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The possibility that large marine reptiles and other Mesozoic vertebrates produced nekton-fall communities similar to those of modern cetaceans is presently receiving increased attention in the literature. The author describes a rare ichthyosaur carcass-fall community from the Posidonia Shale (lower Toarcian) of Germany, which provides insights into the role played by large marine vertebrates in determining regional benthic ecology during this period. It is demonstrated here that, while more important than previously thought, there is little evidence to suggest that ichthyosaur carcasses played a substantial role in structuring the benthic ecology of the European Toarcian epeiric sea. In general, within the shallow waters of the Posidonia Shale, conditions conducive to the creation of carcass-fall communities were rare, and when present, resulted in localized magnification of background taxa and higher local biodiversity, rather than a unique community of epibiont organisms. The community which developed is ecologically similar to modern whale-fall communities, but differs in important ways, particularly with regard to the presence/absence of the chemosynthetic faunas which are most intensively described in the literature.
A nesting trace preserved in alluvial floodplain deposits in the Upper Cretaceous Two Medicine Formation at the Willow Creek anticline in north-central Montana contains four crushed theropod eggs referable to the oospecies Continuoolithus canadensis. These eggs immediately overlie the lower surface of a 35-cm-long × 7-cm-thick, dark-green mudstone lens, surrounded by reddish-purple mudstone. The long axes of three eggs are parallel to one another and to the lower boundary of the lens, whereas the fourth egg lies at a 30° angle to the others. A thin, 1-cm-thick organic horizon overlies the eggs, suggesting they were buried with some vegetation. Geometric modeling of the slightly asymmetrical C. canadensis eggs yields a volume and mass of approximately 194 cm3 and 205 g for each egg. This method provides a more accurate estimation for the surface area than allometric equations that are based on modern bird eggs because of the elongate shape of many non-avian theropod eggs. Pore density and water vapor conductance (GH2O) calculated from one egg in the trace and five additional C. canadensis eggs from the Willow Creek anticline vary across three regions. High, moderate, and very low GH2O characterize the equatorial zone, blunt, and tapering poles, respectively. The average GH2O for all eggs exceeds that of an avian egg of similar mass by 3.9×, thus supporting sedimentologic evidence of substrate burial during incubation.
Clypeasteroid echinoids can be common in both siliciclastic and carbonate sedimentary sequences of the Miocene of the Mediterranean and Paratethys areas where they frequently form highly fossiliferous deposits. Two echinoid assemblages dominated by clypeasteroids from the early-middle Miocene of Logudoro region (Northern Sardinia) are compared using a detailed sedimentological and taphonomic analysis along with functional morphological considerations. The assemblages differ widely with respect to taxonomic composition, sedimentological features, and taphonomic signatures, such as the orientation and degree of abrasion, encrustation, and bioerosion. The first assemblage (Ardara) is characterized by a low-diversity echinoid fauna, consisting exclusively of the two clypeasteroid genera, Amphiope and different morphotypes of Clypeaster. This deposit represents a proximal tempestite originating in a high-energy, shallow-water, shoreface setting. Higher diversity characterizes the second assemblage (Ittiri) with clypeasteroid, spatangoid, and cassiduloid echinoids. This deposit originated by multiple in situ reworking events within a deeper, low-energy, sublittoral environment. The difference in faunal diversity between these assemblages reflects ecological factors such as substrate characteristics and food supply. Differences in preservation potential of the different echinoid tests also influence their representation within the deposits. These described echinoid deposits represent two examples of a wide spectrum of clypeasteroid-dominated assemblages from the Miocene of Sardinia, which are compared with respect to taxonomic makeup, sedimentological and taphonomic features, and depositional environments.
Accurate paleoenvironmental reconstruction relies on the correct interpretation of the postmortem history of skeletal remains in shelly assemblages. In contrast to marine settings, actualistic taphonomic studies are lacking for shell-rich concentrations in freshwater riverine systems. In particular, the taphonomic pathways and the origins of taphonomic signatures that are recorded in bioclasts from fluvial settings are poorly known. In this study, we addressed this issue by comparing the taphonomic signatures and shell-damage profiles among shells of freshwater mollusks recorded both in death and in fossil assemblages from the same fluvial environment. Our data indicated that dissolution was the most pervasive taphonomic process leading to the destruction of the shells. The loss of taphonomic information extended beyond shell dissolution in the riverbed, or the early diagenesis in the sedimentary record. The loss of biological information from the living community through the death assemblage, until the incorporation of shells as fossils, mainly occurred during the time the shells were in the sediment-water interface. Though this destruction affected primarily dead shells, reworked fossils also became vulnerable because they were carried out into the river load again by channel avulsion. A model that included the main taphonomic pathways followed by the molluscan shells in the fluvial Touro Passo Formation (Pleistocene–Holocene) is discussed. In this model, two main destructive domains were recognized, which were the biological, physical, and chemical processes operating at the taphonomically active zone ( = TAZ domain) and the pedogenetic domain.
We report the flooding and transport of glaucous-winged gull (Larus glaucescens) nests and eggs in response to high tides at a nesting colony in Washington State. Affected nests were located on the beach bordering a small marina. Flooding and transport most commonly occurred during very high tides. Nests with eggs, nests without eggs, and eggs without nests were observed floating offshore. Our observations have implications for the interpretation of fossilized amniote nests and eggs, namely that a given fossil nest cannot be assumed a priori to have been preserved in situ without careful consideration of nest structure and lithological features of the preservation site. Also, prior to preservation, a fossil egg or egg clutch may have been transported by a floating nest functioning as a transport vessel, even if at the time of discovery evidence for the nest has disappeared.
Abundant, well-preserved Zoophycos is common in the lower and middle Permian paleotropical neritic limestone of South China and in the middle Permian glaciomarine lithic wackestone of southeastern Australia. Zoophycos from both regions is composed of a marginal tube and a tongue-like spreiten complex, the latter itself consisting of primary lamellae in planar view and backfill structures (dark and light menisci) in cross-sectional view. The Zoophycos tracemaker is interpreted to have periodically collected and fed on the surrounding nutrient-enriched sediments within a shallow depth of the seafloor. The dark menisci may correspond to the burrowing phase, whereas the light menisci may be related to a multiple-behavior phase, including dwelling, feeding, farming, resting, and excreting. Symbiotic microorganisms (e.g., sulphate-reducing bacteria) may have been closely involved with the Zoophycos tracemaker in producing the complex structures of the spreiten, based on the abundant pyrite framboids that were found in the Zoophycos spreiten. We suggest that Zoophycos is not simply a biogenic sedimentary structure formed by the motion of the tracemaker; rather it represents a set of complex and elaborate biogenic structures formed by a succession of life behaviors of the tracemaker along with its symbiotic microorganisms. The complete formative process of Zoophycos is reconstructed and linked to its morphology, based on this interpretation.
Small shorebird-like footprints have been discovered in Cenicero (La Rioja, Spain). They are preserved in a sandstone block of the transition unit between the Nájera and Haro Formations from the Ebro Basin. This level is positioned in the Y2 local zone (MN2), of Agenian age (early Miocene). The footprints are small, tridactyl or tetradactyl, with slender and proximally unconnected digit impressions. They have phalangeal pads and claw marks, and there is no evidence of a web or metatarsal pad. The footprints are compared with other shorebird-like ichnotaxa and assigned to the Cretaceous ichnotaxon Koreanaornis isp., which is herein identified for the first time in the Cenozoic. Other shorebird-like footprints from the late Eocene to early Miocene in the Ebro Basin and the early Miocene ichnotaxa Aviadactyla media and Aviadactyla vialovi are also related to this ichnotaxon. In addition to the shorebird-like footprints, the Cenicero tracksite has invertebrate traces and sedimentary and organic structures typical of the Scoyenia ichnofacies, suggesting a mud-dominated floodplain in a central-distal alluvial fringe as the paleoenvironment. The morphology, habitat, and behavior inferred from the shorebird-like footprints in the Cenicero tracksite are similar to other fossil footprints found in the Mesozoic and Cenozoic as well as to modern shorebird tracks. This is likely due to morphological, ecological, and behavioral convergences among different avian clades from the Early Cretaceous to the present.