Archaeocyatha are now confidently considered as a Class within the phylum Porifera. Regulares and Irregulares are no longer a systematic division, but have only ecological significance. Topographic relief, biogenic framework and a wave-resistant capability are in favour of the reefal nature of archaeocyathan bioconstructions. Several types of boundstones are recognized, based on the relative proportion of the different framework builders. The same basic plan, characterized by component domains occupied by an association of lime mud, archaeocyaths calcimicrobes, growth cavities, shows little changes throughout the Early Cambrian. A four-stage ecological succession can be inferred, according to some documented cases of distinct biological zonation in archaeocyathan reefs. There is no consensus about the question of photosynthesis in those reefs. They follow the transgression of carbonate facies in sub-tropical areas during the Early Cambrian.
Les Archaeocyatha sont maintenant considérés unanimement comme une classe du phylum Porifera. Les anciennes distinctions entre Regulares et Irregulares n’ont qu’une valeur écologique et n’ont plus de valeur systématique. Le relief topographique, la charpente biogénique et une certaine capacité de résistance aux vagues font que les bioconstructions à archéocyathes sont de véritables récifs. La proportion relative des différents calcimicrobes et des archéocyathes dans le cœur du récif permet de distinguer plusieurs types de
Les premières bioconstructions à métazoaires contenaient un groupe d’affinités incertaines, les archéocyathes. Après les découvertes d’éponges corallines actuelles, les études comparatives ont montré que les Archaeocyatha formaient une seule classe dans le phylum Porifera
Suivant les variations morphologiques des récifs, on distingue les kalyptrae (monticules) simples (
La répartition spatio-temporelle des récifs à archéocyathes (
En conclusion, même si les récifs du Cambrien inférieur ont été surtout le fait de l’action constructrice des calcimicrobes, les archéocyathes ont participé à leur établissement comme support des calcimicrobes, comme toit des cavités internes et, plus rarement, comme constructeurs de la charpente principale. Dans les zones péri- et inter-biohermales, ils ont agi comme filtreurs et contribué au bon fonctionnement du biotope récifal au Cambrien inférieur.
Les exemples cités dans cet article ont été étudiés sur le terrain par Max et Françoise Debrenne.
The earliest skeletal, metazoan-built reef ecosystem starts at the beginning of the second stage of the Early Cambrian, in Siberia. It spreads around the world and decreases at the end of the Early Cambrian, essentially corresponding to the temporal and spatial distribution of a pioneer group of sponges, the Archaeocyatha. This first episode of reef building by metazoans covers a time span equivalent to the Palaeocene, i.e. 11 Myr.
They were called by authors “archaeocyathan reefs”, but for a long time, the systematic position of archaeocyaths was controversial and, in the other hand, the definition of reefs differs according to biologists, geologists, palaeontologists or sedimentologists.
This paper is a review of literature and is mainly focused on the role of archaeocyatha in the establishment of Cambrian bioconstructions.
Archaeocyaths were marine sessile organisms, appearing in the Tommotian (521 Myr) and almost extinct in the Toyonian (510 Myr). Their biological affinities were much debated from their first find in Labrador in the mid-19th century to the discovery of living aspiculate sponges in the late 20th century
The use of the word
Archaeocyatha were interpreted, since the beginning, as being responsible for constructions comparable with the Great Barrier Reef
Between 1960 and 1966, Zhuravleva
Kalyptrate reefs are typical of Lower Cambrian reefs. Nevertheless, an archaeocyath–calcimicrobe consortium is able to construct massive, non-kalyptrate reefs as the 600-m-wide, 90-m-thick build-up of Schrimp Lake in Yukon Territories, northwestern Canada
Other exceptional individual bioherms are massive non-kalyptrate reefs (
Specific investigations on structure and composition of Lower Cambrian reefs were not carried out until the late 1970s. The Russian authors
The studies of reefs from different localities and age show that they have the same basic plan and can be categorised into component domains occupied by associations of lime mud, archaeocyaths, calcimicrobes and/or cement, with opportunities for cavity development. Domains reflect microenvironmental variations (
Most of the Lower Cambrian reefs were built by a consortium of calcimicrobes and archaeocyaths. Archaeocyaths themselves did not generally produce a real framework, but were an obligate substrate for dominant calcimicrobes, for cement, and provided additional opportunities for cavity development. Archaeocyaths were relatively abundant in bioherms built by calcimicrobes of the
The carbonate platform of South Australia was intensively examined to investigate as many different palaeoenvironments as possible with their influence on archaeocyathan reef development
Characteristics of archaeocyathan reefs are based on core components. Then, the archaeocyath–calcimicrobial reefs can be classified according to the type of dominant builder. Among the archaeocyathan reefs now listed among the Cambrian rocks of the world, the following examples given below are those that my husband Max and I had the opportunity to visit or from which I have received material for study.
It is a complex intergrowth of different calcimicrobes. Few or no archaeocyaths are found in their core. They occur in inner shelf locations under low-energy environments.
In some rare cases archaeocyaths may be the dominant reef framework. Their bioconstructions are generally limited to some individual bioherms.
Riding and Zhuravlev [45] found, at Ulakhan-Sulugur, just above the base of the Tommotian stage, the oldest known reef on the Siberian Platform in which individual cups of
The most frequent framework builder of Cambrian reef is
In rarer cases, archaeocyaths are dominant or equally associated with a consortium of calcimicrobes often being cryptobionts. They have an open shelf, middle- to high-energy environment setting and are often associated with
This reefal type consisted of complex intergrowths of calcimicrobes dominant and archaeocyaths, in which
They are associated with the main builders, but volumetrically not significant, such as the calcarean sponges
Archaeocyathan-calcimicrobial bioconstructions have topographic relief, a biogenic framework, and a certain capacity for wave resistance. Reefs were presumably initiated during episodes when the mud input slowed down or ceased, thus allowing localised cementation or stabilisation of seafloor mud
In matrix or in peri-reefal beds of some Lower Cambrian reefs are locally found trilobites; disarticulated valves of ostracodes, valves of inarticulate brachiopods, hyoliths, salterellids, helcionelloids, echinoderm ossicles, chancelloriids, sponge spicules
The Early Cambrian reef ecosystem was largely composed of generalists and opportunistic passive filter- and active suspension-feeders, which were dependent upon a relatively high supply of nutrients. Many reefs were dominated by just one or two archaeocyathan species, implying that these communities were the result of rapid colonisation and subsequent growth from one or a few larval spat falls
Lower Cambrian reefs were long time thought not to display vertical zonation
A. Zhuravlev
Despite their small size, a number of Early Cambrian reefs represent complete ecological successions. Others were initiated and dominated by calcimicrobes, with archaeocyaths as subordinate dwellers, contributing only modestly to reef construction.
If not all archaeocyaths occur in limestones, as a few examples are found in siliciclastic facies, the vast majority live in carbonate-dominated environment facies and their palaeogeographic distribution coincides with the extension of marine carbonate deposits and of shallow marine habitats. Their association with calcimicrobial/stromatolitic reefs and oolite shoals and the proximity of evaporitic basins are in favour of an adaptation to warm waters, limited to intertropical areas
Palaeogeographic reconstructions indicate that the regions farthest from the palaeoequator were Morocco and southern Europe to the south, possibly the Yangtze (South China) and an enigmatic (though non-confirmed) locality in Korea to the north
The archaeocyathan proliferation followed the continued transgression initiating the carbonate accumulation
Even when reefs are dominated by calcimicrobes, subordinate archaeocyaths contribute as dwellers to the reef construction since the Tommotian, as support for the builders and as cavity roofs. They were active bafflers at the periphery and the top of reefs and constitute an abundant fauna in the inter-reef domains. The basic metazoan-calcimicrobial construction did not change significantly through the Early Cambrian, even if archaeocyaths became more modular, so more significantly reef builders. It may be that the archaeocyathan-calcimicrobial competitive interactions, in which dendritic renalcids were dominant, contributed to the eventual near-total elimination of archaeocyaths from the Cambrian reef biota in the Toyonian stage. Recent researches have corrected some common ideas. For example, the Lower Cambrian reefs were not all small kalyptrae individual or compound. Calcimicrobe and archaeocyath bioconstructions may be massive, ecologically zoned, associated with oolite shoal. A consensus has not yet been found concerning photosynthesis: if absent in the Cambrian, then the archaeocyath reefs would be different from Mesozoic and Cainozoic reef communities. It is the next challenge for archaeocyathan reefs to become integrated in the model of the Recent.
I am indebted to Noël James, who initiated me to reef studies, to all colleagues who accompanied us on their field areas, to the two reviewers for their helpful suggestions and to Zoe Brasier for re-reading this text, and, last but not least, to my husband Max for his constant help on field and research work Photos in
Septal type:
Fig. 1. Type septal :
Taenial type:
Fig. 2. Type taenial :
Titiriktek stream, Lena River tributary, a single kalyptra reef.
Fig. 3. Rivière Titiriktek, affluent de la Lena, un récif formé d’une kalyptra isolée.
Geographic distribution of Early Cambrian archaeocyathan reefs, bottom of map: (
Fig. 4. Répartition géographique des récifs à archéocyathes du Cambrien inférieur, à la base de la carte : (
(
Fig. 5. (
Oy-Muran complex stacked kalyptra reef; Lena River bank.
Fig. 6. Oy-Muran : récif complexe formé de kalyptres empilées ; berges de la Léna.
Stewart's Mill(Nevada): mudshales of Campito Formation overlaid by successive individual boundstones of specific composition, separated by mudshale facies (tidal channel) and capped by an oolite shoal [47 (p. 130)].
Fig. 7. Récif complexe de Stewart's Mill (Nevada) :
Salaany Gol (Mongolia): massive non-kalyptra reef.
Fig. 8. Salaany Gol (Mongolia) : récif massif sans distinction de kalyptres.
Sampling method: square grid marked on the flank of a Serra Scoris bioherm, Sardinia.
Fig. 9. Méthode d’échantillonnage selon une grille tracée sur le flanc d’un bioherme à Serra Scoris, Sardaigne.
Various spatial relationships between different archaeocyathans in kalyptrae, which make up the complex reefs of Labrador. The primary association is the most commonly observed with less common, but recurring, secondary associations (from
Fig. 10. Les différentes relations spatiales dans la répartition des archéocyathes dans les kalyptres qui composent les récifs complexes du Labrador. L’association primaire est la plus fréquente ; les associations secondaires sont moins fréquentes, mais récurrentes (d’après
The different types of calcimicrobes–archaeocyath reefs and their distribution according to the energy spectrum; from
Fig. 11. Les différents types de récifs à calcimicrobes et archéocyathes et leur répartition en fonction de leur position et de l’énergie, d’après
Humoristic sketch of a Tommotian Ulakhan-Sulugur reefal community with large cups of frame building archaeocyaths, delimiting cavities with cryptic small monocyathid cups and renalcids; presence of in-situ bafflers (hyoliths, left) and reef dwellers molluscs; from
Fig. 12. Schéma humoristique d’une communauté récifale d’Ulakhan-Sulugur (Tommotien), charpente formée d’archéocyathes évasés délimitant des cavités habitées par des monocyathides cryptiques et des renalcides; présence de
Archaeocyath boundstone in Matoppa reef (Sardinia), coll. F. & M. Debrenne. MNHN MT6.
Fig. 13.
The tabulate coral
Fig. 14.
Archaeocyath boundstone from Sonora (Mexico); reconstruction of the
Fig. 15.
Archaeocyath boundstone from Labrador;
Fig. 16. Boundstone à archéocyathes du Labrador ; calices modulaires de
Fig. 17.
Ecological succession on an Early Cambrian reef, reconstitution from
Fig. 18. Reconstitution de la succession écologique dans la construction d’un récif du Cambrien inférieur d’après
Facies zonation and interpreted depositional environments in the massive reef complex of Stewart's Mill (Nevada); from
Fig. 19. Zonation et interprétation des environnements des faciès dans le récif massif de Stewart's Mill (Nevada) ; d’après