Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity

Moczyd?owska, M., Schopf, J.W. & Willman, S. 2009: Micro‐ and nano‐scale ultrastructure of cell walls in Cryogenian microfossils: revealing their biological affinity. Lethaia, Vol. 43, pp. 129–136. Recently established protocols and methods in advanced microscopy and spectrometry applied to studies of ancient unicellular organic‐walled microfossils of uncertain biological affinities (acritarchs) provide new evidence of the fine ultrastructure of cell walls and their biochemistry that support the interpretation of some such microfossils as photosynthesizing microalgae. The micro‐scale and nanoscale ultrastructure of the cell walls of late Cryogenian sphaeromorphic acritarchs from the Chichkan Formation (Kazakhstan) revealed by the advanced techniques and studied originally by Kempe et al. (2005) is here further analysed and compared with that of modern microalgal analogues. On the basis of such comparison, we interpret the preserved cell wall ultrastructure to reflect original layering and lamination within sub‐layers of the fossil wall, rather than being a result of taphonomic and diagenetic alteration. The outer thick layer represents the primary wall and the inner layer the secondary wall of the cell, whereas the laminated amorphous sub‐layers, 10–20 nm in thickness and revealed by transmission electron and atomic force microscopy, are recognized as trilaminar sheath structure. Because two‐layered cell walls, trilaminar sheaths and the position of the TLS within the fossil cell wall are characteristic of the mature developmental state in cyst morphogenesis in modern microalgae, we infer that the Chichkan sphaeromorphs are probably resting cells (aplanospores) of chlorophyceaen green microalgae from the order Volvocales. □Biological affinity, cell wall, Cryogenian, microfossils, ultrastructure.     

Early Cambrian small carbonaceous fossils (SCFs) from an impact crater in western Finland

We describe an assemblage of small carbonaceous fossils (SCFs) and acritarchs from cored siltstones of the Lappajärvi impact structure, west‐central Finland. Previous studies had detected a depauperate acritarch biota ascribed to a deep Proterozoic origin—this age, however, was based on recovery of long‐ranging poorly age‐diagnostic sphaeromorphs. To resolve the age and provenance of these crater sediments, we applied low‐manipulation processing techniques optimized for retrieval of larger organic‐walled microfossils. Our study revealed a previously undetected assemblage containing numerous metazoan SCFs consisting of flattened ‘protoconodonts’ (grasping spines assignable to total group Chaetognatha) and a distinctive fossilised chaeta, possibly representing the oldest known annelid remains. Phylogenetically problematic fossils include various acritarchs (large Leiosphaeridia sp., Tasmanites tenellus, smaller sphaeromorphs, Synsphaeridium, Archaeodiscina and Granomarginata) and filamentous forms (Palaeolyngbya‐ and Rugosoopsis‐like filaments, Siphonophycus), likely representing prokaryotic or protistan grades of organisation. As well as adding new diversity to an emerging SCFs record, these data substantially refine the age of these sediments by more than half a billion years, to an early Cambrian Terreneuvian age. More specifically, the assemblage is equivalent to that of the Lontova Formation from the Baltic States and northwest Russia, but is previously unreported from Finland. Identification of Lontova‐type SCFs/organic‐walled microfossils at Lappajärvi further constrains the poorly resolved extent of maximum flooding during the early Cambrian in Baltica. Renewed attention should be directed to strata that have thus far produced only biostratigraphically long‐ranging or ambiguous palynological assemblages—‘SCF‐style’ processing can reveal hitherto undetected, age‐informative microfossils that are otherwise selectively removed in conventional palynological studies.     

The nature and origin of nucleus‐like intracellular inclusions in Paleoproterozoic eukaryote microfossils

The well‐known debate on the nature and origin of intracellular inclusions (ICIs) in silicified microfossils from the early Neoproterozoic Bitter Springs Formation has recently been revived by reports of possible fossilized nuclei in phosphatized animal embryo‐like fossils from the Ediacaran Doushantuo Formation of South China. The revisitation of this discussion prompted a critical and comprehensive investigation of ICIs in some of the oldest indisputable eukaryote microfossils—the ornamented acritarchs Dictyosphaera delicata and Shuiyousphaeridium macroreticulatum from the Paleoproterozoic Ruyang Group of North China—using a suite of characterization approaches: scanning electron microscopy (SEM), transmission electron microscopy (TEM), and focused ion beam scanning electron microscopy (FIB‐SEM). Although the Ruyang acritarchs must have had nuclei when alive, our data suggest that their ICIs represent neither fossilized nuclei nor taphonomically condensed cytoplasm. We instead propose that these ICIs likely represent biologically contracted and consolidated eukaryotic protoplasts (the combination of the nucleus, surrounding cytoplasm, and plasma membrane). As opposed to degradational contraction of prokaryotic cells within a mucoidal sheath—a model proposed to explain the Bitter Springs ICIs—our model implies that protoplast condensation in the Ruyang acritarchs was an in vivo biologically programmed response to adverse conditions in preparation for encystment. While the discovery of bona fide nuclei in Paleoproterozoic acritarchs would be a substantial landmark in our understanding of eukaryote evolution, the various processes (such as degradational and biological condensation of protoplasts) capable of producing nuclei‐mimicking structures require that interpretation of ICIs as fossilized nuclei be based on comprehensive investigations.     

鄂尔多斯盆地三叠系延长组疑源类的荧光特征

在鄂尔多斯盆地三叠纪湖相沉积的延长组中,发现保存很好的藻类化石,荧光显微镜下显示很好的荧光;我们发现同层位产出的不同藻类化石具有不同的荧光强度,对这些藻类荧光强度的观察,符合以往国外学者提出的化石荧光与化石类别相关的结论。这也是我国首次用化石藻类荧光与生物属性关系进行研究的尝试,为今后利用荧光进行化石藻类分选和超微结构研究作了有益的探索。     

Fluorescence intensity studies of Triassic acritarchs from the Yanchang Formation in Ordos basin,northwestern China

Fluorescence properties of Early Cambrian acritarchs were investigated using Leica das Mikroskop (DM)microscopy with a mercury lamp.Well-preserved autoflurescence properties show a correlation between acritarchs morphology and the intensity of emitted fluorescence.In accordance with the fluorescence intensity of organic cell walls,two groups ofmicrofossils were distinguished.Results of observation in this study,which are consistent with those of the previous foreign studies,are in good agreement with regular difference in autofluorescence intensity among palynomorphs reported by McPhilemy (1998).Spores and algae,including Botryococcus,have very bright fluorescence while acritarchs often show less intense fluorescence.Dark brown microfossils have been reworked,and have little or no fluorescence.     

Palynostratigraphic,palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).     

Effects of a transient oxic period on mineralization of organic matter to CH4 and CO2 in anoxic peat incubations

Rates of organic matter mineralization in peatlands, and hence production of the greenhouse gases CH₄ and CO₂, are highly dependent on the distribution of oxygen in the peat. Using laboratory incubations of peat, we investigated the sensitivity of the anoxic production of CH₄ and CO₂ to a transient oxic period of a few weeks’ duration. Production rates during 3 successive anoxic periods were compared with rates in samples incubated in the presence of oxygen during the second period. In surface peat (5–10‐cm depth), with an initially high level of CH₄ production, oxic conditions during period 2 did not result in a lower potential CH₄ production rate during period 3, although production was delayed ～1 week. In permanently anoxic, deep peat (50–55‐cm depth) with a comparatively low initial production of CH₄, oxic conditions during period 2 resulted in zero production of CH₄ during period 3. Thus, the methanogens in surface peal—but not in deep peat—remained viable after several weeks of oxic conditions. In contrast to CH₄ production, the oxic period had a negligible effect on anoxic CO₂ production during period 3, in surface as well as deep peat. In both surface and deep peat, CO₂ production was several times higher under oxic than under anoxic conditions. However, for the first 2 weeks of oxic conditions, CO₂ production in the deep peat was very low. Still, deep peat obviously contained facultative microorganisms that, after a relatively short period, were able to maintain a considerably higher rate of organic matter mineralization under oxic than under anoxic conditions.     

Vibrio coralliilyticus Search Patterns across an Oxygen Gradient

The coral pathogen, Vibrio coralliilyticus shows specific chemotactic search pattern preference for oxic and anoxic conditions, with the newly identified 3-step flick search pattern dominating the patterns used in oxic conditions. We analyzed motile V. coralliilyticus cells for behavioral changes with varying oxygen concentrations to mimic the natural coral environment exhibited during light and dark conditions. Results showed that 3-step flicks were 1.4× (P = 0.006) more likely to occur in oxic conditions than anoxic conditions with mean values of 18 flicks (95% CI = 0.4, n = 53) identified in oxic regions compared to 13 (95% CI = 0.5, n = 38) at anoxic areas. In contrast, run and reverse search patterns were more frequent in anoxic regions with a mean value of 15 (95% CI = 0.7, n = 46), compared to a mean value of 10 (95% CI = 0.8, n = 29) at oxic regions. Straight swimming search patterns remained similar across oxic and anoxic regions with a mean value of 13 (95% CI = 0.7, n = oxic: 13, anoxic: 14). V. coralliilyticus remained motile in oxic and anoxic conditions, however, the 3-step flick search pattern occurred in oxic conditions. This result provides an approach to further investigate the 3-step flick.     

Fluorescence intensity studies of Triassic acritarchs from the Yanchang Formation in Ordos basin, northwestern China

Fluorescence properties of Early Cambrian acritarchs were investigated using Leica das Mikroskop (DM) microscopy with a mercury lamp. Well-preserved autoflurescence properties show a correlation between acritarchs morphology and the intensity of emitted fluorescence. In accordance with the fluorescence intensity of organic cell walls, two groups of microfossils were distinguished. Results of observation in this study, which are consistent with those of the previous foreign studies, are in good agreement with regular difference in autofluorescence intensity among palynomorphs reported by McPhilemy (1998). Spores and algae, including Botryococcus, have very bright fluorescence while acritarchs often show less intense fluorescence. Dark brown microfossils have been reworked, and have little or no fluorescence. __________ Translated from Acta Micropalaeontologica Sinica, 2006, 23(3): 309–312 [译自: 微体古生物学报]     

Absence of biomarker evidence for early eukaryotic life from the Mesoproterozoic Roper Group: Searching across a marine redox gradient in mid‐Proterozoic habitability

By about 2.0 billion years ago (Ga), there is evidence for a period best known for its extended, apparent geochemical stability expressed famously in the carbonate–carbon isotope data. Despite the first appearance and early innovation among eukaryotic organisms, this period is also known for a rarity of eukaryotic fossils and an absence of organic biomarker fingerprints for those organisms, suggesting low diversity and relatively small populations compared to the Neoproterozoic era. Nevertheless, the search for diagnostic biomarkers has not been performed with guidance from paleoenvironmental redox constrains from inorganic geochemistry that should reveal the facies that were most likely hospitable to these organisms. Siltstones and shales obtained from drill core of the ca. 1.3–1.4 Ga Roper Group from the McArthur Basin of northern Australia provide one of our best windows into the mid‐Proterozoic redox landscape. The group is well dated and minimally metamorphosed (of oil window maturity), and previous geochemical data suggest a relatively strong connection to the open ocean compared to other mid‐Proterozoic records. Here, we present one of the first integrated investigations of Mesoproterozoic biomarker records performed in parallel with established inorganic redox proxy indicators. Results reveal a temporally variable paleoredox structure through the Velkerri Formation as gauged from iron mineral speciation and trace‐metal geochemistry, vacillating between oxic and anoxic. Our combined lipid biomarker and inorganic geochemical records indicate at least episodic euxinic conditions sustained predominantly below the photic zone during the deposition of organic‐rich shales found in the middle Velkerri Formation. The most striking result is an absence of eukaryotic steranes (4‐desmethylsteranes) and only traces of gammacerane in some samples—despite our search across oxic, as well as anoxic, facies that should favor eukaryotic habitability and in low maturity rocks that allow the preservation of biomarker alkanes. The dearth of Mesoproterozoic eukaryotic sterane biomarkers, even within the more oxic facies, is somewhat surprising but suggests that controls such as the long‐term nutrient balance and other environmental factors may have throttled the abundances and diversity of early eukaryotic life relative to bacteria within marine microbial communities. Given that molecular clocks predict that sterol synthesis evolved early in eukaryotic history, and (bacterial) fossil steroids have been found previously in 1.64 Ga rocks, then a very low environmental abundance of eukaryotes relative to bacteria is our preferred explanation for the lack of regular steranes and only traces of gammacerane in a few samples. It is also possible that early eukaryotes adapted to Mesoproterozoic marine environments did not make abundant steroid lipids or tetrahymanol in their cell membranes.     

Early evolution of large micro‐organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic‐walled microfossils

The Strelley Pool Formation (SPF) is widely distributed in the East Pilbara Terrane (EPT) of the Pilbara Craton, Western Australia, and represents a Paleoarchean shallow‐water to subaerial environment. It was deposited ~3.4 billion years ago and displays well‐documented carbonate stromatolites. Diverse putative microfossils (SPF microfossils) were recently reported from several localities in the East Strelley, Panorama, Warralong, and Goldsworthy greenstone belts. Thus, the SPF provides unparalleled opportunities to gain insights into a shallow‐water to subaerial ecosystem on the early Earth. Our new micro‐ to nanoscale ultrastructural and microchemical studies of the SPF microfossils show that large (20–70 μm) lenticular organic‐walled flanged microfossils retain their structural integrity, morphology, and chain‐like arrangements after acid (HF‐HCl) extraction (palynology). Scanning and transmitted electron microscopy of extracted microfossils revealed that the central lenticular body is either alveolar or hollow, and the wall is continuous with the surrounding smooth to reticulated discoidal flange. These features demonstrate the evolution of large micro‐organisms able to form an acid‐resistant recalcitrant envelope or cell wall with complex morphology and to form colonial chains in the Paleoarchean era. This study provides evidence of the evolution of very early and remarkable biological innovations, well before the presumed late emergence of complex cells.     

Diverse small spinose acritarchs from the Ediacaran Doushantuo Formation, South China

The Ediacaran Doushantuo Formation in South China is underlain by the Cryogenian Nantuo Formation (glacial rocks) and overlain by the late Ediacaran Dengying Formation. It is characterized by well-preserved, large (normally >100 μm in size) spinose acritarchs (LSAs), which have been shown to be probably the only useful biostratigraphic tool for the global correlation of the early- and middle-Ediacaran. Acritarchs are organic microfossils normally known as single-celled eukaryotic organisms (protists). Although recent research suggests that some large spinose acritarchs may represent diapause egg cysts of metazoans, the biological affinities of the Ediacaran spinose acritarchs, especially for those displaying remarkable size ranges, are still debatable.Recently, smaller specimens of the Ediacaran spinose acritarchs have been found in cherts and phosphorites of the Doushantuo Formation in South China. Many described Ediacaran spinose acritarch taxa display large size variation (from tens to hundreds of microns in vesicle diameter), but some taxa only have smaller (<70 μm) specimens. The morphological comparison with Paleozoic counterparts indicates that some Ediacaran spinose acritarchs may have phylogenetic affinity to eukaryotic algae. More evidence, including wall ultra-structure, geochemical analysis and comparison with modern analogs, is needed to understand the biological affinity of the Ediacaran spinose acritarchs. The remarkable radiation of planktonic protists, characterized by abundant, diverse spinose acritarchs, occurred as early as in the late Neoproterozoic, i.e., 40–60 million years earlier than previously thought.     

Kinetic and Thermodynamic Analysis During Dissimilatory γ-FeOOH Reduction: Formation of Green Rust 1 and Magnetite

The oldest sedimentary rocks on Earth, the 3.8‐Ga Isua Iron‐Formation in southwestern Greenland, are metamorphosed past the point where organic‐walled fossils would remain. Acid residues and thin sections of these rocks reveal ferric microstructures that have filamentous, hollow rod, and spherical shapes not characteristic of crystalline minerals. Instead, they resemble ferric‐coated remains of bacteria. Modern so‐called iron bacteria were therefore studied to enhance a search image for oxide minerals precipitated by early bacteria. Iron bacteria become coated with ferrihydrite, a metastable mineral that converts to hematite, which is stable under high temperatures. If these unusual morphotypes are mineral remains of microfossils, then life must have evolved somewhat earlier than 3.8 Ga, and may have involved the interaction of sediments and molecular oxygen in water, with iron as a catalyst. Timing is constrained by the early in fall of planetary materials that would have heated the planet's surface.

Because there are no earlier sedimentary rocks to study on Earth, it may be necessary to expand the search elsewhere in the solar system for clues to any biotic precursors or other types of early life. Evidence from Mars shows geophysical and geochemical differentiation at a very early stage, which makes it an important candidate for such a search if sedimentation is an important process in life's origins. Not only does Mars have iron oxide‐rich soils, but its oldest regions have river channels where surface water and sediment may have been carried, and seepage areas where groundwater may have discharged. Mars may have had an atmosphere and liquid water in the crucial time frame of 3.9–4.0 Ga. A study of morphologies of iron oxide minerals collected in the southern highlands during a Mars sample return mission may therefore help to fill in important gaps in the history of Earth's earliest biosphere.     

Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

Constraints on Precambrian ocean chemistry are dependent upon sediment geochemistry. However, diagenesis and metamorphism can destroy primary biosignatures, making it difficult to consider biology when interpreting geochemical data. Modern analogues for ancient ecosystems can be useful tools for identifying how sediment geochemistry records an active biosphere. The Middle Island Sinkhole (MIS) in Lake Huron is an analogue for shallow Proterozoic waters due to its low oxygen water chemistry and microbial communities that exhibit diverse metabolic functions at the sediment–water interface. This study uses sediment trace metal contents and microbial abundances in MIS sediments and an oxygenated Lake Huron control site (LH) to infer mechanisms for trace metal burial. The adsorption of trace metals to Mn‐oxyhydroxides is a critical burial pathway for metals in oxic LH sediments, but not for the MIS mat and sediments, consistent with conventional understanding of Mn cycling. Micronutrient trace metals (e.g., Zn) are associated with organic matter regardless of oxygen and sulfide availability. Although U and V are conventionally considered to be organically complexed in suboxic and anoxic conditions, U and organic covary in oxic LH sediments, and Mn‐oxyhydroxide cycling dominates V deposition in the anoxic MIS sediments. Significant correlations between Mo and organic matter across all redox regimes have major implications for our interpretations of Mo isotope systematics in the geologic record. Finally, while microbial groups vary between the sampling locales (e.g., the cyanobacteria in the MIS microbial mat are not present in LH sediments), LH and MIS ultimately have similar relationships between microbial assemblages and metal burial, making it difficult to link trace metal burial to microbial metabolisms. Together, these results indicate that bulk sediment trace metal composition does not capture microbiological processes; more robust trace metal geochemistry such as isotopes and speciation may be critical for understanding the intersections between microbiology and sediment geochemistry.     

Influence of Microbial Composition on the Carbon Storage Capacity of the Mangrove Soil at the Land-ocean Boundary in the Sundarban Mangrove Ecosystem,India

Spatiotemporal assessment of the mangrove soil of Indian Sundarban revealed that decomposition rate of the organic matter was significantly lower in the anoxic condition than that of the oxic condition. Higher degree of enzyme activity in the oxic soil than the anoxic condition suggested that slower biomineralization in anoxic condition would facilitate long-term storage of organic matter in that particular ecosystem. Microbial population of nitrifying bacteria, phosphate solubilizing bacteria, cellulose degrading bacteria and fungi showed significant reduction in anoxic incubation than that in oxic incubation. In contrary, sulfate reducing bacteria and free living N2 fixing bacteria showed higher population in anoxic incubation indicating their preference for anaerobic condition. Soil CO₂ emission rate decreased with the increase in anoxicity and was largely dependent on the soil redox potential, organic carbon and microbial population of the mangrove soil.     

Viruses, prokaryotes and DNA in the sediments of a deep-hypersaline anoxic basin (DHAB) of the Mediterranean Sea

Viral and prokaryote abundance were investigated in a deep-hypersaline anoxic basin of the Eastern Mediterranean Sea (DHAB Atalante basin at c. 3000 m depth). This system was compared with two nearby deep-sea sites characterized by oxic conditions. Viral abundance and virus to prokaryote abundance ratio in hypersaline anoxic sediments displayed values close to those reported in oxic sites. The analysis of vertical profiles of viral abundance in the Atalante basin revealed the lack of significant changes with depth in the sediment, suggesting that benthic viruses in these anoxic and hypersaline conditions are preserved or resistant to decay. The anoxic basin displayed also very high concentrations of labile organic components (proteins and lipids) and extracellular DNA. These findings suggest that the DHAB sediments represent a reservoir for long-term preservation of benthic viruses and nucleic acids.     

A SERIES OF MONOGRAPHS AND MEMOIRS (ISSN 0300-9491) TO SUPPLEMENT LETHAIA

Acid-resistant organic microfossils are described from outcrops and a boring through the middle and upper units of the VisingsÖ Beds in southern Sweden. 21 previously described acritarch species have been identified, and five new species are erected. The acritarchs include simple, although characteristically ornamented sphaeromorphs, one polygonomorph species and one acanthomorph species. Their mode of occurrence and possible lithofacial relationships are discussed. The acritarchs from the middle unit of the Visingso Beds indicate a Late Riphean age. The upper unit contains acritarchs previously known from Vendian and Lower Cambrian strata in the U.S.S.R., China and elsewhere. A Vendian age is suggested for this unit. A glacial origin is suggested for boulder beds in the middle unit of the VisingsÖ Beds.     

Early post-mortem calcified Devonian acritarchs as a source of calcispheric structures

Uniquely preserved Late Devonian calcispheres were found in a core of the deep borehole Sosnowiec IG-1 (Upper Silesia, southern Poland). These enigmatic calcareous microfossils are interpreted here as acritarchs that underwent an early post-mortem calcification. Remnants of organic walls preserved in the calcispheres suggest that they represent various acanthomorphic acritarchs, characteristic members of the Palaeozoic marine phytoplankton. Taphonomic analysis combined with the light microscope and scanning electron microscope (SEM) observations of mineral and organic components of the investigated calcispheres suggest that a complex multi-stage process led to calcification of their in vivo non-mineralized acritarch forerunners. The ubiquity of acanthomorphic calcispheres in many Devonian shallow-water limestones is a testimony to little, thus far, documented acritarch crops that must have existed over extensive areas of carbonate-producing epicontinental seas. The scarcity of acritarchs described from Devonian shallow-water limestones may thus represent a taphonomic bias rather than real rarity or absence.     

Infrared video microscopy—an efficient-method for the routine investigation of opaque organic-walled microfossils

Summary Dark or even opaque organic-walled microfossils are frequently represented in palynological residues which are derived from sediments of high maturity. Such particles may be studied surficially by SEM or incident light, but internal details remain obscured. It has been known for some time that transparency may be achieved by applying infrared microscopy, but the method never became routine. We have now assembled an IR equipment which is based on a regular transmitted light microscope and tested our setup on various kinds of palyno-morphse.g. spores, acritarchs, chitinozoans, scolecodonts as well as phytoclasts and zooclasts. Our examples are from several localities of different stratigraphic and palaeogeographic position within the Palaeozoic. The studies revealed a high potential for IR video microscopy to become an important tool in the study of thermally altered organic particles in palynological slides.     

Biostratigraphy,biodiversity and palaeogeography of late Silurian chitinozoans from A1-61 borehole (north–western Libya)

High resolution biostratigraphic investigation of well preserved and highly diversified chitinozoan assemblages from core samples collected in the upper part of the Alternances Gréso-argileuses Formation (late Silurian) in well A1-61 (Tripolitania, north–western Libya) is undertaken. The chitinozoan assemblages document the range of several Pridolian taxa selected as index species for the global Silurian chitinozoan biozonation. Distribution of important accompanying species is also discussed. The abundance and the diversity of these chitinozoans are quantified and collated with the local environmental trends as deduced from the lithology, sedimentary features and from the associated organic walled microfossils (i.e. scolecodonts, eurypterid fragments, acritarchs, spores, leiospheres, cuticles and plant debris). The palaeogeographic distribution of late Silurian chitinozoans is discussed with a particular attention paid to a very distinctive chitinozoan assemblage characterising the middle part of the Pridoli of the Ibarmaghian Domain in northern Gondwana regions. Systematic notes, including detailed biometric investigations and SEM illustrations, are provided for selected taxa of taxonomic interest and useful for regional or more long distance correlation.