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Cyanobacteria are ubiquitous in a variety of modern habitats, and siliciclastic sediments in particular are home to a wide diversity of microbial communities. Benthic microbial mats, typically established by cyanobacteria on modern Earth, were likely prevalent on Archean Earth, yet explicit traces of their ancestors in Archean siliciclastic rocks are difficult to detect. To understand the taphonomy of benthic microbial mats in sandy, subaquatic environments, cyanobacterial mats were incubated for five months under a range of temperatures representative of ambient (25°C) and eogenetic conditions (37°C, 70°C, and 100°C). Cyanobacterial materials including trichomes, sheaths, and extracellular polymeric substances (EPS) were analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and micro Raman spectroscopy. Textures were permineralized in all temperature regimes with phases that included mixed silicates, Na-carbonate, clays, gypsum-anhydrite, pyrrhotite, anatase, akaganeite, magnetite, natrojarosite, and ankerite. Pigments including chlorophyll, β-carotene, and scytonemin were identified in the lower temperature regimes, but were not easily detected in the samples incubated at 100°C. The morphological characteristics of trichomes and sheaths were maintained to some degree in all temperature regimes, but there was a higher relative abundance of EPS as temperatures increased. The profusion of EPS obscured the absolute differentiation between individual trichomes and sheaths at higher temperatures. The results indicate that over time, morphological, mineralogical, and carbonaceous features that formed at the end of these incubation experiments could collectively create the laminations characteristic of fossilized microbial mats found in sandstones throughout the geologic record. In Archean sandstones, where very little is preserved, these collective features may prove to be especially important in the detection of ancient life.
The late Albian Solaris dinosaur tracksite in Istria (Croatia), which is characterized by an abundance of theropod and sauropod footprints, was surveyed using a combination of GNSS georeferencing, terrestrial laser scanning, and photogrammetry. The entire outcrop was digitally captured with millimeter-scale resolution and high spatial accuracy, allowing both outcrop-scale observations and analysis of fine morphological features at the scale of individual footprints. Quantitative ichnological data acquired from the digital model is equivalent to published results based on traditional methods. Inspection of the digital outcrop model has revealed a number of new tridactyl footprints previously undocumented at the Solaris tracksite. All of the newly identified prints are exceptionally shallow and bear characteristics typical of medium-sized bipedal theropod dinosaurs. The study testifies to how approaches involving digital modelling are able to supplement and improve upon traditional methods of field observation and help revise previous ichnological studies. They can be a particularly effective solution for studying complex, heavily trampled tracksites with highly variable print depths and preservation.
Repaired fossil skeletons provide the opportunity to study predation rates, repair mechanisms, and ecological interactions in deep time. Trilobites allow the study of repaired damage over long time periods and large geographic areas due to their longevity as a group, global distribution, and well-preserved mineralized exoskeletons. Repair frequencies on trilobites from three sites representing offshore marine environments in the Iberian Chains (Spain) show no injuries on 45 complete redlichiid thoraces from Minas Tierga (Huérmeda Formation, Cambrian Series 2, Stage 4), or 23 complete Eccaparadoxides pradoanus thoraces from Mesones de Isuela (Murero Formation, Cambrian Series 3, Drumian). Ten injuries on 69 E. pradoanus thoraces from Purujosa (Murero Formation, Cambrian Series 3, Drumian) were noted. There is no evidence for laterally asymmetric predation or size selection on the trilobites in this study. Weak evidence for selection for the rear of the thorax is documented. A series of injured trilobites illustrates four stages of the healing process. Analysis of injury locations and frequency suggests that injuries to these trilobites are predatory in origin. Semilandmark analysis of previously described exoskeletons with unrepaired damage assigned to the ichnotaxon Bicrescomanducator serratus alongside newly collected damaged exoskeletons from Purujosa (Mansilla and Murero Formations, Stage 5, Drumian), Mesones de Isuela (Murero Formation, Drumian), and Minas Tierga (Huérmeda Formation, Stage 4) found that shapes of biotic and abiotic breaks could not be distinguished.
The lower Silurian Brassfield Formation crops out throughout eastern midcontinent North America as a prominent carbonate deposit representing a variety of shoal to shallow shelf environments. The upper portion of this unit throughout Ohio and Kentucky is characterized by the exceptional abundance of distinctive crinoid columnals historically referred to as ‘beads'. Although recognized for more than a century, the taxonomic identity of the crinoid responsible for producing these elements remains unknown; this is due largely to the taphonomic state of ‘beads', which are nearly always preserved as isolated ossicles not commonly discovered as articulated segments and never in direct association with identifiable calyx plates or arm segments. Recently discovered material from east-central Kentucky allows these columnals to be confidently attributed to the morphogenus Floricolumnus (col.) Donovan and Clark. These elements are now recognized as components of a coiled dististelar attachment structure. There is also an association between ‘beads' and crown material, namely small, uniserial, pinnulate brachials. Collectively, this material supports the interpretation that the producer of these columnals was a large diplobathrid camerate, almost certainly a dimerocrinitid.
The Cryogenian was a time of climatic extremes, with two extended and severe global glaciations bracketing hothouse conditions. The effect of these extreme climate conditions on ocean chemistry and the marine biosphere remain poorly understood. Most of the previous studies of the fossil record from this interval focus on benthic organisms, with few examples of organisms with an inferred planktonic lifestyle and no firm evidence for photosynthetic organisms. Here, we present helically coiled, straight, and curved fossils composed of fine crystalline or framboidal pyrite in limestone samples from the Ikiakpuk formation of Arctic Alaska. These structures are morphologically identical to fossils ofObruchevella, a cyanobacterial form genus reported in pre-Sturtian and post-Marinoan strata, but not in deposits from the Cryogenian non-glacial interlude. We interpret fossils of the Ikiakpuk formation as planktonic cyanobacteria based on their morphology, which is identical to that of some modern planktonic cyanobacteria. Further evidence for a planktonic lifestyle comes from the preservation of these pyritized fossils in deep-water facies that lack evidence of microbial lamination. They provide the first direct evidence for bacterial primary productivity in the pelagic realm during the Cryogenian non-glacial interlude.
We present taphonomic analyses of the Standing Rock Hadrosaur Site (SRHS), a vast Edmontosaurus annectens bonebed in the Maastrichtian Hell Creek Formation of South Dakota, which yields important insights into hadrosaurid paleobiology and environmental settings recorded by basal Hell Creek strata. Though Edmontosaurus bonebeds have been described from other Late Cretaceous formations in the Western Interior, namely the Lance, Prince Creek, and Horseshoe Canyon formations, our study provides the first thorough description of an Edmontosaurus bonebed from the Hell Creek Formation. SRHS is also the first formally described bonebed of E. annectens. Taphonomically, representation of every skeletal element, horizontality of most bones, and rarity of weathering and abrasion suggest brief preburial exposure and transport with minimal sorting bias. Near-universal disarticulation and disassociation, localized orientation of bones, and infrequent preburial breakage indicate moderate flow energy during deposition. Additional fauna, though rare, are indicative of a fluvial-coastal setting, and palynofloral analyses signify deposition in a small, shallow floodplain lake surrounded by cypress forests. Cumulatively, these data indicate that a herd of primarily subadult and adult Edmontosaurus died in a nearby fluvial setting in a mass mortality event and, following brief decay and scavenging by theropods, their bones were buried in a shallow floodplain pond by a flooding event/crevasse splay. Our findings provide supporting evidence for the hypotheses of gregarious herding behavior in hadrosaurids and age structuring of Edmontosaurus herds.