). Ideas as to the affiliation of Ediacaran organisms There is probably no other group of fossil organisms that has generated such a diversity of opinion, concerning their affinities, as has the Ediacaran fauna. There has been reasonable agreement as to the coelenterate affinities of the medusoid organisms that left impressions in the late Proterozoic sediments, although Se ilache r (1992) expressed the opinion that these organisms were very different from the contemporary jellyfish that they superficially resemble. H e coined the term PsammocaraJlia, or sand-corals and wrote (p.611): " In the new inte rpretation, these fossils are regarded as internal sand skeletons of coelenterates comparable to actinians. They were built of sand grains that e nte red the gastric cavity and the n became phagocytized and deposited in place of a mesogloea between ecto and endoderm" . In an earlier paper (Seilacher 1989) proposed the informal ta xon V e ndozoa to accommodate remaining elements of the Ediacaran fauna. He believed that all these showed "a unique, quilted type of biological construction that had no counterpart in the modern , or even the Phanerozoic biosphere". In 1992 he went one step further, and reassigned the Vendozoa to a new Kingdom, the Vendobionta, which he defined as follows (p. 607): "Immobile foliate organisms of dive rse geo­ metries that were only a few millimetres thick, but reached severa l d ec imetres in s ize. A shared characteristic is the serial or fractal quilting of the flexible body wall, which stabilized shape, maxi­ mized externa l surface and compartmenta lized the living content. Since no organs can be recognised, this content is thought to have been a plasmodial fluid rather than multicellular tissue. Inc luded are the Petalona mae (Pflug 1972b) and a variety of forms prev iou s ly interpreted as soft-bodied a ncestor s of metazoan phyla. Range : Vendian. C la imed Cambrian survivors seem to show different preservational properties." In a later development of this concept, Buss and Seilacher (1994) put up the hypothes is that the Vendobionta should be regarded as a phy lum, constituting a monophyle tic s ister group of the Eumetazoa. They speculate that the Vendobionta are c nidarian-like organisms that lacked stinging ce ll s or cnidae and that cnidarians arose later through acquistion of c nidae by symbiosis w ith microsporidians. In this view, Ediacaran foss ils are not thought to have been ancesto rs of living cnidarians. The res istant quilted cons truc tion of many Ediacaran organisms has also recently impressed Retallack (1994). He suggests that the resistance to compaction shown by these quilted organisms is s imilar to that of woody fossils preserved in sandstones and has made a case that the Ediacaran organisms were, in fact, lichens. He concludes that " the diversity of Ediacara n body pl ans can be compared with the variety of form in fungi, algae and lichens". As might have been suspected, the pro posal has elicited a good dea l of critical comment (eg. Waggoner 1995). In the course of his studies of quilted organisms from the Nama sandstones in Southern Namibia, Pf lug (1972a) concluded that mos t Edlacaran fossils should be accommodated in a new phylum, the Petalonamae, "distingui shed by a characteristic feathered surface structure" (p. 134 ). As early as 1971 , G laessne r a rgued for the incorporation of the various compone nts of the Ediacaran fauna in existing phyla. This proposal has been accepted by various subsequent workers, with some modifications. For instance, in the ir rev iew of Proterozoic metazoan body fossils, Runnegar and Fedonkin (1 992) assign all known fo rms to the following phyla: Pe ta lonamae, Trilobozoa (as proposed by Fedonkin , 1985), Cnidaria, " Ve rmes" ( inc luding molluscs a nd ex tinc t phyla) , Arthropoda, Echinodermata and itncertae sedis. To thi s li st , a new Phy lum , Tribrachidia, has been proposed by Jenkins (1992) to accommodate Tribrachidium from the Flinders Ra nges. As an example of the extreme divergence of views as to the affiliations of Ediacaran organisms, the case of Dickinsonia can be cited. First described by Sprigg in 1947, Dickinsonia was a flat, quilted organism, oval in outline, with prominent ribs radi~ting from a partial midline. The larges t specimens appear to have been about one metre in length (Jenkins 1992). Taxonomic- affiliations suggested thus far are as follows: KINGDOM ANIMALIA Phylum Cnidaria, Class Dipleurozoa Class Actinozoa Phylum Platyhelminthes Phylum Annelida Phylum Petalonomae Phylum Vendobionta KINGDOM VENDOBIONTA (Harrington & Moore 1955) (Valentine 1992) (Term ier & T e rmier 1968 Fedonkin, 198 1) ' ( Wade 1972, Runnegar, 1982(c), Glaess ne r 1984, Jenkins 1992) (Pflug 1972(b), Runnegar & Fedonkin 1992) (Buss & Seilacher 1994) (Se ilacher 1992) KfNGDOMS PLANTAE/FUNGI, Lichens (Retallack 1994) While most workers on the Ediacaran fauna have considered the organisms involved to have been heterotrophs , McMenamin (1986, 1987), with his concept of " The Garden of Ediacara" visualised them as green photoautotrophs and subsequently as osmotrophs as well (McMenamin 1993). Questions about the Cambrian radiation There i s no doubt that the early to middle Cambrian Period was a time when a large variety of organisms made their first appearance in the fossil record; there is debate, nevertheless, as to whether their first fossil appearances marked the actual start of their lineages - a discussion that has been unde~way since the time of Darwin 's Origin of Spectes. Following the radiation of Ediacaran organisms during the late Proterozoic, there was a lull in the appearance of new metazoans in the fossil record, followed by a build up of the "small shelly faunas " (eg. Matthews & Missarzhevsky 1975; Bengtson et al. 1990), including the very numerous sponge-like archaeocyathids (eg. Debrenne 1992). The fossil residues of early Cambrian limes tones are ofte n dominated by calcareous tubes, as well as spicules and sclerites from larger armoured invertebrates, such as the halkeriids , that had re lationships to molluscs, brachiopods and annelids (Conway Morris & Peel 1995). Then folJowed a rapid increase in further appearances, during early Mid-Cambrian times, as manifested in the Cambrian radiation proper. Valentine et al. (1990) have documented the first and last appearances of ordinal-level foss il taxa from Vendian and Cambrian sediments as well as 3 ordinal-level standing diversity, which they list as follows (p. 289): Lower Vendian 1: 0; Vendian II: 15 (the Ediacaran radiation) ; Vendian III: 6; Vendian IV:l4; Tommotian: 36; Atdabanian: 61; Botomian: 68; Early Middle Cambrian: 59; Middle Middle Cambrian: 86; Late Middle Cambrian: 51, and Late Cambrian: 63. A rather similar increase in the record of trace fossil taxa across the Proterozoic-Cambrian boundary has been shown for a number of assemblages (Crimes 1992), particularly from the White-lnyo Mountains in California, the Cassiar Mountain s in British Columbia, the Burin Penin sula , Newfoundland and Tanafjord in Norway. Some of the best-known foss il evidence for the Cambrian radiation comes from the Burgess Shale as it is exposed on Fossil Ridge, close to Mount Field in the Canadian Rockies. Charles D. Walcott, who was Secretary of the Smithsonian Institution (Yochelson 1967), was spending the summer of 1909 exploring Proterozoic and Cambrian rocks in the Canadian Rockies, when he came upon superbly preserved animal fossils. He returned for intermittent excavations at the site between 1910 and 1 924, taking back 65000 specimens to the Smithsonian Institution in Washington. In hi s foreword to a recent book on The Fossils of the Burgess Shale (Briggs et a/.1994), Whittington wrote: "It is to Charles D. Walcott 's lasting credit that h~ found the Burgess Shale, amassed a huge collectiOn of these remarkable fossils, and made the outlines of his find known to paleontologists the world over. How significant his discovery was is only now beginning to be recogni sed, as not a local aberration to be noted in passing, but as a unique revelation of the extraordinary wealth of Cambrian biota." Following Walcot 's initial work, a resurgence of interest in the fossil assemblage was promoted by the work of Harry Whittington (eg. 1971 , 1985) and of his research students at that time, Derek E.G. Briggs and Simon Conway Morris. In the interim, other assemblages of Cambrian soft-bodied fossil organisms have been found in various parts of the world (Conway Morris 1985), in particular the Chengjiang fauna which is somewhat earlier than its Burgess Shale equivalent. It was discovered in 1984 in Yunnan, Soulh China (Zhang & Hou 1985) and has produced a wide variety of fossil ?rganis_ms, reminiscent of the Burgess Shale fauna, tncludtng the rece nt record of a Pikaia-like chordate (Shu et al. 1996). A Lower Cambrian fossil lagersUitte in the southern hemisphere has recently been reported (McHenry & Yates 1993), - the Emu Bay Shale on Kangaroo Island, South Australia. From it a new species of Anomalocaris, the larges t predator in the Burgess Shale assemblage, has been described (Nedin 1995). 4 As mentioned above, it has long been suspected that the sudden appearance of animals in the fossil record , during the Ediacaran a nd Cambrian radiations, did not necessarily represent the starting points of the lineages that they represented. Although pre-Ediacaran metazoans have hardly been found as fossi Is at aJ l, their suspected presence has been linked to the observed decline in the abundance and divers ity of stromatolites during the Proterozoic, throug h grazing and burrowing activities (eg. Awramik 1971). On this topic, Walter and Heys (1985) wrote as follow s : " Those s tromatolites which formed in qui e t subtidal environments began to decline in both abundance and diversity about 1 Ga ago. This was followed by a gene ral decline in all stromatolites beginning 0.7-0.8 Ga ago. Then during the Cambrian there appeared the first thrombolites, stromatolites with a clotted, unlaminated fabric. All three events can be related to the evolutionary hi story of the Metazoa, and in fact shed new light on that history. We consider that the thrombolites owe the ir origin to and record the first macroscopic burrowing and poss ibly boring by animals, an interpretation consi stent with the we ll known first abundant occurrence of vertical burrows in the earlies t Cambrian. The 0.7-0.8Ga decline can be attributed to the first w idespread grazing by a nimals (an inte rpretation made earlie r by other a uthors and supported by our review of new information). The decline of subtidal stromatolites which began about 1Ga ago records the early, subtle effects of grazing, presumably by small acoelomate animals." Ev idence of a different kind for the early existence of animal lineages, prior to the Ediacaran/Cambrian radiations is rapidly coming to light through new researc h into "molecular c locks" a nd genetic developme ntal mechanism s. T he concept of a " molecular clock" is based on the assumption that mutational changes, either in the DNA sequences of genes or the amino acids that are assembled on the instructions of suc h genes, will have arisen at a constant rate over the past history of an organism. In an early application of thi s technique , Runnegar ( 1982a&b) made use of the oxygen-carrying proteins, hae moglobins and myog lobins in living vertebrates. For instance, li ving jawless fish have hae mog lobins composed of identical myoglobin­ like molecules, but this is not true for sharks and other higher verte brates. These have tetrameric haemoglobin formed by two pa irs of different globins called a and B chains, presumably because the genes for haemoglobin were duplicated in the ancestors of the jawed fi sh. Once thi s happened, the two forms of globin were able to evolve separately, even in the same a nima l, resulJ ing in a 60% difference in the amino acid sequences of the two globins in Jiving vertebrates, irrespective of their evolutionary g rade . Modern sharks and hi ghe r vertebrates last shared a common ancestor in late Ordovician or Silurian fish that had inherited the recently acquired duplicate genes. The haemoglobin clock may be calibrated by comparing observed perce ntage sequence differe nces with probable times of separation, judged from the isotopically dated fossil record (Runnegar 1982a). Since there is good stati s tical agreement be twee n these two independent data sets for the last 450 million years, it should be poss ible to use the same technique to estimate when the invertebrate phyla separated, and hence to date the f irst major radiation of the animals. When Runnegar wrote this pioneering paper, few inverte brate globins had been sequenced . Those that had , proved to be about 80% different from each other and from vertebrate globins, leading to the conclusion that the initial radiation of the lower animals probably occurred about 900-1000 Ma ago. Since 1985, a good deal of work has been done (eg. Raff et al. 1996) on the use of DNA sequences in unravelling the Cambrian radiation of animal phyla. A study by Wray et al. (1996) made use of sequences from seven genes from a large sample of species in several phyla and concluded that the lines Leading to the c hordates and the major protostome phyla diverged about 1,2 Ga ago, or a little earlier than Runnegar had surmised. Such a view appears to be ga ining considerable support (Be ll 1997). Yet it should be mentioned that not all molecular clocks appear to keep the same time. Doolittle et a /. ( 1996) made use of amino acid seque nce data from 57 different e nzymes to de te rmine the dive rgence times of the major biolog ica l g roupin gs . They estimate that deuterostomes and protostomes split about 670 Ma ago, while plants, animals and fung i last shared a common ancestor about 1 Ga ago - considerably less tha n other estimates. Despite s uc h major di sc re pancies, a ll molecular estimates agree that animal lineages orig inated long before their first appearances in the foss il record. New evidence, now c oming to lig ht on the antiquity of developmental control , or hox, genes is sure to further elucidate this question (Erwin et al. 1997). Thus it is clear tha t molecular techniques can provide an enormous amount of information about the branching pattern of animal lineages, and the timing of such branching. But as Conway Morris ( 1994) has pointed out, palaeontology still remains an indispensable source of information on the na ture and s tructure of the ancient animals themselves. But if ancestral invertebrate lineages had been in ex istence for so long before they appeared as fossils, why did they not fossilize? A novel exp lanation has recently been proposed by Davidson eta/. ( 1995), in which they postulate that representatives of the various inverte brate lineages ex isted prior to the Cambria n radiation in the form of microscopic larvae , cons tituting "a cryptic , pre -E diaca ran evolutionary phase that left no fossil record (or at leas t no ne so far recovered)". Then, in early Cambri an times, the genetic programmes were evolved that allowed progression form such minute larvae to adult invertebrates, as are known today, showing large s ize and resistant elements favourable for survival as fossi ls. This theory, as have many before it, presupposes some striking emironmental fac tor m aking its sudde n appearance in early Cambrian times. Most commonly cited was the oxygen conce ntration in the late Prote rozoic atmosphere that crossed some critical threshold (eg. Nursall 1 959; Berkner & Marshall 1965). Runnegar (1982c) for instance estimated that the Ediacaran organism, Dickinsonia, would have required 6-10% of the present atmospheric level to have survived. There is a good deal of current research on the topic of oxygenation of the earth's atmosphere (eg. Des Marais et al 1992; Canfield & Teske 1996; Thomas 1997), postulating a rise in atmospheric oxygen concentrations to at least 5-18% of present levels, between 0,6 and 1,0 Ga ago, a change that may have triggered the evolution of animals. Numerous other suggestions have been made as to influences that mediated the metazoan radiations. These include changes in ocean chemis try that promoted the development of skeletons (eg. Daly 1907, Kempe & Degens 1985); the termination of the particularly intense and worldwide late Proterozoic glaciation (eg. Hambrey & Harland 1985); the advent of predation (Stanley 1973) and the premium it placed on the rapid evolution of protective skeletons (eg. Vermeij 1989) and the evolu tion of sex ual reproduction which served to accelerate the tempo of evolution ( Stanley 1976). SOME RECENT DEVELOPMENTS CONCERNING NAMA SEDIMENTS The Nama basin covers a very large area in southern Namibia, extending from lOOkms south of Windhoek in the north to the Orange River in the south and from the western escarpment to beyond the South African border in the east. The basin extends from the Orange River to the vicinity of Springbok and then into the Vanrhynsdorp area. G eological in ves tiga tion s of the Nama basin sediments commenced in early German colonial days when Schenk (191 0) correlated the Nama with the Otav i Sys te m on the bas is of the unmetamorphosed limestones and dolomites fo und in both successions. Soon afterwards Range (19 12) provided the first detailed description of the Nama sequence and proposed the fo llowing subdivisions based on exposures in the Bethanie district: Fish Ri ver Series Schwarzrand Series Schwarzkalk Series Kuibi s Series and Basal Beds 5 Mart in (l950) concluded that the Nam a sequence was younger than the Otavi counterpart and, in 1965 included the Huns limestones in the Schwarzrand Series . The South African Committee for Stratigraphy (1980) set up the Nama Group with three subdivisions: The lowerm ost Kuibis Subgroup , the Schwarzrand Subgroup and the uppermost Fish River Subgroup, a nomenclature c urre ntl y in use. Severa l other related groups , together with the Nama, were regarded as com­ prising the Damara Supergroup; these are, among others, the Gariep and Witvle i Groups in the southern half of Namibia and the Nosib, Otavi and Mulden Groups further north. The history of the various compone nts of the Damara Supergroup is closely related to that of the cratons on which they rest: the Gariep, Witvle i and Nama Groups lie on the Kalahari Craton, whil e the Nosib, Otavi and Mulde n Group sedime nts were deposited on the Congo Craton to the north. An understanding of the relative movements of these cratons, together with that of a South American plate, then s ituated immediately to the west, is crucial to the under­ standing of the development of the Nama basin. According to Hoffmann (1992), rifting occurred between the Kalahari and Congo cratons at about 820 Ma ago, into which depression sediments of the Nosib Group were deposited. Marine transgres­ sion followed from the ancient Proto-Atlantic, or Adamastor Ocean, with the deposition of the lower carbonates of the Otavi Group. This deposition was interrupted by a glacial episode, represented by the C huos a nd Blaubeker Formatio ns (Otav i and Witvlei Groups respecti vely) at about 720-7 10 Ma ago. Continental breakup fo llo we d with the deposition of further dolomites of the Otavi and Witvlei Groups in the space between the cratons. Northward subduction foJlowed at about 660 million years, followed by a reversal in the relative movements of the Congo and K a lahari c ratons , such that they once again approached each other at the time of renewed glaciation , recorded in the Numees tillite , which pre-dates the onset of Nama and Mulden Group sedime ntation. In response to mountain-building in the northwest, a forela nd basin developed in which the sediments of the Nama group were deposited. Actual continental collision occured during Schwarzrand depo itional times, with further mountain building leading to the deposition of the uppermost Fish River Subgroup sediments as a molasse. Two doctoral projects have provided detailed information about the lower Nama Group in Namibia (Germs 1972c) and the more southerly Vanrhynsdorp Group, as it has become known (Gresse 1986) , and s tratigraphic correlations between the two groups were then unde rtaken (Germs & Gresse 1991). Germs and Gresse (1993) have also provided detailed information about the evolution of the Nama foreland basin, bounded to 6 the northwest by the Damara orogen and to the west by the Gariep orogen and which was divided into the Zaris and Witputs basins in Namibia (separated by the 0 is ridge) and the Vanrhy nsdorp basin , separated from the first two by the Kamieskroon ridge. Only during the deposition of the upper Fish River beds were the basins continuous. Originally, the lower Nama sediments were derived from the Kala hari c raton to the eas t, while later in the seque nce the sourc e was the newl y eleva ted mountains to the north and west. In 1992, a deta iled stratigraphic study of the s tr ati g raphy o f the Kuibi s a nd Sch wa rzrand Subgroups in the Witputs area was started by Beverly Saylor of the Massachusetts Institute of Technology (Saylor 1992/93). This study was des igned to apply the concepts of sequence stratigraphy to ide nti fy unconfo rmity-bo unded depositio na l seque nces which can be used as time lines for inte rbasina l correlation. The study confirmed the presence of two sequence boundaries in the Kuibis Subgroup sedime nts, and five in the Schwarzrand Subgroup. In additi on, the presence of 11 volcanic ashbed in the sequence was reported, which have the potenti al for radiometric dating. This study has been developed considerably (Saylor et al. 1995; Grotzinger et a /. 1995) and , together with earlier investigations, has ensured that the Nama basin sequence is among the best documented te rminal Proterozoic successions in the world . T wo issues deserve special mention: the use of isotope studies for late Pro te rozoic correlation and the implications of rad iometric dating of the Ediacaran radiation. Isotope studies and Late Proterozoic glacial episodes A good deal of attention has been given recently to the secular vari ati on in stable carbon isotope ratios in late Proterozoic successions from various parts of the world (eg. Knoll et al 1986; Kaufman et al 199 1; Kn o ll 19 91 ; Knoll & W a lte r, 19 92; Kaufman et al. 1993; Kaufman & Knoll , 1995) and these variations have been shown to be very useful in corre la ting wide ly separated s tra ti g ra phic sequences. (The numeric value of carbon isotope ratios is conveniently reported as 13C. The number represents the difference between the ratio of 13C to 12C found in a particular sample and the ratio that exists in a uni ver al standard , expressed as a per mill (%o) deviation from the standard). Late Pro te rozoic sedime ntary seque nces have shown 13C va lues to be gen e ra ll y pos iti ve , indicating gene ra lly high rates of organic carbon burial during their deposition. However there have been episodes in the record when the enrichment in 13C declined rapidly and the 13C values became strikingly negative. Such deviations were found to coincide with glacial episodes, such as the 600Ma Varanger glaciation, ev idence for which has been fo und to unde rli e s tra ta containing E diacara n fossils in various parts of the world including the Nama bas in . Immediate l y be neath the Nam a seque nce lies the Numees ti ll ite of the Gariep group, apparentl y with an unconformable contact (Kroner & Germs 197 1 ), thoug h M artin (1965 , p. 105) found the contact to be conformable in the area west of Witputs. In the course of a detai led study of carbo n isotope ratios of Damara S upe rgroup carbonates, Kaufman et al. (1991) have shown that at the base of the N ama group, 13C values ri se dramaticall y f ro m -4 to +5 %o, within a sho rt s tratig raphic interval. A similar trend was observed in dolomites of the Otavi Group immediately above the Chuos tillite , which was co1Te lated in this study with the Ka igas tilli te of the Gari ep G ro up a nd the Blaubeker ti11ite of the W itv lei Group. Thus, one of the partic ula rl y signi f icant conc lusions of the carbo n isotope study o n Nam ibian Pr o terozoic carbona tes is that t he trends in iso to pe concentra tio ns a llow fo r corre la ti o ns w ith equivalent sediments in other basins of the world, as we ll as between carbonate on the adjacent Congo and Kalahari cratons. It has been observed (eg. Hambrey & Harland 1985; Hambrey 1992) that the Varanger glaciation, to which the pre-Nama N umees tillite bears witness, was not only the most inte nse in the the history of the earth , but also spread throughout low equatorial latitudes. It is a vexing question as to what circ umstances would have been required to allow for such low latitute glaciation, but new ev idence suggests that this may have been a feature of other Proterozoic glaciations as well. For instance, a new study on glaciogenic depos its of the much older (2 ,2 Ga) T ransvaal S upergroup on the Kaapvaal Craton in South Africa (Evans et al 1997), show that these were deposited well within the tropics. Evidence for two other glacia l episodes within the Nama Group sediments has been reported. Over 50 years ago , Sch wellnus (1 941) f ound glac ial pavements, tillite and other fea tu res of g lacia l activity in the Klein Karas mountains. With some reservations, this evidence was accepted by Martin ( 1965) w ho re-examined th e ex posure a nd concluded that f loating and gro unded ice-flows, rather than true glaciers, had been responsible for t he o bserved effects. Germs ( 1972c) a lso reassessed the field ev idence and fo und that the g lacial indications, whic h he did not q ues tio n , occured close to the base of the Schwarz rand S u bgr o up , betwee n the N uda us and U ru s is Formations. He a lso fo und further ev idence for glacial activity close to the top of the Schwarzrand Subg ro up . T his took the form of s teep-s ide d channels, partially fiiJed with conglomeritic debris, which had been c ut afte r the depos ition of the Spitskopf limestone. More recently Saylor (1992/3) q ues tio ned the g lac ia l inte rpre tat io n of these features, though she did not rule it out. Support for a glacial episode at, or close to, the Proterozoic­ Cambrian boundary , comes from a marked negative Carbon isotope excursion at this time, reported from elsewhere in the world (Knoll & Walter 1992), though not shown in the Namibian curve, probably as a result of the time-hiatus represented by the Spitskopf-Nomtsas unconformity (Grotzinger et al. 1995).- The various lines of evidence discussed above combine to show that the terminal Proterozoic time period was an e nvironmentally turbule nt one. T here is worldwide evidence of tectonic activity and mountain-building; of increased hydrothermal activity (Knoll 1991) ; of high rates of organic carbon burial that would have increased the oxygen concentration of the atmosphere, and there are indications of extremely severe glacial episodes, e xtending into tropical latitudes. It was in such circumstances that the first radiation of metazoans appears to have occurred in the Nama basin and elsewhere. Precise dating of the Ediacaran radiation U ntil recently , the dating of Nama Group sedime nts has been somewhat imprec ise. For instance, measurements on detrital white micas in upper Schwarzrand sediments gave an age of about 6 30Ma, w hile micas in younger Fish River sedime nts s ugges ted a pprox im ately 5 30Ma (Ahrendt et al. 1978). T he post-tectonic Kuboos and Bremen igneous suites, which are intrusive into the Kuibis and Schwarzrand Subgroups yielded ages of between 500 and 550 Ma (Allsopp et at. 1979), suggesting a minimum age of 500Ma for the uppermo s t Schwarz rand Subgroup. F urthe r determinations b y Hors tma nn e t al. ( 1990) of detrital white mica, narrowed the sedimentation t ime of uppe r Schwarzrand a nd Fish Riv e r Subgroups to between 570 and 500Ma. As mentioned above, the fieldwork undertaken by Sa-y lor ( 1992/3) in the Nama basin confirmed the presence of a number of volcanic ashbeds in the sequence. Four of these were selected for dating by B o wring at t he M assac husetts Ins titute of T echnology, using the uranium- lead m ethod applied to single zircon crystals (Grotzinger et a!. 1995). The samples were positioned in the Nama stra tig raphic column as fo llows : 1. From the northern subbasin, a 30 em-thick ashbed that lies in the Hoogland carbonate member , 270 m above the base of the Kuibis Subgroup; 2. In the southern subbasin, a 50 em-thick ashbed in the lower part of the Spitskopf carbonate member at Witputs; 3. a 20 em-thick ash in the uppermost Spitskopf limestone at Swartpunt, 13 5 m above sample 2; 4. The stratigraphicall y hig hes t a sh , in the lowermost Nomtsas Formation at Swartkloofberg. The best age estimates for these samples are as fo llows: 7 Basal Nomtsas Formation (sample 4): 539.4 ±1 Ma Upper Spitskopf M ember (sample 3) : 543.3 ±1 Ma Lower Spitskopf Member (sample 2): 545.1 ±l Ma Lower Hoogland Member (sample 1): 548.8 ±1 Ma These results cons train the Precambrain­ Cambrian boundary in Namibia to be younger than 543.3 ±1 Ma and older than 539.4 ±1 Ma, in good agreement with a date from the lowermos t Cambrian strata in Siberia. Grotzinger et a / . ( 1995) report the fi nding of body fossils of Pteridinium and "a Dickinsonid-like fossil that may be Nasepia or possibly a new taxon" near the top of the Spitskopf Member, 90-100 m above the upper Spitskopf ash. This extends the stratigraphic range of suc h fossils in the Nama Group upwards considerably and shows they existed until virtually the Cambrian boundary. The fact that the Nama dates can be tied to the carbon isotope curve, which has world-wide application, means that the time span of o ther Ediaca ran assemblages, such as that in the Flinders Ranges of Australia, can be estimated with greater assurance than in the past. The general conclusion is thus reached that the most diverse Ediacaran fossil assemblages are no more than about 6 million years older than the Precambrian-Cambrian bo undary. However, the minimum lower limit for the age range of the Ediacaran fauna is 565 Ma, as has been m easured at Mis taken Point , Newfoundland. Generally, Ediacaran fossils have not been thought to occur below the Varanger glacial beds, although simple disk and ring-shaped impressions from the Tw itya Formatio n of northwestern Canada (Hofmann et al. 1990) suggest that macroscopic metazoans may predate the glaciation. If this is so, the Ediacaran radiation has a time range of at least 55 million years, though the span of the lower Nama sed iments is considerably shorter than had been est ima te d previous ly. An important implication of the new Nama dates is that the upper limit of the Ediacaran radiation is brought to the Cambrian boundary. This, taken with the fact that new dates for early Cambri an strata in Siberia (Bowring et a/. 1993) show that that the initial phases of the Cambrian Period were much briefer than previously thought, accentuates the rapidity of faunal diversification leading to the main Cambrian radiation itself. SOME COMMENTS ON THE NAMAN FOSSIL ASSEMBLAGES Martin Pickford (1995) has recently provided a comprehensive overview of the foss il fauna from Nama Group sediments. T he majo rity of taxa, being members of the soft-bodied Ediacaran fauna , are listed by him under "Vendozoa", as follows: 8 Pteridinium simplex Gtirich 1930, originally spelt Pteridium Rangea schneiderhohni Gtirich 1929 Orthogonium parallelum Gtirich 1930 Ernietta plateauensis Pflug 1966 Ausia fenestra Hahn & Pflug 1985 Paramedusium africanum Gtirich 1933 Nasepia altae Germs 1973 Cyclomedusa davidi Sprigg 1947 Velancorina martina Pflug 1966 Petalastroma kuibis Pflug 1973 In addit ion to these Vendozoan texa , Pflug (1972a) described the following taxa from the farm s Plateau a nd Aar, which are regarded by Runnegar & Fedonkin (1992) as synonyms of Ernietta plateauensis: Ernietta aarensis , Ernietta tschanabis, Erniobaris baroides, Erniobaris epistula, Emiobaris gula, Erniobaris parietalis, Erniaster apertus, Erniaster patellus, Erniobeta forensis, Erniocarpus carpoides, Erniocarpus sermo , Erniocentris centriformis, Erniocoris orbiformis, Erniodiscus clip eus, Erniodiscus rutilus, Erniofossa prognatha , Erniograndis paraglossa, Erniograndis sandalix, Ernionorma abyssoides, Ernionorma clausu/a , Ernionorma corrector, Ernionorma peltis, Ernio­ norma rector , Ernionorma tribuna/is, Erniopelta scrupu/a and Erniotaxis segmentrix. Other possible synonyms of Ernietta p/ateauensis are Namalia villiersiensis Germs 1968, and Kuibisia glabra Hahn & Pflug 1985. Other faunal taxa listed by Pickford (1995) are these: Hagenetta aarensis Hahn & Pflug 1988. This was a smalJ almost circular fossil , about 5mm in diameter, with an upper valve of roughly triangular outline. Runnegar & Fedonkin ( 1992) consider H . aarensis to be a synonym of Beltanelli­ f ormis brunsae, from Vendian sediments of the Russian platform. It was possibly a sessile organism that lived on sandy s ubstrates and fed on phytoplankton. Protoechiurus edmondsi Glaessner 1979. This was a cigar-shaped organism , about 7 em long from the Kuibi s quartzite on Plateau farm. Glaessner reconstructed as an echiurid worm with a spatulate proboscis and e ight longitudinal muscle bands. Runnegar & Fedonkin (1992) regards it to be a dubiofossil. Spriginnidae ? Germs (1973b) described the imprint of a flat, elongate foss il about 10 em long and 1 em wide from shale underl ying the Mooifontein limestone Member on the farm Buchhol zbrunn 99. The organi s m appeared to have had at least 34 segments, with lateral ribs and possible intestinal trac t a nd protos tomium. Unfortunate ly the Figure I. The calcareous tube o f Claudina sp. from the Mooifonte in limes tone of the Nama Group, seen as a SEM image. Claudina i the earl iest known organism to have made use of biomineralisation, a process cruc ial to the late r development of skeletonised anima ls. First described from the Nama Group by G. J. B . Ge rms, Claudina has s ince been found in many parts of the world and is regarded as a terminal Proterozoic index foss il. Photo: C. K. Bra in. s pecimen , that h ad been lodged in the State Museum, Windhoek, has since been lost. Germs compared this specimen to Spriggina ovata from Ediacara in south Australia. Archaeichnium haughtoni Glaessner 1963. Haughton (1962) described tubular structures on two s labs of Kuibi s Subg ro up quartzi te from Grtindoorn, in SE Namibia and regarded these as archaeocyathids. He noted that they consisted of long tubes, conical at one end and open at the other, showing "a number of elongate pustulate ridges". On the basis of plaster casts of these slabs, Glaessner (1963) named these foss il s Arc haeichnium haughtoni, which he considered as worm tubes or worm burrows. Glaessner ( 1977) later made a close examination of the original specimens in the South African Museum , and cone I uded that the impressions consist of "agglutinated sand grains and a re similar to tubes m ad e actively by living polychate worms . . . . . In the a bsence of any taxonomicall y s ig ni f icant c ha racters of tube construction the systematic position of the originator of these fossils remains uncertain". Cloudina hartmannae and C. riemkeae Germs 1972 Germs (1972a) described these two species of calcareous tube-building organi sms, that occur in limestones throughout the Kuibis and Schwarzrand subgroups, but are particularly numero us in the " bioherm" that Germs described from Driedoring­ vlakte, near Schlip on the northern margin of the Nama basin. The two species were designated on the basis of tube-size, with the larger species, C.harmannae, 2,5-6,5 mm wide as opposed to 0,3-1,3mm for C.riemkeae (Fig ure 1) . The tubes were found to consist of a cone-in-ceRe structure, c losed at one end, but flared at the open ends, where space existed between each of the eccentric layers . Germs considered that Cloudina sho uld possibly be placed in the Class Cribricyathea, of the P hyl um Archaeocyatha. Subsequently Hahn a nd Pflug (1985) described Cloudina from Brasil and set up the Family Cloudinidae. Grant ( 1990) undertook a detailed study of the shell structure and distribution of Cloudina, after which he maintained the two species described by Germs and s howed that representatives of the genus have now been found in many parts of the world , inc luding Antarctica. He concluded that C loudina can be regarded as an index fossil for the terminal Proterozoic. He cons idered that Sinotubulites from C hina, Nevadatubulus a nd Wyattia from North America are "either closely related to or congeneric with Cloudina." Grant regarded the phylum, c lass, order and family as uncertain and concluded that " the organism that secreted these shells appears to have had to move up the tube between the deposition of successive she ll layers implying a musculature and internal complexity consistent with a diploblastic or even triploblastic animal. Thus the Cloudina organism was probably a metazoan of at least a cnidarian grade of organisation." The particular significance of Cloudina is that it represents the earliest known metazoan to have made use of biomineralisation, a process crucial to the later evolution of skeletonised animals. As was the case with Ediacaran body fossils, it was the Nama sediments that first yielded evidence of this an imal , late r fo und to occur in many other countries. In addition to the actual fossil taxa listed above, at least 18 taxa of trace- or ichnofossils are known from the Nama sediments as reported by Germs ( J 972b) and Crimes a nd Germs (1982). T hese include the trace fossil Phycodes pedum, which is considered to be an important Cambrian indicator a nd has been found to occur in the Nomtsas Formation and the overlying Fish River Subgroup beds (Saylor et a! 1995). Apart from fossils of animals discussed above, the Nam a sediments have produced remains of 9 organic-walled microfossils (Germs et a!. 1986) and probable calc if ied metaphytes (Grant et al. 1991 ). For the last three years the present writer has been investigating Nama limestones, particularly at ashbed-chert/limestone interfaces for the presence of preserved micro-organis ms. The ai m is to reconstruct the community of which Cloudina formed a part. This community must have included representatives of the invertebrate lineages whose later members made their fossil appearance during the Cambrian radiation. The micro-organisms are being investigated in thin-sections and by way of SEM images of silicified remains that are picked from acetic acid residues. One such image is that of a Cloudina from the Mooifontein limestone shown in Figure 1. The results of this investigation wi ll be published separate ly. C ONCLUSIONS As will have become apparent from the overview presen ted above, the Nama basin conta ins a remarkable sequence of sediments, approximately 3000m thick, which cover a critical period of the Ediacaran and early Cambrian radiations. These sediments are largely unaltered and unafffected by tectonism, except along the western margin of the bas in. The seq uence has been studie d in considerable detail and the volcanic ashbeds which occ ur at intervals throughout its dep th have provided precise dates. These can be corre lated with the curve for carbon isotope values that have been determined and which can be used for inter­ basinal correlations on other continents. The palaeontological significance of the Nama basin is very considerable and the potentia l for f urther di scoveries is great. In fact, the unique combination of Naman fossil, sedimentological and chronological information can be used to address almost every facet of the current debate on animal origins. ACKNOWLEDGEMENTS In particu la r , I would l ike to thank Gerard Germ s for introducing me to the pleasures. excitements and frus tations of the ama basin and its fossil s. Throughout thi s recent project , Laura Brain has given me invaluable support. I am also very gratefu l to the following people for help and advice both in the field and laboratory: J. E. Almond, S. Conway Morris, J. G. Gehling, P. G. Gresse, M.A. Fedonkin, W . Hegenberger, K. H . Hoffmann, A. H. Knoll , M,.A. S. McMenamin, M. H. L. Pickford, B. Runnegar, W. J. Schopf, A. SeiJacher and M.R. Walter. Bruce Runnegar and John Almond kindly provided useful comments on the manuscript and I am grateful to Bruce Rubidge for editorial help. The re earch was supported by a g rant from the Foundation for Research Developme nt in Pretoria. 10 REFERENCES AHRENDT, H., HUNZIKER, J. C. & WEBER, K. 1978. Age and degree of metamorphism and time of nappe emplacement along the southern margin of the Damara Orogen, Namibi a. Geol. Rundsch. 67, 7 19-742. ALLSOP, H. L. , KOSTLIN, E. 0., WELKE, H. H. , BURGER, A. J., KRONER, A. & BLIGNAUT, H. J. 1979. Rb-Sr and U-Pb geochrono logy of the late Precambrian- Early Palaeozoic igneous activity in the Richtersveld and southern South West Africa. Trans. Geol. Soc. S . Afr. 82, 185-204. ANDERSON, M. M. & CONWAY MORRIS , S. 1982. A review, with description of four unusual forms of soft-bodied fauna of the Conception and St. John 's Groups (Late Precambrian), Avalon Peninsula, Newfoundland. In: Mamet, B. & Cope land, M. J., Eds, Proceedings of the North American Palaeontological Convention, 1, 1-8. -------- & MISRA, S. B. 1968. Fossils found in the Precambrian Conception Group in southeastern Newfoundland. Nature, 220, 680-681. A WRAMIK, S.M. 1971. Precambrian columnar stromatolite d iversity: reflection of metazoan appearance. Science 174, 825-827 . BELL, M A. 1997. Origin of metazoan phyla: Cambrian explosion or Proterozoic s low burn?. TREE 12( 1), 1-2. BENGTSON, S ., CONWAY MORRIS, S. , COOPER, B. J ., JELL, P. A. & RUNNEGAR, B. N. 1990. Early Cambrian fossils from South Australia. Association of Australasian Palaeontologists, Memoir 9, 1-364. Brisbane. BERKNER, L. V. & MARSHALL, L. C. 1965. His tory of major atmospheric compone nts. Proc. Nat/. Acad. Sci. USA 53, 1215- 1225. BOWRING, S. A., GROTZJNGER, J.P., ISACHSEN, C. E., KNOLL, A. H. , PELECHATY, S.M. & KOLOSOV, P. 1993. Calibrating rates of early Cambrian evolution. Science 61,1 293-1298. BRIGGS, D. E. G., ERWIN, D. H. & COLLIER, F. J. 1994. The f ossils of the Burgess Shale. Washington, Smithsonian Institution Press. BRINK, W. C. 1950. The geology, structure and petrology of the Nuwerus area, Cape Province. Ann. Univ. Stellenbosch 26(3- 11 ) ,97-22 1. BUSS, L. W. & SEILACHER, A. 1994. The Phylum Vendobionta: a s ister group of the Eumetazoa. Paleobiology 20( l) , 1-4. CANFIELD, D. E. & TESKE, A. 1996. Late Proterozoic ri se in atmospheric oxygen concentration inferred from phylogenetic and sulphur- isotope studies. Nature 382: 127- 132. CLOUD, P. & GLAESSNER, M. F. 1982. The Ediacaran Period and System: metazoa inherit the earth. Science 217, 783-792. CONWAY MORRIS, S. 1985. Cambrian Lagerstiitten: their distribution and significance. Phil. Trans. Roy. Soc. London B311, 49-65. -------- 1993. The fossi.l record and the early evolution of the metazoa. Nature 361, 219-225. -------- 1994. Why molecular biology needs palaeontology . Development, 1994 Supplement, 1-13. -------- & PEEL, J. S. 1995. Articulated halkariids from the Lower Cambrian of North Greenland and their role in early Protostome evolution. Phil. Trans. Roy . Soc. London B 347:305-358. COPE, J. C. W., 1977. An Ediacaran fauna from South Wales. Nature 268,624. CRIMES , T. P. 1992. The record of trace fossils across the Proterozoic-Cambrian boundary. In: Lipps, J. H. & Signor, P. W. Eds., Origin and Early Evolution of the Metazoa, pp. 117-202. New York, P lenum . -------- & GERMS, G. J. B. 1982. Trace fossils from the Nama Group (Precambrian-Cambrian) of South West Africa \Namibia. J . Palaeontology 65, 890- 907. DALY , R. A. 1907. The time less oceans of Precambrian time. Am. J . Sci. 23, 93- 11 5. DAVIDSON, E. H. , PETERSON, K. H. & CAMERON, R. H., 1995. Origin ofbilaterian body plans: evolution of developmental regulatory mechanisms. Science 270, 13 19- 1325. DEBRENNE, F. 1992. Diversification of Archaeocyatha. In : Lipps, J. H. & Signor, P. W., Eds.,Origin and Early Evolution of the Metazoa, pp. 425-439. Ne w York, Plenum. -------- & NAUD, G. 198 1. Meduses et traces foss iles supposees precambriennes dans Ia formation de San Vito, Sarrabus, Sud-Est de Ia Sardaigne . Bull. C eo/. Soc. France 23, 23-3 1. DES MARAIS, D. J ., STRAUSS, H., SUMMONS, R. E. & HAYES , J. M. , 1992. Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment. Nature 359,605-609. DOOLITTLE, R. F. , FENG , D. F. , TSANG, S., CHO, G. & LITTLE, E. 1996. Determining divergence times of the major kingdoms of living organisms with a protein clock. Science 271, 470-477. ERWIN, D., VALENTINE, J & JABLONSKI, D. 1997. The origin of an imal body plans. American Scientist 85, 126-1 37. EVANS, D. A., BEUKES, N.J. & KIRSCHVJNK, J. L. 1997. Low-lati tude glaciation in the Palaeoproterozoic era. Nature 386,262-266. FARMER, J.D. , VIDAL, G. , STRAUSS, H. MOSZYDLOWSKA, M., AHLBERG , P. & SIEDLECKA, A. 199 1. Ediacaran medusoids from the Innere lv Member (late Proterozoic) Tanafjorden area of N. E. Finnmark. Geol. Soc. Am. Abstracts 23(5), 97. FEDONKIN, M. A. 1981. White Sea biota of the Vendian. Precambrian non-skeletal fauna of the Russian Platform. Moscow, Nauka. (In Russian). -------- 1985. Precambrian metazoans: the problems of preservation, systematics and evolution. Phil. Trans. Zoot. Soc. London B311, 27-45. -------- 1990. Systematic description of the Vendian metazoa. In: Sokolov, B. S. & Iwanowski, A. B. Eds., The Vendian System, Vol. l (Palaeontology) pp. 77- 120. New York, Springer Verlag. -------- 1992. Vendian faunas and the early evolution of metazoa. In: Lipps, J. H. & S ignor, P. W., Eds. , Origin and Early Evolution oft he Meta=oa , pp. 87- 129. New York, Plenum. FORD, T. D. 1958. Pre-Cambrian fossils from Charnwood Forest. Proc. Yorkshire Geol. Soc. 31,211-21 7 . GERMS, G. J. B. 1968. Discove ry of a new fossil in the Nama System , South West Africa. Nature 219, 53-54. -------- 1972a Ne w she lly foss ils from the Nam a Group, South West Africa. Amer. J . Sci. 272, 752-761. --------1972b. Trace fossils from the Nama Group, South West Africa. J . Palaeont. 46, 864-870. -------- 1972c. The stratigraphy and palaeontology of the lower Nama Group, South West Africa.Bull. Pre-Cambrian Research Unit, Univ. of Cape Town 12, 1-250. -------- 1973a. A reinterpretation of Rangea schneiderhohni and the discovery of a re lated new fossil from the Nama Group, South West Africa Lethaia 6, 1- fO. -------- I973b. Possible sprigginid worm and a new trace fossil from the Nama Group, South West Africa. Geology 1(2) , 69-70. -------- 1974. The Nama Group in South West Africa and its relation to the Pan-African Geosyncline. J. Geol. 82, 30 1-317. -------- 1983. Implications of a sedimentary facies and depositional analysis of the Nama Group in South West Africa \Namibia. In : M iller, R. Me. Ed. , The Damara Orogen. Special Publication of the Geol. Soc. S. Afr. 11,89-114. -------- 1995. The Neoproterozoic of South West Africa, with emphasis on platform stratigraphy and pa!aeontology. Precambrian Research 73, 137-151. -------- & GRESSE, P. G. 1991. The fore land basin oft he Damara and Gariep orogens in Namaqualand and southern Namibia: stratigra phic correlations and basin dynamics. S. Afr. J . Geol. 4, 159-169. 11 -------- , KNOLL, A. H. & VIDAL, G. 1986. Latest Proterozoic microfoss ils from the Nama Group, Namibia (South West Africa). Precambrian Research 32, 45-62. GEHLING , J . G. 1991. The case for Ediacaran fossil roots to the metazoan tree. In: Radhakrishna, B. P ., ed., The World of Martin Glaessner. Geol. Soc. India Me moir 20, 181 -224. GLAESSNER, M. F. 1963. Zur Kenntnis der Nama-Fossilien SUdwest-Afrikas. Ann. Naturhiscor. Mus. Wien. 66, 113- 120. -------- 1971. Geographic distribution and time range of the Ediacaran Precambrian fauna. Geol. Soc. A mer. Bull. 82, 509-514. -------- L 977. Re-examination of Archaeichnium, a foss il from the Nama Group. Ann. S . Afr. Mus. 74(13), 335-342. -------- 1979. An echiurid worm from the late Precambrian. Lethaia 12, 121 - 124. -------- 1984. The Dawn of Animal Life: a Biohistorica/ Study. Cambridge, Cambridge Univers ity Press. GRESSE, P. G. 1986. The tectono-sedimentary hiStory of the Vanrhynsdorp Group. Unpublished PhD thesis, Univ. Stelle nbosch, 155pp. -------- & GERMS , G. J. B. L 993. The Nama foreland basin: sedimentation, major unconformity-bounded sequences and multis ided active margin advance. Precambrian Res. 63,247-272. GRANT, S. W. F. L 990. Shell structure and distri bution ofCloudina, a potential index fossil for the terminal Proterozoic.Amer. J . Sci. 290, 26 1-294. --------, KNOLL, A. H. & GERMS , G. J. B. 1991. Possible calcified metaphytes in the latest Proterozoic Nama Group, Namibia; orig in, diagenesis and implications . .!. Pa/aeont. 65, 1- 18. GROTZINGER, J.P., BOWRING, S. A. , SAYLOR, B. Z. & KAUFMAN , A. J. 1995. Biostratig raphic and geochronologic constraints on early animal evolution. Science270, 598-604. GURICH , G. 1929. Die altes ten Fossilien SUdafrikas. Z. Prakt Geol. 37, 85. -------- J 930. Die bi slang altesten Spuren von Organhmen in SUdafrika. C. R . 2, 15th Int. Ceo/. Cong. S. Afr. 1929, 670-680. -------- 1933. Die Kuibis-Fossilien der Nama-Formation von SUdwestafrika. Palaont.Z. 15, 137- 154. HAHN, G. & PFLUG, H. D. 1985. Polypenartige Organismen aus dem Jung- Prakambrium (Nama-Gruppe) von Namibia. Geologica Palaeontologica 19, 1- 13. -------- 1988. Zweisc halige organismen aus dem Jung- Prakrambrium (Vendium) von Namibia (S. W. Afrika). Geologica Palaeontologica 22, l - 19. HAMBREY, M. 1992. Secrets of a tropical ice age. New Scientist Feb. 1992, 42-49 -------- & HARLAND, W. B. 1985. The Late Proterozoic g lacial e ra. Palaeogeogr. , Pa/aeoclimaco/, Palaeoecol. 51,255-272. HARLAND , W. B. 1989. Palaeoclimatology. In: Cowie, J. W. & Brasier, M.D. Eds., The Precambrian-Cambrian Boundary. Oxford Monographs on Geology and Geophysics, 12, 199-204. HARRINGTON, H. J. & MOORE, R. C. 1955. Fossil jellyfish from Pennsylvanian rocks and e lsewhere. State Geol. Surv. of Kansas Bull. 114, 153- 164. HAUGHTON, S. H. 1962. A archaeocyatrud from the Nama System. Trans. Roy. Soc. S. Afr. 36,57-60. -------- &MARTIN, H. 1956. The Nama System in South and South West Africa. Congresso lnternat. Mexico El Sistema Cambrico 1, 323-329. HOFFMANN, K. H . 1992. Depositional his tory of the Damara Belt in Namibia: evidence for major ocean closure and continental collis ion in the late Proterozoic. Unpublished ms. HOFMANN, H. J. , NARBONNE, G . M. & AITKEN, J .D. 1990. Ediacaran fossils from inte rtillite beds in northweste rn Canada. Geology 18, I I 99- 1202. HORSTMANN, U. E., AHRENDT, H. CLAUER, N. & PORADA, H. 1990. The metamorphic history of the Damara Orogen based on K/ Ar data of detri tal white micas from the Nama Group, Namibia. Precambrian Res. 48,41 -6 1. HOUZA Y, J. P. 1979. Empre intes attribuables a des meduses dans Ia serie de base de I' Adoudounien (Precambrian termaina de I' Anti -Atlas, Maroc). Geo/. Med. 6, 379- 384. JENKINS , R. F. J. J 98 1. The concept of an "Ediacaran Period" and its stratigraph ic significance in Austra lia. Trans. Roy. Soc. S. Aust.105(4), 179- 194. -------- 1992. Functiona l and ecological aspects ofEdiacaran asse mblages. In : Lipps, J. H. & Signor, P. W ., Eds. Origin and Evolution of the Metazoa, pp. 131 -17 6. New York, Plenum. KAUFMAN, A, J . & KNOLL, A. H. 1995. Neoprote rozoic variations in the C-isotopic composition of seawater: stratig raphic and biogeochemical implications. PrecambrianRes. 73, 27-49 . -------'", HAYES, J . M., KNOLL, A. H. & GERMS, G. J. B. 199 1. Isotopic composit ions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the e ffects of diagenesis and metamorphism. Precambrian Res. 49, 301 -327. --------, JACOBSEN, S. B. & KNOLL, A. H. 1993. The Vendian record of Sr and C isotopic variations in seawater: implications for tectonics and paleocl imate. Earth & Planetary Science Letters 120,409-430. KELLER, B. M. & FEDONKIN , M.A. 1977. New organic fossil f inds in the Precambrian Valdai Series on the Syuzma River.lzv. Akad. Nauk SSSR Ser. Geol. 3, 38-44. KEMPE, S. & DEGENS, E. T . 1985. An early soda ocean? Chern. Geol. 53,95- 108. KNOLL, A. H. & WALTER, M. R . 1992. Latest Prote rozoic s tratigraphy and earth history. Nature 356, 673-678. -------- , HAYES, J. M. , KAUFMAN, A. J. , SWETT, K. & LAMBERT, I. B. 1986. Secular variation in carbon isotope ratios from uppe r Prote rozoic successions of Svalbard and East Greenland. Nature 321, 832-838. KRONER, A. & GERMS, G. J. B. 197 1. A re- interpretation of the Numees-Nama contact at Aussenkjer, South West Africa. Trans. Geol. Soc. S. Afr .. 74, 69-74. -------- McWILLIAMS , M. 0 ., GERMS, G. J. B ., REID, A. B. & SCHALK, K. E. 1980. Paleomagnetism of Late Precambrian to Early Paleozoic mixtite-bearing formations in southe rn Namibia (S.W. Africa); the Nama Group and Blaubeker Formation. Am. J. Sci. 280, 942-968. KULLING , 0. 1972. The Swedish Caledonides. In: Strand, T . & Kulling, 0. Eds.,Scandinavian Caledonides , pp. 147-302. London, Wiley. MARTIN, H. 1950. SUdwestafrika. Ceo/. Rundsch. 38, 6- 14 . -------- 1965. The Precambrian Geology of South West Africa and Namaqualand. The Precambrian Res. Unit, Univ. of Capetown. MATTHEWS, S.C. & MISSARZHEVSKY, V. V. 1975. Small shelly foss ils of Late Precambrian and early Cambrian age: a revie w of recent work. J . Geol. Soc. London 131, 289-304 . Mc HENRY, B. & YATES, A. 1993. First report of the e nigmatic metazoanAnomalocaris from the south ern hemisphere and a trilobite with preserved appendages from the Early Cambrian of Kangaroo Island, South Austra lia. Rec. S. Aust. Mus. 26(2), 77- 86. McMENAMIN, M.A. S ., 1986. The garden of Ediacara. Pa/aios 1, 178- 182. 12 -------- 1987. The emergence of animals. Scientific American 256,94- I 02. -------- 1993. Osmotrophy in fossil protoctists and early animals. Invertebrate Reproduction and Development 23(2-3), I 65-169. -------- 1996. Ediacaran biota from Sonora, Mexico. Proc. Nat/. Acad. Sci. USA 93, 4990-4993. -------- & McME AMI , D. L. S. 1990. The Emergence of Animals: the Cambrian Breakthrough . ew York, Columbia University Press. NED IN , C. 1995. The Emu Bay Sha le , a Lower Cambrian Lagerstatten, Kangaroo Island, South Australia.Mem . Ass. Austra/as . Pa/aeonrols. 18,31-40. NURSALL, J . R. 1959. Oxygen as a prerequisite to the origin of Metazoa. Nature 183, I I 70-1 I 72. PALMER, D. 1996. Ediacarans in deep water. Nature 379, 114. PFLUG, H. D. 1966. Neue Fossilreste aus den Nama-Schichten in SUdwest Afrika. Pa/aeont. Z. 40( 1-2), 14-25. -------- 1970a. Zur fauna der Nama-Schichten in SUdwe t-Afrika I. Pteridinia, Bau und systematische Zugehorigkeit. Pa/aeontographica Al34, 226-262. -------- 1970b. Zur fauna der Nama-Schichten in SUd west-Afrika. H. Rangeidae, Bau und sys tematische Z ugehorigkeit.Palaeontographica Al35, 198-23 I. -------- 1972a. Zur fauna der Nama-Schichten in SUdwest-Afrika. lll. Erniettomorpha, Bau und Systematik. Pa/aeonrographica A139, 134- 170. -------- 1972b. Systematik der jung-prakambrisc he n Petalonamae. Palaeont . Z. 46,56-67. -------- 1973. Zur fauna der Nama-Schichten in SUdwest-Afrika. IV. Mikroskopische Anatom ic der Petalo-organismen.Palaeontographica A l 44, 166-202. PICKFORD, M . H. L. 1995. Re view of the Riphean Vendian and early Cambrian palaeontology of the Otavi and ama Groups, Namibia. Communs. Ceo/. Surv. Namibia 10, 57-8 1. RAFF, R. A., MARSHALL, C. R. & TURBEV ILLE, J . M. 1994. Using D A sequences to unravel the Cambrian radiation of animal phyla. Ann . Rev. of Ecology and Systematics 25, 351-375 . RA NGE, P. 19 12. Geologie des deutschen amalandes. Beitr. C eo/. Erforsch dt. Schutzgeb. II . RETALLACK, G. J. 1994. Were the Ediacaran foss ils lichens? Paleobiology 20(4) , 523-544. RIC HTER, R. 1955. Die altesten Fossitien SUdafrikas. Senckenb. Let h. 36, 243-289. RUN EGAR, B. 1982a. The Cambrian explosion: animals or fossils. J . Ceo/. Soc. Ausr. 29, 395-411. -------- 1982b. A molecular c lock date for the origin of anima l phyla. Lerhaia 15, 199-205. -------- 1982c Oxygen requirements, a biology and phylogenetic signi ficance of the late Precambrian worm,Dickensonia and the evolution of the burrowing habit. Alcheringa 6, 223-239. -------- 1995. Vendobionta or Metazoa? Developments in the unde rs tanding o f the Ediacara ' fauna' .Neues Jahrbuch fiir Geologie und Pali.iontologie Abhandlungen195,305-3!8. -------- & FEDONKIN, M . A. 1992. Proterozoic metazoan body fossils. In : Schopf, J. W. & Klein, C. Eds., The Proterozoic Biosphere. A Multidisciplinary Study. pp. 369-388. Cambridge University Press. SAYLOR, B. Z. 1992/3. Progress report on the sedimentology and stratigraphy of the Kuibis and Schwarzrand Subgroups, Wi tputs area, southwestern Nam ibia. Communs geol. Surv. Namibia 8, 127- 135. -------- , GROTZINGER, J. P. & GERMS, G. J. B. 1995. Sequence s tratigraphy and sedimentology of the Neoproterozoic Kuibis and Schwarzrand Subgroups (Nama Group) southwestern Namibia. Precambrian Research 73, 153-170. SCHENK, A. 19 10. Bemerkungen zur geologischen Karte von Deutsch-SUdwestafrika. Meyer, Kolonialreich II. SCHW ELL US, C. M. 1941. The ama tillite in the Kle in Karas Mountains, South West Africa. Trans C eo/. Soc. S. Afr. 44, 19-33. SEILACHER , A. 1989. Vendozoa: organismic construction in the Proterozo ic biosphere. Lethaia 22,229-239. -------- 1992. Vendobionta and psammocorall ia: lost construct ions of Precambrian evolution. J. Ceo/. Soc. London 149, 607-6 13. S HU, D., CO WAY MORRIS, S. & ZHA G, X. -L. 1966. A Pikaia-like chordate from the Lower Cambrian of China. Nature 384, 157-158. SOKOLOV , B.S. 1975. On palaeontological finds in the pre-Usol deposits at Irkutsk Amph itheatre. Trudy fr1SI. C eo/. Geofi-:. . Sibirsk. Otd. SSSR Akad. Nauk 232, I 12- 117 (in Russian). -------- & FEDONKJN, M.A. 1984. The Vendian as the termina l system of the Precambrian. Episodes 7( I), 12- 19. SOUTH AFRICAN COMMITIEE FOR STRATIGRAPHY 1980. Stratigraphy of South Africa, Ke nt, L. E. , compile r. Handbook Ceo/. Surv. S. Afr. 8, 1-690. SPRJGG , R. C. 1947. Early Cambrian (?) "Je llyfishes" from the Flinders Ranges, South Australia. Trans. Roy. Soc. S. Aust. 71, 2 12-224. STANISTREET, I. G., KUKLA, P. A. &HENRY, G. 199 1. Sedimentary basinal responses to a Late Precambrian Wilson Cycle: theDamara Orogen and Nama Foreland, Namibia.]. Afr. Earth Sci. 13, 141 - 156. STANLEY, S.M. 1973. An ecological theory for the sudden origin of multicellular life in the Late Precambrian. Proc. Nat . A cad. Sci. USA 70, 1486- 1489. -------- 1976. Ideas on the timing of metazoan diversification. Paleobiology 2, 209-219. TERMIER, H. & TERMIER. G. 1968. Evolution et biocinese . Les inverrebres dans I' histoire dumonde viva Ill. Paris , Masson. THOMAS, A. L. R. 1997. The breath of life - did increased oxygen levels trigger the Cambrian explosion? TREE 12,44-45. YAL E TINE, J. W . 1992. Dickensonia as a polypoid organism. Paleobiology 18,378- 382. --------, A WRAMIK, S. M., SIG OR, P. W. & SAD,LER, P. M. 1990.The biological explosion at the Precambrian-Cambrian boundary. Evolutionary Biology 25, 279-356. VERMETJ , G. J. 1989. The orig in of skeletons. Palaios 4, 579-584. YO BAC KSTROM, J. W . 1960. Die geologie van die gebied om ieuwoudville, Kaapprovinsie. C eo/ Surv. Explanation ofsheet241 . WADE, M. 1972. Dickensonia: polychaete worms from the late Precambrian Ediacaran fauna, South Australia. Mem. Queensland Mus. 16(2) , 171- 190. WAGGONER, B. M. 1995. Ediacaran lichens: a critique. Paleobiology 21(3), 393-397. WALTER, M. R. & HEYS, G. R. 1985. Links between the rise of the metazoa and the dec line of stromato li tes. Precambrian Research 29, 149- 174. WHITfiNGTON, H. B. 197 1. The Burgess Shale , hi tory of re search and preservation of fossils . Proc. N. A mer. Paleon. Convem ion 1969, I, I 170- 120 I. ~ -------- 1985. The Burgess Shale. New Haven, Yale Uni v. Press. WRA Y, G. A., LEYINTON, J . S. & SHAPIRO, L. H. 1996. Molecular evidence for deep Precambrian divergences among Metazoan phyla. Science 274, 568-573. 13 Xl G, Y ., & LI U, G. 1979. Coelente rate fossi ls from the Sinia n System of southern Liaoning and its strat igraphic s ignificance. Acta. Geo/. Sinica 53, 167- 172. YOCHELSO , E. L. 1967. Charles Doolittle Wa lcott , 1850- 1927. A biographical memoir. Biographical Memoirs 39, 47 1-540. ZHA G, W. -T. & HOU, X. -G . 1985. Pre lim inary notes on the occurre nce of the unusual trilobiteNaraoia in Asia. Acta Pa/aeontologia Sinica 24(6), 59 1-595.