Significance of lignin and fungal markers in the Devonian (407 Ma) Rhynie Chert.

The Rhynie Chert (Lower Devonian, Scotland) hosts a remarkably well-preserved early terrestrial ecosystem. Organisms including plants, fungi, arthropods, and bacteria were rapidly silicified due to inundation by silica-rich hot spring fluids. Exceptional molecular preservation has been noted by many authors, including some of the oldest evidence of lignin in the fossil record. The evolution of lignin was a critical factor in the diversification of land plants, providing structural support and defense against herbivores and microbes. However, the timing of the evolution of lignin decay processes remains unclear. Studies placing this event near the end of the Carboniferous are contradicted by evidence for fungal pathogenesis in Devonian plant fossils, including from the Rhynie Chert. We conducted organic geochemical analyses on a Rhynie Chert sample, including hydropyrolysis (HyPy) of kerogen and high-resolution mass spectrometric mapping of a thin section, to elucidate the relationship between lignin and the potential fungal marker perylene. HyPy of kerogen showed an increase in relative abundance of perylene supporting its entrapment within the silicate matrix of the chert. Lignin monomers were isolated through an alkaline oxidation process, showing a distribution dominated by H-type monomers. G- and S-type monomers were also detected, preserved by rapid silicification. Polycyclic aromatic hydrocarbons including perylene, a known marker for lignin-degrading fungi, were also concentrated in the kerogen and found to be localized within silicified plant fragments. Our results strongly link perylene in the Rhynie Chert to the activity of phytopathogenic fungi, demonstrating the importance of fungal degradation processes as far back as the Early Devonian.

Rapid encapsulation of true ferns and arborane/fernane compounds fossilised in siderite concretions supports analytical distinction of plant fossils.

Fossilised true ferns (Pecopteris sp.) preserved in siderite concretions from the Mazon Creek Lagerstätte (Illinois) presented a unique opportunity to characterise the organic signatures of these late Carboniferous plants. Localised analyses of true fern fossils showed several highly abundant phytohopanoids and fernane/arborane derived aromatic products, which were present only negligibly within their siderite matrix, as well as from other types of fossilised plants. These terpenoids had been recognised in some extant ferns, but scarcely in sedimentary organic matter and their exact source remained ambiguous. The present fossil biomarker data confirms an ancient true fern origin. Furthermore, the excellent concretion preservation of a series of related terpenoid products provided a rare insight into their diagenetic formation. The benign properties of carbonate concretions could be exploited further for biomarker evidence of other fossilised organisms, with one important caveat being that biomarker signals attributed to isolated fossils be significantly distinct from background organic matter pervading the concretion matrix. For instance, hydrocarbon profiles of seed ferns (pteridosperms) and articulates (horsetails) also preserved in Mazon Creek concretions were indistinguishable from separate analysis of their concretion matrix, preventing biomarker recognition.

Microbially mediated fossil concretions and their characterization by the latest methodologies: a review.

The study of well-preserved organic matter (OM) within mineral concretions has provided key insights into depositional and environmental conditions in deep time. Concretions of varied compositions, including carbonate, phosphate, and iron-based minerals, have been found to host exceptionally preserved fossils. Organic geochemical characterization of concretion-encapsulated OM promises valuable new information of fossil preservation, paleoenvironments, and even direct taxonomic information to further illuminate the evolutionary dynamics of our planet and its biota. Full exploitation of this largely untapped geochemical archive, however, requires a sophisticated understanding of the prevalence, formation controls and OM sequestration properties of mineral concretions. Past research has led to the proposal of different models of concretion formation and OM preservation. Nevertheless, the formation mechanisms and controls on OM preservation in concretions remain poorly understood. Here we provide a detailed review of the main types of concretions and formation pathways with a focus on the role of microbes and their metabolic activities. In addition, we provide a comprehensive account of organic geochemical, and complimentary inorganic geochemical, morphological, microbial and paleontological, analytical methods, including recent advancements, relevant to the characterization of concretions and sequestered OM. The application and outcome of several early organic geochemical studies of concretion-impregnated OM are included to demonstrate how this underexploited geo-biological record can provide new insights into the Earth's evolutionary record. This paper also attempts to shed light on the current status of this research and major challenges that lie ahead in the further application of geo-paleo-microbial and organic geochemical research of concretions and their host fossils. Recent efforts to bridge the knowledge and communication gaps in this multidisciplinary research area are also discussed, with particular emphasis on research with significance for interpreting the molecular record in extraordinarily preserved fossils.

Fossil Biomarkers and Biosignatures Preserved in Coprolites Reveal Carnivorous Diets in the Carboniferous Mazon Creek Ecosystem.

The reconstruction of ancient trophic networks is pivotal to our understanding of ecosystem function and change through time. However, inferring dietary relationships in enigmatic ecosystems dominated by organisms without modern analogues, such as the Carboniferous Mazon Creek fauna, has previously been considered challenging: preserved coprolites often do not retain sufficient morphology to identify the dietary composition. Here, we analysed n = 3 Mazon Creek coprolites in concretions for dietary signals in preserved biomarkers, stable carbon isotope data, and macromolecular composition. Cholesteroids, metazoan markers of cholesterol, show an increased abundance in the sampled coprolites (86 to 99% of the total steranes) compared to the surrounding sediment, indicating an endogenous nature of preserved organics. Presence of unaltered 5α-cholestan-3ß-ol and coprostanol underline the exceptional molecular preservation of the coprolites, and reveal a carnivorous diet for the coprolite producer. Statistical analyses of in situ Raman spectra targeting coprolite carbonaceous remains support a metazoan affinity of the digested fossil remains, and suggest a high trophic level for the coprolite producer. These currently oldest, intact dietary stanols, combined with exquisitely preserved macromolecular biosignatures in Carboniferous fossils offer a novel source of trophic information. Molecular and biosignature preservation is facilitated by rapid sedimentary encapsulation of the coprolites within days to months after egestion.