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<jats:title>Abstract</jats:title> <jats:p>The preservation of organic biosignatures during the Proterozoic Eon required specific taphonomic windows that could entomb organic matter to preserve amorphous kerogen and even microbial body fossils before they could be extensively degraded. Some of the best examples of such preservation are found in early diagenetic chert that formed in peritidal environments. This chert contains discrete domains of amorphous kerogen and sometimes kerogenous microbial mat structures and microbial body fossils. Our understanding of how these exquisite microfossils were preserved and the balance between organic degradation and mineral formation has remained incomplete. Here, we present new insights into organic preservation in Proterozoic peritidal environments facilitated through interactions among organic matter, cations, and silica. Organic matter from Proterozoic peritidal environments is not preserved by micro- or cryptocrystalline quartz alone. Rather, preservation includes cation-rich nanoscopic phases containing magnesium, calcium, silica, and aluminum that pre-date chert emplacement and may provide nucleation sites for silica deposition and enable further chert development. Using scanning electron microscopy and elemental mapping with energy dispersive X-ray spectroscopy, we identify cation enrichment in Proterozoic organic matter and cation-rich nanoscopic phases that pre-date chert. We pair these analyses with precipitation experiments to investigate the role of cations in the precipitation of silica from seawater. Our findings suggest that organic preservation in peritidal environments required rapid formation of nanoscopic mineral phases through the interactions of organic matter with seawater. These organic-cation interactions likely laid the initial foundation for the preservation and entombment of biosignatures, paving the way for the development of the fossiliferous chert that now contains these biosignatures and preserves a record of Proterozoic life.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The carapace and plastron bones of fossil turtles are often characterized by bone modification features such as pits, grooves, and holes. The significance, origin, and frequency of these features remains unclear because they have not been described from the bones of modern turtles. Taxon-specific description and analyses of defleshed turtle shell is essential for assessing the paleoecological significance of bone modification features. This study focuses on bone modification features on carapace and plastron bones of the emydid turtle Trachemys scripta elegans. Four subadult and 14 adult turtle shells were examined for non-ontogenetic features such as pits, grooves, holes, wounds, abrasions, and pathological growth structures. Bone modification features were lacking on subadult specimens but observed on each adult. Shallow, circular to subcircular pits (similar to the ichnotaxon Karethraichnus lakkos), and to a lesser extent pit clusters, are the most common feature noted on T. scripta elegans shells. Although they occur on both the plastron and the carapace, they proved far more common on the lower shell. Sparsely distributed ring-shaped grooves similar to the ichnotaxon Thatchtelithichnus holmani were present on approximately half of the turtle plastra studied. Amorphous surface etching was observed on several turtles, most commonly near the plastron midline (posterior portion of the hyoplastron/anterior portion of the hypoplastron). Pathological responses to these marks are lacking, but were noted on two turtles in response to sustained injuries. The occurrence of circular and subcircular pits, pit clusters, and ring traces on the external surface of every adult turtle analyzed in this study (regardless of sex or geographical occurrence), their discrete size and shape, and the lack of evidence of a systemic pathological response by the host suggests parasites, possibly leeches, as the etiological agent responsible for these features.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A new bioclaustration of a symbiont is here described from the mantle cavity of the strophomenatan brachiopod Clitambonites schmidti. It is the second bioclaustration in brachiopods known from the Kukruse Regional Stage (Sandbian) of Estonia. It shares affinities with the bioclaustrations Burrinjuckia and Haplorygma. The outgrowth in the ventral valve interior was secreted by the brachiopod around a symbiont. Most likely the symbiont was a suspension feeder that collected food particles from the brachiopod's mantle cavity. The symbiont was either a kleptoparasite or fed on the brachiopod's feces (coprophagy). The majority of symbiosis cases in brachiopods in the Ordovician of Baltica involve clitambonitids as the hosts. Thus, clitambonitid brachiopods were more likely hosts for symbiosis than other brachiopods in the Ordovician of Baltica.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The earwigs, Dermaptera, are a group of insects which have been present since the Mesozoic. They have a relatively sparse fossil record, yet their life activities on and in soil or sediment leave traces with the potential for long-term preservation. These may include some burrows seen in Quaternary dunes and other sandy substrates. The well-known, cosmopolitan, sand-dwelling species Labidura riparia is examined as a potential model and reference for dermapteran tracemakers there and elsewhere in the geological record, through experimentally produced shelter burrows and trackways from wild-caught, laboratory-raised specimens. Shelter burrows were typically U-shaped with a pair of surface entrances, and these U-shapes could be additionally modified into Y-shapes or linked together to form a network. Trackways of L. riparia generally resembled those of other insects but may show features consistent with dermapteran anatomy such as tail-drag impressions produced by cerci.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Skeletobionts, organisms that attach to or bore into the skeleton of a host, provide a useful system to observe biological interactions over geological timescales. We examined skeletobionts on brachiopod hosts from a stratigraphic section in western New York State that spanned the Lower Kellwasser and Upper Kellwasser events, the two pulses of the Frasnian–Famennian (Late Devonian) mass extinction. The fossils are largely preserved as molds, and even endoskeletobiont borings are often visible with minimal preparation. At least seven major groups of skeletobiont are present including microconchids, stenolaemate and ctenostome bryozoans, hederelloids, and various borings attributed to sponges. The total frequency of skeletobiosis declined significantly across the first extinction pulse (Lower Kellwasser Event), and relative abundance patterns shifted, although the biotic and/or abiotic drivers of these changes require further study. Multivariable logistic regression indicates that large host body size was a strong and consistent predictor of skeletobiosis. Endoskeletobionts were more common in coarser lithologies, reflecting either an ecological preference for sands over muds or a bias against preservation in mudstones. Endoskeletobionts were also more common on ribbed/costate host shells.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Studies dealing with the colonization window typically emphasize two major features: duration (short term vs. long term) and frequency of colonization (episodic vs. continuous). However, our understanding of tide-influenced meander loops requires consideration of an additional feature, the architecture of the colonization window, which comprises not only the spatial dimension and geometry of the colonization surface, but also its evolution through time. Tide-influenced meander-loop systems show a heterogeneous trace-fossil distribution that reflects the variety of processes operating along the point-bar and overbank colonization surfaces. Ichnofabric analysis of tide-influenced meander-loop deposits from the Upper Cretaceous Tremp Formation (Pyrenees, Spain) provides valuable insights into the sedimentary and ichnological dynamics of these marginal-marine systems and allows the importance of stratal geometry controlling the colonization window to be evaluated. Six ichnofabrics are identified in point bars and associated overbank deposits. These ichnofabrics differ in bioturbation index (e.g., higher in the upper part than the lower-middle parts of point bars), preservation of primary sedimentary fabric (typically preserved in the lower-middle parts of point bars), inferred behavior and trophic types (e.g., dominance of dwelling or feeding structures in the lower-middle and upper parts of point bars, respectively), and other features such as depth of penetration, ichnotaxonomic composition, presence or absence of root trace fossils and/or mottling, or number of superimposed suites. The key environmental factor controlling the nature and distribution of ichnofabrics is the morphology of the point-bar lateral-accretion surfaces and their evolution through time. The architecture of the colonization window is here linked to the helicoidal flow and discharge changes in meandering channels, and the successive development of lateral accretion units with time.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Microbially induced sedimentary structures (MISS) are abundant in Ediacaran and lower Cambrian successions. However, the relationship between MISS distribution and facies has not been thoroughly explored in Ediacaran–Cambrian successions in South America. This study documents the occurrence of MISS and other potential biogenic structures from the late Ediacaran Serra de Santa Helena Formation in the Bambuí Group of eastern Brazil. This unit overlies the Cloudina-bearing Sete Lagoas Formation and is a mixed carbonate-siliciclastic succession devoid of macroscopic body fossils. Potential microbial structures include wrinkled structures such as “elephant-skin” and Kinneyia-like textures, as well as pustular structures and abundant positive epirelief discoidal structures. Another putative biogenic structure is a mm-wide meandering groove resembling a simple locomotion trail of a small vagile benthic metazoan. Microbial surface textures (i.e., “elephant skin” and Kinneyia-type wrinkles) were mainly observed in heterolithic deposits, usually at the interface between sandstone and siltstone/shale. On the other hand, discs show a facies-independent distribution, observed in heterolithic as well as carbonate and marl deposits. Petrographic analyses of these discs suggest that they have complex origins and some of them may be diagenetic structures. Thus, while facies may have strongly controlled the preservation of MISS-related structures and textures in the Serra de Santa Helena Formation, their abundance and diversity in tidal flat deposits indicate the wide distribution of matgrounds in these shallow marine paleoenvironments. Also, we demonstrate how detailed description and classification of simple features, such as discoidal structures, is an important task for paleoenvironmental reconstruction of marine ecosystems at the Ediacaran–Cambrian transition when the microbially bounded substrates played important roles in the dynamics of coastal environments.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>The sedimentary environments and redox conditions of the Lower Triassic Osawa Formation in the Southern Kitakami Terrane were reconstructed based on lithofacies, trace fossils, and other paleontological content. The muddy and sandy lithofacies of the Osawa Formation lack evidence of storm waves despite the presence of storm-induced turbidites, suggesting that the oldest deposits of the Osawa Formation were deposited in the proximal part of the outer shelf. In turn, water depth increased from the lower to upper part of the formation, ultimately recording the distal part of the outer shelf. In addition to sandy lithofacies caused by turbidity and traction currents, multiple sandy layers within the muddy lithofacies would have originated via supply into the prodelta setting from a fan delta system. Collapses of the delta front or river system floods could have generated hyperpycnal flows, resulting in abundant supplies of mud and organic matter. Trace fossil analyses revealed that the degree of bioturbation (ichnofabric indices) dramatically decreased as water-depth increased, indicating a steep oxic-dysoxic gradient along the onshore-offshore transect. Diagenetic pyrite framboids indicative of dysoxic/anoxic benthic conditions are abundant at greater water depths. Abundant pyrite framboids less than 6 μm in diameter suggest intermittent euxinicity. With an increase in global weathering, abundant sediment supply including organic matter from the fan delta system could have contributed to the development of ocean redox stratification, which appears to have impacted on the adaptation of both nektic and benthic animals of this area during the Early Triassic.</jats:p>