Marcelo G. Carrera

Early ordovician microbial-sponge-receptaculitid bioherms of the precordillera, western Argentina

SummaryReef-mounds built by benthonic microbes, lithistid sponges, and the receptacultidCalathium occur near the base of the Early Ordovician San Juan Formation of the western Argentine Precordillera. Biostratigraphic information suggests a Lower Early Ordovician age for these buildups, which formed in a subtidal, shallow ramp setting.Lateral and vertical coalescence of individual domal bioherms 2–4 m in diameter, resulted in a reef-mound complex 4 m in thickness. The reef-mounds exhibit two main rock types:(a)massive skeletal and lithoclastic wackestone/packstone; most of the sponges andCalathium occur in a para-autochthonous position, withGirvanella tubules encrusting and bounding sponges and bioclasts; lithoclastic wackestones to grainstones were deposited on the irregular mound surface;(b)microbial boundstones, including stromatolitic structures, non-laminated peloidal micritic boundstone andGirvanella crusts. The reef-mounds are dissected by conspicuous channels filled with coarse crinoidal grainstone and lithoclastic rudstone. The mounds developed distinct flanking beds.The bioherm growth resulted from a relative higher rate of skeletal production with respect to the surrounding area, the baffling action of conical and cylindrical sponges,Calathium and crinoids, and the binding activity of microbial mats, encrusting algae and locally tabulate sponges.The structures here described are remarkably similar to other Early Ordovician bioherms reported from North America.

Biogeography of Ordovician sponges

Sponges have an unrealized potential importance in biogeographic analysis. Biogeographic patterns determined from our analysis of all published data on distribution of Ordovician genera indicate Early Ordovician sponge faunas have relatively low diversity and are completely dominated by demosponges. Early Ordovician (Ibexian) faunas are characterized by the widespread co-occurrence of Archaeoscyphia and the problematic Calathium. This association is commonly found in biohermal structures. Middle Ordovician faunas show an increase in diversity, and two broad associations are differentiated: Appalachian faunas (including Southern China and the Argentine Precordillera) and Great Basin faunas. Late Ordovician faunas show important changes in diversity and provincialism. Hexactinellid and calcareous sponges became im- portant and new demosponge families appeared. Four Mohawkian-Cincinnatian associations are recognized here, including: 1) Mid- continent faunas; 2) Baltic faunas; 3) New South Wales faunas; and 4) Western North American (California and Alaska) faunas. However, two separate biogeographic associations are differentiated based on faunal differences. These are a Pacific association (western North American and New South Wales) and an Atlantic association (Midcontinent Laurentia and Baltica). Distribution of sponge genera and migration patterns are utilized to consider paleogeographic dispositions of the different continental plates, climatic features, and oceanic currents. Such an analysis points to close paleogeographic affinities between the Argentine Precordillera and Laurentian Appalachian faunas. However, significant endemicity and the occurrence of extra-Laurentian genera suggest a relative isolation of the Precordillera terrane during the Late Ibexian-Whiterockian. The study also shows a faunal migration from the Appalachian region to South China during the Middle Ordovician and the migration of faunas from Baltica to Laurentia in the Late Ordovician. The occurrence of Laurentian migrants in New South Wales during the Late Ordovician could be related to inferred oceanic current circulation between these two areas, although other paleogeographic features may be involved.

Evolutionary History of Cambrian Spiculate Sponges: Implications for the Cambrian Evolutionary Fauna

Abstract Broad-scale analyses of Cambrian spiculate sponges are scarce. The apparent differences between Cambrian and Ordovician sponge faunas were included in Sepkoskis concept of evolutionary faunas; in these, sponges were regarded as minor contributors to the Paleozoic and modern faunas and insignificant in the Cambrian Evolutionary Fauna. More recent published occurrences of Cambrian and Ordovician spiculate sponges and the inclusion of archaeocyaths in the phylum Porifera, however, have altered our understanding of the significance of sponges among Cambrian faunas. The majority of Cambrian occurrences appear to be segregated into two major associations: lower Cambrian sponges in China, and middle Cambrian sponges in North America, primarily British Columbia and Utah. The main associations of spiculate sponges are in siliciclastic deposits from middle-to-deep muddy shelf and basin environments, whereas orchoclad demosponges are associated with shallow carbonate environments. Four main aspects of sponge biology are considered potential factors dictating the distribution of sponges in the Cambrian: their trophic requirements, hydrodynamic constraints, possible biogeochemical constraints, and the sponge-sediment relationship. A series of critical steps in sponge evolutionary history occurred during the interval from the Proterozoic-Cambrian boundary to the middle– late Ordovician. The lower–middle Cambrian faunas are considered to be a Cambrian evolutionary sponge fauna, with archaeocyaths and diverse monaxonid demosponges as distinctive components. There was a transitional fauna in the upper Cambrian–Lower Ordovician, with orchoclad lithistids dominating shallow environments. Hexactinellids began to colonize nearshore siliciclastic settings during this time. The third interval, Middle–Upper Ordovician, corresponds to the Paleozoic Evolutionary Fauna, which is the interval during which lithistids diversified in several suborders and families and the stromatoporoid and sphinctozoan calcified sponges experienced their first radiation.

Early Ordovician (Arenig) faunal assemblages from western Argentina: biodiversification trends in different geodynamic and palaeogeographic settings

Abstract A survey of early Ordovician faunal assemblages from different geodynamic and palaeogeographic settings from the west of Argentina has been carried out. The distribution and dominance of four fossil groups (rhynchonelliform brachiopods, trilobites, sponges, and bivalves) are analysed and compared in three distinct basins: a passive-margin carbonate platform (Precordillera), a volcanic-arc island platform located at intermediate latitude (Famatina) and a siliciclastic pericratonic epeiric platform placed at intermediate–high latitude (Cordillera Oriental). Scales of analysis range from the level of the patterns exhibited by individual clades to the level of the assemblages along the onshore–offshore gradient. Overall taxonomic diversity (at genus and family levels), alpha or within habitat diversity, and ecospace utilisation were evaluated and contrasted among bathymetric zones and among different basins. On this basis a mosaic of five distinctive type assemblages is characterised: (1) a demosponge–brachiopod type assemblage (Precordillera), (2) a brachiopod–bivalve type assemblage (Famatina), (3) a brachiopod–trilobite type assemblage (Famatina), (4) a trilobite–brachiopod type assemblage (Cordillera Oriental), and (5) a trilobite type assemblage (Cordillera Oriental). Possible large-scale controls in the configuration of these type assemblages are assessed. The structure and distribution of the type assemblages from the west of Argentina are interpreted to be largely controlled by environmental dynamics at each geodynamic setting coupled with the latitudinal position of the basins. Both large-scale factors regulate a number of regional parameters such as sedimentary regime, volcanic activity, oceanic circulation, temperature, etc. In particular temperature appears as a critical control over the food supply and primary productivity as well as seasonality of the resources. According to the evidence analysed in the Argentine basins, the latter variables might have played a key role in the diversification of suspension-feeding organisms that, in turn, promoted the development of the Palaeozoic evolutionary faunas and the Ordovician radiation. Our case study from the west of Argentina further represents a small-scale model of the broad spectrum of the possible regional conditions that promoted the differential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.

Epizoan-sponge Interactions in the Early Ordovician of the Argentine Precordillera

Abstract Evidence for biotic interaction is not obtainable easily in the fossil record. Epizoans cemented to the skeletons of host organisms provide an opportunity to study possible host–epizoan interactions and the ecological structure of hard substratum communities. One such possibility is sponges and their associated encrusters, but few such occurrences have been documented from the fossil record. Early Ordovician limestones of the San Juan Formation in the Argentine Precordillera bear an important sponge fauna consisting mainly of demosponges. This Early Llanvirn assemblage has the highest diversity, and most sponges were subjected to epizoan encrustations. A total of nineteen species included in eleven genera of sponges were examined to record epizoans encrustations. Type and relative abundance of epizoans were compiled for each sponge species. Five kinds of epizoans were recorded: bryozoans, stemmed echinoderms, brachiopods, other sponges, and scars and borings of undetermined organisms. Crinoids and blastozoans are widespread common epizoans on sponges. Small brachiopods are attached to sponges showing no location preference; rarely, juvenile sponges appear on conspecific adults. Sinuous scars and worm-like organisms are found in almost all the sponges. Deep borings, like those found in Devonian or younger sponges, are absent. The analysis of epizoan distribution on each sponge species shows a fairly homogeneous distribution of echinoderms on sponges but host specificity of bryozoans. Here, surface texture seems to have been a significant controlling factor. Almost all the trepostome bryozoa adhere to Rhopalocoelia tenuis Carrera. This taxon is a cylindrical sponge whose smooth surface texture contrasts sharply with the rough and open textured spiculation observed among other species in the fauna. This feature seems to have been a key factor for the mutual relationship. Rhopalocoelia tenuis shows a reaction to epizoans, modifying the shape of growth and changing the direction of the spicular structure in the vicinity of the epibiont, providing evidence that encrustation occurred while the sponge was alive. Diversity, composition and encrusting strategies of the Argentine epizoan community reflect the early evolutionary stage of this biotic relationships in the Early Paleozoic. This contrasts with the more varied and diversified relationships among organisms that were able to encrust, bore, and chemically perforate hard parts after the Devonian. The Early to Middle Ordovician was a time of rapid evolutionary diversification among hard substrate communities. The Argentinean epizoan community appears to be transitional between the first hard substrate encrusters (Late Cambrian), which were eocrinoid dominated and were not accompanied by macroborers or bryozoans, and the later Paleozoic communities, in which bryozoans were the most prolific utilizers of attachment and macroborers were common.

Darriwilian bryozoans from the San Juan Formation (Ordovician), Argentine Precordillera

Abstract. A detailed study is carried out on the taxonomy of the bryozoan fauna from the upper levels of the San Juan Formation (Darriwilian) in the Talacasto and Cerro Viejo sections, Argentinean Precordillera. Three bryozoan genera are described and two new species are erected: Lamottopora multispinosa sp. nov., Aostiporn sanjuanensis sp. nov., and Phylloporinidae sp. indet. The paleoecological significance of the bryozoan fauna is discussed. The ramose bryozoans Lamottopora and Aostipora dominate in the Talacasto section. Some colonies probably lived attached directly to the substrate, while others occur attached to the surface of the abundant sponge fauna in the section. The majority of bryozoans at the Cerro Viejo section occur as epibionts, with bases attached to the surface of sponges. Bryozoans collected in the matrix are scarce compared to the Talacasto section, and comprise a few fragments of ramose bryozoans and two small fragments of the reticulate Phylloporinidae sp. indet were recorded exclusively in this locality. The presence of this delicate form in Cerro Viejo is further evidence of the quiet water conditions suggested for these levels. Only the laminar form Nicholsonella occurs in slightly older levels (Floian) of the San Juan Formation. The low diversity recorded here shows a slight decoupling with the global pattern. The first local radiation (three genera) is minor compared with the important radiation that the phylum experienced elsewhere.

Chapter 8 Cambrian, Ordovician and Silurian non-stromatoporoid Porifera

Abstract The Cambrian, Ordovician and Silurian distributions of non-stromatoporoid sponges are reviewed. The earliest Cambrian faunas contain mostly hexactinellids, with protomonaxonids dominating middle Cambrian assemblages. There are no obvious palaeobiogeographical patterns, with many genera being found widely. Vauxiids, lithistids and heteractinids are apparently confined to low latitudes, but this may be due to a poor fossil record. Most known Ordovician faunas are from low latitudes, although some high-latitude faunas are known, which contain reticulosan hexactinellids and protomonaxonids. There is some division of faunas within Laurentia, into eastern and western provinces, with the western assemblage extending across low northern latitudes during the Late Ordovician. During the Silurian Period, sponge diversity was very low during the Llandovery Epoch, probably partly owing to lack of habitat for taxa restricted to carbonate facies, and also because of sampling bias. There was a dramatic increase in diversity through the Silurian Period, mostly owing to an apparent diversification in the demosponges; however, there are many ghost lineages, indicating that their fossil record is poorly known. Non-lithistid sponges are very poorly known, with few recorded outside Euramerica. The currently available data for Early Palaeozoic sponges are too incomplete to allow any reliable palaeobiogeographical inferences. Supplementary material: the compilation of Silurian sponge occurrences is available at: http://www.geolsoc.org.uk/SUP18666.

Ordovician sponge spicules from Spitsbergen, Nevada and Newfoundland: new evidence for hexactinellid and demosponge early diversification

Unusually well-preserved spicule assemblages from three Lower to Middle Ordovician localities are examined: Spitsbergen (Floian-Dapingian), Nevada (Vinini Formation, Dapingian) and Newfoundland (Cow Head Group, Tremadocian-lower Darriwilian, and Table Head Group, mid-Darriwilian). The recorded diversity increases the knowledge of sponge spicules in the Lower Ordovician and their palaeogeographical distribution. Hexaster type microscleres and scopules, equinate hexactins, pinular hexactins and inflated pentactins and hexactins are the main spicule types amongst the Hexactinellida. Hemidiscs and a possible amphidisc are recorded and can be evidence of the presence of the hexactinellid subclass Amphidiscophora. Oxyasters, C-shaped sigmata and trianes are the main spicule types amongst demosponges. The presence of sigmata and oxyasters can also be evidence of the presence of two main orders of extant demospongids: Sigmatophora (now Spirophorida) and Astrophora. One modified octactinellid spicule type is found from Calcarea. The presence of echinate and pinulate hexactins amongst megascleres suggests a protective functional morphology. These features can be associated with an evolutionary tendency which is seen mainly amongst hexactinellids and demosponges for an efficient occupation of shallower settings and/or a defensive armoury strategy. A newly recorded flat-top pinulate form is also a suitable element for reinforcement or protection of any external surface. Some of the microscleres found can be considered ancient homologues of recent hexasters, sceptrules, hemi/amphidiscs and oxyasters. This work suggests that the microscleres of hexactinellid and demosponge subclasses were already present and significantly diversified in the Early Ordovician. The presence of definite axial canals in scopules provides key evidence for this statement. This finding is significant and confirms the presence of the Sceptrulophora in the early Palaeozoic. The microscleres comes from widely separated Lower Ordovician localities and possess similar forms to those found in Mesozoic microscleres, implying a conservative morphology.

Hierarchy of Factors Controlling Faunal Distribution: A Case Study from the Ordovician of the Argentine Precordillera

Abstract Limiting factors affecting faunal distribution in the Ordovician rocks of the Precordillera basin, western Argentina, are analyzed and tentatively established. Eight biotic intervals ranging from the Tremadoc to the Ashgill are defined based upon the distribution in time and space of articulate brachiopod, trilobite, sponge, and bryozoan genera. High magnitudes of turnover are recognized on the basis of vertical ranges of taxa; this observation also is supported by low indices of carryover and holdover. Times of high faunal change define boundaries between the biotic intervals. Abiotic constraints largely prevail over ecological factors in controlling most substantial shifts in faunal composition and distribution at the interval boundaries. Taxonomic composition through different intervals is consistent with a change in paleobiogeographic affinities throughout the Ordovician, from markedly Laurentian affinities in the lowermost Ordovician to increasing Gondwanic affinities towards the Middle and Late Ordovician. At a smaller scale, the development of successive intervals is overprinted strongly by changes in facies and tectonic-induced setting. The composition and structure of assemblages were modified dramatically in the late Arenig-early Llanvirn with the changeover from sponge and brachiopod-dominated assemblages to brachiopod and trilobite-dominated assemblages with abundant bryozoans. This change occurs in consort with a shift from carbonate facies to mixed carbonate-siliciclastic and exclusively siliciclastic facies, and is considered to be the result of sea-level rise and the development of subsiding depocenters in the northern and southern parts of the Precordillera basin. On paleoecological grounds, this episode has allowed the biotic intervals to be grouped in two distinct stages. The lower stage includes intervals 1 to 3, whereas the upper stage includes intervals 6 to 8; intervals 4 and 5 represent the diachronous arrangement of both types of assemblages due to the gradual southward flooding of the basin. Abiotic factors acted at different temporal and spatial scales. Hence, their effect on faunal composition is explained best by distinguishing hierarchical levels of incidence. On this basis, three major hierarchical levels of factors are proposed. The general context in which faunas developed is provided by the shifting position of the Precordillera terrane throughout the Ordovician; accordingly, a first-order hierarchical level is represented by paleogeographic position. The second-order level is represented by the combined effects of rapid sea-level rise and basin deepening, with a high paleoecological impact on the fauna. The third-order level includes single factors producing minor changes in otherwise stable community structures (e.g., minor sea-level changes, fluctuations in water temperature, different facies types).

A lowermost Ordovician tabulate-like coralomorph from the Precordillera of western Argentina: a main component of a reef-framework consortium

Abstract. Although putative corals of uncertain affinities occur in the early Cambrian, the earliest definite tabulate corals have not been described prior to the Early Ordovician in North America. This paper reports a new finding of a tabulate-like coralomorph forming part of biostratigraphically well-constrained reef mounds in the latest Cambrian—Early Ordovician La Silla Formation in the Argentine Precordillera. The oldest record of the coralomorph genus Amsassia is reported and a new species, A. argentina, is erected. The discovery of this genus in the lowermost Ordovician modifies the previously proposed paleogeographic distribution and patterns of origination and migration routes of this coral-like organism. Amsassia argentina n. sp. constitutes a main framework builder together with a complex microbial consortium. This oldest occurrence of Amsassia as a reef builder represents a new record of a skeletal organism in the gap of metazoan reef constructors after the demise of archaeocyaths in the late early Cambrian.