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Using abundance data, this study explores quantitative patterns from marine benthos, including implications for paleogeography, depositional environment, stratigraphic position, taxonomic groups (brachiopod or mollusc), substrate preferences, and ecological niches. Twenty-nine brachiopod- and bivalve-dominated fossil assemblages from the Pennsylvanian and Early Permian of North and South America, Thailand, and Australia were analyzed from carbonate-platform environments; specifically, Nevada, Kansas, Oklahoma, Texas, Utah, New Mexico, Venezuela, Kanchanaburi (Thailand), and Queensland (Australia). Samples were categorized by paleogeographic location, depositional environment, and age to help differentiate factors controlling the faunal patterns. Pooled from primary and summary literature resources, 336,321 specimens were identified to genus level and classified in terms of taxonomic membership, substrate preference, and ecological niche. Data were analyzed using detrended correspondence analysis (DCA) and multi-response permutation procedure cross-validated a-priori categories (e.g., paleogeography, depositional environment, stratigraphic position, and specimen ecology). Multivariate analyses indicate that the separation between genera and the orthogonal trends implies that paleoecological patterns within the studied late Paleozoic faunal associations were influenced strongly by the abundance of sessile versus mobile faunal components.
The Great Bank of Guizhou (GBG) is an isolated Late Permian to Late Triassic carbonate platform in the Nanpanjiang Basin of Guizhou Province, southwest China. A faulted syncline exposes a cross section of the platform margin, including a well-preserved Anisian (earliest Middle Triassic) reef complex approximately 1 km wide and 800 meters thick. Geochronologic constraints from associated basin-margin strata indicate that reef development initiated late in the Early Triassic, making it the oldest-known platform-margin reef complex of the Mesozoic Era. The reef framework consists primarily of microspar-filled tubes ∼100 μm wide and up to a few cm long that are embedded in irregular to branching, mm-scale masses of micrite, traditionally assigned to the problematic genus Tubiphytes. Based on preserved sporangia, the Nanpanjiang structures are interpreted as microbially induced micritic precipitates that formed in association with an otherwise uncalcified alga. A low-diversity metazoan and algal community also occurs within the reef complex, but these organisms did not contribute significantly to the reef framework or to the accretion of the reef complex. Rather, reef development is interpreted to have resulted largely from the stabilization of platform-margin sediments by algae and associated microbial mats. Only gradually, through the Middle and Late Triassic, did framework-building metazoans evolve to occupy and then construct reefs on the margins of carbonate platforms.
Although onshore to offshore retreat of brachiopods, in terms of their community-level abundance, took place through the Mesozoic and Cenozoic, this study shows that comparable trends also occurred repeatedly on a short time scale and mainly were driven by variations in sediment and nutrient supply. In the Kössen Formation (Upper Triassic), brachiopods retreated to offshore habitats during nutrient-rich, siliciclastic regimes and expand to onshore habitats during nutrient-poor, carbonate regimes. Epifaunal bivalves occupied onshore and offshore habitats during both siliciclastic and carbonate regimes. Infaunal suspension-feeding bivalves expanded to offshore habitats during nutrient-rich, siliciclastic regimes and retreated from offshore habitats during nutrient-poor, carbonate regimes. Thus, the onshore to offshore retreat of brachiopods and the offshore expansion of infaunal bivalves repeatedly coincided with the switch from a nutrient-poor, carbonate regime to a nutrient-rich, siliciclastic regime. Because brachiopods and epifaunal bivalves were abundant in micrite-rich, soft-bottom habitats, the replacements between infaunal and epifaunal communities cannot be explained by variations in substrate consistency alone.
Differences in guild structure between siliciclastic and carbonate regimes and onshore to offshore replacements indicate that distribution of bivalves and brachiopods is related to their differential response to low nutrient supply, turbidity, and, possibly, oxygen levels. Based on actualistic evidence, brachiopods are able to thrive in nutrient-poor conditions due to low metabolic demands and are less tolerant of high-turbidity conditions than bivalves. Epifaunal bivalves that co-occur with brachiopods in nutrient-poor habitats may have been characterized by higher clearance rates in contrast to infaunal bivalves with similar metabolic requirements. Although higher biogenic sediment disturbance or other biotic interactions could play a significant role in the retreat of brachiopods to offshore habitats, this study highlights the importance of varying nutrient supply and turbidity in governing onshore to offshore replacements on short time scales.
No temporal trend in the intensity of drilling by naticids on Glycymeris yessoensis can be recognized during the late Cenozoic. Drilling sites shifted from the umbo to the center of the valve during the late Cenozoic. This shift might reflect the change of predators from Glossaulax in the Miocene to Cryptonatica or Euspira in the Pleistocene. Borehole sites in the middle Pleistocene were more stereotyped than in the early Pleistocene population despite the same predators. Edge drilling, which is a faster drilling method, first appeared in the population showing high drilling intensities in the early Pleistocene. Because a prediction of the hypothesis of escalation is that changes in predators' behavior developed through time, these changes in drilling location may be the results of escalation. In contrast with the stereotypic trend of borehole sites, correlation coefficients of predator-prey size decreased from the early to the middle Pleistocene.
Years of over-fishing combined with increased nutrient pollution have had a catastrophic effect on the ecology of the Chesapeake Bay. The Holocene record of bay mollusks may provide a baseline for ecological restoration, but the effects of taphonomic bias on these assemblages first must be assessed. In this study, a live-dead comparison was carried out on four sites distributed in the main channel of the upper bay. Molluscan death-assemblage data were obtained from replicate box-core samples from which whole specimens and fragments were sorted, identified, and counted. Data on live communities at the same sites, sampled over the past twenty years, were provided by the Chesapeake Bay Program, making it possible to examine the degree to which death assemblages reflect long-term changes in the live community. Traditional live-dead metrics document a strong agreement between live-community and death-assemblage estimates of species composition, richness, and abundance—77% of the species in the live community are found in the death assemblage, and 99% of the individuals of species found in the death assemblage are found in the live community. Correlations between live and dead estimates of species richness are not statistically significant, although they do improve with longer-term sampling of the live community. Rank abundance of taxa in the death assemblage is correlated strongly and significantly with live rank abundance regardless of the duration of live sampling. These results suggest that Holocene molluscan assemblages may provide useful estimates of richness and abundance for Chesapeake Bay restoration.
Substrate properties, such as grain size, water content, shear strength, and content of organic mucus, influence the life activity of benthic organisms and their trace-fossil record. This study deals with actualistic experiments using small crustaceans (amphipods and isopods) moving mainly over plaster of Paris surfaces in various stages of hardening. Several consistencies, such as semi-fluid, very soft, soft, soft-stiff, stiff, and very stiff are distinguished. The morphology of surface lebensspuren shows a broad variety that directly depends on stiffness of the substrate and the capability of the organisms to cope with it. Semi-fluid substrates hinder the organisms in their motility— they move by plowing, whereby sediment flows back behind the animal and refills the furrow, leaving an indistinct line on the surface. With increasing stiffness, traces acquire additional morphological details such as levees and median furrows. On stiff and very stiff substrates, the crustaceans do not penetrate into the sediment, but move by jumping, with the consequence of producing jumping traces instead of furrows. Some lebensspuren obtained in the experiments, especially furrows, are similar to some trace fossils attributed to several non-arthropod animal groups such as annelids, bivalves, and gastropods. This study helps clarify the interpretation and taxonomy of trace fossils and in reconstruct of substrate properties during their formation.
A transitional Scoyenia–Mermia ichnocoenose from the Saint John River, Fredericton, New Brunswick, Canada, is dominated by elements of the Mermia ichnofacies, with traces comparable to Curvolithus, Helminthopsis, Gordia, Spirophycus, and Lockeia. Environmental characteristics are, however, more typical of the Scoyenia ichnofacies, with an emersion event providing conditions favorable to viewing traces preserved in a sand-softground substrate. Observation of in situ trace-making behavior allowed traces to be attributed to their progenitors, which include unionid and sphaeriid bivalves. An omission assemblage of vertebrate tracks also was present, comprising gull, raven, and mink. Oichnus-like borings were observed in some unionid shells. The shallow-tier trace assemblage created in a high-energy river channel may be expected to have a poor preservational potential, with loss of trace definition observed at the water margin during emersion and subsequent deterioration by eolian sediment transport.