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The latest Neoproterozoic Ediacara biota are a collection of enigmatic soft-bodied eukaryotes that have been variously interpreted as both diploblasts and triploblasts, and which are thought to have been preserved as pyritic ‘death masks'. We perform decay experiments on sea anemones (Condylactis gigantea) and mollusks (Dolabella auricularia) under both ‘normal' (i.e., baseline) and Ediacaran-style conditions; this allows us to construct decay indices for both these organismal groups, test the influence of Ediacaran taphonomic scenarios on rates and patterns of decay, and potentially constrain the phylogenetic affinities of Ediacaran fossils. We find that in both types of organisms the most labile tissues are preferentially lost, and tissue type exerts a stronger control on preservation than overall diploblastic versus triploblastic organization. Thus, in sum, there is not a large differential of preservation between diploblastic and triploblastic organisms. Geochemical analyses of sediment from around the carcasses indicate that pyritic ‘death mask'-style preservation is dependent on the availability of Fe. Perhaps most importantly, experiments under Ediacaran-style taphonomic scenarios exhibit more rapid decay of labile tissues, such as anemone tentacles and the internal gut system of mollusks, than in baseline experiments. This finding highlights potential biases against the preservation of metazoan characters in the late Neoproterozoic, which may have influenced the interpretation of many iconic Ediacaran organisms.
We propose a new method to evaluate the diversity of ichnofossils from outcrop. Ichnodiversity (defined here as the number of ichnotaxa) characterizes paleoenvironmental conditions. However, the apparent numbers of ichnotaxa observed in outcrops are significantly affected by differences in areas of exposed outcrops. This study proposes a new method to evaluate ichnodiversity, independent of outcrop exposure bias, by using an image-resampling technique combined with the shareholder quorum subsampling method. In this method, the relationship between observed and detected numbers of ichnotaxa is estimated by subsampling from existing outcrop images. The relative diversity of ichnotaxa is obtained at a given value of the estimated coverage parameter, representing the ratio of the observed number of ichnotaxa to the actual diversity. The method was verified by analyzing artificial images of ichnoassemblages, and the method successfully estimated reasonable values of relative diversity of ichnotaxa. It was also suggested that the spatial distribution patterns of ichnofossils on the bedding planes does not affect the estimated intensity of ichnodiversity when using this method. This method was also applied to field data pertaining to deposits of the submarine channel-levee complex in the Oligocene Izaki Olistolith of the Nichinan Group, southwest Japan. As a result, the ichnodiversity of the successions in the Izaki Olistolith was reconstructed to be relatively high in channel deposits and low in levee deposits.
Terpenoids are hydrocarbons, comprising isoprene as their fundamental building blocks, many of which serve defensive functions in plants and protect them from potential enemies in the environment. Preservation of volatile terpenoids in amber is unusual and rarely detected in fossilized remains. In the present study, a remarkable preservation of monoterpenoid constituents are detected in Pliocene–Pleistocene ambers collected from the Siwalik strata of eastern Himalaya. Amongst the monoterpenoids, eucalyptol, borneol, α-pinene and p-cymene are detected in significant abundance. The sesquiterpenoid fraction comprises both biomolecules and geomolecules. Biological compounds include copaene, selinene, OH-bearing compounds such as spathulenol, globulol and α-cadinol as well as isomers of elemene and muurolene. Calamenene and dihydro-ar-curcumene are the diagenetically altered sesquiterpenoids recorded in the samples. High abundances of biotriterpenoids like β-amyrin and α-amyrin as well as moderate concentrations of altered products such as double-bonded ketones and alcohols generated from the parent triterpenoids are detected in the ambers. The results demonstrate that the monoterpenoids, which are particularly susceptible to degradation, could survive exhaustive diagenesis and alteration over a long span of time on rare occasions. The biosynthetic pathways of the secondary metabolite terpenoids, particularly the monoterpenoids, evolved in plants long back in time and are present in the angiosperm clades that evolved much later in Earth history. This indicates the unique efficiency of these volatile terpenoid compounds as defense tools in more evolved and complex biota.
Sclerochronology uses shell growth lines or bands for the construction of environmental time-series and the measurement of organism growth, but more study is needed to constrain the triggers of the dark cessation bands observed in many bivalve groups. We constructed a database of direct observations of modern growth seasonality across the class Bivalvia and compared the occurrence of seasonal growth bands to environmental data including latitude, temperature, and chlorophyll-a concentration. Bivalves with cold-season (winter) cessations are more common towards the poles, with logistic regression showing that temperature, followed by latitude of occurrence, displays the strongest relationship with occurrence of winter cessation. Remotely sensed and directly measured chlorophyll-a concentration show no significant relationship. Summer cessations are sparse and only weakly associated with environmental controls but are concentrated at the subtropical latitudes among temperate bivalves at their equatorial extremes. The rarity of summer cessations can be explained by the limited annual ranges of temperature in the tropics, combined with the exponential relationship of metabolic rate to temperature leading to a narrow window between normal functioning and mortality at high temperatures. This data suggests that, unless annual temperatures have low variability like in equatorial or polar regions, the season of growth cessation across bivalves is primarily a function of temperature tolerance through restriction of scope for growth. At most latitudes, growth bands can be interpreted as being primarily triggered by temperature stress, rather than seasonal starvation.