The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ∼ 100–650 BP). In addition, age-distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record.

Exceptionally preserved fossil feathers and feather-like integumentary structures provide valuable insights into the early evolution of feathers and flight, but taphonomic biases often make interpretations at the microstructural and ultrastructural levels ambiguous. Maturation experiments have been demonstrated to be useful for investigating the taphonomic alterations of soft tissues, including feathers, during diagenesis. However, experimentally matured feathers resembling fossil feathers preserving keratinous matrix have not yet been obtained. Here we employ experimental maturation to obtain feathers corresponding to different degradation stages, and compare these matured feathers with untreated feathers and fossil feathers at the macroscopic, microstructural, and ultrastructural levels. Results show that several features of thermally matured feathers are similar to those found in fossil feathers. The fusion of barbules that occurred in thermally matured feathers suggests that such a process could occur during diagenesis, making barbules difficult to identify in fossil feathers. Under the most extreme experimental condition, the keratinous matrix can partially survive when the whole feather is turned into ash-like remains and many melanosomes are exposed. Moreover, our results show that the keratinous matrix immediately surrounding melanosomes appears to be more resistant to degradation than the unpigmented keratinous matrix, supporting the hypothesis that melanin can act as a fixative agent to prevent the degradation of keratin.

Two types of unusual concretions with similar biotic contents but markedly different shapes and distributions were found in close stratigraphic proximity within the Lower Jurassic Whitmore Point Member of the Moenave Formation in St. George, Utah. Both types of concretions formed in lacustrine sediments and contain abundant ganoid fish scales, numerous ostracode carapaces, and apparent rip-up clasts. Elongate, cylindrical concretions developed in parallel and regularly spaced rows in one horizon, and comparatively flat and irregularly shaped and distributed concretions formed in an overlying layer only a few centimeters above. Microprobe and Raman analyses of concretion samples reveal abundant hematite in both concretions as well as groundmass minerals dominated by silica in the cylindrical concretions and dolomite in the flat concretions. The abundance of fish skeletal debris in concretions from two consecutive horizons may suggest recurring fish mass mortality in ancient Lake Dixie, the large lake that occupied the St. George area during the Early Jurassic. We propose a model for the formation of the concretions based on their shapes, distributions, and chemistry. In this model, accumulations of disarticulated fish debris were colonized and consolidated by microbial mats and shaped by oscillatory flow (in the case of the cylindrical concretions) or lack thereof (in the case of the flat concretions). Then, after burial, groundwater chemistry and possibly the metabolic activities of microorganisms led to the precipitation of minerals around and within the masses of fish material. Finally, diagenetic alteration changed the mineral makeups of the cylindrical and flat concretions into what they are today.

Analytical questions relating to the influence of sedimentation on the preservation states of Carboniferous plant fossils are seldom addressed in the literature. Here we address specifically the influence facies differences have on preservation states and suggest how they can be analyzed. The case study involves the seed fern Neuropteris ovata (Hoffmann) that occurs as opaque pinnules in the roof shale and as transparent pinnules in an associated crevasse-splay of the basal Cantabrian in age, Point Aconi Coal Seam, Sydney Coalfield, Canada. The color differences imply different molecular pathways for organic matter transformation over geological time, which resulted in production of compression fossils in the roof shale and fossilized-cuticle in the crevasse-splay, respectively. Fourier transform infrared spectroscopy methods are used to quantify functional groups, and the derived data are chemometrically evaluated. Results indicate that the compressions are, as anticipated, characterized in the crevasse-splay facies by a predominantly aromatic composition. The fossilized-cuticles, however, are mainly characterized by oxygen-containing aliphatics, confirming the influence of facies changes on preservation states of the species studied. Implications for preservation, taxonomy, and paleoecology are emphasized.