Vol. 8, No. 2 February 1998 INSIDE • G. K. Gilbert, p. 16 GSA TODAY • Forests as Carbon Sinks, p. 22 • Rocky Mountain A Publication of the Geological Society of America Section Meeting, p. 27
The Ediacara Biota: A Terminal Neoproterozoic Experiment in the Evolution of Life Guy M. Narbonne Department of Geological Sciences, A B Queen’s University, Kingston, ON K7L 3N6, Canada, [email protected]
ABSTRACT The Ediacara biota is a distinctive assemblage of large, soft-bodied organisms that characterizes terminal Neoproterozoic (latest Precambrian) strata worldwide. Some Ediacaran organisms apparently were the root- stock for the Phanerozoic evolution of animals; other bizarre forms may rep- resent a failed experiment in Precam- brian evolution. The Ediacara biota and its nonactualistic preservation and ecosystem characterized the final 20 m.y. of the Proterozoic, and disap- peared near the beginning of the Cambrian “explosion” of shelly and burrowing animals. Figure 1. Photograph and artist’s reconstruction of the holotype of Swart- puntia germsi from Namibia. For simplic- INTRODUCTION ity, only three petaloids are shown. Met- The terminal Neoproterozoic was a ric scale. After Narbonne et al. (1997, Figs. 6 and 8). period of fundamental change in Earth history. Major changes included the breakup of the supercontinent Rodinia and the subsequent collision of some of the fragments that were to form Gond- wana (Hoffman, 1992; Unrug, 1997), a stone beds (Fig. 1). The Ediacara biota con- Phanerozoic systems (cf. Sepkoski, 1981). succession of at least four global glacia- tains a few forms that might be familiar to Prior to the appearance of the Ediacara tions, and some of the largest known a modern beachcomber, alongside bizarre biota, Mesoproterozoic to mid-Neopro- changes in the oceans and atmosphere taxa whose place in the evolution of mod- terozoic (1600–600 Ma) benthic communi- (Knoll and Walter, 1992). It is against this ern life is uncertain. A few “Ediacaran sur- ties had been dominated by prokaryotic backdrop that we see the appearance of vivors” have been reported from the Cam- microbes along with some sheetlike and abundant, large organisms, including the brian, but most of the archetypical forms ribbonlike algae (Knoll, 1992). Evidence first definite fossil animals in Earth his- disappeared abruptly near the Cambrian from molecular phylogeny suggests that tory, the Ediacara biota. “explosion” (Grotzinger et al., 1995). microscopic animals could have evolved The Ediacara biota typifies terminal The unique biology, ecology, and preser- sometime prior to the appearance of the Neoproterozoic rocks worldwide (Glaess- vational mode of the Ediacara biota effec- Ediacara biota, but the exact timing ner, 1984; Fedonkin, 1992; Jenkins, 1992; tively marks the end of the Proterozoic remains debatable (Wray et al., 1996; Runnegar and Fedonkin, 1992). It was Eon, and may herald the beginning of Conway Morris, 1996). The oldest known first described from Newfoundland and the Phanerozoic. megascopic Ediacara-type remains occur Namibia, but the name is derived from the in the Twitya Formation of northwestern superb assemblages of these fossils discov- EVOLUTIONARY HISTORY Canada (see Fig. 3, locality 22) immedi- ered at Ediacara in the Flinders Ranges of ately below tillites correlated with the The Ediacara biota occupies a pivotal South Australia by Sprigg (1947). In con- Marinoan-Varanger glaciation and position in the evolution of life on Earth, trast with the shelly fossils that character- believed to be about 600 m.y. old (Hof- between the largely microbial (especially ize the Phanerozoic, the Ediacara biota mann et al., 1990). The structures consist stromatolitic) communities that charac- consists almost exclusively of the remains of centimeter-scale rings and discs that are terize the classic “Precambrian” and the of soft-bodied organisms, which are typi- shelly biotas of the Cambrian and younger cally preserved as impressions on sand- Ediacara Biota continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 8, No. 2 1998 The Ediacara Biota: A Terminal Rock Stars—G. K. Gilbert ...... 16 Neoproterozoic Experiment in the GSAF Update ...... 18 GSA TODAY (ISSN 1052-5173) is published monthly Evolution of Life ...... 1 Washington Report ...... 20 by The Geological Society of America, Inc., with offices at 3300 In Memoriam ...... 2 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box Forests as Carbon Sinks ...... 22 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage Memorial Preprints ...... 2 Penrose Conference Reports paid at Boulder, Colorado, and at additional mailing offices. 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Ervin Brown MS 917, National Center, Reston, VA 22092 Managing Editor: Faith Rogers Production & Marketing Manager: James R. Clark Production Editor and Coordinator: Joan E. Manly Graphics Production: Joan E. Manly, Leatha L. Flowers Ediacara Biota continued from p. 1 North America (Fig. 3). The known strati- graphic range of Ediacara-style biotas is ADVERTISING: Classifieds and display: contact Ann Crawford, (303) 447-2020; fax 303-447-1133; [email protected] similar to the simplest (most primitive?) approximately 55 m.y. (ca. 600–544 Ma), geosociety.org. elements of the Ediacara biota worldwide. but diverse and complex fossils are known Issues of this publication are available as electronic Acrobat A global rise in atmospheric oxygen in only from the final 20 m.y. of the Neopro- files for free download from GSA’s Web Site, http://www. the Late Proterozoic may have been the terozoic (Fig. 2). At least three stratigraphi- geosociety.org. They can be viewed and printed on various trigger that permitted animals to achieve cally restricted assemblages have been personal computer operating systems: MSDOS, MSWin- dows, Macintosh, and Unix, using the appropriate Acrobat megascopic size at this time (Canfield and recognized, each more diverse than its reader. Readers are available, free, from Adobe Corporation: Teske, 1996). predecessor (Narbonne et al., 1994). It was http://www.adobe.com/acrobat/readstep.html. Ediacaran biotas diversified rapidly formerly believed that the Ediacara biota This publication is included on GSA’s annual after the end of the Neoproterozoic gla- disappeared several tens of millions of CD-ROM, GSA Journals on Compact Disc. ciations (Fig. 2) and are now known from years before the beginning of the Cam- Call GSA Publication Sales for details. 50% Total Recoverd Fiber some 30 localities on five continents, brian, but precise U-Pb dates of fossilifer- Printed in U.S.A. using pure soy inks. 10% Postconsumer including at least seven distinct regions of ous strata in Namibia indicate that the
2 GSA TODAY, February 1998 In my view, any inter- Aitken, 1990; Fedonkin and Runnegar, pretation that unites the 1992). disparate fossils of the Edi- The second are the nonresistant acara biota into a single tax- “discs” such as Ediacaria and Cyclomedusa onomic group, be it jelly- (Fig. 5). These were originally interpreted fish, protists, lichens, or as jellyfish (medusoids), but considera- vendozoans, should be tions of the morphology and preservation viewed with suspicion. Edi- of these discs have progressively elimi- acaran fossils range from nated taxa to the point that no undoubted centimeter-scale blobs to “jellyfish” remain among the Ediacara complexly segmented discs biota! All appear to have been bottom- and fronds more than 1 m dwelling (benthic) organisms, and many Figure 2. Late long. Symmetry ranges seem to have been permanently attached Neoproterozoic from bilateral to trigonal to to the sea bottom (Seilacher, 1982; Nar- stratigraphic tetragonal to pentameral to bonne and Aitken, 1990; Gehling, 1991; column, showing radial. Some were soft and Fedonkin and Waggoner, 1997). The cen- the position of the Ediacara biota. jellylike; others were highly timeter-scale disc to hemisphere Beltanelli- resistant to both mechani- formis (Fig. 6), perhaps the most common cal stress and decomposi- and widely distributed Ediacaran fossil tion. Different taxa lived worldwide, is strikingly similar to the base under conditions ranging of some anemones and especially to the Ediacara biota persisted to near the from shallow, sunlit shelves to the deep bases of Paleozoic burrows generally beginning of the Cambrian “explosion” seafloor. Several distinct groups can be rec- attributed to anemones (Narbonne and (Grotzinger et al., 1995). Possible explana- ognized; I focus here on the three most Hofmann, 1987; Fedonkin and Runnegar, tions for the abrupt disappearance of the common and familiar. 1992). Larger discs such as Ediacaria, some Ediacara biota include elimination of a The least equivocal are the trace fos- of which are as large as a dinner plate, are preservational “window” (Fedonkin, sils, which are represented almost exclu- more controversial. Most show no evi- 1992), competition and predation by early sively by simple subhorizontal burrows dence as to the nature of their upper sur- skeletal animals (McMenamin, 1986), and (Fig. 4). These show evidence of mobility face, but several taxa show evidence of global geochemical perturbations (Bartley and imply concentration of sensory hairlike markings extending radially from et al., 1998). organs at a “head” region. All previous a central disc (e.g., Fig. 5A) which might workers have regarded these simple trace alternatively be regarded as the tentacles AFFINITIES fossils as the work of bilaterian animals of a polyp or the rootlike structures of a (e.g., Seilacher, 1989; Narbonne and In a little more than a decade, the Ediacara Biota continued on p. 4 affinities of the Ediacara biota have gone from being a well-documented “fact” to becoming one of the great controversies in paleobiology. Prior to the mid-1980s, virtually all workers emphasized similari- ties in two-dimensional structure between Ediacaran fossils and living groups of jellyfish, soft corals, annelid worms, and arthropods, and concluded that the Ediacara biota represented the direct ancestors of these modern groups. Glaess- ner (1984) aptly termed this “The dawn of animal life.” In contrast, Seilacher (1982, 1989, 1992) believed that most of the Ediacara biota was unrelated to modern organisms, a “failed experiment” in the history of life on Earth. Seilacher suggested that Ediacaran organisms be referred to his new phylum or kingdom “Vendozoa,” which consists of quilted organisms that lacked mouths and guts and received energy by absorbing dis- solved organic molecules or by harboring photosynthetic or chemosynthetic sym- bionts. The rival classifications of Glaess- Figure 3. Global distribution of principal Ediacara-type fossil assemblages and environments. Data from ner and Seilacher represent two end- Glaessner (1984, Fig. 1.8, localities 1–23), Hofmann (1987, Fig. 13, localities 24–25) and recent litera- member views that have generated lively ture (localities 26–29). Glaessner’s localities 8 (calcareous tubes), 16 (pseudofossils), and 18 (Cambrian) debate in the literature over the past have been omitted, as have several low-diversity assemblages of discs and problematica described subse- decade (Gehling, 1991; Runnegar, 1995). quently. Australia: 1—Flinders Ranges, Adelaide “Geosyncline”; 2—Officer Basin; 3 and 4—Amadeus Basin; 5—Mt. Skinner; 6—Georgina Basin. Africa: 7—Namibia; 28—Algeria (Bertrand-Sarfati et al., Seilacher (1992) subsequently restricted 1995). Europe: 9—White Sea, Russia; 10—Podolia, Ukraine; 11—Urals; 19—Charnwood Forest, England; his original concept of the “Vendozoa” 29—Finnmark, Norway (Farmer et al., 1992). Asia: 12—Yenisey River, Siberia; 13—Lake Baikal, Siberia; (which he now terms the “Vendobionta”) 14—Anabar and Olenuk, Siberia; 15—River Maya, Siberia; 17—Yangtze Gorge, China; 23—Iran. North and specifically removed taxa that he now America: 20—Avalon Peninsula of Newfoundland; 21—North Carolina; 22—Mackenzie Mountains, regards as true animals. northwestern Canada; 24—Wernecke Mountains, northwestern Canada; 25—Rocky Mountains, British Columbia; 26—Cariboo Mountains, British Columbia (Ferguson and Simony, 1991); 27—Spring Range, Nevada (Horodyski, 1991; Waggoner and Hagadorn, 1997).
GSA TODAY, February 1998 3 AB
Figure 4. Simple burrows following a carbona- ceous layer in the Wernecke Mountains, north- western Canada. Scale in millimeters. After Nar- bonne and Hofmann (1987, Fig. 10a). Figure 5. Ediacaran discs from northwestern Canada. Scale in millimeters. A: Hiemalora from the Mackenzie Mountains. Note pustular surface with carbonaceous coating, and radiating tentacle-like markings to left. After Narbonne (1994, Fig. 3.1). B: Ediacaria (Beltanella) with overlying stem (right side of disc) from the Mackenzie Mountains. After Narbonne and Aitken (1990, Pl. 1, Fig. 1).
Ediacara Biota continued from p. 3 holdfast (Runnegar, 1995). Others are overlying sand bed and thus are preserved attached to tubelike structures (Fig. 5B) as ridges on its sole (Figs. 5, 6), whereas or even complete fronds (Jenkins, 1992), most quilted fossils (e.g., Dickinsonia) did and are best interpreted as holdfasts. not decompose until after cementation of A third group are potential true the sand and are preserved as indentations Vendobionta (sensu Seilacher, 1992). on the base of the bed (Fig. 7). Resistant Seilacher (1989, 1992) has shown that organisms can even be preserved within their distinctive quilted structure (Figs. 1, and on top of storm beds (Fig. 9), but this 7) is constructed of parallel, linked tubular is extremely unusual for nonresistant segments in which the walls were more discs. resistant than the ceilings. After the organ- One might expect exquisite preser- ism died, segments could become deflated vation of soft organisms under a layer or even imbricated. These organisms were Figure 6. The probable anemone Beltanelliformis of submarine volcanic ash such as at Mis- formerly regarded as representing the from the Bernashev Member, Ukraine. Metric scale. taken Point, but why do some thick storm ancestors of several modern groups rang- deposits and proximal turbidites faithfully ing from soft corals to annelid worms preserve delicate structures on Ediacaran (Glaessner, 1984). However, close similar- fossils despite the obvious evidence of dated at 565 ± 3 Ma (U-Pb on zircons; ity of the quilted constructional elements high-energy conditions during deposition? Benus, in Landing et al., 1988), providing among architectures that range from erect The answer seems to lie in the warty, pus- a precise date for this “Ediacaran Pom- multifoliate “fronds” (e.g., Swartpuntia; tular texture that marks most fossiliferous peii.” The most common mode of preser- Fig. 1) to flat reclining sheets (e.g., Dickin- Ediacaran surfaces (Figs. 4–7). Russian vation of the Ediacara biota worldwide is sonia; Fig. 7A) implies that these taxa were paleontologists refer to this as “old ele- on the soles of storm or turbidite beds more similar to each other than to any phant skin,” and it provides the search (Fig. 8B), again reflecting instantaneous modern organisms (Narbonne et al., image for Ediacaran fossils on at least deposition of sand on the muddy seafloor. 1997). The Vendobionta appears to be three continents; it is rarely found in beds Wade (1968) recognized two preserva- a distinctive clade (monophyletic group) that do not contain Ediacaran fossils, and tional modes that reflected resistance to of Neoproterozoic organisms, but its taxo- these fossils are almost never found in decomposition. In particular, most discs nomic position is uncertain; suggestions beds that lack this texture. Gehling (1987) decomposed prior to cementation of the range from an extinct group within the phylum Cnidaria (which also includes modern corals and anemones) to an extinct phylum or even kingdom unre- AB lated to the subsequent Phanerozoic evo- lution of animals (Buss and Seilacher, 1994).
PRESERVATION If almost all Ediacaran organisms were soft-bodied, how were they preserved so abundantly worldwide? First and fore- most, Ediacara-type fossils are preserved in event beds. These soft-bodied organ- isms lived on the mud bottom, and their impressions were preserved when they were catastrophically covered by a bed of sand-sized debris (an event). One famous example is the felsic tuff that covers the Figure 7. Dickinsoniids preserved as indentations on bed soles from Ediacara, Australia (A), and the fossil surface at Mistaken Point in New- Mackenzie Mountains, northwestern Canada (B). Metric scale. A: Two specimens of Dickinsonia costata. foundland (Fig. 8A). This tuff has been South Australia Museum P13760. B: Windermeria aitkeni. After Narbonne (1994, Fig. 3.2).
4 GSA TODAY, February 1998 first recognized the similarity between these surfaces and modern microbial mats and has concluded that these mats were instrumental in stabilizing the mud sur- Figure 9. Living face and perhaps providing a “death assemblage (A) and mask” on the organism itself. In most preservation (B) of cases, the distinctive texture is all that Swartpuntia germsi remains of the former microbial mat, but from Namibia. Spec- in northwestern Canada mats are pre- imens lived on the mud bottom and served as black carbonaceous sheets that are preserved within coat the fossil surfaces and that are locally and on top of the ripped up to form intraformational con- bed of hummocky glomerates of carbonaceous sheets, some cross-bedded sand- still bearing fossil impressions (Fig. 10). stone. After Nar- bonne et al. (1997, The prevalence of microbial mats Fig. 11). throughout the subtidal realm represents a nonactualistic feature of Neoproterozoic seas (Seilacher and Pfluger, 1994; Mac- Naughton et al., 1997). Bioturbation, crop- ping, and competition resulted in severe reduction of these mats in the Early Cam- as the Burgess Shale, implying that the dis- brian and, thus, less likelihood of preserv- appearance of the Ediacara biota was not ing soft-bodied organisms on sandstone solely preservational. soles. However, with a few notable excep- tions (see e.g., Conway Morris, 1993), ECOLOGY complex Ediacara-type organisms are also As discussed above, recent studies absent from Cambrian Lagerstätten such suggest that the Ediacara biota consists mainly of benthic organisms, most of which are in their original life position. Shallow-water deposition characterizes the vast majority of Ediacaran fossil sites (Fig. 3), including the classic sites in Australia, Namibia, and northern Russia Figure 10. Conglomerate of carbonaceous rip-up (Gehling, 1988; Narbonne et al., 1997; clasts, with a specimen of Spriggia preserved on a Grazhdankin and Ivantsov, 1996). Fossilif- clast, from the Wernecke Mountains of northwest- ern Canada. GSC 83030. erous sandstones in all these regions con- tain wave ripples and hummocky cross- stratification, but they typically lack true desiccation cracks and other evidences of lithospheric plate. Strata were deposited in emergence. Most fossiliferous strata were shelf and slope settings on the margin of deposited between wave-base and storm ‰ the opening proto–Pacific Ocean. Shelf wave-base, and were probably within the facies in the Wernecke Mountains com- euphotic (photosynthetic) zone. A handful prise mainly storm-deposited sandstones of deep-water occurrences of the Ediacara A that were laid down in shallow, wave- biota are also known (Fig. 3), principally agitated, and probably euphotic environ- in the Avalon Zone of eastern North ments (Narbonne and Hofmann, 1987). America and England (Jenkins, 1992). In contrast, slope facies in the Mackenzie Shallow-water features have not been Mountains lack shallow-water features found in any of these successions, and and are characterized by turbidites, con- process sedimentology studies (such as tourites, and mass-flow deposits that accu- those carried out at Mistaken Point in mulated in 1.0–1.5 km water depth on the Newfoundland; Landing et al., 1988) continental slope (Dalrymple and Nar- imply that the strata were deposited on a bonne, 1996). Ediacara-type fossils in both deep-sea turbidite fan, but the subsequent regions appear to be in their original life tectonic history of these regions makes ‰ positions. The principal difference in biota calculations of absolute depth impossible. between these two environmental regimes Diversity is moderate to high, but only is the extreme abundance of probable one taxon (Charnia) from Mistaken Point anemone Beltanelliformis in the shallow- and Charnwood Forest is also known from water assemblages of the Wernecke Moun- shallow-water successions. However, it is tains and its complete absence from the uncertain whether the unusual composi- deep-water assemblages in the nearby tion of these assemblages reflects the deep- B Mackenzie Mountains. This mirrors global water setting or paleogeographic isolation trends—Beltanelliformis occurs profusely in of the Avalonian microcontinent during Figure 8. Preservation of Ediacaran fossils in shallow-water settings worldwide but has the terminal Neoproterozoic. North America. Arrows show their positions. Ruler not yet been found in any deep-water suc- The Windermere Supergroup of is 15 cm long. A: Felsic tuff (565 ± 3 Ma) covering cession. Few other differences in composi- the fossil surface at Mistaken Point, Newfound- northwestern Canada provides a unique tion are evident, suggesting that the land. B: Cyclomedua plana on sole of a turbidite in natural laboratory to study the depth dis- the Sheepbed Formation at Sekwi Brook, north- tribution of the Ediacara biota on a single western Canada. Ediacara Biota continued on p. 6
GSA TODAY, February 1998 5 Ediacara Biota continued from p. 5 Buss, L. W., and Seilacher, A., 1994, The phylum Knoll, A. H., and Walter, M., 1992, Latest Proterozoic Vendobionta: A sister group of the Eumetazoa?: stratigraphy and Earth history: Nature, v. 356, Paleobiology, v. 20, p. 1–4. p. 673–678. depth-related zonations that characterize Canfield, D. E., and Teske, A., 1996, Late Proterozoic Landing, E., Narbonne, G. M., and Myrow, P., editors, Phanerozoic and modern seas were not rise in atmospheric oxygen concentration inferred from 1988, Trace fossils, small shelly fossils and the Precam- strongly developed in the terminal Neo- phylogenetic and sulphur-isotope studies: Nature, brian-Cambrian boundary: New York State Museum proterozoic (Narbonne and Aitken, 1990). v. 382, p. 127–132. Bulletin 463, 81 p. In some ways, the Ediacaran ecosys- Conway Morris, S., 1993, Ediacaran-like fossils in MacNaughton, R. B., Dalrymple, R. W., and Narbonne, Cambrian Burgess Shale–type faunas of North America: G. M., 1997, Early Cambrian braid-delta deposits, tem differed significantly from all modern Palaeontology, v. 36, p. 593–635. Mackenzie Mountains, northwestern Canada: Sedimentology, v. 44, p. 587–609. systems. Microbial mats covered most Conway Morris, S., 1996, Molecular clocks: Defusing marine surfaces and exerted a major influ- the Cambrian “explosion”: Current Biology, v. 7, McMenamin, M. A. S., 1986, The garden of Ediacara: ence on sedimentation patterns. Burrow- p. R71–R74. Palaios, v. 1, p. 178–182. ing organisms were sparse and mostly Dalrymple, R. W., and Narbonne, G. M., 1996, Conti- Narbonne, G. M., 1994, New Ediacaran fossils from ineffective bioturbators, and no animals nental slope sedimentation in the Sheepbed Formation the Mackenzie Mountains, northwestern Canada: (Neoproterozoic, Windermere Supergroup), Mackenzie Journal of Paleontology, v. 68, p. 411–416. seem to have been capable of preying on Mountains, N.W.T.: Canadian Journal of Earth Sci- Narbonne, G. M., and Aitken, J. D., 1990, Ediacaran fos- ences, v. 33, p. 848–862. the largely soft-bodied and immobile Edi- sils from the Sekwi Brook area, Mackenzie Mountains, acaran organisms. McMenamin (1986) Farmer, J., Vidal, G., Moczydlowska, M., Strauss, H., northwestern Canada: Palaeontology, v. 33, p. 945–980. Ahlberg, P., and Siedlecka, A., 1992, Ediacaran fossils has referred to this as “The garden of Edi- Narbonne, G. M., and Hofmann, H. J., 1987, Ediacaran from the Innerelv Member (late Proterozoic) of the biota of the Wernecke Mountains, Yukon, Canada: acara,” and in many ways it reflects the Tanafjorden area, northeastern Finnmark: Geological Palaeontology, v. 30, p. 647–676. last vestiges of the Precambrian lifestyle Magazine, v. 129, p. 181–195. Narbonne, G. M., Kaufman, A. J., and Knoll, A. H., that had characterized the preceding 3 b.y. Fedonkin, M. A., 1992, Vendian faunas and the early 1994, Integrated chemostratigraphy and biostratigraphy evolution of Metazoa, in Lipps, J., and Signor, P. W., of Earth history. However, the presence of of the Windermere Supergroup, northwestern Canada: eds., Origin and early evolution of the Metazoa: anemones (Gehling, 1988) and sponges Implications for Neoproterozoic correlations and the New York, Plenum Press, p. 87–129. (Gehling and Rigby, 1996) among the Edi- early evolution of animals: Geological Society of Fedonkin, M. A., and Runnegar, B., 1992, Proterozoic America Bulletin, v. 106, p. 1281–1292. acara biota implies that ecological niches metazoan trace fossils, in Schopf, J. W., and Klein, C., Narbonne, G. M., Saylor, B. Z., and Grotzinger, J. P., in the Ediacaran seas included microcarni- eds., The Proterozoic biosphere: A multidisciplinary 1997, The youngest Ediacaran fossils from southern study: Cambridge, UK, Cambridge University Press, vores and filter feeders. Ecological tiering Africa: Journal of Paleontology, v. 71, p. 953–967. p. 389–396. included, for the first time, three feeding Runnegar, B., 1995, Vendobionta or Metazoa? Fedonkin, M. A., and Waggoner, B. M., 1997, The levels: an elevated level in the water col- Developments in understanding the Ediacara “fauna”: Late Precambrian fossil Kimberella is a mollusc-like Neues Jahrbuch für Geologie und Paläontologie, umn occupied by the fronds; a seafloor bilaterian organism: Nature, v. 388, p. 868–871. Abhandlungen, v. 195, p. 303–318. level with anemones, sponges, and a vari- Ferguson, C. A., and Simony, P. S., 1991, Preliminary Runnegar, B., and Fedonkin, M. A., 1992, Proterozoic ety of other organisms; and the shallow report on structural evolution and stratigraphic correla- metazoan body fossils, in Schopf, J. W., and Klein, C., tions, northern Cariboo Mountains, British Columbia: subsurface where wormlike bilateria bur- eds., The Proterozoic biosphere: Cambridge, UK, Cam- Geological Survey of Canada Paper 91-1A, p. 103–110. rowed beneath microbial mats. These eco- bridge University Press, p. 369–388. Gehling, J. G., 1987, Earliest known echinoderm— logical innovations laid the groundwork Seilacher, A., 1982, Late Precambrian and Early Cam- A new Ediacaran fossil from the Pound Subgroup of brian Metazoa: Preservational or real extinctions? in for the massive ecological changes of the South Australia: Alcheringa, v. 11, p. 337–345. Holland, H. D., and Trendall, A. F., eds., Patterns of Cambrian “explosion,” and thus marked Gehling, J.G., 1988, A cnidarian of actinian grade from change in Earth evolution: Berlin, Springer-Verlag, the beginning of our Phanerozoic world. the Ediacaran Pound Subgroup, South Australia: p. 159–168. Alcheringa, v. 12, p. 299–314. Seilacher, A., 1989, Vendozoa: Organismic construction ACKNOWLEDGMENTS Gehling, J. G., 1991, The case for Ediacaran fossil roots in the Proterozoic biosphere: Lethaia, v. 22, p. 229–239. to the metazoan tree: Geological Society of India Seilacher, A., 1992, Vendobionta and Psammocorallia: Memoir 20, p. 181–224. Interactions and discussions with Lost constructions of Precambrian evolution: Geologi- members of the IUGS Working Group on Gehling, J. G., and Rigby, J. K., 1996, Long expected cal Society of London Journal , v. 149, p. 607–613. sponges from the Neoproterozoic Ediacara fauna of a Terminal Proterozoic System and IGCP Seilacher, A., and Pfluger, F., 1994, From biomats South Australia: Journal of Paleontology, v. 70, to benthic agriculture: A biohistoric revolution, in Project 320 (Neoproterozoic Events and p. 185–195. Resources) helped me in writing this Krumbein, et al., eds., Biostabilization of sediments: Glaessner, M. F., 1984, The dawn of animal life: A bio- Oldenberg, Germany, Bibliotheks und Information- paper. Line drawings are the work of John historical study: Cambridge, UK, Cambridge University system der Universitat Oldenberg, p. 97–105. Press, 244 p. Glew, Rob MacNaughton, and Ela Rusak. Sepkoski, J. J., 1981, A factor analytic description of the Long-term funding has been provided by Grazhdankin, D. V., and Ivantsov, A. Yu., 1996, Recon- Phanerozoic marine fossil record: Paleobiology, v. 7, the Natural Sciences and Engineering structions of biotopes of ancient Metazoa of the Late p. 36–53. Vendian White Sea biota: Palaeontological Journal, Sprigg, R. G., 1947, Early Cambrian (?) jellyfishes from Research Council of Canada (NSERC v. 30, p. 674–678. Research Grant 2648); the Geological the Flinders Ranges, South Australia: Royal Society of Grotzinger J. P., Bowring, S. A., Saylor, B. Z., and Kauf- South Australia Transactions, v. 71, p. 212–224. Survey of Canada and Lithoprobe also man, A. J., 1995, Biostratigraphic and geochronologic Unrug, R., 1997, Rodinia to Gondwana: The geody- constraints on early animal evolution: Science, v. 270, provided support during critical periods. namic map of Gondwana supercontinent assembly: p. 598–604. I thank Molly Miller for editorial advice GSA Today, v. 7, no. 1, p. 1–6. Hofmann, H. J., 1987, Precambrian biostratigraphy: and Michael Gibson and an anonymous Wade, M., 1968, Preservation of soft-bodied animals Geoscience Canada, v. 14, p. 135–154. reviewer for helpful reviews. This paper in Precambrian sandstones at Ediacara, South Australia: is dedicated to the memory of Bob Hofmann, H. J., Narbonne, G. M., and Aitken, J. D., Lethaia, v. 1, p. 238–267. 1990, Ediacaran remains from intertillite beds in Waggoner, B. M., and Hagadorn, J. W., 1997, Ediacaran Horodyski, who found the first Ediacaran northwestern Canada: Geology, v. 18, p. 1199–1202. fossils from western North America: Stratigraphic and fossils ever discovered in the western Hoffman, P. F., 1992, Did the breakout of Laurentia paleogeographic implications: Geological Society of United States. turn Gondwanaland inside out? : Science, v. 252, America Abstracts with Programs,v. 29, no. 6, p. A-30. p. 1409–1412. Wray, G. A., Levinton, J. S., and Shapiro, L. H., 1996, REFERENCES CITED Horodyski, R. 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6 GSA TODAY, February 1998