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A taphonomic study was conducted on disarticulated micromammalian bones and isolated teeth from a late Eocene (Priabonian) assemblage in the Headon Hill Formation (Solent Group, Hampshire Basin, Isle of Wight, United Kingdom). The aim was to understand accumulating mechanisms and additional postmortem agents that influenced and potentially biased the assemblage. Skeletal elements (N = 4296) belong to three marsupials, two glirid and two theridomyid rodents, two chiropterans, three nyctitheres, two omomyid primates, one pantolestid, and one apatothere. Surface modifications (i.e., etching, breakage, splitting, and impact marks) occur on bone, enamel, and dentine. Qualitative and quantitative analyses of surface modifications of each individual taxon resulted in recognition of several taphonomic agents. Predation of two theridomyid and at least two marsupial species by the mammalian carnivore Cynodictis cf. lacustris (also present in the fauna) is indicated by high proportions of etched remains, broken bones, and puncture marks. Variations within the assemblage in proportions of modified specimens or numbers of individuals are interpreted as reflecting differences in cause of death, habits, locomotor adaptations, and predator-prey relationships. The rodents, marsupials, and probably the nyctitheres are judged to have been living close to the site of deposition, justifying their previous use for isotope-based paleoclimate studies and demonstrating their participation in the local paleocommunity.
Fossil gastropod egg capsules have rarely been reported from the fossil record. This paper describes gastropod egg capsules preserved exclusively on cardiniid bivalves from Lower Jurassic (Hettangian) deltaic deposits of the Holy Cross Mountains area, central Poland. In most instances, only the bases of the egg capsules are preserved as dark outer rings with hollow cavities inside. Some specimens, however, are preserved as dark circular spots or even shallow depressions on the bivalve molds. The general occurrence, morphology, size and composition of the egg capsules as revealed by Energy Dispersive Spectroscopy (EDS) analyses and elemental mapping point to neritimorphs as the most probable producers. The egg capsules are composed primarily of organic matter with some calcium. Recent neritid gastropods possess aragonitic spherules within the egg capsule walls composed of conchiolin or chitin, which strengthen them and increase their chance of fossilization compared to egg capsules of other gastropods. Recent neritids lay their egg capsules on mollusk shells as well. The occurrence of Early Jurassic egg capsules only on redeposited cardiniid bivalve shells may suggest that the shells were the best medium for egg-capsule deposition. The occurrence of egg capsule bases together with dark circular spots, interpreted as compressed unhatched capsules, suggests that these egg capsules either hatched or were deposited at different times in the same place. The Early Jurassic egg capsules were preserved due to the strengthening role of the carbonate phase within the wall and rapid burial coupled with limited bioturbation in deltaic settings. The prodelta-delta front depositional environment, with high rates of sedimentation by rivers and sediment redistribution by wave and current action, was favorable for preserving these fossils.
Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone bedding surfaces in the Cincinnati Ordovician, are sometimes associated with moats, which are sediment depressions or gutters ∼5-mm wide surrounding the commissure. Moats are interpreted as trace fossils, excavated by water expelled as the valves snapped shut. Other specimens vary from nearly horizontal to nearly vertical with the hinge line down and commissure up. Meniscate backfill beneath the anterior shell margin traces an arcing path formed as the shell rotated upwards around the posterior hinge line. Rotational tracks are interpreted as trace fossils, recording movement from an initial position buried horizontally to an inclined position as the brachiopod tried to escape burial. The traces form a continuum. Specimens lying flat on the bed surface have moats but no rotational tracks. Inclined shells are associated with deeper burial by obrution events and a greater arc of rotation. The moat shape is inconsistent with differential compaction. The precise association between moats and commissures and the independence of these structures on shell azimuth are inconsistent with current scour. If moats formed by rapid expulsion of water during valve snapping, then rotational tracks may have formed by a similar process. These traces are interpreted as fugichnia formed in response to catastrophic burial, but some moats could be equilibrichnia, formed by adjustment to minor sedimentary events. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar tracemaking abilities, then other strophomenates probably shared these abilities.
The predominantly unknown nature and magnitude of the total taphonomic biasing that affects fossil assemblages commonly limits the testing of paleoecological hypotheses by the quantitative comparison of fossil assemblages. Multiple regression analyses of a suite of taphonomic characteristics against a measure of the total taphonomic biasing of a fossil assemblage can be used to generate a taphonomic model that identifies and quantifies many of those taphonomic processes that are important in biasing fossil assemblages. By examining the parameters of regression-based quantitative taphonomic models, key elements of the taphonomic history of a fossil assemblage can be reconstructed and such histories compared between fossil assemblages. We discuss the methodology pertinent to the building of quantitative taphonomic models and illustrate the effectiveness of such an approach using a case study considering two samples (857 specimens) of an Oligocene mammalian fossil assemblage from the Scenic Member of the Brule Formation, South Dakota. It is possible to quantify all of the taphonomic biasing affecting bone abundances in each of the two sampled assemblages using a small number of taphonomic characteristics. The most important of these characteristics is bulk density. The control of biasing by density is attributed to a combination of the increased resistance to physical damage of dense bones as well as the decreased probability of transport. Element shape and surface area to volume ratio are shown to have a significant but lesser control on assemblage biasing. Comparison of the taphonomic models for each sampled assemblage using currently viable methods demonstrates that the two assemblages are not statistically isotaphonomic. This interpretation may change with the gathering of additional data.
Late Triassic marine vertebrates occur in the Hound Island Volcanics exposed on the eastern shore of Hound Island, southeastern Alaska. This fossil assemblage derived from slope deposits within an intra-arc basin of a volcanic island arc complex. It is associated with the allochthonous Alexander Terrane prior to its northern translation from tropical latitudes to its current latitude. Fossils occur within a 13-m-thick succession of interbedded calcareous shale and volcaniclastic-rich bioclastic limestone. The limestone layers are skeletal (vertebrate and invertebrate) packstones to wackestones and are interpreted as episodic thin-bedded turbidite and debris flow deposits that resulted from redeposition of slope sediments. Forty-seven vertebrate fossils were collected that consist of complete and fragmented cranial and postcranial elements belonging to Eosauropterygia, Thalattosauria, and Ichthyosauria, including the genus Shonisaurus. Bones are frequently disassociated and show predepositional abrasion and breakage. Another less common taphonomic mode is observed, characterized by several large associated elements, representing a single individual, found parallel to bedding and draped by shale. Rare Osteichthyes and Chondrichthyes teeth are also present. Bivalve and conodont fossils indicate a middle Norian age for the assemblage.
Coupled dual-beam focused ion beam electron microscopy (FIB-EM) has gained popularity across multiple disciplines over the past decade. Widely utilized as a stand-alone instrument for micromachining and metal or insulator deposition in numerous industries, the submicron-scale ion milling and cutting capabilities of FIB-EM systems have been well documented in the materials science literature. These capacities make FIB-EM a powerful tool for in situ, site-specific transmission electron microscopy (TEM) ultrathin foil preparation. Recent advancements in the field-emission guns (FEGs) of FIB-EM systems have provided spatial resolution comparable to that of many high-end scanning electron microscopes (SEM), thus providing enhanced imaging capacities with material deposition and material removal capabilities. More recently, FIB-EM preparation techniques have been applied to geological samples to characterize mineral inclusions, grain boundaries, and microfossils. Here, we demonstrate a novel method for analyzing three-dimensional (3-D) ultrastructures of microfossils using FIB-EM. Our method, FIB-EM nanotomography, consists of sequential ion milling, or cross sectioning, and concurrent SEM imaging. This technique with coupled dual-beam systems allows for real-time, 3-D ultrastructural analysis and compositional mapping with precise site selectivity and may provide new insights in fossil ultrastructures. Using the FIB-EM nanotomography method, we investigated herkomorphic and acanthomorphic acritarchs (organic-walled microfossils) extracted from the ≥999 Ma Mesoproterozoic Ruyang Group of North China. The 3-D characteristics of such important but controversial acritarch features as processes and vesicularly enclosed central bodies are described. Through these case studies, we demonstrate that FIB-EM nanotomography is a powerful and useful tool for investigating the three-dimensionality of microfossil ultra- and nanostructures.