calcified metazoans in thrombolite-stromatolite reefs of the terminal

q 2000 The Paleontological Society. All rights reserved. 0094-8373/00/2603-0003/$1.00

Paleobiology, 26(3), 2000, pp. 334–359

Calcified metazoans in thrombolite-stromatolite reefs of theterminal Proterozoic Nama Group, Namibia

John P. Grotzinger, Wesley A. Watters, and Andrew H. Knoll

Abstract.—Reefs containing abundant calcified metazoans occur at several stratigraphic levels with-in carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia.The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefsare composed of thrombolites (clotted internal texture) and stromatolites (laminated internal tex-ture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefsranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occurin depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefsoccur broadly across the ramp profile and are well developed as pinnacle reefs in downdip posi-tions.

The three-dimensional morphology of reef-associated fossils was reconstructed by computer,based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens andadditionally supported by observations of over 90 sets of serial sections. Most variation observedin outcrop can be accounted for by a single species of cm-scale, lightly calcified goblet-shaped fos-sils herein described as Namacalathus hermanastes gen. et sp. nov. These fossils are characterized bya hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular openingwith a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlyingbiological structure. The goblets lived atop the rough topography created by ecologically complexmicrobial-algal carpets; they appear to have been sessile benthos attached either to the biohermalsubstrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phy-logenetic affinities of Namacalathus are uncertain, although preserved morphology is consistentwith a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unminerali-zed, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appearto be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans.Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming mi-croorganisms antedate the Cambrian Period.

John P. Grotzinger and Wesley A. Watters. Department of Earth, Atmospheric and Planetary Sciences, Mas-sachusetts Institute of Technology, Cambridge, Massachusetts 02139. E-mail: [email protected] and E-mail:[email protected]

Andrew H. Knoll. Botanical Museum, Harvard University, Cambridge, Massachusetts 02138. E-mail:[email protected]

Accepted: 8 March 2000

Introduction

Siliciclastic successions of terminal Prote-rozoic age contain moderately diverse, com-monly problematic fossils of soft-bodied or-ganisms, collectively known as the Ediacaranbiota. Many rocks that host Ediacaran assem-blages also contain a limited diversity ofmetazoan trace fossils. In contrast—and instrong contradistinction to Cambrian andyounger successions—terminal Proterozoiclimestones have been thought to yield onlylimited evidence of animal life. A major ex-ception to this pattern is provided by Cloudina,a globally distributed calcified fossil knownalmost exclusively from uppermost Protero-zoic carbonate rocks. But should Cloudina be

regarded as the exception that proves the ruleor as a hint that more diverse animals inhab-ited carbonate platforms before the dawn ofthe Cambrian?

Here, we report evidence for a paleoecol-ogically distinctive assemblage of calcifiedmetazoans in thrombolite-stromatolite reefsand associated facies of the terminal Protero-zoic Nama Group, Namibia, where Cloudinawas originally discovered. Particularly abun-dant are goblet-shaped fossils described inthis paper as Namacalathus hermanastes gen. etsp. nov., and interpreted as lightly mineral-ized, attached benthos comparable to simplecnidarians in bodyplan.

Archaeocyathans and corallimorph meta-zoans formed persistent ecological associa-

335CALCIFIED PROTEROZOIC METAZOANS

FIGURE 1. Geological map of southern Namibia, show-ing the distribution of major sedimentary and tectonicelements, including the Nama Group, discussed in thetext.

tions with microbial communities in Early(but not earliest) Cambrian reefs (e.g., Ridingand Zhuravlev 1995), and stromatolite-throm-bolite reefs have been reported from upper(but not uppermost) Proterozoic successions(e.g., Turner et al. 1993, 2000). Therefore, inaddition to contributing evidence of diversityamong terminal Proterozoic skeletons, thefossiliferous bioherms reported here help tofill a gap in our understanding of the sedi-mentary transition from Proterozoic reefs ac-creted by physical and microbial processes toPhanerozoic structures in which mineralizedalgae and animals play important construc-tional roles.

Geological Setting of Nama Reefs

The geology of southern Namibia, includ-ing the distribution of the Nama Group, isshown in Figure 1. The Nama Group has beeninterpreted as a foreland basin fill (Germs1983; Gresse and Germs 1993) related to con-vergence along the western and northern mar-gins of the Kalahari craton and overthrustingin the Gariep and Damara orogens (Miller1983). The general stratigraphy of the NamaGroup (Fig. 2) was outlined by Martin (1965)and developed in a series of papers by Germs(1972, 1974, 1983). Regional isopachs and fa-cies distributions define two subbasins; theWitputs subbasin, located in southern Namib-ia, thickens toward the Gariep orogenic belt,while the more northerly Zaris subbasin

thickens northward toward the Damara oro-genic belt. The Osis Arch represents a site ofdepositional thinning of all Nama units thatseparates the two subbasins (Fig. 1).

In general, the Nama Group consists of anumber of marine-shelf siliciclastic and car-bonate sequences (Kuibis and SchwarzrandSubgroups) overlain by alluvial to shallowmarine molasse (Fish River Subgroup) thatdocuments unroofing of the Damara/Gariephinterlands. Near the subbasin axes, thick-nesses are on the order of 2–3 km, thinning toless than 1 km farther onto the craton towardthe Osis Arch (Germs 1974, 1983). Thrombol-ite-stromatolite reefs are well developed in theKuibis Subgroup of the northern, Zaris sub-basin, and in the Huns platform of the south-ern, Witputs subbasin (Fig. 2).

Geochronologic constraints (Grotzinger etal. 1995) are provided by U-Pb zircon ages onseveral units within the Nama Group (Fig. 2).An ash bed within the northern Nama basinyields an age of 548.8 6 1 Ma for the middleKuibis Subgroup (Grotzinger et al. 1995). Insouthern exposures of the Nama, the overly-ing Schwarzrand Subgroup contains ash bedsthat yield, in ascending order, ages of 545.1Ma, 543.3 Ma, and 539.4 Ma (all 61 Ma) (Grot-zinger et al. 1995). The Proterozoic/Cambrianboundary in Namibia is bracketed by the 543.3Ma and 539.4 Ma ages, although this also in-cludes a significant unconformity; on a globalbasis the Proterozoic/Cambrian boundary iscurrently regarded to be on the order of 543Ma.

Zaris Subbasin—Kuibis Platform

The basal unit of the Nama Group, the Kui-bis Subgroup is regionally widespread, con-sisting of a thin, basal, transgressive sand-stone that grades upward into a carbonateplatform 150–500 m thick (Figs. 2, 3A). In theZaris subbasin, Kuibis rocks form a well-de-veloped carbonate ramp that thickens fromthe Osis Arch northward to the NaukluftMountains (Germs 1983; Saylor et al. 1998;Smith 1998). This change in thickness is ac-companied by a gradation from shallow-waterfacies in the south to deeper-water, basinal fa-cies near the Naukluft Mountains (Germs1983). Germs (1972b, 1974, 1983) subdivided

336 JOHN P. GROTZINGER ET AL.

FIGURE 2. Generalized stratigraphy of the Nama Group for the Zaris (north) and Witputs (south) subbasins, show-ing major lithostratigraphic, chemostratigraphic, biostratigraphic, and sequence stratigraphic attributes. Note thepositions of thrombolite reef complexes above sequence boundaries.


the Kuibis platform into two members(Omkyk and Hoogland; Fig. 2) that corre-spond approximately to stratigraphic se-quences (Grotzinger et al. 1995; Saylor et al.1995, 1998; Smith 1998). The surfaces definingthe member boundaries, however, representflooding events rather than sequence bound-aries (Smith 1998). Detailed mapping of thesesequences (Smith 1998) shows that the reefsdescribed here, including the Driedoornvlagtebioherm identified by Germs (1972a, 1974), allnucleated within the Transgressive Systems toearly Highstand Systems Tract (TST to HST)at the base of their respective sequences (Fig.2). This implies that the most favorable con-ditions for reef growth occurred during timesof increased accommodation and lowered sed-iment flux on the platform, similar to reefs ofolder Proterozoic (Grotzinger 1989; Grotzin-ger and James 2000) and Phanerozoic age (So-reghan and Giles 1999).

In updip sections, biostromes form contin-uous sheets (within the Zebra River farm) thatbreak up into patch-reef bioherms toward andwithin the Donkergange farm (Fig. 3B,C). Fur-ther downdip, large pinnacle reefs are devel-oped at Driedoornvlagte in a position of max-imum accommodation within the deposition-al profile (Fig. 3A).

Witputs Subbasin—Huns Platform

The Huns Member consists of a thick sec-tion (up to 500 m) of platform carbonates inthe middle of the Nama Group, within theWitputs subbasin of southern Namibia (Fig.2). Germs (1972b, 1974, 1983) first recognizedpinnacle reefs on the Swartkloofberg farm;subsequent work (Grotzinger et al. 1995; Say-lor and Grotzinger 1996; Saylor et al. 1998) hasshown that these reefs are associated withdrowning of the platform. Fifty to 70 m wideat their base and up to 50 m high (Fig. 4), Hunsreefs are blanketed by shales deposited at orbelow wave base.

Facies and Diagenesis

The thrombolite biostrome facies consists ofmassive thrombolites, stratiform thrombolitesand stromatolitic thrombolite-forming col-umns, branching columns, and heads anddomes of decimeter-scale dimensions and re-

lief (Fig. 5A). In general, the cores of domesand columns are characterized by a throm-bolitic texture, whereas the margins becomeprogressively more stromatolitic in nature, inthe sense that they exhibit crude lamination(Fig. 5A; equivalent to the stromatoliticthrombolites of Kennard and James 1986). Inthe Kuibis reefs, columns are consistentlyelongated with an azimuth of 2708–3108. Cal-cified macrofossils occur within thrombolitedomes and columns, and the intrachannel fillbetween domes and columns consists oftrough cross-bedded, fossiliferous packstoneand grainstone containing simple tubes, morecomplex cups and goblets, and their bioclasticdetritus (Fig. 5A). Figure 7 shows the close re-lationship between fossil content and throm-bolitic texture.

The thrombolite biostrome facies character-istically developed as broad, laterally contin-uous reef complexes that became discontinu-ous down depositional dip to form isolatedbioherms and pinnacle reefs. Energy condi-tions were high, as demonstrated by thetrough cross-bedded intrachannel grainstone,the elongation of the stromatolitic thrombolitecolumns, and the association of this facieswith the grainstone facies. The consistent ori-entation of the thrombolite elongation is com-parable to that of other platforms where waveaction is inferred to have been responsible forthe elongation of stromatolites (Hoffman 1969;Cecile and Campbell 1978; Grotzinger 1986;Bartley et al. 1998), analogous to the devel-opment of ‘‘spur and groove’’ structure inmodern coral reefs. This interpretation sup-ports other evidence that the Kuibis ramp wasa wave- and storm-dominated system, wherethe dominant currents were induced by strongwave-generated flows.

The internal texture of the thrombolite reefsis characterized by mesoclots that range froma few millimeters to a few centimeters in di-ameter (Figs. 5A,B, 6). Nama mesoclots are re-stricted to simple ovoid, globose, and colli-form morphologies and do not include thecomplex digitate or dendriform fabrics foundin younger thrombolites (e.g., Kennard andJames 1986; Riding and Zhuralev 1995). Namamesoclots formed an open framework that re-sulted in development of abundant cavities


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FIGURE 3. Reefs of the Zaris subbasin. A, Landsat TM image showing pinnacle reef (Driedoornvlagte bioherm) onthe Driedoornvlagte farm, Zaris subbasin. The reef is developed on platform carbonates (dark-blue colors) and isoverlain by deep-water shales (orange-red colors). The platform carbonates unconformably overlie much olderquartzites (mauve-gray colors at top of photograph). A road runs along Quaternary alluvium (white, cream colors)that covers the stratigraphically lowest shales. Color in the TM image indicates composition—the reef has a gray-lavender color that reflects high dolomite content, whereas the dark-blue color of shallow-water platform carbonatesreflects dominance of limestone. Rocks have a structural dip of about 408 toward the south. B, Laterally continuousbiostrome at the top of the Omkyk Member within Donkergange passes laterally into discrete patch reefs, eachindicated by an ‘‘R.’’ Reefs are overlain by deep-water shales, which in turn pass upward into a grainstone shoalcomplex. C, Close-up of a representative patch reef showing stratigraphic position immediately above a sequenceboundary, which separates reef facies from underlying thick-bedded grainstone facies. The reef passes laterally intorelatively thin-bedded shales and fine calcarenites. Note how bedding thickness decreases immediately above thesequence boundary, indicating a regime of increasing accommodation.

(Figs. 5B, 6). Mesoclots typically were over-grown by a synsedimentary crust of fibrousmarine cement that now consists of calcite butis interpreted to have replaced original ara-gonite (Fig. 6). Marine cement crusts weresubsequently covered by geopetal sediment,which partially filled framework pores (Fig.5B, 6). Remaining porosity was occluded by adolomite rim cement, followed by infillingwith blocky calcite spar (Fig. 6). Recrystalli-zation commonly makes interpretation ofgrowth processes difficult, but sporadicallydistributed patches with relatively good fabricpreservation show evidence that coccoidal mi-croorganisms, expressed as poorly preservedspheroids in mesoclot cores, contributed toreef accretion (Grotzinger in press).

Early lithification of thrombolites and stro-matolites in Nama pinnacle reefs resulted inthe development of penetrative networks ofneptunian dikes (Fig. 5C). These are best ex-pressed in the Huns reefs and Driedoornvlag-te bioherm, where fractures that cut sharplyacross reefs are filled with fibrous marine ce-ments, interpreted as calcite-replaced arago-nite (Fig. 5C). Neptunian dikes are formedonly in the larger reefs, where gravitationallyinduced failure of the reef was likely. A relatedeffect includes failure of the sides of the pin-nacle reefs, which resulted in down-slopetransport of slide breccias. These breccias arecomposed of angular blocks up to one meterin diameter with thrombolite and stromatolitetextures. Voids between breccia blocks arefilled with massive lime or dolomite mud-stones, or fibrous marine cements.

Calcified Metazoans of the Nama Group

Within thombolitic facies, fossils occurabundantly both in the domal and columnarstructures that make up individual reefs andin trough cross-bedded intrachannel fill. Thefossils—millimeter- to centimeter-scale cal-careous cups, goblets, and tubes—are partic-ularly abundant in thrombolites and throm-bolitic stromatolites, evidently preferring thefirm substrate and bathymetric elevation thatthese structures offered (Fig. 7). Intrachannelfill consists of skeletal packstone and grain-stone containing goblets, cups, cylindricaltubes, Cloudina, and their bioclastic detritus.

Morphology. Figure 8 shows a number ofcalcified goblet-shaped fossils. Two dimen-sional slices provided by outcrops, polishedslabs, and thin-sections reveal a moderate de-gree of assemblage variability, ranging fromclearly defined goblet-shaped forms (Fig. 8E,originally illustrated in Grotzinger et al.1995), to rounded cups without ‘‘stems,’’ tobulbous structures showing apparent hexag-onal symmetry in cross-section.

Exposed specimens reflect some combina-tion of intraspecific, interspecific, and tapho-nomic variation, but the relative contributionsof these factors cannot be evaluated in the ab-sence of information about the three-dimen-sional morphology of constituent fossils. Be-cause the fossils are preserved as calcitic void-fill in a calcite matrix, individual specimenscannot be freed by conventional techniques—posing a challenge for morphological and,hence, systematic interpretation. Our responsehas been to develop a computer-based, ‘‘to-mographic’’ reconstruction technique to de-


FIGURE 4. Thrombolite-stromatolite pinnacle reefs at top of Huns platform, Witputs subbasin, southern Namibia.Reefs are formed of a core of coalesced thrombolite mounds, overgrown by a stromatolitic mantle. A, Overview ofmultiple pinnacle reefs formed at top of Huns platform. Reefs have been exhumed by erosion of overlying shales.B, Cross-section of representative pinnacle reef showing its foundation of stromatolite sheet facies and remnantsof overlying shales that have been eroded to expose reefs. A sequence boundary separates the stromatolite sheet-and pinnacle reef facies; thus, the pinnacle reefs formed during a time of increasing accommodation space withina Transgressive System Tract.


termine their true three-dimensional geome-try.

In brief, appropriate samples were selected,the position of each sample and its containedfossils calibrated with respect to an externalreference system, 25 m ground from the pol-ished sample surface, and the ground surfacephotographed using a digital camera. In thisstudy, the last two steps were iterated severalhundred times for samples taken from reefs atVioolsdrift (Witputs subbasin) and Donker-gange (Zaris subbasin). Contours of the fossilsin each cross-sectional image were obtainedusing a battery of image-processing tech-niques, including a binary thresholding pro-cedure. Ultimately, the three-dimensional sur-face of our models is determined by the shapeof these contours, placed adjacent to one an-other in the third dimension and connectedusing Delaunay triangulation to interpolatethe surface points between contour layers. Inthis process, segments in a given contour layerare connected to the vertices in an adjacentlayer, ultimately allowing the complete sur-face to be described by a set of triangular tiles.Multiple views of one complete and eight par-tial tomographic reconstructions are shown inFigure 9. Additional morphological data areprovided by six additional reconstructionsand more than 90 sets of serial sections notused in generating reconstructions. (A de-tailed description of the digital reconstructiontechnique used here is presented in Watters2000 and Watters and Grotzinger in press.)

All complete or nearly complete recon-structed specimens have a stem and an out-ward-flaring cup. The cup, a few millimetersto 2.5 cm in maximum dimension, has a broadcircular opening at the top with a lip thatcurves inward. The cup is perforated by six orseven holes of similar size and shape and hasa matching number of regularly arranged sidewalls. Cups taper to a shallow base fromwhich a hollow cylindrical stem extends.Some specimens contain an additional holenear the cup base. Stems are commonly longerthan maximum cup dimension and are openat both ends.

In constructing a model morphology fromreconstructed individuals, we took note ofvariation in the following parameters: (1) the

radial profile of the cup; (2) the aspect ratio ofthe cup (i.e., maximum diameter to totalheight); (3) the size and shape of the cup’s in-ner lip; (4) the relative size of the cup’s circularopening; (5) the curvature of cup walls (wallcurvature varies with height, in some casesflattening midway up the cup wall and in oth-ers remaining rounded); (6) the size, shape,and position of holes (e.g., oval or circular, lo-cated at the top, middle, or base of the cup;and with or without an inward-turning lip);(7) the thickness of walls; (8) the number ofsides and holes (i.e., six or seven); (9) thelength of the stem; (10) the size of the cup; and(11) the trajectory and radial profile of thestem.

In developing a mathematical model for themorphology of these fossils, only the featureslisted above were assumed. Individual tomo-graphic reconstructions were used to obtainvalues for all 11 parameters, enabling us togenerate individual models. Figures 10A and10C show sample cross-sections of tomo-graphic reconstructions that were used tomeasure profiles of the cup’s radius and in-ward lip, the cup’s aspect ratio, and the num-ber of holes and side-wall sections. The cor-responding mathematical models are shownin Figures 10B and 10D.

An exhaustive database of randomly orient-ed synthetic cross-sections was obtained fromthe models depicted in Figure 10. These syn-thetic cross-sections reproduce the vast major-ity of shapes observed in rocks, a subset ofwhich is depicted in Figure 11. Because we canaccount for most observed variation by a com-bination of random sections through the mor-phologies reconstructed in Figure 10 and sim-ple taphonomic alterations (e.g., physical dis-tortion) observed in our samples, we interpretmost of the variation seen in outcrop as rep-resenting a single species population. Figure12 illustrates the variation in cup size (maxi-mum diameter) and shape (aspect ratio 5maximum diameter/height) for a samplepopulation of reconstructed specimens from asingle rock sample.

Mineralization. Cloudina was only lightlymineralized, apparently by the precipitationof calcite in an organic template (Grant 1990).The same is true of the new taxon, Namacala-


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FIGURE 5. Representative photographs of microbialites and associated textures. A, Stromatolitic thrombolite col-umns forming a laterally continuous biostrome at the top of the Omkyk Member, Zaris subbasin. The core of eachcolumn has a predominantly thrombolitic texture, whereas margins have better-laminated stromatolitic thrombolitetexture. Columns are strongly elongate normal to the trend of the Omkyk carbonate ramp, indicating persistenthigh-velocity, wave-generated flows. Arrows point to goblet- and tube-shaped fossils that are abundant within theintercolumn fill sequences, but also occur within the thrombolitic texture. B, Well-preserved thrombolite texture ina pinnacle reef at the top of the Huns platform, Witputs subbasin. Reefal framework is created by dark-coloredmesoclots; these create shelter pores that are partially filled by light-colored geopetal sediment and are ultimatelyfilled by fibrous marine and later blocky burial cements. Inset shows fabric details: T 5 thrombolite mesoclot; GS5 geopetal sediment; SP 5 shelter porosity filled with cement. Coin is approximately 1.2 cm in diameter. Insetshows enlargement of region near center of photograph. C, Neptunian dike transecting thrombolite mound, Drie-doornvlagte bioherm, Zaris subbasin. Dike is infilled with botryoidal fibrous calcite cement, interpreted to havereplaced primary aragonite. Cements fill fractures within the reef, providing evidence for early lithification of thereef. Coin is approximately 1 cm in diameter. Inset shows enlargement of region below coin.

thus hermanastes, reported here. Like Cloudina,Namacalathus had thin walls (little more than100 m thick in best-preserved specimens) thatdeformed flexibly during compaction (Fig.13). Petrographically, most fossils are pre-served as casts of void-filling calcite, with rareevidence of thin, organic-rich wall structures(Fig. 13D–F). Such fossils might be interpretedas casts and molds of unmineralized organ-isms, were it not for fragmental remains thatwere almost certainly mineralized before frac-ture and dispersal. Original carbonate min-eralogy is difficult to determine because of ex-tensive dissolution or replacement, but com-mon overgrowth of shells by euhedral calcitecrystals supports the interpretation of a calciteprecursor for Namacalathus. Overgrowthsshow strong preference for the outer walls ofthese fossils; inner walls tend to be smooth(Fig. 13G). Overgrowths are not observed onthe simple tubes or on Cloudina.

Evidence of calcite mineralization is in-triguing because field and petrographic stud-ies show that aragonite was favored as thecommon marine cement in Nama environ-ments, accreting ooids, filling neptunian dikesthat cross-cut pinnacle reefs, growing amongthrombolitic mesoclots in reefs, and liningprimary void space in grainstones (Grant1990; Grotzinger unpublished data). This im-plies that Namacalathus, like Cloudina (Grant1990), had already evolved the physiologicalcapacity to regulate mineral precipitation.

Paleoecology. Namacalathus populations areclosely associated with thrombolitic textures,but they do not appear to have participated inreefal frame construction or sediment baf-

fling. The ‘‘stem and cup’’ morphology of theNama fossils suggests that they were benthicorganisms attached to, rather than nestling onor within, their substrate; however, most spec-imens are oriented horizontally and not in in-ferred life position (Fig. 12A).

Goblets may have been attached directly toreef surfaces or anchored by an unmineralizedbasal holdfast to locally accumulating sedi-ments. It is also possible, however, that Na-macalathus individuals were attached to sea-weeds or other soft-bodied macrobenthos thatcolonized accreting microbialites—much asliving stauromedusae of the scyphozoan Ha-lyclystus auricula attach to algae and sea grass-es by means of disks at the ends of their ex-umbrellar stalks (Ruppert and Barnes 1994;Knoll personal observation). Invertebrate epi-bionts of sea grasses are well known from Cre-taceous and Tertiary rocks, although theplants themselves are rarely preserved (Bra-sier 1975). More to the point, diverse andabundant macroscopic algae occur in terminalProterozoic successions of South China (Chenet al. 1994; Steiner 1994; Xiao et al. 1998), andthallose algae grow profusely on subtidal mi-crobialite surfaces today in places like the Ba-hama Banks (Feldmann and McKenzie 1998)and Shark Bay, Australia (Knoll and Grotzin-ger personal observations).

Whatever their mode of attachment, Nama-calathus far outnumbered Cloudina and othercalcified metazoans in Nama reef communi-ties (Fig. 12B,C).

Paleobiological Interpretation. Among meta-zoans, stalked radially symmetrical organ-isms are associated with sessile epibenthos.


FIGURE 6. Thin-section photomicrograph of thrombolite fabrics within a pinnacle reef at top of Huns carbonateplatform, Witputs subbasin. Thrombolite mesoclots (Throm) are encrusted with a fringe of fibrous calcite (FibrousMarine Cement) interpreted to have replaced original aragonite (note square crystal terminations). This was fol-lowed by deposition of a layer of crystal silt (Geopetal Sediment). Occlusion of remaining shelter porosity beganwith precipitation of an isopachous fringe of Dolomite Rim Cement, followed by a final infill of blocky calcite spar.Note that mesoclots are composed of distinct aggregates of smaller clots. These smallest fabric elements most likelyare formed by the early cementation of coccoid microorganisms. Note the accumulation of Geopetal Sediment (crys-tal silt), which smoothes out the rugged relief created by clots and encrusting Fibrous Marine Cements.


FIGURE 7. Proportional distribution of calcified fossilswithin broadly defined lithologic facies of the NamaGroup (A) and within microbial facies of the NamaGroup (B). Data were collected by recording every bedwithin a single measured section, and noting their faciesand fossil content. If any fossils were observed, the bedwas counted as ‘‘fossiliferous’’ and recorded. Resultsshow that fossils are very strongly correlated with mi-crobial facies in general, and with thrombolitic facies inparticular. Fossils are less abundant in packstones andgrainstones, which are composed principally of intra-clasts and peloids, and are rare in mudstones. The sim-plest interpretation of this distribution is that the fossilorganisms were benthic, with a strong preference formicrobially colonized substrates. In addition, the pref-erence for thrombolitic over stromatolitic substratessuggests further paleoecologic control on their distri-bution. See text for discussion.

Phylogenetically, goblet morphology might beexpected to occur among the Porifera, the Cni-daria, or early stem branches of the Bilateria(before the evolution of strongly bilateral sym-metry). Namacalathus could, thus, in principlebe viewed as the remains of simple asconidsponges with thin body walls and a relativelylarge spongocoel. Such an interpretationwould be consistent with reports of Ediacaranimpression (Gehling and Rigby 1996), bio-marker (McCaffrey et al. 1994), and spicule(Brasier et al. 1997) fossils that suggest abroader importance of this group in terminalProterozoic communities, and, more general-ly, with the phylogenetic expectation thatsponges should have appeared early (Nielsenet al. 1996). On the other hand, the walls of theNama fossils contain no evidence of pores orspicules. The former could, of course, havebeen obliterated during diagenesis, and thelatter is not conclusive insofar as not allsponges synthesize mineralized spicules. Theobservations are made to underscore the pointthat Nama goblets contain no features beyondoverall shape that suggest affinities withsponges. The symmetric distribution of side

holes is not easily reconciled with poriferanfunctional biology.

Chalice- or goblet-like morphologies can befound among the Cnidaria—for example, inhydrozoan polyps, the asexual scyphopolypsof some scyphozoans, and sessile scyphozoanstauromedusae in which exumbrellar surfacesare elongated into stalks (Ruppert and Barnes1994). The side holes characteristic of Nama-calathus fossils can be reconciled with a cni-darian bodyplan, especially if these holes areinterpreted as diagenetic features whose per-sistent occurrence and regular arrangementreflect underlying biological structure. In anumber of specimens, sediment within gob-lets is distinct in color or texture from exter-nally encompassing sediments. The boundarybetween sediment types is commonly just assharp across side holes as it is across pre-served walls (see Fig. 11), suggesting that wallmaterial once existed where holes are foundtoday. In contrast, mixing of internal and ex-ternal sediments suggests that the gap at thetop of the cups was open at time of burial.

Features potentially capable of governingthe positions of lateral holes include (unmi-neralized) tentacles at some distance from themouth (Brusca and Brusca 1990), planuloidbuds arising from surfaces of goblet-shapedscyphopolyps (e.g., Cassiopea [Lesh-Laurie andSuchy 1991: Fig. 59]), or radially distributedbiological structures such as septa and gonadswhose decay contributed to localized disso-lution of wall carbonate. Viewed in this way,Namacalathus can be interpreted in terms ofcnidarian structure and function: sessile or-ganisms with a mouth that opened into a largegastrovascular cavity.

Other interpretations are less attractive.Stem groups to the bilaterian (or ctenophore1 bilaterian [Ax 1989; Eernisse et al. 1992;Nielsen et al. 1996]) clade might have had gob-let-like morphologies, but the large internalcavity characteristic of Nama specimens isn’teasily incorporated into such a view.

Calcification occurs within the red, green,and, to a limited extent, brown algae, but noknown living or fossil forms (Wray 1977;Fritsch 1965; Graham and Wilcox 2000) aresimilar in morphology to the Nama fossils.Namacalathus does not display the internal cel-


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FIGURE 8. Assemblage of calcified Namacalathus fossils within reefal and related thrombolitic horizons within theNama Group. A, Namacalathus packstone illustrating a range of cross-sectional morphologies. B, Single large Na-macalathus, over 2 cm in diameter. Note overgrowth of euhedral calcite on shell wall (see Fig. 13G for photomicro-graph of similar fringe). C, Cross-sections of two Namacalathus fossils. The first defines a well-developed hexagon,while the second, which came to rest inside the first, is composed of three segments. D, Another specimen showinghexagonal geometry. E, Original goblet-shaped fossil described by Grotzinger et al. (1995). Note adjacent cup-shaped fossils, which also represent cross-sections of Namacalathus. All specimens in A–E can be explained as ran-dom sections through a single three-dimensional morphology (see text).

lular structure that is ubiquitous among cal-cified rhodophytes; nor is the goblet mor-phology of this fossil approximated by knownred algae. Calcifying green algae in the ordersCodiales and Dasycladales precipitate arago-nite intercellularly within thalli, so that pre-served skeletal carbonates include hollowmolds of the complex filaments that underpinthallus morphology; no such internal struc-ture marks the Nama fossils. Some Paleozoicdasyclads had a bulbous morphology, buttheir external profile reflects interlocking ele-ments that arise from a central stalk ratherthan a continuous sheet of cellular material.Namacalathus is unlikely to be an alga.

Still less attractive are comparisons withbroadly vase-shaped protists such as tintin-nids, allogromid foraminifera, and testateamoebae. Known living and fossil specieshave volumes as much as three orders of mag-nitude less than those of the Nama fossils (Leeet al. 1985).

In addition to comparisons with living or-ganisms, one can ask how Namacalathus com-pares with other fossils. The cuplike structureof N. hermanastes is at least broadly reminis-cent of the small, radially symmetrical im-pressions found abundantly in some Ediacar-an assemblages (e.g., Narbonne and Hofmann1987; Sokolov 1997). It calls to mind, as well,specimens of the problematic calcareous fossilKhatsaktia and rare corallimorphs from LowerCambrian carbonates in Siberia and elsewhere(Debrenne et al. 1990). The hexagonal sym-metry of many Namacalathus specimens fur-ther recalls the triradiate symmetry character-istic of some Ediacaran organisms (e.g., Tri-brachidium and Albumares [Fedonkin 1990])and the calcareous anabaritids, tubular fossilsfound in uppermost Proterozoic and LowerCambrian successions (Qian and Bengtson1989; Bengtson et al. 1990). The weakly calci-

fied Nama fossils do not, in general, appear tobe phylogenetic precursors of the diverse,skeletonized bilaterian animals found in Cam-brian and younger rocks.

Evidence of Further Diversity. Germs (1972a)described two species of Cloudina, C. reimkeaeand C. hartmannae. Namacalthus hermanastesadds to the diversity of calcified Nama ani-mals but does not exhaust it. Approximately10% of the forms observed on thromboliticsurfaces cannot be ascribed to Namacalathus orCloudina, and we believe that these representone or more additional, if poorly character-ized, taxa. Calcified Nama fossils certainly in-clude additional populations of simple tubes.These fossils, which commonly show a gentlecurvature, are typically 1–2 mm wide. Maxi-mum lengths are harder to estimate becausethe fossils are almost always broken, but seg-ments up to 3 cm long have been observed.Shell walls are very thin and appear to reflectcarbonate impregnation of an organic matrix.At least some of the tubes are closed at thebase (Fig. 14D), differentiating them from Na-macalathus stems. Smooth walls, as seen in pet-rographic thin-section, further differentiatethese tubes from Cloudina (see discussion, be-low). Monospecific tube assemblages occur inmicritic carbonates that accumulated on levelbottoms on the Nama platform.

Like other Nama fossils, the simple tubesare problematic. Functional considerationssuggest that their makers were benthic organ-isms that lived erect on the seafloor, perhapsfilter feeders analogous to if not necessarilyhomologous with modern fan or feather-dust-er polychaete (annelid) worms (Brusca andBrusca 1990). Calcareous polychaete tubes areknown from limestones as old as Ordovician(Steele-Petrovich and Bolton 1998), and un-mineralized polychaetes occur in the Burgess


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FIGURE 9. Tomographic reconstructions of the calcified fossil Namacalathus hermanastes. First described as havinga ‘‘goblet’’ shape (Grotzinger et al. 1995), the fossil has a well-defined stem that is attached to an outward flaringcup perforated by six or seven incurved holes, with an upper circular opening lined by an inward-turning lip. Partialspecimens A through H are shown in inferred growth position (top) and also viewed from above (bottom). Notebasal stem (or what is left of it) attached to a cup (A, C–G). The cup has six or seven holes, and these are apparentwith a regular symmetry in the majority of cases. Note also the single, circular opening (or part of it) in the inferredtops of each specimen. Complete specimen I is shown from several perspectives.

Shale and its Lower Cambrian counterparts(Conway Morris 1998).

Exceptionally preserved populations allowfresh observations of C. riemkeae, the smallerand lesser known of the two Cloudina speciesin Nama rocks. The highest concentrations ofthis taxon occur in association with a floodingsurface developed atop the Driedoornvlagtebioherm. Like the larger C. hartmannae, C. ri-emkeae has been reconstructed as a series ofstacked cones (see Grant 1990; Bengtson andZhao 1992); however, primary marine cementsthat line a continuous inner surface emphasizethat the ‘‘cones’’ are funnel-like flangesformed episodically during the growth of acylindrical tube. The outer part of each flangediverges from the tube wall, whereas the innerpart merges with the bases of earlier-formedflanges (Fig. 14A–C). Several workers (Germs1972a; Glaessner 1976; Conway Morris et al.1990) have noted points of structural similar-ity between Cloudina and tube-forming anne-lids such as the serpulid Merceriella (Fauvel1923). The morphologies illustrated in Figure14 also resemble the (unmineralized) tubes ofsome modern pogonophorans (Ivanov 1963),as well as diagenetically mineralized vesti-mentiferan worm tubes recovered from Cre-taceous vent deposits in California (Little et al.1999). (Pogonophorans and vestimentiferansare considered to nest phylogenetically withinthe Annelida [McHugh 1997].) Alternatively,C. riemkeae might be compared to the episod-ically accreting (unmineralized) sheaths ofbudding scyphozoan polyps (Werner 1967).Whatever its affinities to living organisms, C.riemkeae does resemble Saarina, organicallypreserved tubes from the Vendian of the Rus-sian platform that are comparably organized(Sokolov 1997). Some Early Cambrian anabar-itids also have well developed flanges (Bengt-son et al. 1990; Qian and Bengtson 1989).

The Nama Group thus contains at least five

taxa of calcified metazoans: C. hartmannae andC. riemkeae (Germs 1972a; Grant 1990), N. her-manastes, simple closed tubes, and the unchar-acterized forms in reef assemblages. Moretaxa may be recognized in continuing work;for example, fragmental remains in skeletalpackstones hint at additional diversity. In ab-solute terms, observed diversity is modest,but, then, only about a dozen species of ca-nonical Ediacaran fossils have been reportedfrom Nama sandstones (Richter 1955; Pflug1970a,b, 1972; Germs 1972b,c; Runnegar 1992;Narbonne et al. 1997).

Globally, the Ediacaran biota contains fewerthan 100 taxa (Fedonkin 1990; Runnegar1992). Additional species of calcified metazo-ans have been described from terminal Pro-terozoic beds in Brazil (Hahn and Pflug 1985),and small mineralized fossils (foraminifer-ans?) have been claimed to occur in correlativeunits in Uruguay (Gaucher and Sprechmann1999). Phosphatized tubes in the terminal Pro-terozoic Doushantuo Formation, China, mayor may not be of animal origin (Li et al. 1997).Regardless of uncertainties surrounding theUruguayan and Chinese structures, calcifiedfossils constitute at least five percent of knownEdiacaran diversity—not very different fromthe proportional representation of mineral-ized species in the Middle Cambrian BurgessShale (Conway Morris 1998). The confirmationthat a number of Proterozoic animals wereable to precipitate mineralized skeletons lendssupport to the hypothesis that physiologicalpathways integrated in skeletal biominerali-zation evolved early in the history of animals,if not earlier (Westbroek and Marin 1998). Inconsequence, the subsequent Cambrian diver-sification of well-skeletonized animals likelyreflects increasing levels of predation ratherthan physiological innovation per se (Bengt-son 1994).

Despite expanding a distinct taphonomic


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FIGURE 10. Mathematical models, based on 11 geometric parameters that capture the essential morphology of Na-macalathus hermanastes (B and D) and cross-sections of tomographic models upon which these are based and whichwere used to obtain parameter values (A and C). A (from model B in Fig. 9), Cross-sections, through center of cup,showing regular holes sliced horizontally (relative to growth position) (top), and vertically through growth axis(upper-middle); and wire-frame views of circular opening at inferred top (lower-middle) and of the cup in profile(bottom). B, Mathematical model informed by the images in A. C, The same cross-sections and wire-frame diagramsfor Model I in Figure 9. D, Mathematical model based upon the images in C. Specimen in A-B was collected froma locality in the Zaris subbasin; the specimen in C-D is from the Witputs subbasin. The two models are primarilydistinguished by (1) the number of holes (six in the case of A-B, seven in the case of C-D); (2) the radial profile ofthe cup (reaching maximum diameter near the top of the cup for C-D and near the middle in the case of A-B); (3)the wall curvature (nearly flat at the cup’s midheight in C-D while still slightly round in A-B); and (4) the size ofthe inward-turning lip that lines the cup’s upper circular opening.

window on early animal evolution, calcifiedNama fossils do not reveal Proterozoic rootsfor the skeletonized bilaterians that radiatedacross carbonate platforms in the PaleozoicEra. On the contrary, like the Ediacaran as-semblages preserved in contemporaneous sil-iciclastic rocks, calcified Nama assemblagesappear to document a distinctively Protero-zoic fauna.

Systematic Paleontology

Genus Namacalathus n. gen.

Type Species. Namacalathus hermanastes n.sp.

Diagnosis. Centimeter-scale, chalice- orgoblet-shaped fossils consisting of a calcare-ous wall less than 1 mm thick; a basal stemopen at either end connects to a broadly sphe-roidal cup perforated by six or seven holeswith slightly incurved margins distributedregularly around the cup periphery and sep-arated by lateral walls; the cup contains an up-per circular opening lined by an incurved lip.

Etymology. From the Nama Group and theGreek kalathos, denoting a lily- or vase-shapedbasket with a narrow base or, in latinizedform, a wine goblet.

Namacalathus hermanastes n. sp. (Figs. 8–11)

Diagnosis. A species of Namacalathus dis-tinguished by cups 2–25 mm in maximum di-mension, with aspect ratio (maximum cup di-ameter/cup height) of 0.8–1.5.

Description. Goblet-shaped calcified fos-sils; walls flexible, ca. 100 m thick (originalwall dimensions commonly obscured by dia-genetic cement growth); basal cylindricalstem, hollow and open at both ends, 1–2 mm

wide and up to 30 mm long, attached to sphe-roidal cup; cup with maximum dimension 2–25 mm, broad circular opening at top with in-ward-curving lip, perforated by six to sevenslightly incurved holes of similar size andshape distributed regularly about cup periph-ery. Specimens preserved principally by void-filling calcite, with rare preservation of pri-mary, organic-rich wall.

Etymology. From the Greek, herma, mean-ing sunken rock or reef, and nastes, meaninginhabitant.

Material. More than 1000 specimens frombiohermal carbonates of the Kuibus andSchwarzrand Subgroups, terminal Proterozo-ic Nama Group, Namibia.

Type Specimen. Our understanding of Na-macalathus hermanastes derives principallyfrom virtual fossils modeled from seriallyground surfaces. Systematic practice, howev-er, requires that real fossils be designated astypes. Accordingly, the specimen illustrated inthe lower right corner of Figure 8E is desig-nated as holotype for the species. The typespecimen is to be reposited in the paleonto-logical collections of the Museum of the Geo-logical Survey of Namibia, as collection No.F314. Representative specimens are alsohoused in the Paleobotanical Collections ofthe Harvard University Herbaria (HUHPCNo. 62989).

Type Locality. Reefal biostrome developedat the top of the Omkyk Member, Zaris For-mation, Kuibis Subgroup, exposed along theZebra River near the boundary between Don-kergange and Zebra River farms, south ofBullsport, Namibia.


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FIGURE 11. Diverse cross-sections of Namacalathus hermanastes might be interpreted as different taxa (upper pho-tos); however, virtual slices through the mathematical models shown in Figure 10 produce a diverse array of syn-thetic cross-sections (lower images) that can be used to estimate species richness. Most of the diverse cross-sectionsof calcified fossils in Nama rocks can be explained in terms of a single morphology with some minor variations.The cross-sections shown in the upper photos were obtained from 40 specimens, only two of which were used togenerate tomographic reconstructions (Fig. 9E,F).

Thrombolite-Stromatolite-Metazoan Reefs

Thrombolite/invertebrate associations charac-terize Early Cambrian and some younger Pa-leozoic reefs (Soja 1994; Kruse et al. 1995; Ri-ding and Zhuravlev 1995). Nama biohermsshow that the close ecological relationship be-tween invertebrate and microbial populationswas established before the end of the Prote-rozoic Eon, prompting questions about the or-igins of this distinctive reef association.

Microbial mat populations have contributedto stromatolitic reef accretion since the Arche-an (Hofmann et al. 1999; Grotzinger and Knoll1999), but bioherms characterized by throm-bolitic textures are rare before the latest Pro-terozoic Eon (Aitken 1967). Partly on the basisof this stratigraphic distribution, Walter andHeys (1984) interpreted thrombolitic clots asordinary stromatolitic laminae disrupted byburrowing. Kennard and James (1986), how-ever, subsequently demonstrated that thom-bolitic textures are primary, confirming theoriginal inference of Aitken (1967). Kennardand James argued specifically that the diag-nostic clotted fabric of thrombolites is gener-ated by in-situ calcification of coccoid-domi-nated microbial communities. Coccoid cya-nobacteria form rough, irregular mats thatmay, upon calcification, yield a clotted texturewith only crude lamination (Hofmann 1975),and recent work supports this interpretationfor some Neoproterozoic thrombolites (Turn-er et al. 1997). Such an interpretation, however,leaves open an important question. Mats builtby coccoidal cyanobacteria are common in Pa-leo- and Mesoproterozoic tidal flats (Golubicand Hofmann 1976; Sergeev et al. 1995), butthrombolitic fabrics are essentially absent.How can this discrepancy be explained?

Feldmann and McKenzie (1998) have re-cently proposed that ancient thrombolites re-flect the participation of higher algae in mat-building consortia. This inference is based on

actualistic observations on the Bahama Banks,where cyanobacteria-dominated mat commu-nities accrete well-developed laminae in inter-tidal environments, whereas eukaryotic com-munities produce poorly laminated, throm-bolitic textures in open-marine subtidal envi-ronments. Coccoid spores thought to beproduced by dasycladalean green algae play aparticularly important role in fabric genera-tion.

The observations of Feldmann and Mc-Kenzie (1998) are consistent with paleoenvi-ronmental research that locates Paleozoicthrombolites in subtidal environments (Ait-ken 1967; Bova and Read 1987; Pratt and James1986). They are also congruent with biostrati-graphic data that bracket the expansion ofthrombolites between the initial diversifica-tion of green algae and the appearance ofwell-skeletonized dasyclads (Wray 1977; Knoll1992).

Observations of Nama thrombolites are alsoconsistent with predictions of the Feldmannand McKenzie (1998) model. Nama thrombol-ites formed in open-marine, wave-swept en-vironments, often associated with grainstoneshoals. Well-laminated stromatolites are rarein the Nama Group, but so are intertidal fa-cies. The best-developed Nama stromatolitesform columns within a sheetlike biostromalunit at the top of the Omkyk Member (Fig.3B,C)—a unit interpreted to have formed in avery shallow subtidal environment. Signifi-cantly, thrombolitic textures are most abun-dant and best developed in pinnacle reefs atthe top of the Huns platform and at Driedo-ornvlagte. These reefs are composed of largemounds, which are rarely elongate; whereelongation is developed, it is defined by a lowlength-to-width ratio, consistent with low cur-rent velocities in deeper-water subtidal set-tings. Thrombolite textures developed in thisenvironment are similar to those found in Pa-


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FIGURE 12. Statistics on the orientation, diversity, and geometry of calcified fossils in the Nama Group. A, Per-centage of Namacalathus fossils in three categories of orientation for specimens within a single sample from theWitputs subbasin. ‘‘Vertical orientation’’ describes fossils with a cross-section parallel to bedding that show the sixor seven regular holes at the cup’s midsection (as in Fig. 8C,D). This orientation spans approximately 228, measuredfor the angle formed by the cup’s symmetry axis and the bedding plane, at the cup’s geometric center. ‘‘Horizontalorientation’’ describes specimens with cross-section parallel to bedding, simultaneously showing the cup’s uppercircular opening and the basal hole to which the stem attaches. This orientation spans an angle of approximately258, measured in the same way. ‘‘Oblique orientation’’ describes fossils that fit neither of the above categories; thesespan an angle of 438. It follows that if there were no preferred orientation, 28% of fossils would have a verticalorientation, 24% horizontal, and 38% oblique. Instead, we find that the vast majority (69%) have a horizontal ori-entation. B, Percentage of fossils by type found in a sample from the Zaris subbasin. C, Percentage of fossils by typefound in a sample from the Witputs subbasin. D, Size distribution of Namacalathus cups within a sample from theWitputs subbasin (size 5 average of cup diameter and height). Mean size 5 0.61 cm. E, Distribution in aspect ratio(maximum cup diameter to cup height) from the same sample. Mean aspect ratio 5 1.18.

leozoic rocks (compare Fig. 5B with Fig. 2B ofKennard and James 1986).

As noted above, Namacalathus and otherNama metazoans lived in specific associationwith thrombolitic substrates, forming bothreefs and level-bottom accumulations. Despitetheir greater abundance in reefs, these ani-mals did not participate in any substantialway in reef building, either as framebuildersor sediment bafflers. With this in mind, it isstraightforward to visualize a paleoecologicreconstruction in which the calcified organ-isms were loosely attached to the accretingthrombolitic substrate, much as calcispongesand green algae are in modern Bahamianthrombolites. Alternatively, Namacalathus mayhave been an epibiont attached to seaweeds orsoft-bodied animals that colonized the throm-bolitic substrate. Upon death or dislocation ofthe algal components, calcified organismswould have collapsed to the seafloor as detri-tal particles to be swept into the depressionsbetween thrombolite columns and mounds, oroccasionally to be trapped in random posi-tions within accreting mats. This interpreta-tion also provides a mechanistic basis for un-derstanding the locally great abundance of de-tached, horizontally oriented fossils in thin (afew cm) beds of thrombolitic laminites thatform spatially extensive sheets within deepersubtidal facies. Before the radiation of meta-zoan macrograzers, broad expanses of the ter-minal Proterozoic seafloor may have beencovered by microbial-algal carpets, with cal-cified metazoans attached to a (tiered?) algallawn. Animals assumed a constructional role

in bioherm accretion only with the evolutionof heavily skeletonized, attached epibenthos.

Conclusions

Nama bioherms document the emergence ofa distinctive reef association. Animals thatlived in the reef environment include some ofthe first organisms able to form skeletons bythe enzymatic precipitation of CaCO3 on anorganic template. In important ecological andphysiological respects, then, Nama biohermsand the fossils they contain foreshadow Paleo-zoic biology. At the same time, however, Na-macalathus hermanastes and other calcifiedNama fossils are morphologically, and per-haps phylogenetically, distinct from Paleozoicskeleton formers. At present the significanceof these similarities and differences is notcompletely known, but continuing study ofthe vast and beautifully exposed carbonate ac-cumulations of the Nama Group promises toclarify our understanding of terminal Prote-rozoic biology and geobiology.

Acknowledgments

We gratefully acknowledge the GeologicalSurvey of Namibia for providing a field vehi-cle and logistical support. W. Hegenberger, C.Hoffman, and R. Swart are thanked for helpin providing an introduction to the geology ofNamibia, and for providing helpful guidanceand advice. R. Swart and NAMCOR arethanked for providing the Landsat TM imagein Figure 3A. Special thanks go to R. and M.Field and to R. Magson for access to theirfarms, Zebra River and Donkergange, and for


FIGURE 13. Taphonomic aspects of Namacalathus. Most of the photographs indicate the relatively flexible nature ofthe wall, suggesting only light mineralization at time of death. A–C, Deformational features include folds and wrin-kles in walls and stems; squashed and flattened cups; cups without stems, presumably because of postmortembreakage; wall ruptures, holes, and impressions caused by adjacent objects; and disintegration of walls. D, Cross-section through a goblet-shaped fossil, illustrating replacement by void-filling calcite. Arrow points to section en-larged in Figure 13E. E, Void-filling calcite indicates decay/dissolution of primary wall. Note crystal size enlarge-ment toward center of fossil wall. F, Rare preservation of primary shell structure (arrow) showing calcite-impreg-nated, dark organic matrix. In most specimens, this material has decayed/dissolved, with secondary void spacefilled by void-filling calcite (as in Fig. 13D,E). G, Wall of cup-shaped fossil. Note that the inner part of wall is smooth(white arrow), while the outer part is overgrown by a relatively thick layer of replacive calcite. Note crystal ter-minations extending out into and replacing matrix. Black arrows point to replaced intraclasts in original matrix.This suggests that nucleation and orientation of mineral overgrowths were controlled by primary crystal orientationwithin the fossil wall.


FIGURE 14. Other calcified fossils in the Nama Group. A–C, Thin-section photomicrographs of Cloudina riemkeaeshowing unusually good preservation of shell structure. In all three cases, note the straight inner wall, highlightedbest in B and C, but also visible in A. A and B show external wall structure indicating en echelon stacking of flanges,which have largely been recrystallized in C. D, Tube-shaped fossils constituting a distinct population of shapes,distinct from both Cloudina and Namacalathus. E, Tomographic reconstruction of a tube-shaped fossil, showing itsclosed base and flared top.

their hospitality. A.-C. Auclair helped greatlyin acquiring contour and image data. A. John-ston guided us through the cnidarian collec-tion of the Museum of Comparative Zoologyat Harvard. Support for this research was pro-vided by National Science Foundation grantEAR-9628257 and the National Aeronauticsand Space Administration Astrobiology Insti-

tute. Finally, S. Bengtson and an anonymousreviewer are thanked for their thoughtful re-views, which helped improve the manuscript.

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