Siliceous Sponges Research Articles

Occurrence of the siliceous sponge spicule Konyrium (Hexactinellida) in the upper Cambrian of the Mackenzie Mountains, northwestern Canada

The fossil record of siliceous sponges—Hexactinellida and demosponge “Lithistida”—hinges upon both body fossils plus isolated spicules mostly recovered from limestones by acid digestion. The earliest record of siliceous sponge spicules extends back to the late Neoproterozoic of Hubei, southern China (Steiner et al., 1993) and Mongolia (Brasier et al., 1997), and body fossils attributed to the hexactinellids have been described from the Ediacaran of South Australia (Gehling and Rigby, 1996); thus they are the oldest-known definite representatives of extant animal phyla. The Early Cambrian saw a remarkable diversification in spicule morphology, with the appearance of an essentially “modern” array of forms (Zhang and Pratt, 1994). While a diversity decline may have occurred with the late Early Cambrian extinction(s), the subsequent Paleozoic and Mesozoic fossil record of spicules shows a relatively consistent range of morphologies (e.g., Mostler, 1986; Bengtson et al., 1990; Webby and Trotter, 1993; Kozur et al., 1996; Zhang and Pratt, 2000). However, because spicule form is not restricted to individual taxa and many sponge species secrete a variety of spicule shapes, it is difficult to gauge true siliceous sponge diversity and to explore their biostratigraphic utility using only isolated spicules.

OCCURRENCE OF THE SILICEOUS SPONGE SPICULE KONYRIUM (HEXACTINELLIDA) IN THE UPPER CAMBRIAN OF THE MACKENZIE MOUNTAINS, NORTHWESTERN CANADA

The fossil record of siliceous sponges—Hexactinellida and demosponge “Lithistida”—hinges upon both body fossils plus isolated spicules mostly recovered from limestones by acid digestion. The earliest record of siliceous sponge spicules extends back to the late Neoproterozoic of Hubei, southern China (Steiner et al., 1993) and Mongolia (Brasier et al., 1997), and body fossils attributed to the hexactinellids have been described from the Ediacaran of South Australia (Gehling and Rigby, 1996); thus they are the oldest-known definite representatives of extant animal phyla. The Early Cambrian saw a remarkable diversification in spicule morphology, with the appearance of an essentially “modern” array of forms (Zhang and Pratt, 1994). While a diversity decline may have occurred with the late Early Cambrian extinction(s), the subsequent Paleozoic and Mesozoic fossil record of spicules shows a relatively consistent range of morphologies (e.g., Mostler, 1986; Bengtson et al., 1990; Webby and Trotter, 1993; Kozur et al., 1996; Zhang and Pratt, 2000). However, because spicule form is not restricted to individual taxa and many sponge species secrete a variety of spicule shapes, it is difficult to gauge true siliceous sponge diversity and to explore their biostratigraphic utility using only isolated spicules.

Coal deposition in the Noric Depression (Eastern Alps): raised and low-lying mires in Miocene pull-apart basins

Pull-apart basins often form during the late stage of continental collision. Provided appropriate climatic conditions, these basins may contain thick coal seams. Two such examples are the Leoben and Fohnsdorf Basins in Austria, which formed in Miocene times during the post-collisional stage of the Alpine orogene. Both basins contain more than 10 m of sub-bituminous coal. Despite similar ages, tectonic settings, and similar stratigraphic position of the seams within the sedimentary sequence, the properties of the coals differ significantly. The Leoben coal is quite clean, whereas Fohnsdorf coal is rich in ash and sulfur. Petrographical and geochemical methods were applied to reconstruct the environmental factors that controlled coal properties in these basins. Coal formation in the Leoben Basin commenced in an eutrophic, low-lying mire. This was followed by deposition of the main body of coal, which formed in an ombrotrophic raised mire. Coal deposition was only shortly interrupted by deposition of volcanic ashes. Due to an acid paleo-environment, the volcanic material within the raised mire was altered to kaolinite. Thick vitrain bands representing large trees are found in the lower part of the seam only. Siderite is the main inorganic constituent in the main coal body. Lakes within the raised mire were present during late stages of peat accumulation and were filled with sapropelic coal. The high ash yields of this facies are due to abundant siliceous sponges, siliceous crusts, and tuffogenic minerals. Finally, high subsidence rates caused the flooding of the raised mire and sapropelic shales were deposited in the newly formed lake. The iron, which formed siderite in the raised mire, and biogenic and abiogenic silica present in the lakes within the raised mire, are derived from volcanic ashes. The high-ash Fohnsdorf coal was formed in a low-lying mire close to a brackish lake. The seam geometry is controlled by the progradation of a fluvio-deltaic system into the brackish lake, and its subsequent drowning. The roof of the seam is formed by coquinoidal limestones and sapropelic shales. The brackish influence reduced the acidity of the mire, which resulted in the transformation of volcanic glass into montmorillonite. The low acidity also promoted high bacterial activity, which together with the high sulfate content of brackish waters, explains the high sulfur and pyrite content of the coal. Its high content of biogenic (?) methane is probably another effect of bacterial activity. Low-ash, low-sulfur coal is restricted to thin layers. These might represent local raised mires, which formed after the withdrawal of a fluvial system.

Why is stromatactis so rare in Mesozoic carbonate mud mounds?

The sedimentary–diagenetic structure stromatactis is widespread in Palaeozoic spiculitic carbonate mud mounds, but occurs only sporadically in Mesozoic sponge carbonate mud mounds. Comparative analysis of Palaeozoic and Mesozoic stromatactis limestones suggests that this variation results from the degree of siliceous sponge skeletal rigidity and the amount of internal sediment accumulation in the original cavity network. Partial to entire filling by internal sediment resulted in a continuum, from a small amount of internal sediment and large amount of cement (stromatactis, common in the Palaeozoic), to only internal sediments (aborted stromatactis, common in the Mesozoic). These observations match independent lines of evidence concerning the siliceous sponge evolution and sediment recycling (e.g. bioerosion) across the Palaeozoic to Mesozoic biotic revolution.

Bioconstructions a spongiaires siliceux dans le Lias terminal du Bassin lusitanien (Portugal); stratigraphie, sedimentologie et signification paleogeographique

Abstract The Upper Liassic series in the western border of Iberia (Lusitanian Basin, Portugal), show an important lutitic sedimentation, characterized generally by a monotonous marl/limestone alternation. Small scale siliceous sponge mudmounds occur in these deposits from Middle Toarcian to Lower Aalenian age. The scope of this work is to pinpoint the stratigraphical and sedimentological context and to characterize controlling factors of the spongioliths. Stratigraphic and facies analysis. Relevant sections were observed and investigated in different locations of the Lusitanian Basin (e.g., Alvaiazere, Porto de Mos, Rabacal, Coimbra and Cantanhede) (fig. 1). The siliceous sponge facies correspond to the upper part of the S. Giao Unit and to the lower part of the Povoa da Lomba Unit (fig. 2). Considering the sequential scheme of Duarte [1997], the sediments correspond to groups of third-order depositional sequences MST3 and MST4 (mainly in the upper part of this sequence: MST4B). The sedimentary evolution of these units shows a stacking pattern composed of shallowing upward sequences deposited in an outer homoclinal ramp setting, dipping northwestwards. Both units increase in thickness from south to north (fig. 3) and their vertical facies associations correspond to a very bioturbated (Chondrites, Zoophycos, Planolites and Thalassinoides) marl/limestone succession (figs. 4 and 5). MST3 is demonstrably more marly than MST4B. The base of MST4 [MST4A in Duarte, 1997] corresponds to a marl/marly limestone alternation, very poor in siliceous sponge mudmounds. The first unit (MST3) which includes sponge mudmounds is dated as uppermost Bifrons zone through the base of the Bonarellii zone. The majority of the siliceous sponge mudmounds occur within this time slice. These mounds are characterized by a great diversity of accompanying fauna mainly composed of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves. The initial growth of the sponge build-ups can be correlated basin-wide to the intra Bifrons regional flooding surface (MST2/MST3 boundary). The second unit (MST4), particularly its upper part (MST4B), corresponds to the top of the Meneghinni-Opalinum interval and is related to a carbonate progradational phase. In the eastern part of the basin, the calcareous facies of MST4B are more bioclastic. Siliceous sponge mudmounds. The Toarcian mudmounds of the Lusitanian Basin are usually only a few decimetres thick and most display irregular knob-like to flat lenticular morphologies. Some build-ups are round and can reach 1,5 metres in thickness and ten metres in diametre. Also worth mentioning is a siliceous sponge biostrome developed at the base of MST3 in the Porto de Mos section (figs. 3 and 6). The upper mound surface is normally rough and uneven. In both sequences they are always related laterally with carbonate beds, which corresponds to the top of fourth order sequences. The mudmounds consist of mostly brownish iron-rich calcified siliceous sponges and a greyish, sometimes peloidal allochthonous micritic matrix. In general, the sponges themselves consist of dense leiolitic microbolites [automicrites sensu Reitner and Neuweiler, 1993]. The sponge spicules are diagenetically transformed into calcite. The great majority of the sponge specimens belong to the Hexactinosa (Class Hexactinellida) and are unknown and undescribed to date. "Lithistides" (polyphyletic desma-bearing demosponges) are very rare and only occur as forms encrusting Hexactinosan sponges. The benthic macrofauna is abundant and consists of monospecific crinoids, rhynchonellids, terebratulids and bivalves (mainly pectinids and ostreids). Encrusting organisms are serpulids, bryozoans and foraminifera, as well as "Lithistids" mentioned above. They are entirely restricted to the stratonomical surfaces of the siliceous sponges. The sponge bioherms consist of several microfacies types (wackestones, packstones, floatstones and boundstones). All of them are micrite dominated and represent low energy environments. They differ mainly in the amount of siliceous sponges, micrite, microbialites and the accompanying fauna. Palaeoenvironmental significance. The amount of microbial induced carbonate clearly mirrors the importance of microbial activity in respect of the reef building potential. Furthermore, three other controlling factors played an important role in the initiation of the siliceous sponge mudmounds of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation rate. The role of the first two factors is evident because the siliceous sponge mudmounds are particularly important (abundance and volumetric expression) in the eastern part of the basin (Rabacal-Alvaiazere region). They are practically absent towards the west (essentially in MST4B) where the series show hemipelagic sedimentologic features (figs. 7 and 8). Reduced sedimentation rate is a precondition for the settlement of siliceous sponges and Hexactinosa in particular. Compared to all other Toarcian sequential units, MST3 and MST4B are the thinnest and therefore reflect the lowest sedimentation rates (fig. 8).

Silicification in the Belt Supergroup (Mesoproterozoic), Glacier National Park, Montana, USA

Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal‐CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica‐secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.

Secular Distribution of Biogenic Silica through the Phanerozoic: Comparison of Silica‐Replaced Fossils and Bedded Cherts at the Series Level

Abstract The modern marine silica cycle is dominated by silica‐secreting phytoplankton, principally diatoms, but this cycle has evolved considerably during the Phanerozoic. We analyzed the temporal distribution of silica‐replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies. Trends in silica replacement of fossils match faunal radiations during the Ordovician and Siluro‐Devonian, with peaks in silica replacement in the Late Ordovician and Middle Devonian corresponding to peaks in the abundance and diversity of siliceous sponges. Sharp drops in the abundance of silica‐replaced fossils and/or bedded cherts coincide with four of the five major mass extinctions. No discernible decrease marks the extinction at the end of the Triassic. We expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling. Orogenic episodes, which may trigger continental glaciati...

Upper Devonian Sponges from the Holy Cross Mountains, Central Poland

The rich fauna of Late Devonian (Late Frasnian) siliceous sponges from the Holy Cross Mountains, Poland is composed of 15 species and 11 genera. Both astylospongid demosponges (lithistids) and hexactinosan hexactinellids are present. The following new genera and/or species are proposed: D regulara Rigby and Pisera sp. nov., Jazwicella media Rigby and Pisera gen. et sp. nov., Astyloscyphia irregularia Rigby and Pisera gen. et sp. nov., A. turbinata Rigby and Pisera gen. et sp. nov., Astylotuba modica Rigby and Pisera gen. et sp. nov., Paleoregulara cupula Rigby and Pisera gen. et sp. nov., Paleoramospongia bifurcata Rigby and Pisera gen. et sp. nov., Cordiospongia conica Rigby and Pisera gen. et sp. nov., Paleocraticularia elongata Rigby and Pisera gen. et sp. nov., P gigantia Rigby and Pisera gen. et sp. nov., Polonospongiadevonica Rigby and Pisera gen. et sp. nov., P fistulata Rigby and Pisera gen. et sp. nov., Urnospongia modica Rigby and Pisera gen. et sp. nov., and Conicospongia annulata Rigby and Pisera gen. et sp. nov. The investigated fauna contains the youngest astylospongiids known and the oldest well‐preserved, and most diversified Palaeozoic hexactinosans. The sponge fauna constituted a significant element of a brachiopod‐coral‐sponge assemblage that inhabited a deep slope of the local Dyminy Reef structure, during its final phase of growth, in a clearly hemipelagic setting. This fauna is limited to the intrashelf depression within an incipiently drowned carbonate platform.

Codon usage in the siliceous sponge Geodia cydonium: highly expressed genes in the simplest multicellular animals prefer C- and G-ending codons

en

Marine carbonate facies in response to climate and nutrient level: The upper carboniferous and permian of central spitsbergen (Svalbard)

Carbonate-dominated successions of the Gipsdalen and Tempelfjorden Groups from Svalbard record a significant shift from Photozoan to Heterozoan particle associations in neritic settings during the late Palaeozoic. During the Bashkirian, benthic particle associations which included photoautotrophs such as phylloid algae (Chloroforam Association) characterised shallow subtidal environments. Most depositional settings which endured siliciclastic terrestrial input exhibited poorly diversified associations dominated by brachiopods, bryozoans and siliceous sponges (Bryonoderm Association). During the Moscovian to Asselian, highly diversified associations typified by various calcareous green algae,Palaeoaplysina, Tubiphytes, fusulinids, smaller and encrusting foraminifers (Chloroforam Association) prevailed in carbonate sediments from supratidal to shallow subtidal environments. During the Sakmarian and Early Artinskian, oolitic carbonate sands (Chloroforam Association) typified intertidal flats, whereas shallow subtidal environments were occupied by moderately diversified associations with fusulinids, smaller foraminifers, echinoderms and bryozoans (Bryonoderm-extended Association) and poorly diversified associations with echinoderms, brachiopods and bryozoans (Bryonoderm Association). During the Late Artinskian to Kazanian, poorly diversified associations characterised by brachiopods, echinoderms and bryozoans (Bryonoderm Association), and sponge-dominated associations (Hyalosponge Association) reigned within siliceous carbonates of intertidal and shallow subtidal environments.

Reservoir characteristics of Devonian cherts and their control on oil recovery: Dollarhide field, west Texas

Approximately 70 million bbl of oil have been produced from the chert-dominated Thirtyone Formation at Dollarhide field. The Thirtyone Formation is Devonian in age and contains two reservoir intervals, an upper dolomite and a lower chert, separated by nonporous limestone. The chert reservoir contains approximately 83% of the original oil in place (OOIP) and consists of two different facies--laminated microporous chert and burrowed chert. The laminated microporous chert was apparently deposited as sponge spicule sands (grainstones) in channels and fans on the slope of the Tobosa basin. The burrowed chert facies was apparently deposited as burrowed mixtures of sponge spicules, siliceous mud, and carbonate mud in broad slope environments between fans and channels. Spicules are thought to be derived from disaggregation of siliceous sponges living on the slope. Early marine and/or meteoric diagenesis dissolved many sponge spicules and reprecipitated the silica as microcrystalline chalcedony and quartz, resulting in spicule-moldic porosity and microporosity in the originally siliceous strata. The burrowed chert underwent small-scale differential compaction that produced short, discontinuous fractures, 1-10 mm in length and 0.1-0.5 mm in width. Production from the field has been related to amounts and characteristics of the pore systems. The thickness of the porous chert ranges laterally from 0 to 80 ft (0-24 m), and pore volume (fh) ranges from 0 to 20 pore ft (0-6 pore m). The laminated microporous chert is very homogeneous and has high porosity (25-35%) and uniform permeability (5-30 md). Areas dominated by laminated microporous chert had moderate primary recovery (approximately 200,000 bbl of oil per well), excellent waterflood production (1-2.5 (Begin page 36) million bbl of oil per well), and poor 20-ac (8.1 ha) infill production (<20,000 bbl of oil per well). The burrowed chert has more heterogeneous porosity (5-30%) and permeability (<1-100 md). The dolomite also has heterogeneous pore networks that have porosities of 3-15% and measured permeabilities of less than 1-200 md. Wells having substantial amounts of porous dolomite and/or burrowed chert had moderate to good primary recovery (200,000-300,000 bbl of oil per well), moderate waterflood recovery (300,000-1,100,000 bbl of oil per well), and moderate 20-ac (8.1 ha) infill recovery (50,000-100,000 bbl of oil per well). Oil recovery from the CO2 flood is expected to average approximately 250,000 bbl per well, having slightly higher recovery (300,000 bbl) in areas that have thick laminated microporous chert and/or burrowed chert. Oil recovery from the homogeneous, laminated microporous chert during the CO2 flood is improved by large amounts of residual oil and good sweep efficiency but decreased by the previously efficient sweeping of original mobile oil by the waterflood. The CO2 recovery from the heterogeneous, burrowed chert is improved by significant amounts of unswept mobile oil but decreased by poorer sweep efficiency. As a field, primary recovery is estimated at 13% of OOIP, waterflood at 30% of OOIP, infill at 3.5% of OOIP, and CO2 flood at 11% of OOIP.

Discovery of a “Living Dinosaur”: Globally unique modern hexactinellid sponge reefs off British Columbia, Canada

Globally unique hexactinellid sponge reefs occur on the continental shelf off British Columbia, Canada. They cover about 425 km2 of seafloor on the continental shelf off British Columbia (Canada) in water depths between 165 and 240 metres and occur on a low-angle deep shelf, iceberg scoured seafloor, characterized by very low sedimentation rates and very stable environmental conditions. The sponge bioherms are up to 19 metres high with steep flanks, whereas the biostromes are 2–10 metres thick and many kilometres wide. They all consist of dense populations of only seven hexactinellid species. Three of them, all hexactinosan species (Aphrocallistes vastus, Heterochone calyx, Farrea occa) are the main frambuilders, composing a true rigid framework of sponge skeletons encased in a organic rich matrix of modern clay baffled by the sponges. Growth rates of hexactinosan sponges range in the order of 0–7 centimetres per year. The base of the oldest sponge reefs date from approximately 9000 years b.p. Different invertebrate and fish faunas occupy the reefs than occur on adjacent seafloor areas and some species appear to use the sponge reef complex structures as refugia where they can hide. Sidescan sonar data and direct observation by manned submersible clearly show that large areas of sponge reefs have been heavily damaged by seafloor trawling in the past decade. These unique extant siliceous sponge reefs can be used as a modern analogue for a better understanding and interpretation of fossil siliceous sponge reefs, known from many ages and many locations world wide.

Sponge paraphyly and the origin of Metazoa.

In order to allow critical evaluation of the interrelationships between the three sponge classes, and to resolve the question of mono- or paraphyly of sponges (Porifera), we used the polymerase chain reaction (PCR) to amplify almost the entire nucleic acid sequence of the 18S rDNA from several hexactinellid, demosponge and calcareous sponge species. The amplification products were cloned, sequenced and then aligned with previously reported sequences from other sponges and nonsponge metazoans and variously distant outgroups, and trees were constructed using both neighbour-joining and maximum parsimony methods. Our results suggest that sponges are paraphyletic, the Calcarea being more related to monophyletic Eumetazoa than to the siliceous sponges (Demospongiae, Hexactinellida). These results have important implications for our understanding of metazoan origins, because they suggest that the common ancestor of Metazoa was a sponge. They also have consequences for basal metazoan classification, implying that the phylum Porifera should be abandoned. Our results support the upgrading of the calcareous sponge class to the phylum level.

A VARIED MIDDLE ORDOVICIAN SPONGE SPICULE ASSEMBLAGE FROM WESTERN NEWFOUNDLAND

Abundant and morphologically fairly diverse siliceous sponge spicules of Darriwilian (Middle Ordovician) age were collected from the Table Cove Formation of Port au Port Peninsula, western Newfoundland. Without co-occurrence of appropriate sponge body fossils, it is difficult to refer these disarticulated sponge spicules to existing or to new taxa. However, representatives of demosponges and hexactinellids are recognized. While marked differences are noted between older and younger lithistid dendroclones, for example, spicule form in these Middle Ordovician specimens is similar overall to previously known assemblages of various ages, demonstrating the morphologically conservative nature of the spicular skeleton.

A varied Middle Ordovician sponge spicule assemblage from western Newfoundland

Abundant and morphologically fairly diverse siliceous sponge spicules of Darriwilian (Middle Ordovician) age were collected from the Table Cove Formation of Port au Port Peninsula, western Newfoundland. Without co-occurrence of appropriate sponge body fossils, it is difficult to refer these disarticulated sponge spicules to existing or to new taxa. However, representatives of demosponges and hexactinellids are recognized. While marked differences are noted between older and younger lithistid dendroclones, for example, spicule form in these Middle Ordovician specimens is similar overall to previously known assemblages of various ages, demonstrating the morphologically conservative nature of the spicular skeleton.

ABSTRACT: Upper Jurassic Cotton Valley Reef Types and Porosity Distribution, East Texas Basin

Abstract Detailed petrologic study of the Cotton Valley Limestone and application of Upper Jurassic reef analogues from Europe and North Africa has resulted in the development of an integrated exploration model specific to the East Texas Basin. A variety of reef types exhibiting multiple stages of reef growth occur along the Cotton Valley Reef trend. Cotton Valley reefs grew along the shelf margin from the Late Kimmeridgian to the Kimmeridgian - Tithonian boundary. The reefs are composed of corals, calcareous algae, calcareous and siliceous sponges, and microbolites that formed a progression from shallow to deeper water. The reefs were composed of stacked biostromes or low-relief bioherms. Their pinnacle character or prominent elevation above the sea floor was manifested because reef growth occurred at a time of strongly starved sedimentation within the basin that was associated with arid or semi-arid climatic conditions. Multi-stage diagenesis has affected porosity, and structural variability along the trend further complicates the geology. Coral-rich reef facies that originally had greater amounts of metastable aragonitic skeletal material were deposited in shallower water depths. These skeletons were more susceptible to leaching and the creation of secondary biomoldic pores. These reefs also had greater potential for subaerial exposure and freshwater diagenesis that resulted in the partial leaching and the creation of matrix microporosity. Those reefs that were located in structurally favorable positions for more frequent or prolonged periods of exposure appear to have thicker and more numerous porous zones. Substantial macroporosity was occluded by late burial cementation; however, early hydrocarbon migration helped to preserve porosity.

Environmental changes in pre-evaporitic Late Miocene time in the Lorca Basin (SE Spain): diatom results

Abstract During the Late Miocene the Lorca Basin, which was an integrated part of the Betic Strait, underwent desiccation. The present investigation was completed to determine the response of the diatom flora to environmental changes before this event. Near the city of Lorca (Province of Murcia, SE Spain) the outcrop area around the Serrata ridge was studied and a stratigraphic section of 125m has been measured and sampled (115 samples). The Serrata ridge is capped by a 25m thick gypsum bed (Gypsum Member of the Serrata Formation). The profile below these gypsum layers comprises marl, sandstone, gypsum and partially laminated diatomite (Varied Member of the Serrata Formation). The frequency of radiolaria, siliceous sponge spicules, silicoflagellates, ebridians, diatoms and diatom species has been determined quantitatively. For the lower 87.5m of the section, which contains sufficiently well preserved diatoms, an age of latest Miocene (6.6-5.3 Ma; Magnetic Epoch 6 and 5) can be assigned by the presence of the diatom species Asterolampra acutiloba and Nitzschia reinholdii , applying the stratigraphic ranges of the species used in the North and Equatorial Pacific and in the Atlantic. In total, 116 diatom species and species-groups have been identified. However, the number of species per sample varies only between 13 and 36 species (a minimum of 300 valves counted). This low diversity, together with a dominance of one species-group, Thalassionema nitzschioides (up to 80%), which tolerates considerable ecological changes (e.g. in salinity) indicate that environmental stress must have prevailed during the sedimentation of the Varied Member of the Serrata Formation. Abundance fluctuations of this species-group, as well as changes in the composition of the remaining diatom assemblage and decreases in abundances of the siliceous microfossil groups, reflect repeated periods of interruption of the still marine influenced deposits before the deposition of thick gypsum layers. Several repetitive changes are indicated, where each cycle is characterized by an increase in meroplanktonic species (e.g. Paralia sulcata, Actinoptychus senarius ) and by a contemporary decrease in the holoplanktonic species towards the top, suggesting progressive environmental stress and a shallowing of the basin. Five of these cycles are clearly distinguished characterizing the initial stages of the evaporitic desiccation event in the Lorca Basin. Benthic diatoms are present only in low abundances (2–4%) and show no correlation with these fluctuations.

Siliceous sponge Spicule - like forms from the Neoproterozoic Bhander Limestone, Maihar Area, Madhya Pradesh

Siliceous sponge Spicule - like forms from the Neoproterozoic Bhander Limestone, Maihar Area, Madhya Pradesh

Decline in Mesozoic reef-building sponges explained by silicon limitation

Several unrelated clades of siliceous sponges proliferated on the shelves of the Jurassic Tethys Sea, becoming prominent builders in reefs and near-shore mounds1,2,3,4. Many of these builders are characterized by massive, rock-like skeletons made of spicules with a characteristic terminal hypersilicification4,5. Such hypertrophied spicules are generically known as desmas, irrespective of their phylogenetic origin5. Desma-bearing sponges virtually disappeared from reefs and other neritic environments during the Cretaceous and the Early Tertiary1,2,4,6, but have subsisted in relict populations in deeper, bathyal waters5,7,8. The causes of the decline and bathymetric shift of these sponges remain obscure. Here we show experimentally that the concentration of silicic acid in seawater modulates the phenotypic expression of the various spicule types genetically available in a sponge species. We also show that the concentration of this nutrient in Recent surface waters is insufficient for this species to secrete its desmas. These findings indicate that silicon limitation, probably aggravated in shallow waters by the diatom burst around the Cretaceous–Tertiary boundary9,10, may have forced neritic sponges with desmas to either lighten their skeletons or move to deeper, silicon-rich environments.

Cambrian Radiolaria from the Georgina Basin, Queensland, Australia

Well-preserved Middle Cambrian radiolarian were recovered from the Beetle Creek Formation, Inca Formation, and Devoncourt Limestone, Georgina Basin, Australia. Unlike previously reported Early Paleozoic radiolarians, these Middle Cambrian Radiolaria have two diagnostic characteristics in common with siliceous sponge spicules that occur in the same faunas: their skeletal elements consist of hollow tubes, and when a spicule is present, all spicular skeletons, including the internal spicule, are point-centered. The faunas contain 6 new genera and 18 new species; most of them have one or more spicular skeletons. Two new families are established. One family has a spherical shell wall with one or more point-centered internal spicule(s); it is named here Archeoentactiniidae n. fam. and consists of three new genera, Archeoentactinia, Fungomacula, and Spongomassa. The other new family, Palaeospiculumidae n. fam., consists of a point-centered spicular skeleton without a spherical shell capsule; the family is composed of the single genus Palaeospiculum n. gen. The taxa belonging to this family might perhaps be considered to be sponge spicule, but they differ from sponge spicule because their skeleton has the same basic structure as in Palaeoscenidiidae, that is, the spicule consists of apical and basal rays. In addition to these genera belonging to the two families, two other genera are present. They are uncommon and are of uncertain taxonomic position at the family level. One of the two, Aitchisonellum n. gen., has a spherical shell without internal spicule, but it belongs to neither Inaniguttidae Nazarov and Ormiston 1984 nor Palaeoactinosphaeridae Noble 1994; it seems to fit best in the informal group of Palaeoactinomiids Holdsworth (1977). The other genus, Protobiramus n. gen., has no typical internal spicule, but consists of a biaxially arranged four-rayed spicule and repeatedly bifurcated branches of those rays; the branch tips are not merged. The difference in faunal compositions among the three Middle Cambrian formations examined is not great: the Beetle Creek Formation, Inca Formation, and Devoncourt Limestone faunas consist of 7, 23, and 8 species, respectively.