Registered users receive a variety of benefits including the ability to customize email alerts, create favorite journals list, and save searches.
Please note that a BioOne web account does not automatically grant access to full-text content. An institutional or society member subscription is required to view non-Open Access content.
Contact firstname.lastname@example.org with any questions.
Minute fossils from a variety of different metazoan clades, collectively referred to as small shelly fossils, represent a distinctive taphonomic mode that is most commonly reported from the Cambrian Period. Lower Triassic successions of the western United States, deposited in the aftermath of the end-Permian mass extinction, provide an example of small shelly style preservation that significantly post-dates Cambrian occurrences. Glauconitized and phosphatized echinoderms and gastropods are preserved in the insoluble residues of carbonates from the Virgin Limestone Member of the Moenkopi Formation. Echinoderm plates, spines and other skeletal elements are preserved as stereomic molds; gastropods are preserved as steinkerns. All small shelly style fossils are preserved in the small size fractions of the residues (177 to 420 μm), which is consistent with the size selection of small shelly fossils in the Cambrian. Energy-dispersive X-ray spectra of individual fossils coupled with X-ray diffraction of residues confirm that the fossils are dominantly preserved by apatite and glauconite, and sometimes a combination of the two minerals. The nucleation of both of these minerals requires that pore water redox oscillated between oxic and anoxic conditions, which, in turn, implies that Lower Triassic carbonates periodically experienced oxygen depletion after deposition and during early diagenesis. Long-term oxygen depletion persisted through the Early Triassic, creating diagenetic conditions that were instrumental in the preservation of small shelly fossils in Triassic and, likely, Paleozoic examples.
Nanoscale details of original aragonite crystals and organic inclusions are preserved in shells from the Pennsylvanian Buckhorn Asphalt of Oklahoma, USA. Exceptional preservation occurred because, either during or shortly after deposition, oil migrated along wrench faults generated during the simultaneous Ouachita Orogeny. The early sealing by oil (later converted into asphalt) prevented diagenetic alteration of shell material. Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Detection (EBSD), and Atomic Force Microscopy (AFM) reveal a striking high fidelity of preservation, including the oldest known unaltered nacre tablets in gastropods, bivalves, and cephalopods. These nacre tablets are indistinguishable from modern representatives in nanoscale morphology and crystallographic orientations. Fossils from the Buckhorn Asphalt show that by the Pennsylvanian Period, nacre and crossed lamellar were the dominant microstructures in the inner shell layer of the Mollusca. Calcitic microstructures and loosely organized horizontal bundles of aragonite fibers were common among Cambrian mollusks and problematic lophotrochozoans (e.g., hyoliths). Through the early to middle Paleozoic the dominance changed to more fracture-resistant textures nacre and crossed lamellar. This transition reflects the importance of these two types of shell microstructure in deterring predation, and it is clear that the ability to produce crossed lamellar and nacreous microstructures contributed to molluscan success during the Mesozoic and Cenozoic eras.
The structure and composition of rhodoliths in two regions of the Brazilian shelf, Abrolhos Continental Shelf (ACS) and South Espírito Santo State (SES) were examined and compared. Rhodoliths were sampled at depth ranges of 10–20 m and 50–60 m in SES, and 20–30 m and 50–75 m in ACS. Rhodoliths in SES are algal boundstones, built mainly of melobesioid corallines, with subordinate bryozoans and encrusting foraminifers. They show high porosity and the sediment infill of borings and voids contains a relatively high amount of siliciclastics (up to 8%). Rhodoliths from ACS are formed by a structureless carbonate mass covered by a thin veneer of encrusting coralline algae. The massive interior was produced by multiphase boring and infilling of an original boundstone. The infillings consist of a micritic matrix with bioclasts and low amounts of siliciclastic grains. Coralline assemblages are reduced to fragments of thin crusts. Rhodoliths from shallow depths are small (< 8 cm) whereas rhodoliths from the deeper zone have a wide size range (1 to 17 cm). The innermost parts of deeper rhodoliths in ACS yield radiocarbon ages of ∼ 7,000 years BP (75 m) and ∼ 2,000 years BP (65 m). Rhodoliths from the deep zone in SES are younger (less than 700 years BP). Siliciclastic sediment influx reaching rhodolith beds promotes burial of rhodoliths, determining the small size of shallow SES rhodoliths and the relatively young ages of the deeper ones. Scarce siliciclastic influx at the ACS rhodolith beds favors long residence times of rhodoliths on the seafloor, resulting in a thorough destruction of the original coralline/invertebrate boundstone by bioerosion.
Actuopaleontological studies of echinoids and their predators in modern ecosystems can augment our ability to identify and interpret the fossil record of predation. Here, we examine present-day interactions between the spatangoid Meoma ventricosa and the drilling gastropod Cassis tuberosa from a shallow tropical marine habitat (San Salvador Island, Bahamas) to assess the impact of drilling predation on the fossilization potential of echinoids, estimate drilling frequency, characterize drill hole morphology, and evaluate size and site selectivity of predators. Cassids produced recognizable drill holes ranging from 2–14 mm in diameter. A comparison of drilled versus live individuals suggested that predation was size-selective with preference toward smaller prey (p << 0.001), whereas landmark morphometrics revealed no evidence of stereotypy in specific attack site, drill holes were preferentially located on the oral side of the test (85.2%). Although the annual mortality of M. ventricosa attributed to C. tuberosa was 0.021 individuals per m2, the drilling frequency in samples of dead echinoid tests was 96.8%. This exceedingly high drilling frequency most likely reflects the fact that echinoids killed by C. tuberosa—a predator that immobilizes, drills, kills, and consumes M. ventricosa buried in the sediment—have a greater chance of preservation than echinoids that died due to other causes (e.g., fish predation). These results not only suggest that the frequency of drilled prey specimens may greatly exaggerate the importance of certain species of drilling predators, but also indicate that certain predators may play a critical role in enhancing the fossilization potential of their prey.