Pattern of Evolution of Cambrian Benthic Communities: Environments of Carbonate Sedimentation

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 displacement 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 deposition. 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 based 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 comparable 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 itie s 335

BENTHIC PREDATORS poloeoscolecidons polymeroid trilobites?

BENTHIC MACROFAUNA

lingulote brochiopods hexoctinellids gostropods? miomeroid trilobites colcoreons (Yuwenio) polymeroid trilobites demosponges

BENTHIC PREDATORS

5E55tLt : MOBILE: priopulids cnidorionr non-trilobile orthropods z polychoetes îrilnhiice r'l^h^n^.1c" c.

NTHIC MACROFAUNA Uc. I S non-trilobite orthropods t B Ò non-lrilobife orlhropods echinoderms hexoctìnellides molluscs = lrilobiles trilobites demosponges hemichordote lingulote brochiopods hyoliihomorph hyoliths choncelloriids polychoeies priopulids

,NET' PHYTOPTANKON ocritorchs

?DEMERSAL PREDATORS BENTHIC PREDATORS toconodonts (?) borers lholkieriids/sochilids(?)

BENTHIC MACROFAUNA ( D B .,, "Aldonotrcto F Atdanelo onobotitids holklerlids/sochiiids (?) Lof /lis Be m / Ano b / lgo r/ N oc h t /Wo îommo.tiids (?) All/Exl Lod/Loc/Tut/Unil hyoliihomorph hyoliths holkieiiids/sochitids (?) choncellorlids orchoeocyoîhs lommotiids (?) coleolids Cysîicyothus (?) burrowers hyoliîhelminths (^i^r rl^+ó

BENTHIC CYANOBACTERIA I Dann/ni<\

WATER COLUMN DETRITUS 336 A.Yu. Zburaolea & F. Debrenne

reover this conditions eliminate the predators that con- demise of the "Tommotian" Fauna by the end of the sume grazers, then species adapted to such environment Early Cambrian, trilobites and lingulate brachiopods can dramatically inhibit the development of stromatoli- dominated until the Middle Ordovician, in Laurentia tes. Under normal marine conditions, various ecophy- and on the Siberian Platform (Sepkoski & Sheehan, siological adaptations, such as oxygen and HzS coexi- 1983; Sukhov & Pegel', 1986) (but also in Australia, stence in aiternative laminae of oxygenic and anoxygenic China, Kazakhstan). Brachiopods and less commonly photosynthetic cyanobacteria, production of extracellu- hyoliths, graptolites and pterobranchs occupied two su- lar slimes and nitrogen fixation are more significant to spension-feeding levels, while trilobites were deposit-fee- prevent grazing. ders and possibly carnivores. During Middle Cambrian - The first possible chitons appeared during the Middle Ordovician, a reorganisation of communities Late Cambrian (Trempealeauan) (Bergenhayan, 1960:' proceeded by addition of new elements, especially ga- Runnegar et aI., 7979; Stinchcomb & Darrough, 1995). stropods, rostroconchs, cephalopods and articulate bra- Stromatolite-thrombolite dominance persisted during the chiopods, beginning in the Trempealeauan. In the Dre- Early and early Middle Ordovician (Toomey, 1981; Pratt sbachian-Franconian, trilobites account for two-thirds of the species present. By the Trempealeauan Early Er James, 1989). A widespread intrusion of metazoans to Ord- into the stromatolitic-thrombolitic consortium started to ovician interval, palaeocommunity compositions are occur during the Middie Ordovician (but also in lower split more or less eveniy between trilobites and mol- iuscs (gastropods, cephalopods, and rostroconchs). In Early Ordovician) with the coral Lichenaria (Pratt & Ja- mes, 1.982, 1989). The success of new Ordovician meta- Northern China, the within-community diversity of zoan reef-builders (tabulates, stromatoporoids, bryozo- molluscs exceeded that of trilobites (Chen & Teichert, ans) in the algal-cyanobacterial framework reefs lies in 1983). Finally, during the Middle Ordovician interval, their modular and encrusting type of growth. the number of trilobite species were maintained at the same level, while the number of other species increased b) Level-bottom community. For taxonomic and to about one-third in their prior number of species. trophic aspects, the Early and early-Middle Cambrian Thus, decline in the relative importance of trilobites in nearshore environments was a case as spe- level-bottom communities resembled the coeval reefal as- of dilution, sociations but differed in the absence of heavily calcified cies of new clades appeared, rather than an actual displacement (Westrop et al., 1995). organisms. In both cases, filter- and suspension-feeders dominate both in number and in diversity. Typically level-bottom communities were more di- c) Hardground community. This community was a verse on the open shelves facing the ocean on silicicla- late innovation appearing only during the Late Cam- stic rather than on carbonate platforms. The Middle brian and becoming widespread during the Ordovician. Cambrian Phyllopod Bed and \Wheeler Formation fau- Although hardgrounds existed since Late Atdabanian at nas (Laurentia), by the exceptional soft parts preserva- least (e.g., Sellick Hill Formation, South Australia) tion, provide an unique insight into the structure of a (Alexander & Gravestock, 1990), they were lacking a di- bottom-ievel community (Conway-Morris, 1986; Robi- stinct fauna (A. Zhur. pers. observation). Only much son, 1991) (Fig. 1.2). Because of the presence of soft-bo- later, the early Franconian - Trempealeauan Snowy Ran- died animals, the characterization of Cambrian life of ge flat pebble conglomerates of Laurentia exhibit evi- the Phyllopod Bed community does not correspond en- dence of early lithification. The resulting hardground tirely with the classical one based on shelly components was locally inhabited by solitary fíter / suspension-fee- only. An unsuspected ecological complexity appears, ding organisms, namely eocrinoids and other pelmatozo- with a finer niche partitioning: low levels were occupied ans, orthid brachiopods, and spicular demosponges mostly by brachiopods and rare edrioasteroids but spon- (Brett et al., 1983). The Cambrian hardground commu- ges together with rarer pelmatozoan echinoderms, domi nity, not yet ecologically partitioned, was dominated by nated at higher levels. More contrasting is the role of solitary forms (like the other Cambrian communities). fleshy algae and.cyanobacterians as primary producers The encrusting (especially bryozoan) and boring faunas and deposit feeding arthropods as consumers. Diverse on the upward-facing hardground surfaces, increased ra- predators are confirmed by the abundance of effectively pidly in diversity in Early and Middle Ordovician oc- soft-bodied carnivores, together with some carnivores cupying the full range of levels above the substratum with hard parts. Similar communities existed since the (Palmea 1982). Open-framework burrow systems from Early Botoman at least, for example the Chinese Chen- the Late Cambrian Peerless Formation of Laurentia are gjiang Biota (Chen et al., 1989). attributed to Thalassinoides, which penetrated firm sub- The shallow level-bottom community underwent strates and, thus, belonged to an intermediate firm sub- most significant changes during the Cambrian. After the strate community (Myrow, 1995). Cambrian Bentbic communities .))/

The rise of stalked echinoderms, starting in the fleshy algae Margaretia as primary producers, spicular Middle Cambrian, was one of the prerequisites for the sponges as fiiter-feeders, hyoliths, lingulate brachiopods development of hardground communities. The easy and eodicids trilobites as suspension-feeders, rare paraga- breakage of echinoderm skeletons into multiple frag- stropods as grazers, palaeoscolecidans and protolenid tri- ments composing up to 30o/o of rock volume, increased lobites as carnivores. Polymeroids with wide thin the amount of calcareous detritus, rapidly lithified on exoskeleton, multiple thoracic segments and enlarged the sea floor through the precipitation of cements (tWil- pleurae, were nektobenthic trilobites adapted to low son et al., L992; Rozhnov, 1993). The echinoderms, oxygen tension (Repina & Zharkova, 1974; Fortey & thus, played an indirect taphonomic role in their own Wilmot, 1991). Volumetrically, trilobites and lingulates habitat, as their debris supported further attached echi- dominated. The latter might have fed on an abundant noderms. Development of Tbalassinoides burrows and but monotypic acritarch flora. A simiiar community oc- hardgrounds during Late Cambrian to Ordovician and curred on the Siberian Platform over the late Early - Middle to Late Cainozoic is notable, coinciding with early Middle Cambrian (Pel'man, 1982). Later, agno- calcitic-sea conditions. Then, the inorganic precipitation stoids and olenids, replaced eodiscids and protoienids re- of carbonates - as mud, allochems, and cements - was spectively. exclusiveiy calcite (Sandberg, 1983; Vilson et a1.,1,992). Thus, the switching from the Early Cambrian aragoni- tic-sea epoch to the Late Cambrian calcitic one may Conclusions. have somehow promoted the development of hard- The Ediacaran benthic fauna was chiefly compo- ground communities. sed of passive fiTter or/and suspension feeders and of a few deposit feeders (Lipps et al.,1992), independently of the systematic interpretation of its members. Although Deeper carbonate seas. this biota is known from shallow-water onshore and off- Two principal deeper water communities, aerobic shore sandstones, deep-water turbidites and shallow-wa- and dysaerobic, occupied well andlor poorly oxygenated ter carbonates, it may be attributed to a single commu- sites. They did not changed significantly during the nity. A iarge diversity of communities was deployed du- Ordovician preserving relicts of the Cambrian Fauna ring the Early Cambrian; some others were added by (Cook &. TayIor, 1977). the Late Cambrian. Relatively diverse Early Cambrian benthos represented by the Tommotian Fauna was repla- a) Aerobic community. Several fossil assemblages ced during the Middle Cambrian by a simpler, trilobite- are recognised in the late Middle Cambrian (Mayan Sta- dominated Cambrian Fauna s.s. which, in turn, was re- ge) of the Siberian Platform. Pterobranchs, eocrinoids placed by the Palaeozoic Fauna during the Late Cam- (?), graptolites, Brooksella?, hyoliths, abundant trilobites brian. This process is obvious in the reefal and level-bot- and brachiopods (mainly iingulates) are described from tom communities. The proximal communities were the outer slope, open-sea facies which occurred distally on most conservative; the distal, deep water communities the Siberian Platform, Olenek Province, from the Zele- grew permanently by adding of new elements, and the notsvet, Dzhakhtar and Siligir Formations (Fedorov et intermediate shallow shelf subtidal communities were a1., 1986; Durman 6c Sennikov, 1993). The Late Cam- the most changeable in taxonomic composition. The di- brian Hales Limestones of Laurentia and the Chopko splacement of communities was a common phenome- Formation of the Siberian Platform contained faunas li- non: the earliest deep water communities were derived ving probably below 350 m, below the thermocline from former shallow water elements; a trilobiteJingulate (Cook & Taylor, 1977;Barskov & Zhuravlev, 1988; Var- community, which occupied normal marine conditions lamov & Pak, 1993). The triiobite fauna is not diverse, during the Middle Cambrian, at first appeared in dysae- the burrows and the infauna sparse, but the presence of robic Early Cambrian basins. palaeoscolecidans, benthic bradoriids, conodonts, and A typical Cambrian food chain, acritarch / bacte- abundant sponge spicules suggests the presence of filter rioplankton - hyolith / lingulate? / arthropod - priapu- and suspension-feeders and carnivores. lid /"iobopodian"- anomalocarid is shon and simple, in spite of the presence of second order consumers, and is b) Dysaerobic community. A typical Early Cam- comparable with diatom - anchovies / krill - tuna / sea brian example is studied by Zhuravlev and Wood (1995) bird - marine mammals' web of Recent eutrophic areas. from the Botoman Sinsk Formation of the Siberian Trace fossil data of Crimes (L992\ indicate that the Cam- Piatform (Fig. 1.3). The Sinsk biota was represented by brian biota consisted of 60% of deposit-feeders, 15olo of the calcified bacterium Obruclteoella and abundant green filter- and suspension- feeders and 30o/o of probable car- 338 A.Yu. Zburavlev & F. Debrenne

nivores and scavengers. Volumetric analysis of single ning of the filter - /suspension-feeding niche, the early communities, however, indicates bacterial primary pro- Phanerozoic shallow epicontinental warers, indeed, ducers, filterers, suspension-feeders and carnivores. De- might have been turbid and nutrifient-rich. posit-feeders and grazers were less common. Such a ratio of guilds is typical of communities which have thrived in eutrophic conditions fiean & Thouzeau, 1995), This Aknouledgements. fact does not imply that each Cambrian community This paper is the result of researches carried out by A.Yu Zhsravlev during his stay as associate professor of MNHN, Paris proliferated in such conditions but it reveals that Cam- the and by F. Debrenne, URA 12 CNRS as a pan of the IGCP 366 brian communities were potentially adapted to them. In Program. The authors thank B. Pratt for fruitful scientific comments the absence of terrestrial vegetation, and at the begin- and linguistic improvements.

REFE,RENCES

Aiexander E.M. & Gravestock D.I. (1990) - Sedimentary fa- Chen Junyuan, Hou Xianguang & Erdtmann B.M. (1989) - cies of the Sellick Hill Formation, Fleurieu Peninsula, New soft bodied fossil fauna near the base of the Cam- Australia. In Jago J.B. & Moore P.S. (Edt - The Evolu- brian System at Chengjiang, Eastern Yunnan, China. tion of a late Precambrian-early Palaeozoic rift com- Intern. GeoL Congr., 1989,'V/ashíngton D.C., USA, pp. plex: The Adelaide Geosyncline. Geol. Soc. Australia, 265-278, Science Press, Beijìng, China. Spec. publ., v. 16, pp. 269-289, Sidney. Conway Morris S. (1986) - The community structure of the Aitken l.D. (1967) - Classification and environmental signifi- Middle Cambrian Phyllopod Bed (Burgess Shale). Pa. cance of cryptalgal limestones and dolomites with illu- laeontologt, v. 29, pp. 423-467 , I-ondon. strations from the Cambrian and Ordovician of Cook H.E. Ec Taylor M.E. (1977) - Comparison of continen- southwestern Alberta. Journ. Sedim. Petrol., v. 37, pp. tal slope and shelf environments in the Upper Cam- 1163-1178, Tulsa. brian and lowest Ordovician of Nevada. In Cook H.E. Astashkin VA. (1985) - Problematichnye organízmy - porodo- & Enas P. (Eds.) - Deep-\later Carbonate Environ- obrazovateli nizhnem Sibirskoy platformy v kembrii ments. Soc. Econ. Paleontol. Mineral Spec. publ., v. 25, (Problematic Lower rock-forming organisms in the pp. 51-81, Tulsa. Cambrian of the Siberian Platform). Akad. nauk. Crimes T.P. (L992) - Changes in the trace fossii biota across S.S.S.R., Sibirsk. otdel., LG.LG., Trudy 632, pp. 144-149, the Proterozoic-Phanerozoic boundary. J. Geol. Soc. Lon- Novosibirsk (in Russian). don, v. 149, pp. 637-646, I-ondon. Astashkin V.A., Varlamov A.I., Gubina N.K, Ekhanin A.E., Droser M.L. E{ Bottjer D.J. (1988) - Trends in extent and Pereladov V.S., Romenko V.L, Sukhov S.S., Umperovich depth of early Paleozorc bioturbation in the Great Ba- N.V., Fedorov A.8., Fedyanin A.P., Shishkin B.B. & sin (California, Nevada, and Utah). In \leide D.L. & Khobnya E.I. (1984) - Geologiya i perspektivy neftega- Faber M.L. (Eds) - This Extended Land, Geological zonosnosti rifofykh sistem kembriya Sibirskoy platfor- in the Southern Basin and Range. Geol. Soc. my (Geology and hydrocarbon potential of the Cam- Journeys Am., Cord:ileran Section, Field Trip Guidebook, pp. brian reefal systems on the Siberian Platform), 181 pp., lZJ-).)J, ItOUlOer. Nedra, Moscow (in Russian). Drozdova N.A. (1980) - Vodorosli v organogennykh po- Ba]sam \Xl.L. (1974) - Reinterpretation of an archaeocyathid stroykakh nizhnego kembriya Zapadnoy Mongolii (A1- reef: Shady Formation, South Western Yirginia. Sout- gae in Lower Cambrian organogenous buildups of we- heastern geology, v. 16 Q), pp. L2L-L29, Durham. stern Mongolia). 7i. - SorLmestn. Soo. -Mong. paleontol. Barskov I.S. & Zhuravlev A.Yu. (1988) - Myagkotelye orga- eksped., v. 10, 1-140, Moscow (in nizmy kembriya Sibirskoy platformy (Soft-bodied orga- pp. Nauka, Russian). Druce E.C., Shergold Radke B.M. (1982) reasse- nisms from the Cambrian of the Siberian Platform). Pa- J.H. & - A leontol. Zh., n. l, pp. 3-9, Moscow (in Russian). sment of the Cambrian-Ordovician boundary section at Black Mountain, \íestern Australia. In Bergenhayan J.B.M. (1960) - Cambrian and Ordovician lorica- Queensland, Basset M.G. Dean \7.T. (Eds) - The tes from North America. J. Paleontol., v. 34, pp. 168- & Cambrian-Ordo- 178, Lawrence. vician boundary: sections, fossil distribution and corre- Brett C.E., Liddel V.D. Ec Derstler K.L. (1983) - Late Cam- lations. Nat. Mus. lVales, Geol. Ser., v. 3, pp. 193-209. brian hard substrate communities from Montana/Vyo- Cardiff. ming: the oldest known hardground encrusters. Lethaia, Durman P.N. E Sennikov N.V (1993) - A new rhabdopleu- v. 16, pp. 281-289, Oslo. rid hemichordate from the Middle Cambrian of Siberia. Chen Junyuan & Teichen C. (1983) - Cambrian Cephalopo- Palaeontology, v. 36, pp. 283-296, I-ondon. da of China. Palaeontograpbica, Abt. A, v. 181, pp. 1- Fagerstrom J.A. (1987) - The evolution of reef communities. 102, Stuttgart. \7i1ey J. Er Sons, 600 pp., New York. Cambrian Benthic cornmunities 339

Fedorov A.B., Egorova L.I., Pegel' T.V. S{ Popov I.P. (19S6) - Mattes B.rM & Conway Morris S. (1990) - Carbonate/evapo- Analiz stroeniya i opyt stratigraficheskogo raschlene- rite deposition in the Late Precambrian - Early Cam- niya riforykh kompleksov na primere Anabarskogo ri brian Ara Formation of Southern Oman. In Robertson fovogo srednekembriyskogo massiva (Analysis of the A.H.F., Searle M.P. Er Ries A.C. (Eds.) - The Geology structure and essay on the srratigraphic subdivision of and Tectonics of the Oman Region. Geol. Soc. Spec. reef complexes from the example of the Middle Cam- Publ., v. 49, pp. 617-636, London. brian Anabar Reef Massif). In Gurari F.G. & Krasnov Myrow (1995) - Thalassinoides and the enigma of Early Pa- V.I. @ds.) - Regional'nye i mestnye stratigraficheskie po- leozoic open-framework burrow systems. Research Re- drazdeleniy a dlya krupnomas shtabno go geolo gichesko- port, Palaios, v. 10, pp. 58-74. Tulsa. go kartirovaniya Sibiri (R.egional and local strarigraphic Palmer T.J. Q982) - Cambrian ro Cretaceous changes in subdivisions for a large scale geoiogical mapping of Si- hardground communities. Lethaia, v. 15, pp. 3A9-323, berta). Siberian Scíentific-Research Institute of Geology, Os1o. Geoplrysics and Mineral Resources, pp. 4S-58, Novosibirsk Pel'man Yu.L. (1982) - Ekologicheskie gruppirovki fauny v (in Russian). srednekembriyskikh domanikoidnykh otlozheniyakh Fortey R.A. & \flilmot N.V. (1991) - Trilobite cuticle thic- kuonamskoy svity (Sibirskaya platforma, r. Muna) @co- kness in relation to palaeoenvironmenr. Paleontol. Zh., logical groups of fauna in the Middle Carnbrian doma- v. 65, pp. 147-151, Moscow. nicoid sediments of the Kuonamka Formarion (Siberian Geldsetzer H.H.J., James N.P. & Tebbutt E. (1989) - Reefs, Platform, Muna River). Alead. nauk S.S.S.R., Sibirsh. ot- Canada and Adjacent Area. Mem. Can. Soc. Pen Geol., del., I.G.LG., Tntdy 628, pp. 6A-74, 752-1,53, Novosi- v. 1.3,775 pp., Calgary. birsk (in Russian). Hamdi 8., Rozanov A.Yu. Er Zhttravlev A.Yu. (1995) - Larest Pratt B.R. (1989) - Early Ordovician cryptalgal-sponge reefs, Middle Cambrian metazoan reef from northern Iran. Survey Peak Formation, Rocky Mountains, Albena. In Geol. Mag., v. 1.32, pp.367-373, I-ondon. Geidsetzer H.H.J, James N.P. & Tebbutt E. @ds.). Jean F. & Thouzeau G. (1995) - Estimation des variables Reefs, Canada and Adjacent Area, Mem. Can. Soc. Petr. d'état d'un modèle de réseau trophique benthique en Ceol., v. 13, pp. 213-217, Calgary. rade de Brest. C. R. Acad.Scl., sér. 3, v. 318, pp. 145- Pratt B.R. & James N.P. (1982) - Cryptalgal-mefazoan bio- 154, Paris. herms of early Ordovician age in the St George Group, Kennard J.M. & James N.P. (1986) - Thrombolites and srro- western New{oundland. Sedimentolog, v. 29, pp. 543- matolites: Two distilct types o{ microbial srructures. Pa. 569, Oxford. I r41Os, v, r, pp. +'12-)v5, rulsa. Pratt B.R. Ec James N.P. (1989) - Early Ordovician thromboli- Kobluk D.R. 6. James N.P. (1979) - Cavity-dwelling orga- te reefs, St George Group, Vestern Newfoundland. In nisms in Lower Cambrian patch reefs from southern Geldsetzer H.H.J., James N.P. & Tebbuu E. (Eds.) - Labrador. Lethaia, v. 72, pp. 793-218, Oslo. Reefs, Canada and Adjacent Area. Mem. Can, Soc. Petr Kobluk D.R., James N.P. & Pemberton S.G. (1978) - Initial Geol., v. 1,3, pp. 231-240, Calgary. diversification of macroboring ichnofossils and exploita- Repina L.N. & ZharkovaT.M. (1974) - Ob usloviyakh obita- tion of macroboring niche in the lower Paleozoic. Paleo- niya trilobitov v rannekembriyskom basseyne Sibiri biology, v. 4, pp. 163-170, Lawrence. (On the environmental conditions of trilobites in the Korolyuk I.K. (1968) - Biogermnyee obrzovaniya Zapadnogo Early Cambrian basin of Siberia). Akad. nauh. S.S.S.,R., Pribaykal'ya (Biohermal formations of the western Ci- Sibirsk. otdeL., LG.I.G., Trudy 84, pp. 1OO-108, Novosi- sbaikalia). In Smirnov G.A. 6. Klyuzhina M.L. (Eds.) - birsk (in Russian). Iskopaemye rify i metody ikh izucheniya (Fossil reefs Riding R. k Zh,travlev A.Yu. (1995) - Structure and diversity and method of their sttdy). Institut Geologii i geockhi. of oldest sponge-microbe reefs: Lower Cambrian, Aldan mii, Ural'skiy filial, Ahademiya Nauk S55R, pp. 55-71, River, Siberia. Geology, v. 23 V), pp. 649-652, Boulder. Sverdlovsk (in Russian). Robison R.A. (1991) - Middle Cambrian biotic diversity: Kruse P.D., Zhuravlev A.YU. & James N.P. (1995) - Primor- Examples from four Utah Lagerstatten. In Simonetta dial metazoan-calcimicrobial reefs: Lower Cambrian, A'1- A.M. & Conway Morris S. (Eds.) - The Early Evolu- dan River, Siberian Plat{orm. PaLaios, v. 10, pp. 291-32L, tion of Mefazoa and the Significance of Problematic Tulsa. Taxa. Proceedings of the International Symposium, Uni- Lipps J.H., Bengtson S. & Fedonkin M.A. (1,992) - Ecology versity of Camerino, 27-31 March 1989, pp. 77-98, and biogeography. In Schopf J.\f. & Klein C. (Edr) - Cambridge University Press, Cambridge. The Proterozoic Biosphere: A Multidisciplinary Ap- Rozhnov S.V. (1993) - Osvoenie iglokozhimi pridonnogo proach, pp. 437-441,, Cambridge University Press, Cam- sloya vody v rannem paleozoe (Deploying of the water bridge. bottom level by echinoderms in the Early Palaeozoic). Luchinina VA. (1985) - Vodoroslevye postroyki rannego pa- Paleontol. Zh., n.3, pp. t25-127, Moscow (in Russian). leozoya severa Sibirskoy platformy @arly Palaeozoic a1- Runnegar 8., Pojeta J.Jr., Taylor M.E. & Collins D. (1,979) - gal buildups of the Siberian Platform). Ahad. nauk. New species of the Cambrian and Ordovician chitons S.S.S.R., Sibirsk. otdel., LG.I.G., Trudy 628, pp. 45-49, Mattheaia and Cbelodes from \fisconsin and Queen- Novosibirsk (in Russian). sland: Evidence for the early history of polyplacopho- 340 A.Yu. Zburaalep €t F. Debrenne

ran mollusks. J. Paleontol., v.53, PP. I374-L394, birskoy platformy (Upper Cambrian trilobite commu- Lawrence. nities and facies on the North-Vest of the Siberian Sandberg P.A. (1983) - An oscillating trend in Phanerozoic Platform). S tr a tigr afiry a. G e o lo gi c h e s hay a ho rr e ly at s iy a, v. non skeletal carbonate mineralogy. Nature, v. 305, pp. l(Z), pp. 104-110, Moscow (in Russian). 19-22,London. \W'estrop S.R., Tremblay J.V. & Landing E. (1995) - Declining nearshore paìeo- Sepkoski J.J.Jr. & Sheehan P.M. (1983) - Diversification, fau- importance of trilobites in Ordovician nal change, and community replacement during the communities: Dilution or displacement? Palaios, v. I0, r. r^ r I Ordovician radiation. In Tevesz M.J.S. 6. McCall P.L. Pp. /)-/1, Lr.lLSa. (Eds) - Biotic Interactions in Recent and Fossil Benthic \lilson M.A., Palmer T.J., Guensburg T.E., Finton C.D. & Communities. Topics in Geobiology, v. 3, PP. 673-717, Kaufman L.E. (1992) - The development of an Early Plenum Press, New York, London. Ordovician hardground community in response to ra- Stinchcomb B.L. & Darrough G. (1995) - Some molluscan pid sea-floor calcite precipitatíot. Lethaia, v. 25, pp. 19- problematica from the Upper Cambrian-Lower Ordovi- 34, Oslo. 'Wood cian of the Ozark Uplift. I Paleontol., v. 69, pp. 52-65' R., Zhuravlev A.Yu. & Debrenne F. (1992) - Functio- Lawrence. nal biology and ecology of Archaeocyatha. Palaios, v.7, Sukhov S.S. 6c Pegel'T.V. (1986) - Lito- i biofatsial'nyy analiz pp. 131-156, Tulsa. srednekembriyskikh otlozheniy vostoka Sibirskoy lflood R., Zhuravlev A.Yu, & Chimed Tseren A. (1993) - The platformy dlya rekonstruktsii usloviy karbonatonako- ecology of Iower Cambrian buildups from Zuune Arts, pleniya (Lithological and biofacies analysis of the Mid- Mongolia: implications Íor early merazo^n reef evolu- dle Cambrian strata on the East of the Siberian tion. Sedimentology, v. 40, pp. 829-858, Oxford. Platform for the carbonate deposition envlronment re- Zadorozhnaya N.M. (1974) - Rannekembriyskie organogen- construction). In Krasnov V.I. (Ed.) - Paleoekologi- nye postroyki vostochnoy chasti Altae-Sayanskoy skla- cheskiy i litologo-fatsi al' nyy ar,alizy diya obosnovaniya dchatoy oblasti (Early Cambrian organogenous buil- detal'nosti regional'nykh stratigraficheskikh skhem (Pa- dups o{ the eastern part of the Altay Sayan Foldbelt). laeoecological and lithological facies analyses for the Akad. nauk S.S.S.À., Sibirsk. otdel., I.G.I^G., Trudy 84, grounds of detailed regional stratigraphic schemes). .li- pp. 158-186, Novosibirsk (in Russian). birskiy nauchno-issledo'uatel'skiy institut geologii, geofi'ziki Zhtravlev A.Yu. (1996) - Reef ecosystem recovery after the i mineral'nykb resilrsov, pp. 35-50, Novosibirsk (in Rus- Early Cambriar extinction. In Hart M.B. (Ed.) - Biotic sian). Recovery from Mass Extinction Events. Geol. Soc Spec. Toomey D.F. (19s0 - Organic-buildup constructional capabi- Publ., v. fi2, pp. 79-96, London. lity in Lower Ordovician and late Paleozoic mounds. In Zhuravlev A.Yu. & Vood R. fi.995\ - I-ower Cambrian reefal Gray J., Boucot A.J. EÈ Berry V.B.N. (Edt) - Commu- cryptic communities. Palaeontology, v. 38, pp. 443-470' nities of the Past, pp. 35-68, Flutchinson Ross Publis- London. hing Company, Stroudsburg, Pennsylvania. Varlamov A.I. & Pak K.L. (1993) - Soobshchestva trilobitov i fatsii verkhnekembriyskikh otlozheniy severo-zapada Si- Received ApriL 22, 1996; accepted August 1, 1996