Phosphatic Microfossils Research Articles

Assessing chemostratigraphic and biostratigraphic correlation of Ediacaran phosphorites (Bocaina, Khesen and Doushantuo Formations): Diachronic, local signals of the Neoproterozoic phosphogenic-taphonomic event

The Neoproterozoic phosphogenic event records biotic and biogeochemical changes that seem to indicate intrinsic relationships between phosphogenesis and phosphatic fossil preservation. However, chemo-litho and biostratigraphic correlations conducted on fossiliferous, phosphatic successions of the Doushantuo Formation, South China, evidence a highly heterogeneous record, featuring diachronic depositional ages, variable taxonomic diversity and inconsistent, “noisy” δ13C records. Similar detailed chemo-litho and biostratigraphic correlations targeted in high-grade phosphorite strata have not been performed along Ediacaran units elsewhere. Considering the significance and scarcity of Ediacaran high-grade phosphorite strata, establishing a comprehensive correlation framework within and between those units is fundamental to understanding Ediacaran biogeochemical and biotic evolution. This work aims to integrate new δ13C isotope stratigraphy, and existing Doushantuo-Pertatataka acritarch assemblages and geochronological records from Bocaina (Brazil), Khesen (Mongolia) and Doushantuo (China) Formations. Phosphate rich strata in those units feature inconsistent δ13C signatures characterized by negative values showing large bed-to-bed variations, possibly reflecting local modification of original marine signals by phosphogenesis, suggesting caution against the use in global/regional chemostratigraphic correlation. The record of phosphatized acritarch assemblages shows contrastive taxonomic diversities, and a small number of common taxa including Megasphaera. Whereas the presence of biozonal Doushantuo Formation taxa on Khsesen Fm, resonates with growing views of strong environmental resilience and large temporal ranges of acanthomorph acritarchs throughout the Ediacaran-early Cambrian, the scarcity of similar taxa in the Bocaina Fm may question the reliability of Doushantuo Fm acritarch biozones in other paleogeographic or paleoenvironmental scenarios. Alternatively, the low taxonomic diversity recorded in Bocaina and Khesen Fms may also reflect the effects of the late Ediacaran decline in acritarch diversity. Further, combined evidence of selective paleoenvironmental phosphatization, local geochemical signals, narrow time spans and heterogeneous taxonomic diversity found in phosphatized DPK acanthomorph assemblages indicate the local nature of Ediacaran phosphatic fossil preservation events. Finally, evidence for diachronic preservation between high-grade phosphatic strata and phosphatized microfossils show non-causal relationships between Ediacaran phosphogenesis and phosphatic fossil preservation, highlighting the punctuated, intrinsic local nature of those events.

Ediacaran Doushantuo-type biota discovered in Laurentia

The Ediacaran period (635–541 Ma) was a time of major environmental change, accompanied by a transition from a microbial world to the animal world we know today. Multicellular, macroscopic organisms preserved as casts and molds in Ediacaran siliciclastic rocks are preserved worldwide and provide snapshots of early organismal, including animal, evolution. Remarkable evolutionary advances are also witnessed by diverse cellular and subcellular phosphatized microfossils described from the Doushantuo Formation in China, the only source showing a diversified assemblage of microfossils. Here, we greatly extend the known distribution of this Doushantuo-type biota in reporting an Ediacaran Lagerstätte from Laurentia (Portfjeld Formation, North Greenland), with phosphatized animal-like eggs, embryos, acritarchs, and cyanobacteria, the age of which is constrained by the Shuram–Wonoka anomaly (c. 570–560 Ma). The discovery of these Ediacaran phosphatized microfossils from outside East Asia extends the distribution of the remarkable biota to a second palaeocontinent in the other hemisphere of the Ediacaran world, considerably expanding our understanding of the temporal and environmental distribution of organisms immediately prior to the Cambrian explosion.

The cono-dos and cono-dont’s of phosphatic microfossil preparation and microanalysis

Scanning electron microscope (SEM) imaging of fossils allows unlocking ultrastructural information about their skeletal tissues, but sample preparation of biominerals forming their skeletons requires time, patience, and knowledge. SEM and associated analytical methods allow the observation of internal microstructure, shedding light on function, growth and chemistry. Sample preparation is the process by which material is fixed within a medium (e.g. epoxy resin), a transect created and surface defects removed. This step is arguably the most important in any SEM-based analysis, allowing for the acquisition of reliable, high quality data sets. When conducting any SEM-based technique, the presence of a flat surface is needed to collect consistent and reliable data. Surfaces with topography will both induce charging effects but will also compromise the reliability of data acquired. Techniques from material science are continuously adapted to palaeontological applications, in particular with respect to calcareous microfossils. However, similar studies have not been extensively conducted on bioapatite, owing in part to the difficulties faced in sample preparation alongside its susceptibility to electron beam damage. This case study focuses on conodonts, a marine vertebrate group ranging from the late Cambrian to the Late Triassic. They have been chosen as a model due to the abundance of material, complexity of internal tissues and previous work focused on histological features. With these phosphatic microfossils, we attempt to outline the process of sample preparation and provide information on how to avoid and overcome common pitfalls.

Origin of the Ediacaran Weng'an and Kaiyang phosphorite deposits in the Nanhua basin, SW China

The Weng’an and Kaiyang phosphorite deposits are located in Central Guizhou, southwestern China, and formed within the Sinian (Ediacaran) Doushantuo Formation. A systematic investigation of the geology, mineralogy, and geochemistry of these phosphorites was conducted to constrain the redox environment, origin, and genetic mechanisms of Ediacaran phosphorite. In the lower ore layer (layer A), the phosphate minerals are mainly globular phosphate intraclasts, which are characterized by special sedimentary and reworking textures, including microgranular sedimentary, isopachous cement, shrinkage crack, and spongy texture. The phosphate minerals from the upper layer (layer B) have microbial phosphate components, namely embryo- and algae-like microfossils. These phosphatic microfossils have biological structures, and both consist of interior structure and isopachous wall. The P2O5 concentration of phosphorites in layer A reach 32.31%, somewhat higher than that in layer B (28.7%), whereas the P content in globular phosphate intraclasts (40.05%) resemble that of the microbial phosphate component (41.19%). Rocks from layer A have “left-inclining” post-Archean Australian shale (PAAS)–normalized rare earth element (REE) distributions, with higher Ce anomalies (Ceanom) ranging from –0.12 to –0.01 (average –0.07). In contrast, rocks from layer B have “hat-shaped” PAAS–normalized REE distributions, with lower Ceanom ranging from –0.32 to –0.23 (average –0.28). These geochemical characteristics suggest that hydrothermal fluids mixed with normal seawater might have contributed to metallogenesis, and the redox environment transitioned from anoxic to oxic from layer A to B. We conclude that the lower Doushantuo globular intraclasts formed by the mechanical reworking of previous phosphatic sediments, which were dominated by the strong mechanical power of seawater. The upper Doushantuo microbial phosphorite, on the other hand, formed by microbially mediated accretionary growth.

Tubular microfossils from the Ediacaran Weng’an Biota (Doushantuo Formation, South China) are not early animals

The early Ediacaran Weng’an Biota (ca. 609 Ma) of the Doushantuo Formation (Guizhou Province, China) encompasses an abundant and exquisitely preserved assemblage of phosphatic microfossils that have provided unique insight into the origin and early evolution of multicellular eukaryotes. However, the affinities of these early organisms are far from certain, including the tubular microfossils Crassitubulus, Quadratitubus, Ramitubulus, and Sinocyclocylcicus. These taxa have been widely accepted as stem-cnidarians or, alternatively, interpreted as filamentous cyanobacteria, or multicellular algae. We use high-resolution X-ray tomographic microscopy to analyse the structure and development of the four taxa. Our data and analysis allow us to conclude that these four taxa were not biomineralized. Crassitubulus, Quadratitubus, and Sinocyclocylcicus, may be grouped on the basis that they exhibit alternating complete and incomplete cross walls, and bipolar growth; which makes them favourably comparable to filamentous cyanobacteria. In contrast, Ramitubulus exhibits only complete cross walls, unipolar growth and dichotomous branching. These features are difficult to reconcile with a cyanobacterial interpretation. They are, instead, more indicative of multicellular algae-like Cambrian Epiphyton. Thus, the Weng’an tubular microfossils constitute a disparate assemblage of cyanobacteria and algae, but none represents early Ediacaran animals.

Zinc Geochemical Cycling in a Phosphorus‐Rich Ocean During the Early Ediacaran

AbstractDuring the early Ediacaran, there was a large influx of phosphorus into the oceans and a resultant high phosphorus concentration in seawater, where multicellular eukaryotes may have been the primary type of marine productivity. The eukaryotes could play a critical role in regulating Zn cycling and isotopes. To establish Zn geochemical cycling patterns in the phosphorus‐rich ocean, this study investigates Zn isotopic signatures of shallow water phosphorite that contains phosphatized microfossils (Weng'an biota) and deep‐water shale from the Doushantuo Formation. Our results indicate that phosphorite commonly preserves heavier Zn isotope composition, with an average of 0.80‰. The positive δ66Zn values in phosphorites may be ascribed to Zn isotope fractionation associated with the complexation of Zn with phosphate and the adsorption of isotopically heavy Zn onto Fe‐Mn oxides and organism's surfaces. We argue that phosphorite may represent an important sink of isotopically heavy Zn in a phosphorus‐rich ocean during Earth history. Meanwhile, deep‐water organic‐rich shale shows an enrichment of isotopically light Zn with an average of 0.23‰, which may be attributed to sulfide precipitation in mid‐depth environment. The organic‐rich shale may represent an isotopically light Zn sink. In addition, the highest δ66Zn value (0.45‰) in a euxinic black shale may indicate that Zn isotope signal of anoxic deep water is similar to that of modern deep seawater. If that is the case, it suggests that Zn geochemical cycling in the early Ediacaran oceans was similar to that of modern oceans.

Warming after the big one

Climate Change The Chicxulub impact 65 million years ago, which caused the mass extinction at the Cretaceous-Paleogene boundary, also initiated a long period of strong global warming. Using data from phosphatic microfossils, including fish teeth, scales, and bone, MacLeod et al. estimated global average temperature. Immediately after the asteroid strike, temperatures increased by ∼5°C and remained high for about 100,000 years (see the Perspective by Lecuyer). These results are relevant to current climate projections, because the Chicxulub impact perturbed Earth systems on time scales even shorter than the current rate of change. Science , this issue p. [1467][1]; see also p. [1400][2] [1]: /lookup/doi/10.1126/science.aap8525 [2]: /lookup/doi/10.1126/science.aau1690

Postimpact earliest Paleogene warming shown by fish debris oxygen isotopes (El Kef, Tunisia).

Greenhouse warming is a predicted consequence of the Chicxulub impact, but supporting data are sparse. This shortcoming compromises understanding of the impact's effects, and it has persisted due to an absence of sections that both contain suitable material for traditional carbonate- or organic-based paleothermometry and are complete and expanded enough to resolve changes on short time scales. We address the problem by analyzing the oxygen isotopic composition of fish debris, phosphatic microfossils that are relatively resistant to diagenetic alteration, from the Global Stratotype Section and Point for the Cretaceous/Paleogene boundary at El Kef, Tunisia. We report an ~1 per mil decrease in oxygen isotopic values (~5°C warming) beginning at the boundary and spanning ~300 centimeters of section (~100,000 years). The pattern found matches expectations for impact-initiated greenhouse warming.

Experimental precipitation of apatite pseudofossils resembling fossil embryos.

Certain phosphatic grains preserved in the rock record are interpreted as microfossils representing a diversity of microorganisms from bacteria to fossil embryos. In addition to bona fide primary biological features, phosphatic microfossils and fossil embryos commonly exhibit features that result from abiotic precipitation or diagenetic alteration. Distinguishing between abiotic and primary biological features can be difficult, and some features thought to represent biological tissue could instead be artifacts that are unrelated to the original morphology of a preserved organism. Here, we present experimentally generated, abiotically produced mineral precipitates that morphologically resemble biologically produced features, some of which may be observed in the rock record or noted in extant organisms, including embryos. These findings extend the diversity of biomorphic features known to result from abiotic precipitation.

Multi‐jawed chaetognaths from the Chengjiang Lagerstätte (Cambrian, Series 2, Stage 3) of Yunnan, China

AbstractChaetognaths (arrow‐worms) are enigmatic in terms of their phylogenetic position, while the existence of Protosagitta spinosa from the Chengjiang Lagerstätte suggests minimal change in their unique bodyplan since at least the early Cambrian. Apart from rare (and sometimes controversial) soft‐bodied remains, the fossil record of chaetognaths is otherwise almost entirely dependent on early Palaeozoic phosphatic microfossils, some of which are placed amongst so‐called protoconodonts. Fused spine clusters are strikingly similar to the cephalic grasping apparatus of extant forms and are assumed to have had a comparable configuration. Here we report a new chaetognath, Ankalodous sericus gen. et sp. nov., coeval with Protosagitta but with a complex feeding apparatus consisting of multiple bundles of recurved spines whose principal function appears to have been grasping. Like all other chaetognaths a predatory mode of life is likely, but its position relative to the sediment–water interface is less certain. Reduction of the feeding apparatus, from the multi‐jawed arrangement of A. sericus to the grasping spines and associated smaller teeth seen in other chaetognaths, was probably a subsequent development and conceivably was linked with a shift to a pelagic mode of life. We also report a new specimen of Protosagitta. This confirms earlier observations but it possesses hitherto unrecognized features, including a cephalic tentacle and fin rays.

A LA-ICP-MS analysis of rare earth elements on phosphatic grains of the Ediacaran Doushantuo phosphorite at Weng'an, South China: implication for depositional conditions and diagenetic processes

AbstractThe Ediacaran Doushantuo Formation at Weng'an, South China hosts well-preserved phosphatized microfossils known as the Weng'an biota. A laser ablation ICP-MS analysis of rare earth element (REE) characteristics of the fossil-bearing black phosphorite unit of the Doushantuo Formation at Weng'an was conducted, with the aim of unravelling the depositional conditions and diagenetic processes during formation of the phosphorites. Spherical phosphatized microfossils and phosphatic clasts were analysed, and the REE data display middle REE (MREE) -enriched shale-normalized REE patterns. The spherical phosphatized microfossils show an increase in total REE contents (∑REE) from core to rim. Negative correlations between ∑REE and the extent of MREE enrichment over the other REE (indicated by LaN/SmN, YbN/SmN) are observed for analysed spots within individual phosphatic grains, which may be due to complex diagenetic history of the phosphatic grains, with fluctuations in REE sources and chemical parameters in a high-energy shallow-water environment being additional factors. The LaN/YbNand LaN/SmNcharacteristics of the phosphatic grains suggest they were mostly influenced by early diagenetic alteration rather than late extensive recrystallization. The negative Ce anomalies in the samples suggest they formed under oxic-bottom-water conditions. Positive Eu anomalies are present in all samples, and are likely to reflect involvement of hydrothermal fluids rather than changes in redox conditions of porewater. Overall this study has major implications for phosphorites as important archives for the study of geochemical proxies, the Ediacaran period and also evolutionary changes.

The impact of transport processes on rare earth element patterns in marine authigenic and biogenic phosphates

Rare earth elements (REEs) are commonly used proxies to reconstruct water chemistry and oxygen saturation during the formation of authigenic and biogenic phosphates in marine environments. In the modern ocean REEs exhibit a distinct pattern with enrichment of heavy REEs and strong depletion in cerium (Ce). The wide range of REE enrichment patterns found in ancient marine phosphates lead to the proposition that water chemistry has been very different in the Earth’s past. However, both early and late diagenesis are known to affect REE signatures in phosphates altering primary marine signals.Herein we present a dataset of REE signatures in 38 grain specific LA-ICP-MS measurements of isolated phosphate and carbonate grains in three discrete rock samples. The phosphates mainly consist of authigenic phosphates and phosphatized microfossils that formed in a microbially mediated micro-milieu. In addition, isolated biogenic and reworked phosphatic grains are also present. The phosphates are emplaced in bioclastic grain- to packstones deposited on a carbonate ramp setting in the central Mediterranean Sea during the middle Miocene Monterey event. The results reveal markedly different REE patterns (normalized to the Post Archean Australian Shale standard) in terms of total enrichment and pattern shape. Analyses of REE diagenesis proxies show that diagenetic alteration affected the samples only to a minor degree. Grain shape and REE patterns together indicate that authigenic, biogenic and reworked phosphates have distinct REE patterns irrespective of the sample.Our study shows that while REE patterns in phosphates do reflect water chemistry during authigenesis, they are often already heavily altered during reworking, a process, which can occur in geologically negligible timespans. REE patterns are therefore more likely to reflect complex enrichment processes after their formation. Similarities in the REE patterns of reworked and biogenic phosphate further suggest that the frequently observed hat-shaped pattern in biogenic phosphates can result from increased middle REE (Neodymium to Holmium) scavenging during taphonomic processes prior to final deposition. Cluster analysis coupled with sedimentological and previously published geochemical data (bulk carbon isotope and X-ray fluorescence spectrometry) allowed the characterization of REE patterns of phosphates in terms of their formation conditions and depositional history, such as the distinction of phosphates formed in situ from reworked and transported phosphate grains.

Possible algal origin and life cycle of Ediacaran Doushantuo microfossils with dextral spiral structure

In Ediacaran shallow-water dolomites of the Doushantuo Formation (ca. 570 Ma) of southern China, scarce phosphatized microfossils consisting of clusters of coil-like spheroids calledSpiralicellula bulbiferaand co-occurring spherical forms with helically arranged holes namedHelicoforamina wenganicaare interpreted to belong to the same taxon because both have a similar relative abundance and both, uniquely in the assemblage, exhibit a consistent dextral spiral feature—the oldest known fossil examples of fixed asymmetry. Thus, we interpret them as different stages of sexual and asexual life cycles in which the spiral structure was maintained throughout most of the developmental phases. While they can be placed with the acritarchs, we suggest they are a chlorophycean green alga, and like many Ediacaran macrofossils, may represent an extinct clade. This is compatible with the shoal-water marine depositional environment in which they lived, as it would have favored photosynthetic organisms over others kinds of encysting non-metazoan protists. This setting militates against their interpretation as putative embryos which has been put forward for a variety of forms co-occurring in the microfossil assemblage. The multiple affinities of the strikingly diverse biota remain far from resolved, but algal origins warrant further consideration.

A simulation experiment on phosphatized spherical microfossils

The identification and characterization of the phosphate preservation of the spherical microfossils in the Ediacaran Doushantuo time and the Early Cambrian has long been controversial. Modern artificial simulation experiments of phosphate animal eggs can provide scientific evidence to settle the aforementioned disputes. This experiment simulated environmental changes to observe the early morphological changes of the Bluntnose black bream eggs (Megalobrama amblycephala). It was found that the eggs can show a different morphology in the phosphate environment and that these eggs exhibited considerable morphological variations under different conditions. Thus, those eggs cannot be ascribed to the same species based only on the features. The experimental results are expected to provide a useful reference for the identification of phosphatized microfossils and provide a more empirical basis for their genesis.

FTIR microspectroscopy of Ediacaran phosphatized microfossils from the Doushantuo Formation, Weng'an, South China

Phosphatized microfossils from ca. 580Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960cm−1 and 2925cm−1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH3 and aliphatic CH2), and have an additional band of around 1595cm−1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH3/CH2 peak height ratios (R3/2) and aromatic CC/CH2 peak height ratios (RCC/2 values): (1) high-R3/2/low-RCC/2 type (R3/2=~0.2–1.0 and RCC/2~0–2), (2) low-R3/2/medium-RCC/2 type (R3/2=~0.2–0.6 and RCC/2=~1–4); and (3) low-R3/2/high-RCC/2 type (R3/2=~0.2–0.6 and RCC/2~1–8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R3/2 and RCC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria.

The onset of the Permo-Carboniferous glaciation: reconciling global stratigraphic evidence with biogenic apatite δ 18 O records in the late Visean

The δ 18 O values of phosphatic microfossils recovered from NW Ireland are used to determine the timing and magnitude of cooling associated with the onset of the Carboniferous glaciation. Microfossil fish δ 18 O apatite demonstrates a +2.4‰ (V-SMOW) shift, which, once corrected for δ 18 O seawater changes owing to evolving ice volumes, equates to an approximate 4.5 °C reduction in equatorial sea surface temperature between the earliest Asbian and the mid-Brigantian (late Visean). Both conodont and microfossil fish δ 18 O apatite indicate stabilization of an ‘icehouse’ climate during the Brigantian and into the Serpukhovian. Substantial late Visean cooling identified herein is in good agreement with global glacioeustatic records. Supplementary material: Further information on the palaeogeography, lithostratigraphy and palaeoenvironments of the sections examined as well as the sample composition and analytical methods of oxygen isotope analyses is available at http://www.geolsoc.org.uk/SUP18511 .

δ18O composition of conodont apatite indicates climatic cooling during the Middle Pridoli

Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian (Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont colour alteration index of the biogenic apatite did not exceed 1.5 reflecting only a minor thermal alteration. Conodont δ 18O apatite values range from 17.7 to 19.2‰ V-SMOW, with the average values around 18.3‰. Fossil apatite of fish exoskeleton microremains from the same samples have lower δ 18O apatite values ranging from 15.2 to 17.4‰ V-SMOW, with the average values around 16.3‰ V-SMOW. Palaeoseawater temperatures calculated from conodont apatite δ 18O range from 24.8–31.5 °C, those derived from fish δ 18O give approximately 10 °C higher temperatures ranging from 32.6 to 38.1 °C. The lower δ 18O apatite values of fish apatite and thus the unrealistic high palaeotemperatures are interpreted as a result of diagenetic alteration. A significant positive shift in conodont apatite δ 18O of + 1.1‰ at the boundary between the Vievis and Lapės formations is observed, coinciding with a major facies change. It is interpreted as a mid-Pridoli event within the O zarkodina eosteinhornensis Bizone, or the O zarkodina remscheidensis Biozone reflecting a major cooling event, which may have resulted in the formation of an ice sheet in high latitudes and a glacio-eustatic sea-level fall.

ENIGMATIC (BAR-LIKE) MICROSCOPIC PHOSPHATIC ELEMENTS FROM THE LOWER MISSISSIPPIAN OF IRELAND

Enigmatic Bar-like phosphatic microfossils have been isolated from early Mississippian acid insoluble residues of fully marine facies in Ireland where they are usually associated with other phosphatic elements such as conodonts and fish microfossils. Seven Bar-like element morphotypes are described together with a discussion on the possible affinities of these elements. Where coexisting in one sample, a number of the morphotypes resemble the morphology of disarticulated laminae of conulariid exoskeleton. Others differ and appear as discrete elements; it is possible that they represent different morphological parts of a single organism. Comparison of these elements is also made to morphologically similar elements occurring back as far as the Ordovician. However, without articulated material their identification remains in open nomenclature.

ENIGMATIC (BAR-LIKE) MICROSCOPIC PHOSPHATIC ELEMENTS FROM THE LOWER MISSISSIPPIAN OF IRELAND

Enigmatic Bar-like phosphatic microfossils have been isolated from early Mississippian acid insoluble residues of fully marine facies in Ireland where they are usually associated with other phosphatic elements such as conodonts and fish microfossils. Seven Bar-like element morphotypes are described together with a discussion on the possible affinities of these elements. Where coexisting in one sample, a number of the morphotypes resemble the morphology of disarticulated laminae of conulariid exoskeleton. Others differ and appear as discrete elements; it is possible that they represent different morphological parts of a single organism. Comparison of these elements is also made to morphologically similar elements occurring back as far as the Ordovician. However, without articulated material their identification remains in open nomenclature.

ENIGMATIC (BAR-LIKE) MICROSCOPIC PHOSPHATIC ELEMENTS FROM THE LOWER MISSISSIPPIAN OF IRELAND

Enigmatic Bar-like phosphatic microfossils have been isolated from early Mississippian acid insoluble residues of fully marine facies in Ireland where they are usually associated with other phosphatic elements such as conodonts and fish microfossils. Seven Bar-like element morphotypes are described together with a discussion on the possible affinities of these elements. Where coexisting in one sample, a number of the morphotypes resemble the morphology of disarticulated laminae of conulariid exoskeleton. Others differ and appear as discrete elements; it is possible that they represent different morphological parts of a single organism. Comparison of these elements is also made to morphologically similar elements occurring back as far as the Ordovician. However, without articulated material their identification remains in open nomenclature.