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The Cenozoic stratigraphic sequence in the foothills of the Eastern Cordillera of Colombia is mostly fluvial in nature and very thick (∼8000 m), but it contains very few mollusk-bearing horizons. Recent fieldwork discovered a well-preserved molluscan assemblage that occurs near the top of the Carbonera Formation (lower Miocene) in the central foothills of the Eastern Cordillera. This level, named the Huesser horizon, is laterally extensive and can be followed for tens of kilometers. The horizon is 10 m thick and was divided into eight levels, five of them highly fossiliferous. Most of the levels are dominated by the freshwater gastropod Sheppardiconcha, with lower abundances of the bivalves Anodondites and Mytilopsis. The top level is dominated by specimens from the bivalve family Arcidae. The taxonomic composition of the assemblage is similar to that of the Magdalena and Amazonas Basins during the early-to-middle Miocene. Paleoecologic, taphonomic, and palynological analyses indicate that the Huesser accumulated in a freshwater lake system, capped by a marine incursion. The development of a large lake and the subsequent marine event could be related to increasing subsidence coincident with eustatic sea-level rise that has been identified for the basin during the early Miocene.
It has recently been proposed that the northern Adriatic shelf is a living laboratory in which to test the causes of the evolutionary shift from Paleozoic-like, stationary suspension feeders on the sediment surface to modern, infauna-dominated assemblages. The suggestion is that today's “Paleozoic” ecosystems, composed of a modern fauna, are a regular feature in environments of low nutrient levels and predation intensity. We, however, argue that a high-biomass epifauna is not restricted to oligotrophic settings in the northern Adriatic Sea and that predation intensities are instead at Mediterranean levels, which are neither pre-Cenozoic nor similar to those at high latitudes. Environmental requirements of modern, suspension-feeding epifauna do not support the low-nutrient hypothesis, and we suggest that this striking epifauna depends on the presence of stable hard substrata on the seafloor, is very sensitive to sediment input by flood events and storm-induced sediment resuspension, and is related to seasonally high productivity. Elevation above the sediment-water interface has the advantage of feeding from higher-tier levels and helps these organisms to survive hypoxia, which is a typical seasonal feature of the Adriatic shelf and of many ancient epeiric seas. We hypothesize therefore that the gradual disappearance of large, epicontinental seas, along with their low sedimentation rates and frequent bottom-water hypoxia during the Mesozoic, supported the replacement of the archaic epifauna by modern, bivalve-dominated infaunas.
Dinosaur tracks and trackways yield invaluable information as to the identity, size, and gait of the trackmaker and the conditions of the media (=substrate) it traversed. Correctly interpreting tracks requires consideration of their three-dimensional morphology. Laboratory-controlled simulations were conducted to investigate the subsurface track morphology formed from differently shaped feet, as the shape of the footprint deteriorates with depth. A circular, triangular, and a tridactyl dinosaur foot-shaped template, or indenter, were indented vertically into two types of sand, with four moisture contents—dry, 10%, 20%, and saturated. The morphology of all three indenters was preserved most accurately in the moist sand. Tracks in dry and saturated sand were distorted by a greater degree of media deformation. Digit imprints of tridactyl tracks were only clearly discernible in near-surface layers and were deformed by shear zones or inward movement of sediment in dry and saturated sand. The long digits of the template produced the greatest degree of outward displacement, and tracks became wider with depth and deepest in the heel region. This was most distinct in dry sand, where extensive shear zones in cross section demonstrated the outward and upward movement of sediment. All tracks in saturated sand were characterized by considerable downward displacement of sediment and features related to the upward pull of sediment as the templates were withdrawn. These diagnostic features allow vertebrate tracks to be differentiated from nonbiogenic, soft-sediment deformation. Fossil tracks studied from the Middle Jurassic succession of the Cleveland Basin, Yorkshire, demonstrate affinities to the experimental tracks formed in saturated sand.
A diverse assemblage of Australian Ediacaran (late Neoproterozoic) acritarchs from the Centralian Superbasin and Adelaide Rift Complex demonstrates a range of taphonomic degradation. Recognition of taphonomic variants is critical for taxonomic studies and biostratigraphic interpretation. Taphonomic features observed include compression features, folding and tearing of vesicle walls, pitting, perforation, abrasion, exfoliation, shrinking, twisting, splitting, curling, shredding, pyritization, particle entrapment, and thermal maturation effects. The physical and chemical structure of the vesicle wall is instrumental in determining the degree of taphonomic damage. Consistent associations allow identification of degradation series that incorporate previously described individual species and provide a framework for taxonomic revision. Taphonomic associations may also characterize taphofacies, providing an additional tool for basin analysis.
Numerous rhabdoglyphid trace fossils in Cretaceous pelagic limestones from the French and Swiss Alps are ascribed to the ichnogenus HalimedidesLorenz von Liburnau, 1902. The heart-shaped chambers, interpreted as storage chambers, are joined to tunnels in a linear pattern. The burrow system is classified as an agrichnium. Morphological characteristics of the burrows indicate that small infaunal crustaceans were the likely trace makers. Halimedides are deep-sea traces that indicate relatively firm media (substrates)—stiffground to firmground—based on paleoenvironmental and morphological evidence. The usual association of Halimedides with Rhizocorallium and Spongeliomorpha clearly place them in the Glossifungites ichnofacies. Halimedides may be an indicator of subtle gaps in sedimentation. Densely chambered Halimedides are interpreted to indicate lower oxygenation, whereas sparsely chambered burrows indicate higher oxygenation of bottom waters. Consequently, Halimedides may be a useful tool for interpreting water depth, media consistency, sedimentation rate, and seafloor oxygenation.