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The Ischigualasto Formation was deposited in a fluvial system and contains a very well preserved Triassic flora. This flora comprises seven taphofacies: (1) C(St), autochthonous silicified roots of low-statured, woody plants associated with low-sinuosity channels and crevasse-splay deposits; (2) A(Sm/Fm), autochthonous carbonized roots or root impressions of herbaceous plants in crevasse-splay and levee deposits; (3) B(Fsm/Fm), autochthonous root halos of herbaceous plants associated with levee deposits; (4) EI(St), allochthonous silicified tree trunks and charcoal associated with high- and low-sinuosity channel bars; (5) GI(St), leaf cuticles and charcoal associated with trough cross-bedded sandstone; (6) FH(Fl/C), leaf cuticles and impressions associated with palynomorphs in abandoned-channel deposits; and (7) D(Fl), autochthonous silicified stumps in abandoned-channel deposits. Taphonomically, the Ischigualasto Formation can be divided into four parts, and these partially correspond to changes in the environment. The basal part (∼0–45 m) includes the transition from the underlying Los Rastros Formation and is characterized by taphofacies 1 and 2. The fluvial sediments were deposited during tectonic subsidence of the basin, resulting in development of a fluvioaxial system. The next part (∼45–300 m) is characterized by taphofacies 1, 2, and 3 and associated with calcic paleosols that formed under a dry seasonal climate. The middle-upper portion (∼300–600 m) contains all the taphofacies associated with argillic paleosols, which were formed during a time of increasing humidity. The upper portion (∼600–700 m) is characterized by taphofacies 1, 2, and 3, associated with immature paleosols that formed under a dry seasonal climate. The changes in humidity during deposition of the Ischigualasto Formation may have resulted from an increase in rainfall generated on the western side of Pangea by maximal development of the megamonsoon during the middle Carnian Age. The climatic signal in the Ischigualasto Formation was probably modified by the tectonosedimentary development of the basin.
Determining the selectivities, rates, and key agents of destruction in the post-mortem accumulation of skeletal carbonate is important for understanding the possible bias in the fossil record. This is particularly important for tropical settings, since they are areas of high biodiversity today and in the past. Bivalve death assemblages of the San Blas Archipelago, Caribbean Panama, were collected from ten carbonate and siliciclastic shelf environments, across a range of water depths, sediment compositions, grain sizes, water chemistries, biological communities, and porewater chemistries. Taphonomic signature can differ significantly among environments and individual sites. Shells from reefal carbonates display high levels of encrustation, macroscopic bioerosion, edge rounding, and surface alteration, while those from nonreefal carbonates display microboring, patchy to pervasive chalkiness, and pitting. Shells from siliciclastic sediments display extensive staining, moderate-high patchy surface alteration, occasional encrustation, root etching, and microboring. Patterns of damage in these death assemblages reveal strong differences in taphonomic processes among environments that bear additional testing, in particular contrasts between exposure and burial, carbonate versus siliciclastic, and microstructure and organic content. Death assemblages in siliciclastic sites show moderate-low taphonomic damage, whereas carbonate reefs show high levels of damage, and this contrasts with predictions based on temperate siliciclastics, where seasonal dissolution is intense. The differences observed in taphonomic conditions across the range of environments in this study suggest that the fossil records of these environments should be biased relative to each other.
Drill holes in prey skeletons are the most common source of data for quantifying predator-prey interactions in the fossil record. To be useful, however, such drill holes need to be identified correctly. Field emission scanning electron microscopy (FE-SEM) and environmental scanning electron microscopy (ESEM) were applied to describe and quantify microstructural characteristics of drill holes. Various specimens, including modern limpets and mussels drilled by muricid snails in laboratory experiments, subfossil limpets collected from a tidal flat (San Juan Island, Washington state, USA), and various Miocene bivalves collected from multiple European sites, were examined for microstructural features. The microstructures observed are interpreted here as Radulichnus-like micro-rasping marks, or predatory microtraces, made by the radula of drilling gastropod predators. The mean adjacent spacing of these microtraces is notably denser than the spacing of muricid radular teeth determined by measurements taken from the literature. Because the radular marks typically overlie or crosscut each other, the denser spacing of predatory microtraces likely reflects superimposition of scratches from repeated passes of the radula. One incomplete drill hole showed a clear, chemically aided drilling dissolution signature around its outer margin, while a number of other specimens showed similar, but ambiguous, traces of dissolution. The range of organisms examined illustrates the utility of scanning electron microscopy (SEM) imaging for identifying micro-rasping marks associated with predatory drill holes in both modern and fossil specimens. These distinct microtraces offer promise for augmenting our ability to identify drill holes in the fossil record and to distinguish them from holes produced by non-predatory means.
Sediments of late Berriasian–Hauterivian age have been analyzed from DSDP Holes 534A and 603B in the western Atlantic Ocean with respect to calcareous nannofossils and bulk-rock geochemistry (δ13Ccarb, CaCO3). The aim of this study was to obtain a detailed reconstruction of the paleoceanographic conditions in the western Atlantic during the Valanginian positive δ13Ccarb excursion. The well-constrained stratigraphic framework for both sites allows for a supraregional comparison with previous studies in Western Europe. At both sites, the Berriasian-Valanginian boundary interval is characterized by a 20% increase in the relative abundance of calcareous nannofossil species that are indicative of elevated surface-water nutrient levels. These changes in the trophic system coincide with the turning point toward more positive δ13Ccarb values at the M15–M14 magnetochron boundary, leading to the well-known Valanginian positive δ13Ccarb excursion. These changes also correspond with an increase in bulk-rock Sr/Ca ratios, enhanced burial of organic matter, and, at Hole 534A, a decrease in rock-forming nannoconids. Calcareous nannofossil data do not support a substantial cooling in the western Atlantic, as previously suggested by the geochemistry of belemnite rostra, but the observed increase in nutrients is consistent with enhanced upwelling during the Valanginian. Owing to controversial placement of the Valanginian–Hauterivian boundary and different absolute ages for the Valanginian stage, it is still unclear to what extent the Paraná-Etendeka volcanism might have caused the biotic and carbon-cycle perturbations. Our data suggest that the changes documented in the marine plankton system are at least concomitant with the initial, minor phase of the Paraná-Etendeka volcanism.
Enigmatic structures in the shape of inverted rounded cones are very common in a discrete horizon of the upper Stoner Limestone Member (Stanton Limestone Formation, Lansing Group, Upper Pennsylvanian) in southeastern Nebraska. These structures are 100–300 mm in diameter, but rare, very large examples attain 1300 mm in diameter. On bedding planes, individual structures are conspicuously expressed as groups of concentric rings, some of which appear as low ridges (rugae). At least some of these rings correspond to concentric cone sheets (hollow cones) extending downward into the rock mass; these cone sheets are sometimes separated clearly from each other by discontinuous mud linings or drapes. The largest structures usually have smaller ones nested within them. The Stoner Limestone structures differ from described types of nonbiological and biological conelike structures, yet they must have formed in soft sediment prior to burial. Although they have a superficial similarity to such plug-shaped trace fossils as Conichnus and Conostichus, the purported dwelling traces left by cnidarians, several salient points with respect to shape, structure, and occurrence distinguish the Stoner Limestone structures from such traces. A trace-fossil hypothesis for the origin of the structures remains plausible—possible feeding structures—albeit problematic. Study of the Stoner Limestone concentric-conelike structures indicates that careful reexamination of trace fossils and sedimentary structures in Midconinent Pennsylvanian limestones is in order, as well as a reconsideration of how unique physical and diagenetic properties of carbonate sediments in general might affect the preservation of such features.