W

Depth Salinity

Churan -

Bachyk

Bachyk ijranL

Taas thrombolite-Girvanella Epiphyton-archaeocyath Girvanella HZ& Renalcis-archaeocyath

• siliceous sponge archaeocyath

Zhurinskiy I Achchagyy. . .

Churan -

Taas

Zhurinskiy I Achchagyy. . .

Figure 12.3 Lower Cambrian (lower Atda-banian) profile along the present middle courses of the Lena River (Siberia). Shown are peritidal Girvanella reefs (Churan); shallow subtidal Renalcis reefs with rare and nondiverse archaeocyaths and hyolithomorphs (Zhurinskiy Mys); shallow subtidal Renalcis-archaeocy-athan reefs with abundant and diverse archaeo-cyaths (Oi Muran); shallow subtidal Epiphyton

Figure 12.3 Lower Cambrian (lower Atda-banian) profile along the present middle courses of the Lena River (Siberia). Shown are peritidal Girvanella reefs (Churan); shallow subtidal Renalcis reefs with rare and nondiverse archaeocyaths and hyolithomorphs (Zhurinskiy Mys); shallow subtidal Renalcis-archaeocy-athan reefs with abundant and diverse archaeo-cyaths (Oi Muran); shallow subtidal Epiphyton reefs with rare but diverse cryptic archaeocyaths (Bachyk); subtidal (below fair-weather wave base) siliceous sponge-archaeocyathan mud mound with rare but diverse, mainly cryptic, archaeocyaths (Achchagyy Kyyry Taas); subtidal (below fair-weather wave base) burrowed Girvanella-bearing thrombolite reefs (Achchagyy Tuoydakh). The distance between Churan and Achchagyy Tuoydach is 120 km.

Figure 12.4 Reconstruction of Early Cambrian (late Tommotian) reef of the Siberian Platform. Foreground: Cambrocyathellus (Ar-chaeocyathida) thickets (1) on muddy substrate with Nochoroicyathus (Ajacicyathida) sticks (2), hyolithomorph (8) and orthotheci-morph (7) hyoliths, and Chondrites burrows (13). Middle ground: Okulitchicyathus (3)-

Sakhacyathus (Archaeocyathida) (4)-Renalcis (10) boundstone on lithified substrate with cryptic Archaeolynthus (Monocyathida) (5) and Coscinocyathus (Capsulocyathida) (6) and mi-croborings (14). Background: Epiphyton framework (11) with cavity (9) inhabited by cryptic Hydroconus (coralomorph) (9) and micro-burrows (12).

Figure 12.4 Reconstruction of Early Cambrian (late Tommotian) reef of the Siberian Platform. Foreground: Cambrocyathellus (Ar-chaeocyathida) thickets (1) on muddy substrate with Nochoroicyathus (Ajacicyathida) sticks (2), hyolithomorph (8) and orthotheci-morph (7) hyoliths, and Chondrites burrows (13). Middle ground: Okulitchicyathus (3)-

Sakhacyathus (Archaeocyathida) (4)-Renalcis (10) boundstone on lithified substrate with cryptic Archaeolynthus (Monocyathida) (5) and Coscinocyathus (Capsulocyathida) (6) and mi-croborings (14). Background: Epiphyton framework (11) with cavity (9) inhabited by cryptic Hydroconus (coralomorph) (9) and micro-burrows (12).

resilient to clastic input and increased nutrient supply. The ability of Epiphyton and related microbial organisms to inhabit dim cavities argues that they may have been able to withstand sporadic turbidity. By contrast, Middle and Late Cambrian thrombolites and stromatolites, passively constructed by benthic biofilms or mats, seem to have flourished where terrigenous influence was far removed or temporarily suppressed.

Ecological Reconstruction

Early Cambrian reefs exhibit evidence for remarkably complex ecological interactions (figure 12.4). The main guilds developed with the diversification of archaeocyaths and accessory frame-building and dwelling elements, although the constructing guild was still dominated by microbes. Most described examples are patch reefs and do not exhibit a distinct lateral zonation. The areal distribution of radiocyaths in the Toyonian of South Australia (Kruse 1991) suggests that some framework elements responded to directed phenomena, presumably turbulence. Because many archaeocyaths were able to secrete extensive stabilizing exotheca, their relative scarcity on the upper surfaces of microbial reefs is possibly due to competition from the microbes that produced Renalcis.

Cavities hosted a diverse cryptic community (Zhuravlev and Wood 1995). Com petition for space in cavities is demonstrated by chains of individual archaeocyaths or multiple overgrowths, probably because adjacent substrates were occupied by soft-bodied organisms or microbial biofilms that prevented settling. Competition is also seen by distorted archaeocyaths, where the growth of one species has been hampered by another, or even by soft-bodied organisms that have left no other record (Zhu-ravlev and Wood 1995). Mortality was high among archaeocyaths, judging from the number of juvenile cups seen in most frameworks. Such a mortality may be attributed to competitive interactions because most of the juveniles are sealed by their relatives and other organisms. Bioerosion, although locally present, was insignificant as a destroyer of skeletons and as a producer of fine-grained sediment.

A vertical zonation is not obvious in Early Cambrian reefs; however, different ar-chaeocyathan communities seem to be confined to different stages of an individual reef development (Kruse et al. 1995; Riding and Zhuravlev 1995). Nevertheless, these reefs were undoubtedly capable of rapid accretion, given an intuitive judgment of the growth rates of archaeocyaths and rates of microbe calcification (Pratt 1984). Synoptic relief was generally not great, on the order of a meter or less. Early Cambrian reefs became firmly cemented with an unusually high amount of calcite and at times arago-nite, reflecting extensive cavity development.

The complex ecological interactions seen in Early Cambrian reefs vanished by the start of the Middle Cambrian. The Middle and Late Cambrian saw some reefs with an-thaspidellid and axinellid demosponges, but otherwise metazoan frame builders were absent, and the dwelling fauna depauperate by comparison.

Trophic Reconstruction

Only a rather generalized trophic analysis for Early Cambrian reefs is possible (Wood et al. 1993; Kruse et al. 1995; Zhuravlev and Wood 1995; Burzin et al., this volume: figure 10.2), as it is limited by the uncertain affinity of many of the faunal elements. The water column hosted primary producers in the form of acritarchs and bacterioplankton. Benthic microbial communities comprised biofilms of photosyn-thetic cyanobacterial and biodegrading bacteria. Renalcis and Epiphyton were formed by microbial aggregates, but many of these do not exhibit phototaxis; it is possible that if individual cells were photosynthetic, their light requirements were low enough for them to inhabit near-surface cavities. Archaeocyaths and other sponges were epi-faunal filter feeders: most preferred living in cavities or on muddy substrates, a lifestyle perhaps dictated by competition from microbial biofilms. Tabulate-like corals may have been microcarnivores or suspension feeders that captured coarser organic aggregates. Brachiopods and possibly hydroconozoans, chancelloriids, cribricyaths, hyolithomorphs, some orthothecimorphs, and stenothecoids were suspension feeders. Biodegrading and sulfate-reducing bacteria resided in the sediment. Organisms that relied on sedimented detritus and its bacteria include deposit-feeding infaunal worms and epifaunal trilobites and possibly some of the remaining, minor skeletal components, most of which seem to have preferred off-reef areas. There may have been meiofaunal microorganisms, but none has been preserved. Grazers were uncommon.

What is evident from the Early Cambrian reef biota is that almost all the major trophic groups were occupied, as was noted by Kobluk and James (1979). By contrast, Middle and Late Cambrian reefs were ecologically simplified: the benthic community consisted of the microbes, with locally important filter-feeding siliceous sponges and echinoderms, and deposit-feeding worms and trilobites.

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