Preservation potential of Katharina tunicata and Mopalia muscosa (Mollusca, Polyplacophora) on two rocky shores of San Juan Island, Washington, USA

Taphonomic analyses of modern Katharina tunicata and Mopalia muscosa valves (Mollusca, Polyplacophora) collected from San Juan Island, Washington, USA, demonstrate that preservation potential of chitons varies by species and locality. Damage levels observed in the valves reflect differences in extrinsic environmental conditions, but intrinsic characters affect response of the skeletal material to local processes. Although both are more likely to become fossils at the lower energy site, K. tunicata valves in poor condition are more likely to occur than poor-condition M. muscosa valves at either site. Mopalia muscosa valves are least likely to be preserved at the higher energy site. Katharina tunicata valves appear more resistant to destruction than M. muscosa valves, suggesting that the former are more likely to become fossilized. The preservation potential of the three types of valves (normally one head, six medials, and one tail per individual) differs for both species. Medials are more likely to be preserved than either terminal valve of K. tunicata or M. muscosa . Head valves are least likely to be preserved, but K. tunicata heads are less likely to be preserved than M. muscosa heads. The biases are not due to greater frequency of medials, because valves do not occur in the 3:1 ratio seen in living animals. Divergence from expected ratios and variation in taphonomic condition for all three valve types in modern environments agrees with observations in the fossil record. The rarity of fossil chitons may be due to biases against preservation rather than absence in ancient environments.     

Comparison of in vitro decomposition of bryophytic and tracheophytic plant material

The fossil record of bryophytes is generally poor and infertile plants attributable to liverworts and mosses could also be thalloid vascular plant gametophytes or herbaceous lycophytes respectively. The paucity of the bryophytic record could be the result of relatively rapid degradation of bryophytic material in comparison to that of vascular plants, the absence of lignified cells in bryophytes sustaining this belief. However, certain organs of bryophytes are as robust as those of vascular plants (e.g. spores) and the hydroxybenzofuran polymers produced by some mosses could be of similar preservational resistance to lignins. A simplistic experiment to test the relative resistance to decay of bryophytic material has been undertaken. Its findings suggest that bryophytic plant material has a similar preservational potential to that of a selected vascular plant, particularly in organic rich sediments. This result may be construed to imply that bryophytes were indeed less abundant in the preservational environments of the Coal Measures than might be expected on the basis of current palaeoenvironmental reconstructions.     

Identification of lanostanes,A-ring methylated steranes and secosteranes in late Neoproterozoic crude oils by GC×GC-TOFMS: New insights into molecular taphonomy of steroids

The late Neoproterozoic marine succession (Marwar Supergroup) deposited in the Bikaner-Nagaur Basin in western India is an excellent provenance to study steroid biomarkers. Traditional one-dimensional gas chromatography mass spectrometry (GC–MS) and metastable reaction monitoring (MRM) transitions have been previously employed for routine biomarker analyses of crude oils and sediments. The present study with GC×GC-TOFMS (time-of-flight mass spectrometer) demonstrates an improved distribution of the sterane compounds segregated from the co-eluting n-alkanes, cycloalkanes and triterpanes in terminal Proterozoic crude oils. The steranes identified here offer novel insights into the molecular taphonomic alteration of eukaryotic lipids during the late Neoproterozoic. The presence of lanostane and 3β alkyl steranes is probably indicative of a depositional environment stressed by high salinity. To the best of our knowledge, this is the oldest known record of lanostane steroids found in the geosphere. Secosteranes with an open C-ring form as a result of diagenetic cleaving of carbon–carbon bonds. The concomitant presence of 2α-, 3β - and 4α-methyl steranes (A-ring methylated steranes) reflects specific biological input and a distinct palaeo-depositonal environment. The 3β - and 2α-methyl steranes probably form by migration of methyl substituents within the steroid structure. The recognition of a diverse range of steroid compounds by GC×GC-TOFMS advocates its excellent analytical potential in the study of natural products in geological samples. Hence, this state-of-the-art technology will be worth using for re-evaluating and investigating hydrocarbon biomarkers in order to minimize the gaps that exist in the understanding of biotic evolution over geological time scales.     

Formation and Taphonomy of Human Footprints in Microbial Mats of Present-Day Tidal-flat Environments: Implications for the Study of Fossil Footprints

This study concerns the formation, taphonomy, and preservation of human footprints in microbial mats of present-day tidal-flat environments. Due to differences in water content and nature of the microbial mats and the underlying sediment, a wide range of footprint morphologies was produced by the same trackmaker. Most true tracks are subjected to modification due to taphonomic processes, leading to modified true tracks. In addition to formation of biolaminites, microbial mats play a major role in the preservation of footprints on tidal flats. A footprint may be consolidated by desiccation or lithification of the mat, or by ongoing growth of the mat. The latter process may lead to the formation of overtracks. Among consolidated or (partially) lithified footprints found on present-day tidal flats, poorly defined true tracks, modified true tracks, and overtracks were most frequently encountered while unmodified and well-defined true tracks are rather rare. We suggest that modified true tracks and overtracks make up an important percentage of fossil footprints and that they may be as common as undertracks. However, making unambiguous distinctions between poorly defined true tracks, modified true tracks, undertracks, and overtracks in the fossil record will remain a difficult task, which necessitates systematic excavation of footprints combined with careful analysis of the encasing sediment.     

Effects of salinity and light on organic carbon and nitrogen uptake in a hypersaline microbial mat

Utilization of dissolved organic matter (DOM) is thought to be the purview of heterotrophic microorganisms, but photoautotrophs can take up dissolved organic nitrogen (DON) and dissolved organic carbon (DOC). This study investigated DOC and DON uptake in a laminated cyanobacterial mat community from hypersaline Salt Pond (San Salvador, Bahamas). The total community uptake of (3)H-labeled substrates was measured in the light and in the dark and under conditions of high and low salinity. Salinity was the primary control of DOM uptake, with increased uptake occurring under low-salinity, 'freshened' conditions. DOC uptake was also enhanced in the light as compared with the dark and in samples incubated with the photosystem II inhibitor 3(3,4-dichlorophenyl)-1, 1-dimethylurea, suggesting a positive association between photosynthetic activity and DOC uptake. Microautoradiography revealed that some DOM uptake was attributed to cyanobacteria. Cyanobacteria DOM uptake was negatively correlated with that of smaller filamentous microorganisms, and DOM uptake by individual coccoid cells was negatively correlated with uptake by colonial coccoids. These patterns of activity suggest that Salt Pond microorganisms are engaged in resource partitioning, and DOM utilization may provide a metabolic boost to both heterotrophs and photoautrophs during periods of lowered salinity.     

Organic preservation of fossil musculature with ultracellular detail

The very labile (decay-prone), non-biomineralized, tissues of organisms are rarely fossilized. Occurrences thereof are invaluable supplements to a body fossil record dominated by biomineralized tissues, which alone are extremely unrepresentative of diversity in modern and ancient ecosystems. Fossil examples of extremely labile tissues (e.g. muscle) that exhibit a high degree of morphological fidelity are almost invariably replicated by inorganic compounds such as calcium phosphate. There is no consensus as to whether such tissues can be preserved with similar morphological fidelity as organic remains, except when enclosed inside amber. Here, we report fossilized musculature from an approximately 18 Myr old salamander from lacustrine sediments of Ribesalbes, Spain. The muscle is preserved organically, in three dimensions, and with the highest fidelity of morphological preservation yet documented from the fossil record. Preserved ultrastructural details include myofilaments, endomysium, layering within the sarcolemma, and endomysial circulatory vessels infilled with blood. Slight differences between the fossil tissues and their counterparts in extant amphibians reflect limited degradation during fossilization. Our results provide unequivocal evidence that high-fidelity organic preservation of extremely labile tissues is not only feasible, but likely to be common. This is supported by the discovery of similarly preserved tissues in the Eocene Grube Messel biota.