Françoise Debrenne

Functional biology and ecology of Archaeocyatha

A modular organization confers many ecological advantages in reef-building settings, such as indeterminate growth leading to larger size, greater powers of regeneraion, and an ability to encrust and gain secure attachment to substrates. The Archaeocyatha, an early Cambrain group of calcified sponges, were the first skeletal metazoans to develop a modular habit and to be associated with reefs. Archaeocyaths show the predicted ecological changes with the appearance of modularity. Supposed species may be facultatively modular, with modular ecophenotypes always dominating biohermal settings. However, only genera possessing porous septa show any development of modularity, seggesting that an initially well-integrated soft-tissue is a prerequisite for acquisition of the habit

Tabulaconus handfield; microstructure and its implication in the taxonomy of primitive corals

Numerous specimens of Tabulaconus Handfield, 1969, have been collected in carbonate buildups within the Adams Argillite (Early Cambrian, Tatonduk area, Alaska). The wall structure of this form has been investigated, along with contemporaneous archaeocyaths and algae, through the use of polished ultra-thin sections (2–3 μm thick) and scanning electron microscopy. The results of this microstructural comparison indicate that despite diagenetic alteration Tabulaconus has a skeleton that is unlike any presently known and is quite distinct from associated algae or archaeocyaths. It is more elaborate than that found in the archaeocyaths but has not reached the stage of complexity seen in the primitive coral Cothonion Jell and Jell, 1976. The presence of some elongated units may represent an initial step towards the fibrous skeleton typical of Paleozoic corals. This study shows that even though diagenesis alters the original microstructure of calcareous skeletons, the resultant fabrics and detailed structures can be useful in systematic descriptions. Tabulaconus is removed from the Gastroconidae Kordae due to the presence of rudimentary septa and constitution of the tabularium. A number of species assigned to the genus Bacatocyathus Vologdin and included within the Archaeocyatha appear to be examples of Tabulaconus or very close relatives. An emended description of Tabulaconus kordae , the type species, is proposed.

Shackleton Limestone archaeocyaths

The Shackleton Limestone is a thick unit of bedded and allochthonous limestone, with minor dolostone, quartzite, sandstone and siltstone, spanning the central portion of the Transantarctic Mountains. It is a record of Early Cambrian shelf and peri-platform carbonate deposition on the margin of the East Antarctic craton. Slope, shallow marine and intertidal-supratidal conditions prevailed during deposition. Thirty-one species of archaeocyaths described from measured sections in the northern Holyoake Range (Nimrod Glacier area) and at Crackling Cwm (Byrd Glacier area) confirm earlier correlations of Shackleton Limestone faunas with the Botomian stage of Siberia. Archaeocyaths from a section at Mt Egerton (Byrd Glacier area) are of either Botomian or Toyonian age. Ten further archaeocyath species are added to the six species already known to be common to both Antarctica and Australia, confirming the existence of a Gondwana supercontinent in the Early Palaeozoic. New taxa described are the genera Kymbecyathus...

The oldest tabulate coral and the associated archaeocyatha, Lower Cambrian, Flinders Ranges, South Australia

Abstract The oldest known tabulate, Flindersipora bowmani Lafuste gen. et sp. nov. from the Flinders Ranges of South Australia, is described for the first time. The new genus presents all morphological and structural characteristics of Tabulate: cerioid shape, septal ridges, tabulae, wall pores. Its microstructure is nevertheless new, formed by “platelets”. Specimens have been found associated with archaeocyaths and calcified cyanobacteria in Lower Cambrian shelf margin buildups in the Oraparinna Shale and Moorowie Formation, in two different localities: Ten Miles Creek, where they are associated with solitary archaeocyaths, and Moorowie Mine, where they built complex bioconstructions associated with colonial archaeocyaths. The archaeocyaths, comprising 11 species, indicate a middle Botomian age. One species Robertocyathus brenti Debrenne is new. The buildups were among the last to flourish prior to widespread regression in the Early Cambrian in South Australia. With this discovery, the present opinions on the phylogeny of Tabulate Corals must be modified. In comparison with some Cambrian or even Ordovician forms which have been supposed to have affinities with corals, Flindersipora has a less rudimentary morphology and could not be said more primitive than these previously known genera. Because of its advanced favositid architectural type Flindersipora reinforce the idea that the origin of Tabulate is very ancient and must be looked for in the Precambrian.

Extinction of the Archaeocyatha

Archaeocyaths are a group of calcified sponges almost limited to the Early Cambrian. They have undergone a rapid radiation followed by an abrupt decline even more rapid and a total extinction. Rate of this extinction is documented at the generic level and a pattern of extinction is proposed. Changes in palaeotectonics, with the consequent modifications in the environment, and a probable cooling of the climate, are the possible causes of their demise. This extinction involves the disappearance of the first reefs in which Metazoans were, at least partly, implicated.

Lower Cambrian bioconstructions in Northwestern Mexico (Sonora). Depositional setting, paleoecology and systematics of Archaeocyaths

Abstract Investigations on Lower Cambrian carbonates of Caborca (NW Mexico) indicate that in the Puerto Blanco Formation the following two types of archaeocyathan-algal buildups occur: o I -Framework reefs occurring at the base of the carbonate interval of the Puerto Blanco Formation. These consist of an archaeocyathan framework with encrusting renalcids and abundant fibrous cements. Cavities are filled with mudstone and contain rare sponge spicules. Cements are well developed upon internal sediments. The archaeocyathan fauna is dominated by colonial Paranacyathus along with Batenevia and Claruscoscinus with taeniae and curved tabulae. A few regular archaeocyaths are present, including Palmericyatbus and Aulocricus. Lateral to the reef facies the lime mud sediment contains abundant skeletal fragments of trilobites, echinoderms, Chancelloria, hyoliths, and archaeocyaths. These buildups are interpreted to have formed as wave-resistant, framework reefs in a tidally dominated, shelf-margin setting seaward of a carbonate barrier. II -Girvanella-dominated buildups located in the upper part of the carbonate interval of the Puerto Blanco Formation and associated with oolites. These are built by stacked Girvanella crusts, with an important contribution of Renalcis and a less important presence of Epiphyton. Shelter cavities are filled by mudstone often rich in siltsize quartz. Cements are poorly developed and confined to the intraskeletal cavities. Archaeocyaths are scattered in the Girvanella frame either as isolated individuals or as clusters of solitary or colonial forms. Regular archaeocyaths are here abundant, represented by several taxa with an ethmophylloid inner wall, including Palmericyatbus and Cordilleracyathus. The dominant genus is Aulocricus. Colonial species of Arcbaeocyathus and Arrythmocricus are well developed. These latter two genera are now recorded from Mexico to northern Canada. The internal and lateral sediment contains a rich

Upper Cambrian Archaeocyatha from Antarctica

Geological mapping in the Ellsworth Mountains of west Antarctica have established the existence of extensive Middle and Late Cambrian strata. Forms tentatively referred to Archaeocyatha have been examined by F.D. and A.R., who concluded that, despite the commonly held opinion that the phylum became extinct at the boundary of the Lower and Middle Cambrian, the Ellsworth Mountains forms represented Irregularian Archaeocyatha which survived in protected niches into the Late Cambrian. The age of the fauna containing the archaeocyaths is firmly dated by abundant trilobites including species of Homagnostus, Pseudagnostus, Kormagnostella, Erixanium and Onchopeltis.--Modified journal abstract.

Early Cambrian bioconstructions in the Zavkhan Basin of western Mongolia

The Neoproterozoic–Cambrian succession in the Zavkhan Basin of western Mongolia preserves early Cambrian bioconstructions of Nemakit-Daldynian to Botomian age. As elsewhere (Siberia, Morocco), the Nemakit-Daldynian bioconstructions in the upper Tsagaan Oloom Formation to lower Bayan Gol Formation interval were purely calcimicrobial. Spectacular calcimicrobial ‘patch reefs’ of presumed Tommotian age are present in the lower Bayan Gol Formation. In contrast, late Atdabanian–early Botomian bioconstructions are varied and well developed in the upper Salaany Gol Formation including, in addition to calcimicrobial bioherms, Gordonophyton–Razumovskia crusts, radiocyathan—archaeocyathan bioherms and thickets of the ramose archaeocyath Cambrocyathellus . These latter Zavkhan Basin buildups show some ecological and sedimentological features in common with coeval bioconstructions in South Australia, which also developed under a tectonically active regime.

The nature of Radiocyathidsand their relationship to Receptaculitids and Archaeocyathids

Abstract The morphological analysis of the structural plan of the Radiocyathids skeleton implies a closer relationship to Receptaculitids than to Archaeocyathids. As a working hypothesis, they may possibly form a link between part or all of these two groups and use as a test for the validity of the new kingdom Archaeata proposed by I. T. Zhuravleva & E. I. Miagkova.

Cambrian food web: A brief review

Abstract Cambrian trophic web guilds consist of 1/Primary producers: calcified and non calcified bacteria, phytoplankton (acritarchs) and dissolved organic matter and 2/Primary consumers: mainly filter/suspension-feeders, some carnivorous, probable deposit feeders and rare grazers; parasites can only be surmised. Trophic modes are illustrated by examples taken in the different Cambrian communities.