Rivista Italiana di Paleontologia e Stratigrafia volume lul numero 3 pagrne 333-340 Dicembre 1996 PATTERN OF EVOLUTION OF CAMBRIAN BENTHIC COMMUNITIES: ENVIRONMENTS OF CARBONATE SEDIMENTATION A.YU, ZHURAVLEV* & F. DEBRENNE*X Key-uords: Cambrian, Benthic Communities, Carbonate Sedi- makyt-Daldinian (in toto, conformably to the decision mentation. accepted by the IGC of Kyoto in !992, or only its up- Riassunto. Vengono delineati alcuni aspetti delle comunità ben- per part, according to most of the Russian authors, toniche del Cambriano, caratterizzate da sedimentazione carbonatica. Tommotian, Atdabanian, Botoman, Toyonian, Amgan, La relativa ricchezza di queste comunità del Cambriano inferiore Mayan), the Laurentian time scale for the Late Cam- contrasta con le controparti impoverite tipiche del Cambriano Medio brian (Dresbachian, Franconian, Trempealeauan), becau- e dell'inizio del Cambriano Superiore. Tuttavia, si osserva uno stabi- se le aumento della diversità nelle comunità durante il Cambriano. Vie- of their respective richness and well-studied paleonto- ne dimostrato che 1o spostamento delle comunità è un fenomeno co- logical remains. mune e si ipotizza che intere comunità porrebbero migrare dalle loro aree di origine verso condizioni più favorevoli se la comunità prece- dente sof{re di crisi. Evaporite basins. basins, Abstract. Fearures of Cambrian benthic communities that in- Evaporite yielding carbonates and evapori- habited areas characterised by carbonate sedimentation are outlined. tes, were typified by low clastic input and high rate of Relative richness of these Early Cambrian communities contrasts in evaporation. Their coastlines are characterised by chains comparison with their impoverished Middle and Early Late Cam- of islands which shelter hypersaline lagoons with redu- brian counterpans. Nevenheless, a steady increase in the diversity of ced tidal ranges where microbial mats are formed. They communities is observed during the Cambrian. It is shown that di- splacement of communities is a cornmon phenomenon. It is sugge- produced extensive stromatolites; good examples occur sted that entire communities might migrate from their area of origin in the Toyonian Angara Formations of the Siberian into more favourable conditions if the former community suffered a Platform where stratiform and columnar stromatolites crisis. formed low but very wide buildups, up to several kilo- metres in length flanked by ooidal grainstones along lntroduction. their periphery (Korolyuk, 1968). This stromatolite community did not change appreciably during the This paper deals with the diversity of Cambrian Cambrian. Flowever, various molluscs (rostroconchs and communities. Two principai settings, differentiated by possible chiton{ike grazers) occupied barrier complexes the type of sedimentation, existed in the Cambrian: tho- formed under generally higher salinities during the la- se where carbonate sedimentation dominated and those test Late Cambrian in Australia (Druce et al., 1982). which 'were mainly characterized by siliciclastic deposi- tion. The background controls of sedimentation were in Peritidal carbonate environments. general the climate and the size of area available for de- nudation. The Siberian Platform through the entire Peritidal carbonate environments include oolite Cambrian, Laurentia since the middle Early Cambrian shoals, carbonate sand shoals and beaches, and intertidal and Australia during the Early Cambrian exemplified to subtidal flat settings. Since Atdabanian time Opbio- carbonate - dominated habitats which are considered in morpha-Iike trace producers as Awloplrycus Fenton & the present paper. Fenton, L939, occupied mobile lime muds in shoal agita- The various communities are characterised sea- ted back-reef conditions. ward from the inner part of basin. The Siberian time Ophiomorpha-type burrowings represent the inno- scale is chosen for the Early to Middie Cambrian: Ne- vative behaviour due to the ability to produce a pelleted 't Paiaeontological Institute, Russian Academy of Sciences, Profsoyuznaya 123, Moscow 117647, Russia. 'r'r Laboratoire de Paléontologie, URA 12 CNRS, 8 rue Buffon, 75005 Paris, France. 334 A.Yu. Zhuraoleo & F. Debrenne burrow lining, which prevents burrow collapse in sub- zoic, differ greatly from them in ecological aspect strates of relatively low cohesive strength. The Auloplry- fVood et al., 1.992, 1993). They were essentially micro- czs-community persisted through the entire Cambrian: bially mediated mounds in a broad sense, full of lime Atdabanian Nokhoroy lJnit, Botoman Kutorgina and mud and dominated by solitary or low modular soft- Toyonian Keteme Formations of the Siberian Platform substrate dwellers. The analysis of trophic strucrure has and Botoman Poleta Formation and Shady Dolomite of shown an abundance of filter- and suspension-feeders Laurentia (Balsam, 1974; Astashkin, 1985; Droser & and carnivores among the biota (Wood et a1., 1993; Kru- Bottjer,1988). se et al., 1995) (Fig. 1.1). Deposit-feeders and grazers were relatively scarce. The food web was short and ba- sed mainly on bacteria rather than on algae. The low Shallow carbonate seas. diversity of reefal communities (an average of 50-80 spe- Shallow carbonate seas include several carbonate cies per several dozens of km' of outcrops studied) environments, all of which lie at or below fair-weather (Wood et aI., 1,993; Kruse er aL, 1995) reflects a low wave base. A high range of communities inhabits this degree of niche splitting by the Cambrian reefal biota in zone, including reefal, level-botmm and hardground comparison with Palaeozoic (60-400 species) and mo- communities. dern (over 1,200 species) reefal biotas occupying compa- rable space (Fagerstrom, 1987). Contrary to the com- a) Reefal communities. The earliest Cambrian ani- monly held view that the reef fall coincided with the mal-autotroph reefs are calcified cyanobacterial bioherms quasi-extinction of archaeocyaths at the end of the Early in the early Nemakit-Daldynian on the Siberian Cambrian, Middle and early Late Cambrian buildups, Platform and Oman. They locally enclose Anabarites although being purely calcimicrobial, were the largest and Cloudina thickets (Luchinina, 1985; Mattes & framework reefs of the Cambrian (Astashkin et al., Conway Morris, 1990). By the late Nemakit-Daldynian, 1984). A few remaining archaeocyaths, coraiomorphs, bioherms of calcified cyanobacteria were widespread in and problematic calcified sponges, as well as probable the Siberian Platform, Aitay-Sayan Foldbelt and Mongo- calcified algae, are found only in thrombolites. Trilobi- lía (Zadorozhnaya, 1974; Drozdova, 1980; Luchinina, tes were almost the only mgfazoan element of the Mid- 1985). The earliest Tommotian archaeocyath-Renalcis dle Cambrian reefs but both their diversity and abun- reef described from the Siberian Platform (Riding & dance were low (Sukhov & Pegel', 1986). Pure calcimi- Zhuravlev, 1.995), however, was more complex as it con- crobial reefs (dendrolites) lack any meÍ.azoan or algal tained the entire set of reef guilds: constructors (encru- components. The earliest meÍ.azoan reefs to appear aÍfer sting modular archaeocyathidt, binders (monocyathids the demise of archaeocyathan buildups are described and Renalci), bafflers (ajacicyathids, spicular sponges, from the upper Middle Cambrian of the Mila Forma- and some orthothecimorph hyoliths), possible destroyers tion, northern Iran (Hamdi et al., 1995). This reef was (borers), and diverse dwellers. buiit by a branching demosponge together with minor Typical Early Cambrian reefs were reef mounds probable bacterial sheaths. constructed by calcimicrobes and iess by filter-feeding A continuous decline of calcimicrobial diversity metazoans, i.e, archaeocyaths, radiocyaths, and coraio- could have been a direct consequence of elimination of morphs, proliferating from the Tommotian to Toyonian both potential tiny grazers and reef-building metazoans within the palaeoequatorial belt on carbonate platforms, during the Early Cambrian extinction events (Zhura- under normal salinity, in mesotrophic-eutrophic shallow v|ev, 1996). The Middle Cambrian dendrolites were re- 'waters conditions (Vood et aI., 1993). These reefs being placed by thrombolites and stromatolites during the developed on soft mud-dominated substrates, largely, at Late Cambrian (Aitken, 1967;Fedorov et al., 1986; Ken- or below fair-weather wave base act as a hard bottom nard & James, 1986). The availability of space was pro- substratum for relatively rare macroborers (Trypanites- bably the only reason for a spread of stromatolites. Any type) and abundant cryptobionts (Kobluk et al., 1978; attempt to explain stromatoiite decline/ proliferation by Kobluk & James, 1.979; Zhuravlev & \íood, 1995). Me- the development of favourable/unfavourable conditions razoans prefered the soft-bottom reefal periphery or oc- for their grazers fails (Zhuravlev, 1996) because even the cupied reefal cavities. Early Cambrian reefs, although si- extreme conditions, such as restricted lagoons, sabkhas milar sedimentologically to those of the later Phanero- and thermal springs do not exclude grazer pressure; mo- Fio 1 Trophic webs in principal Early Cambrian benthic communities: 1 - reefal archaeocyath-coralomorph-hyolith conmunity (modified after Kruse et al., 7995); 2 - level-bottom open marine priapulid-non-trilobite anhropod-spicular sponge community (modified after Conway Morris, 1986); 3 - level-bottom dysaerobic trilobiteJingulate community. Cambrian B enth i c c o mmun