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Calcareous skeletons evolved as part of the greater Ediacaran–Cambrian diversification of marine animals. Skeletons did not become permanent, globally important sources of carbonate sediment, however, until the Ordovician radiation. Representative carbonate facies in a Series 3 (510–501 Ma) Cambrian to Tremadocian succession from western Newfoundland, Canada, and Ordovician successions from the Ibex area, Utah, USA, show that, on average, Cambrian and Tremadocian carbonates contain much less skeletal material than do post-Tremadocian sediments. Petrographic point counts of skeletal abundance within facies and proportional facies abundance in measured sections suggest that later Cambrian successions contain on average <5% skeletal material by volume, whereas the skeletal content of post-Tremadocian Ordovician sections is closer to ∼15%. A compilation of carbonate stratigraphic sections from across Laurentia confirms that post-Tremadocian increase in skeletal content is a general pattern and not unique to the two basins studied. The long interval (∼40 myr) between the initial Cambrian appearance of carbonate skeletons and the subsequent Ordovician diversification of heavily skeletonized organisms provides an important perspective on the Ordovician radiation. Geochemical data increasingly support the hypothesis that later Cambrian oceans were warm and, in subsurface water masses, commonly dysoxic to anoxic. We suggest that surface waters in such oceans would have been characterized by relatively low saturation states for calcite and aragonite. Mid-Ordovician cooling would have raised oxygen concentrations in subsurface water masses, establishing more highly oversaturated surface waters. If correct, these links could provide a proximal trigger for the renewed radiation of heavily skeletonized invertebrates and algae.
The inception of terrestrial woody plant ecosystems on the Earth is thought to have caused decreases in atmospheric carbon dioxide and water vapor concentrations in the Middle Devonian. Decreased greenhouse gas concentrations enabled a series of long-term glacial-interglacial cycles from the Late Devonian through the Permian. Here we describe the environmental and ecological variability of the earliest known paleosols with evidence of in situ forests (stump casts and attached root systems) from the Appalachian basin. Four examples located in the Manorkill Formation in the Catskill Mountains of New York State, United States, are analyzed using macro- and micromorphological data. Woody plant stump casts and molds on exposed bedding surfaces were mapped at two of the sites and analyzed using nearest-neighbor analysis. This permitted quantification of spatial distribution and ecological conditions of these paleoforests. These fossilized forests exhibit both single and multi-generational growth and formed in both aquic and seasonally well-drained environments, thus indicating established populations and growth adaptations by the represented species. The occurrence of these diverse forested landscapes in the Givetian is concurrent with the onset of the prolonged decrease in atmospheric CO2 concentrations that have been tied to a series of Paleozoic glaciations.
The origin of oxidized iron in Precambrian iron formations has been debated for decades. Direct paleontological evidence for a microbial role in iron oxidation has been sought in the biosignatures in these structures. This study documents how several biosignatures of phototrophic iron-oxidizing communities form in modern hydrothermal iron deposits. The microbes, primary minerals, microfossils, and stromatolitic biofabrics from Chocolate Pots hot springs in Yellowstone National Park were characterized across a range of spatial scales via various types of microscopy, X-ray diffraction, energy dispersive spectroscopy, and total organic carbon elemental analyses. Electron microscopic examination of the cyanobacterial mats reveals the formation of distinct dendritic-like biofabrics. Early-stage iron-permineralized phototrophic microfossils display taxonomic features that allow identification to the genus level. Selected-area electron diffraction analysis indicates that cells were permineralized by iron oxides (2-line ferrihydrite). Although permineralization by silica is considered to result in fossils with the highest cellular fidelity, this investigation suggests that iron permineralization may also produce exceptionally well-preserved microfossils. Characterization of biosignatures in this modern high-iron thermal spring provides a unique opportunity to establish a link between (1) our previous physiological measurements of iron oxidation by a phototrophic community; (2) production of biosignatures by the community; and (3) survival of these biosignatures during the earliest stages of diagenesis in the iron oxides underneath the microbial mats. This fossil evidence linking taxonomy, physiology, and biosignatures may be used to infer the paleobiology and paleoecology of similar fossil benthic microbial communities and may provide a means to assess the microbial contribution to ancient iron deposits.
Three newly discovered bonebeds from the Shishugou Formation of Xinjiang, China, are unusual in preserving vertically stacked and articulated to associated skeletons of at least 18 small, non-avian theropod dinosaurs in pits that are 1–2 m deep. The pits host a soft sediment-deformed mixture of alluvial and volcanic mudstone and sandstone. There is no evidence that the pits were discrete depressions in the topography that filled through time. Rather, they appear to have been highly localized areas of liquefaction caused by large-dinosaur (possibly sauropod) trampling of saturated sediments. Evidence indicates that the small theropods, and some other small vertebrates, became mired and died in these mud-filled pits. High quality skeletal preservation suggests that most individuals were buried within days to months after their deaths. Carcasses were buried successively, coming to rest above previously buried individuals. In some cases, skeletal body parts became separated or were removed, probably during scavenging. Given the large sizes of the pits relative to the small body sizes of the vertebrates contained within them, we conclude that small vertebrates (<3 m long and <1 m tall) were particularly susceptible to miring at these sites. Although the small, presumably herbivorous ceratosaur, Limusaurus inextricabilis, dominates the combined small theropod assemblage from these bonebeds (minimum number of individuals [MNI] = 15), there is no evidence that any biological features other than its small size and a large, and possibly, gregarious local population were responsible for its becoming mired in large numbers. A bias for small theropods in these bonebeds, compared to their relatively low abundance in the overall Shishugou Formation fauna, underscores that small theropods are underrepresented in Mesozoic fossil assemblages collected from other ancient alluvial and paludal settings.
Changes in marine upwelling can affect the radiocarbon content of seawater and thus affect the marine radiocarbon reservoir age, R. These radiocarbon variations are preserved in mollusk shell carbonate. Shell-based estimates of R in a variable-upwelling environment can be biased by (1) changes in molluscan growth rates due to fluctuating environmental conditions and (2) time averaging during sampling due to homogenization of days or weeks of precipitated carbonate. We modeled the growth, radiocarbon content, and radiocarbon sampling of two Peruvian mollusks, Argopecten purpuratus (bay scallop) and Mesodesma donacium (surf clam), to quantify these potential biases. Argopecten purpuratus grows year round, but M. donacium prefers cold conditions and its growth rate decreases in summer. Radiocarbon assays by accelerator mass spectrometry on multiple ∼1 mg samples of a model A. purpuratus shell can capture the full range of annual R variation; similar sampling of a model M. donacium shell only captures ≤75% of this range. Given an annual R variation of 530 14C yr, the mean R calculated using a group of ∼1 mg carbonate samples from an A. purpuratus shell is within 30 14C yr of the actual mean R; that of a M. donacium shell may be skewed up to 140 14C yr older than the actual marine mean. If growth tolerances and parameters of mollusks used for R analyses are considered, it may be possible to correct for these biases and improve the accuracy of marine radiocarbon chronometry.
A single, low-density (1.58 g/cm3), phosphatic coprolite recovered from a fluvial Triceratops site in the Upper Cretaceous Hell Creek Formation of eastern Montana contains small quantities of minute bone or tooth fragments, kerogenized plant residues (pollen, spores, sporangia, and cuticle), hyphae of probable fungal origin, and small detrital mineral grains in a fine-grained, highly porous matrix. Roughly 30% of the matrix, composed almost entirely of microcrystalline francolite (carbonate-fluorapatite), is composed of thin-walled vesicles of roughly spherical shape, 0.5–3 µm in diameter. These vesicles are interpreted as mineral pseudomorphs of organic particles, probably including fecal bacteria, existing in the original scat. This structurally well-preserved coprolite is likely derived from the scat of a bone-digesting carnivorous animal, contains much or all of the autochthonous apatite of the original scat, and lacks permineralization that commonly produces a densely lithified object of low porosity. This is the first detailed description of a coprolite of this type from Mesozoic fluvial deposits. This evidence supports the view that dietary calcium phosphate could precipitate rapidly in the scat of ancient carnivorous animals, providing the structural strength to allow preservation of internal organic forms in great detail.