Emilia Jarochowska

The Paleozoic problematica Wetheredella and Allonema are two aspects of the same organism

Wetheredella is a calcareous encrusting microproblematicum known only from its two-dimensional expression in thin-sections. It occurs in Cambrian through Permian rocks, often in association with Rothpletzella and Girvanella, probable calcifying cyanobacteria. We show that Wetheredella can be produced by sectioning of the Paleozoic incertae sedis sclerobiont Allonema, which has until now been recognized based on its surface, 3D aspect. Therefore, we propose that Wetheredella should be regarded as a junior synonym of Allonema. Structures resembling Wetheredella have also been obtained in thin-sections through another encrusting microproblematicum, Ascodictyon, which, however, differs in the ultrastructure of its wall, which consists of parallel calcite crystals in contrast to radial crystals in Allonema. Allonema specimens used in the study were extracted from the Middle Silurian Mulde Brick-clay Member (Gotland, Sweden), representing an off-platform environment below the photic zone. This, together with the primary calcitic, radial wall structure of Allonema argues against a cyanobacterial or algal affinity, which had been considered for Wetheredella. The taxonomic position of Allonema remains unresolved, but the sclerobiont shares many common characteristics with some Paleozoic encrusting foraminifers. Combining the occurrence data on Allonema in its surface aspect and the “Wetheredella” aspect recognized in thin-sections may shed more light on its affinity and paleoecological functions, particularly its association with cyanobacteria and its role as a framework-builder in Paleozoic reefs.

An acid-free method of microfossil extraction from clay-rich lithologies using the surfactant Rewoquat

Marine rocks characterized by high clay content provide excellent conditions for fossil preservation, particularly for organic-walled microfossils such as retiolitid graptolites and chitinozoans. Nevertheless, the phyllosilicate minerals, which constitute the clay component, make microfossil extractions difficult. The problem results from the tendency of phyllosilicates to form aggregates in low pH values, as standard methods of microfossil extraction employ acids for rock digestion. Consequently, the use of acids for clay-rich rocks is often inefficient and time-consuming. We propose a method of rock disintegration using the surfactant Rewoquat and compare it with two commonly applied approaches: digestion in buffered acetic acid and in HCl-HF. Using examples from the Mulde Brick-clay Member from the Silurian of Gotland and the Daleje Shale from the Devonian of the Prague Basin we observed that disintegration in Rewoquat was faster (days) than digestion in acid (months), and allowed to recover calcareous in addition to organic-walled fossils. The yield and preservation was comparably good, except for conodonts, which were strongly etched after using HCl-HF. Retiolitid graptolites recovered using Rewoquat were preserved in 3D and showed a lower degree of fragmentation. The fossil content of the residue obtained using Rewoquat was higher due to dispersion of clay aggregates. For observation of delicate fossils we recommend to coat the sample with the surfactant. Application of Rewoquat can reveal the most delicate forms and growth stages, and thus provide a better insight into the ontogeny, autecology, and body size distributions of a number of fossil groups.

An unusual microbial-rostroconch assemblage from the Mulde Event (Homerian, middle Silurian) in Podolia, Western Ukraine

The Homerian (middle Silurian) Mulde Event is an extinction event affecting hemipelagic organisms associated with a positive stable carbon isotope excursion, and an increased proliferation of microbial deposits. The Event is recorded in the Bagovytsya section (Podolia, Ukraine), representing a carbonate ramp setting in the East European Craton, and recording the δ13Ccarb excursion with values reaching +5.17%. The associated facies include oncolitic beds, mud-mounds and thrombolitic buildups dominated by cyanobacteria and microproblematica with proposed bacterial affinities. The associated faunal assemblage includes a high proportion of rostroconchs and abundant gastropods and trilobites. The co-occurrence of grazing organisms and microbial sediments suggests that the microbial preservation was not caused by decreased grazing rates.

HIGH-RESOLUTION MICROTAPHOFACIES ANALYSIS OF A CARBONATE TIDAL CHANNEL AND TIDALLY INFLUENCED LAGOON, PIGEON CREEK, SAN SALVADOR ISLAND, BAHAMAS

ABSTRACT Taphonomic processes can substantially affect grain composition and textural parameters of carbonate sediment, but can also produce taphonomic signatures that provide supplemental information on the environment of deposition. The aim of the present study is a microtaphofacies analysis of a tidal channel–metahaline lagoon system (North Branch of Pigeon Creek, San Salvador Island, Bahamas). Samples (39 in total) were collected in 13 sites in three zones located along the channel: in the inlet, in the middle part, and in the inner lagoon. Grain composition and taphonomic features—abrasion, boring, calcification, encrustation, fragmentation, and dissolution—were recorded using (1) examination of loose grains belonging to the sand-size fraction, and (2) point counting in thin sections produced from sediment samples. Proportions of different grain types were not correlated in thin sections and loose sediment samples from the same sites, with the exception of algae, which were systematically recorded in thin section in higher proportions. In examination of loose grains, abrasion, fragmentation, and calcification were the most useful taphonomic features, but not in thin sections; only fragmentation rates were correlated between the two proxies. Taphonomic information did not distinguish new facies in surface sediment, but was useful in refining environmental resolution and broadened the interpretation of the environment of deposition. Discrimination between sites based on compositional and textural characteristics of the sediment was improved more by increasing the number of replicates by site than by inclusion of taphonomic data. Application of recognized microtaphofacies to the fossil record is limited, because very little of grain diversity and taphonomic information can be recovered from analysis of thin sections.

Conodonts in Silurian hypersaline environments: Specialized and unexpectedly diverse

Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.

Wear, tear and systematic repair: testing models of growth dynamics in conodonts with high-resolution imaging

Conodont elements are the earliest mineralized vertebrate dental tools and the only ones capable of extensive repair. Two models of conodont growth, as well as the presence of a larval stage, have been hypothesized. We analysed normally and pathologically developed elements to test these hypotheses and identified three ontogenetic stages characterized by different anisometric growth and morphology. The distinction of these stages is independently corroborated by differences in tissue strontium (Sr) content. The onset of the last stage is marked by the appearance of wear resulting from mechanical food digestion. At least five episodes of damage and repair could be identified in the normally developed specimen. In the pathological element, function was compromised by the development of abnormal denticles. This development can be reconstructed as addition of new growth centres out of the main growth axis during an episode of renewed growth. Our findings support the model of periodic retraction of elements and addition of new growth centres. Changes in Sr content coincident with distinct morphology and lack of wear in the early life stage indicate that conodonts might have assumed their mature feeding habit of predators or scavengers after an initial larval stage characterized by a different feeding mode.

Distinguishing Biologically Controlled Calcareous Biomineralization in Fossil Organisms Using Electron Backscatter Diffraction (EBSD)

Although carbonate-precipitating cyanobacteria are ubiquitous in aquatic ecosystems today, the criteria used to identify them in the geological record are subjective and rarely testable. Differences in the mode of biomineralization between cyanobacteria and eukaryotes, i.e. biologically induced calcification (BIM) vs. biologically controlled calcification (BCM), result in different crystallographic structures which might be used as a criterion to test cyanobacterial affinities. Cyanobacteria are often used as a ‘wastebasket taxon’, to which various microfossils are assigned. The lack of a testable criterion for the identification of cyanobacteria may bias their fossil record severely. We employed electron backscatter diffraction (EBSD) to investigate the structure of calcareous skeletons in two microproblematica widespread in Palaeozoic marine ecosystems: Rothpletzella, hypothesized to be a cyanobacterium, and an incertae sedis microorganism Allonema. We used a calcareous trilobite shell as a BCM reference. The mineralized structure of Allonema has a simple single-layered structure of acicular crystals perpendicular to the surface of the organism. The c-axes of these crystals are parallel to the elongation and thereby normal to the surface of the organism. EBSD pole figures and misorientation axes distribution reveal a fibre texture around the c-axis with a small degree of variation (up to 30°), indicating a highly ordered structure. A comparable pattern was found in the trilobite shell. This structure allows excluding biologically induced mineralization as the mechanism of shell formation in Allonema. In Rothpletzella, the c-axes of the microcrystalline sheath show a broader clustering compared to Allonema, but still reveal crystals tending to be perpendicular to the surface of the organism. The misorientation axes of adjacent crystals show an approximately random distribution. Rothpletzella also shares morphological similarities with extant cyanobacteria. We propose that the occurrence of a strong misorientation relationship between adjacent crystals with misorientation axes clustering around the c-axis can be used as a proxy for the degree of control exerted by an organism on its mineralized structures. Therefore, precisely constrained distributions of misorientations (misorientation angle and misorientation axis) may be used to identify BCM in otherwise problematic fossils and can be used to ground-truth the cyanobacterial affinities commonly proposed for problematic extinct organisms.

The Homerian carbon isotope excursion (Silurian) within graptolitic successions on the Midland Platform (Avalonia), UK: implications for regional and global comparisons and correlations

Abstract New δ13Ccarb data from the most stratigraphically extensive graptolitic sections of Homerian age in the study area are reported from Wenlock Edge and the Ludlow Anticline (UK). These sections, situated upon the Midland Platform (Avalonia), were key in establishing the Homerian graptolite biozonation used within the type Wenlock Series, and are consequently of international importance. Based upon 162 δ13Ccarb samples from four outcrops (Eaton Track, Longville–Stanway Road Cutting, Burrington Section and Mortimer Forest Stop 1), new graptolite collections and a re-evaluation of the original graptolite collections, the onset of both lower (older) and upper (younger) peaks of the Homerian Carbon Isotope Excursion have been calibrated to a revised graptolite biozonation (lundgreni – nassa – praedeubeli-deubeli – ludensis biozones). In addition, high-resolution correlation between the Ludlow Anticline and Wenlock Edge has been achieved by bio-, chemo- and sequence stratigraphic techniques. These correlations suggest a uniformity of depositional rates across the study area and indicate minor diachroneity at the base of the Much Wenlock Limestone Formation. Finally, correlations of the Midland Platform Homerian Carbon Isotope Excursion have allowed for better comparisons with other sections from which high-resolution graptolite and carbon isotope data are available. Such comparisons highlight the pan-regional synchronicity of the Homerian Carbon Isotope Excursion.

First report of Archaeoscyphia rectilinearis (Porifera) from the Wenlock of Gotland, Sweden

Abstract A specimen of the orchocladinid sponge Archaeoscyphia rectilinearis de Freitas (1989) is presented here as the first representative of the Anthaspidellidae from Wenlock strata on Gotland (Sweden). Other coeval occurrences are not yet known from the Silurian of Baltica. This specimen forms a link between the recently described sponge assemblage from Llandovery (Telychian) strata on Gotland and those from Wenlock to Ludlow strata from Arctic Canada. In addition, the specimen fills a gap in the fossil record of Silurian non-stromatoporoid sponges, which are poorly known worldwide.