View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002(Fischer, 1965; Pratt, 1982; Grotzinger, 1990;Riding, 2000). Although not denying the existenceof a Proterozoic decline, Pratt (1982) argued thatit is in part a chimaera: Phanerozoic stromatolitesare widespread but tend to be diluted by the sheerdiversity of reef-building metazoans (Pratt, 1982;Riding, 2000). Unlike their Proterozoiccounterparts, the younger stromatolites havetherefore not been the focus of taxonomic andstratigraphic studies. Walter and Heys (1985),however, found no correlation between stromatolitediversity and number of authors publishing on therespective time interval in the Proterozoic.Nevertheless, it is likely that the stromatolitediversity curves from the Proterozoic reflect notso much real changes in diversity as changes inthe relative abundance of stromatolites. The lowreported diversities of Phanerozoic stromatolites(cf. Awramik and Sprinkle, 1999) may partly reflectthat fact that stromatolite taxonomy is largely apre-Phanerozoic endeavor. Modern stromatoliteseven have a morphological variability similar tothat of Proterozoic ones (Bauld et al., 1992; Walteret al., 1992a), but their more complex fabric andprominent protist components make them pooranalogues of the Proterozoic forms (Riding, 2000).As a measure of possible effects of disruptiveactivities by metazoans, stromatolite abundance inparticular environments may be more significantthan overall “taxonomic” diversity. Walter and Heys(1985) indeed included a measure of abundance,corresponding to the number of basins in which acertain taxon was recorded from a certainstratigraphic interval. Although this gives someinformation on how geographically widespread ataxon is, as a measure of the total relative abundance,the “abundance” as represented in Walter and Heys’s(1985) curves is flawed, as in fact it incorporatesdiversity. The diversity and abundance curves arealmost indistinguishable, and this may be becausethey basically measure the same thing. This “thing”is probably closer to abundance than to diversity.Thus the apparent decline of “taxonomic”diversity in the Proterozoic may be rather an effectof decreasing abundance of well-preservedstromatolites. As such, it may actually be a moredirect measure than true taxonomic diversity offactors that prevent the growth of stromatolites.Measures of stromatolite numbers per unit of rock(“density” of Grotzinger, 1990) or of areal coverof stromatolites in different environments throughtime would be even more appropriate, but thecollection of such quantitative data would be amomentous task.THE CAUSAL CONNECTIONBETWEEN METAZOAN ASCENTAND STROMATOLITE DECLINEDeclining stromatolite diversity in thePhanerozoic had been noted (Fischer, 1965; Cloudand Semikhatov, 1969), and Garrett (1970)proposed that this was due to non-competitiverestriction from grazing and burrowing animals.Awramik (1971) noted a distinct decline in thediversity of columnar stromatolites already in thelate Proterozoic, from a peak in the Upper Riphean(950–675 Ma), and associated this with theevolutionary appearance of bottom deposit feedersand burrowing metazoans in the subtidalenvironment. Data on Proterozoic diversities havesubsequently been improved by various efforts, inparticular those of Walter and Heys (1985), whoincluded also non-columnar stromatolites andcorrected the diversity values for the relativelengths of the stratigraphic intervals and the relativeintensity of study. Their data confirm the patternof late Proterozoic decline, but suggest thatdiversity peaked in the Middle Riphean (1350–1050 Ma), earlier than in Awramik’s 1971 curvebut consistent with his later published curve(Awramik and Sprinkle, 1999) (see Fig. 1).Schubert and Bottjer (1992) noted a briefresurgence of stromatolites in the Early Triassicand attributed this to the dearth of benthic grazersin the aftermath of the end-Permian marineextinction events. A similar effect may be presentfollowing the Late Devonian (Frasnian–Famennian) mass extinction (Schubert and Bottjer,1992; Whalen et al., 1998).Grotzinger (1990) stressed that the data ofWalter and Heys (1985) show the decline of296