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Lake Tanganyika, the world's largest tropical rift lake, is unique among its counterparts in East Africa for the remarkable diversity of mollusk-rich sediments in its littoral zone. Molluscan shell beds are, however, a common feature of ancient lacustrine rift deposits and thus a better understanding of their spatial and temporal development is important. Targeted surveys across the littoral region of the Kigoma Basin reveal three surficial shell-rich facies that differ widely in depositional style and geometry. A unifying characteristic of these deposits is the volume of shells of Neothauma tanganyicense, a large, viviparous gastropod endemic to the lake. Reservoir-corrected radiocarbon dating indicates that Neothauma deposits in these surficial sediments are time averaged over at least the last ∼1600 calendar years BP. Preservation of fossil Neothauma shells in the littoral zone depends on both environmental conditions and on post-mortem shell modifications. Interaction between shells and mobile siliciclastic grains, facilitated by wave action and storms, represents a particularly destructive taphonomic process in the study area. Rank scoring of damage to Neothauma suggests that stromatolitic encrustations or early calcite coatings may help mitigate shell destruction caused by hydraulic fragmentation and abrasion. Persistence of Neothauma in littoral beds has important implications for the structuring of specialized communities of shallow-water benthos, as well as for improving analog models for hydrocarbon reservoirs in lacustrine carbonates.
Taung, South Africa yielded the first Pliocene Hominini fossil, Australopithecus africanus, recovered from a lime quarry in 1924. To identify whether the habitat of the site differed from present-day conditions, dietary preferences of fossil papionins from Taung, including Parapapio antiquus (n = 8), Papio izodi (n = 12), and indeterminate specimens (n = 10) were examined under low magnification to discern patterns of dental microwear. The comparative fossil sample from Sterkfontein Member 4 includes Parapapio broomi (n = 10) and Parapapio jonesi (n = 5). Extant Papio ursinus (n = 20), a savanna-dwelling baboon from South Africa, provides a modern analogue. Six dental use-wear scars on the paracone of the second molar (M2) were recorded and the data analyzed using ANOVA with Tukey's test to detect whether group differences were present for each feature; linear regression identified significant covariation of microwear features. Principal components analysis and discriminant function analysis were utilized to identify species-specific dietary signals. Extant Papio ursinus is separated from the extinct taxa solely by a relatively greater number of fine scratches with respect to the other microwear features. Papio izodi overlaps primarily with extant Papio and secondarily with Parapapio, which forms a more discrete grouping that includes Parapapio antiquus from Taung. A wetter, more closed environment is suggested for Taung and Sterkfontein Member 4 compared to the habitat of present-day central South Africa.
The recovery of an intact, 10 m long fossil baleen whale from the Pliocene of Tuscany (Italy) offers the first opportunity to study the paleoecology of a fully developed, natural whale-fall community at outer shelf depth. Quantitative data on mollusk species from the whale fall have been compared with data from the sediments below and around the bones, representing the fauna living in the muddy bottom before and during the sinking of the carcass, but at a distance from it. Although the bulk of the fauna associated with the fossil bones is dominated by the same heterotrophs as found in the surrounding community, whale-fall samples are distinguishable primarily by the presence of chemosymbiotic bivalves and a greater species richness of carnivores and parasites. Large lucinid clams (Megaxinus incrassatus) and very rare small mussels (Idas sp.) testify to the occurrence of a sulphophilic stage, but specialized, chemosymbiotic vesicomyid clams common at deep-sea whale falls are absent. The Orciano whale-fall community is at the threshold between the nutrient-poor deep sea and the shallow-water shelf, where communities are shaped around photosynthetic trophic pathways and chemosymbiotic specialists are excluded by competition.
The character and quantity of bioturbation preserved on bedding-plane exposures reflect a number of interrelated factors, including sediment accumulation rate, oxygen availability, and benthic community composition. In order to effectively document and compare bioturbation data from bedding planes, precise quantitative methods are needed. Here we present the intersection grid method, a quantitative presence/absence-based technique for accurately and efficiently estimating the percentage bioturbation on bedding-plane surfaces. The intersection grid method, which is based on an established method for estimating vegetative cover, is entirely digital and allows grids to be precisely scaled to match the bioturbation being analyzed. In addition, a record of each grid analysis can be saved as an image file for archiving and use in paleocommunity reconstruction. Testing the intersection grid method on hypothetical bedding-plane images demonstrates that accurate results—within 0.5% of the actual bioturbation percentage—can be obtained efficiently by scaling the grid within a recommended range. Intersection grid method data can be analyzed statistically and combined with descriptive ichnological and sedimentological data to reveal spatial, temporal, and paleoenvironmental patterns in bioturbation and make meaningful comparisons at outcrop, locality, and regional scales.
The chemical composition of well-preserved naraoiids from the Chengjiang, Kaili, and Burgess Shale biotas is compared. Gut diverticulae in samples from all three biotas contain C, P, and Fe, indicating a primary composition of organic carbonaceous material, and the presence of apatite and pyrite as the result of authigenic mineralization in association with decay and early diagenetic processes. Gut traces from Burgess Shale specimens retain apatite and pyrite, as well as clay minerals, reflecting a history involving greenschist-grade metamorphism. Kaili specimens have been subjected to lower-grade metamorphism and, or thermal alteration, but alteration of pyrite pseudomorphs in the gut traces to limonite indicates the effect of weathering. Loss of sulfur and calcium, oxidation of pyrite, and the light color of the Chengjiang samples are the result of a greater degree of weathering than in specimens from the other two localities. As demonstrated here, Kaili samples serve as an important baseline for interpreting specimens from the Burgess Shale (high-grade metamorphism) and the Chengjiang (intense chemical weathering) deposits. Our study shows for the first time that the conservation of organic carbon is the common primary mode of soft-part preservation in naraoiid arthropods from these three signature Burgess Shale-type localities. Differences among these major deposits are a product of later diagenesis and weathering of the authigenic mineralization associated with the preservation of labile structures. Among the major constituents formed during diagenesis—carbonaceous material, calcium phosphate (e.g., apatite), and iron sulfide (e.g., pyrite)—the minerals are more susceptible to chemical weathering via hydrolysis.
In the Middle–Upper Jurassic boundary of the External Subbetic, there are abundant discontinuities with neptunian dikes and sills composed of Callovian–lower Tithonian deposits. In cavities developed on a slope with escarpments, cryptobiontic communities were preserved in life position. These cavities were excavated beneath a hardground covering the upper surface of Bathonian oolitic limestones deposited in very shallow pelagic carbonate platforms. The biogenic crusts are composed mainly of serpulids and Frutexites—laminated dendrolitic microstructures—and, secondarily, by sessile foraminifera. The serpulids were pioneer organisms during colonization of the walls of small cavities in stressed shadow-cryptic environments. Serpulid aggregates then grew downward from the top walls of the cavities. The colonization of serpulid tubes was mainly after the death of organisms, first by microborers, secondly by Frutexites, and later by sessile foraminifera. The preferential colonization of these cryptic environments by serpulids can be interpreted as due to possible photophobic behavior and or the possibility of avoiding space competition or predation.