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Lower Cretaceous (lower Hauterivian) oyster mass occurrences (OMOs) dominated by the gryphaeid small oyster Ceratostreon from the Neuquén Basin (west-central Argentina) are analyzed in terms of taphonomy and paleoecology in order to characterize their origin, reconstruct the oyster-dominated paleocommunity, and assess their paleoenvironmental implications. A laterally extensive oyster-bearing sedimentary interval with high oyster abundance was analyzed at three localities situated along a 75 km N-S transect. Three different types of OMOs were differentiated: biogenic bioherms with dominance of encrusting life habits, biogenic autobiostromes with dominance of soft bottom recliners, and mixed sedimentologic-biogenic parabiostromes with signs of reworking. The development of these different types of OMOs indicates a high nutrient input that favored high oyster proliferation, whereas the different life habits adopted by the oysters indicate a difference in sedimentation rate throughout the studied level: reclining oysters indicate a higher sedimentation rate than cementing ones. This Lower Cretaceous study case is framed in a general context of global shift of OMOs from a Tethyan distribution during the Jurassic to a mainly eastern Pacific distribution in the Cretaceous, occupying both coastal and deep continental shelf. Also, there is a taxonomic shift from primarily gryphaeid OMOs during the Jurassic and Lower Cretaceous to primarily ostreid OMOs during the Upper Cretaceous. Cenozoic OMOs are found in marginal marine environments and are exclusively built by ostreids. An increase in predation pressure from the mid-Mesozoic onwards could have pushed OMOs to marginal marine environments where predators cannot thrive and force a taxonomic shift towards brachyhaline taxa.
This study documents the association of glauconitic pellets and trace fossils at two Cambrian sites: the Reno Member, Lone Rock Formation, in southern Wisconsin and the Lion Mountain Member, Upper Riley Formation, in central Texas. Each site reflects a marine paleoenvironment on the Cambrian Laurentian margin rich in marine life that was affected by shifts in shoreline and wavebase. Both units contain abundant glauconitic fecal pellets, which were mixed with terrigenous quartz, and ichnofauna that characterize siliciclastic sublittoral marine environments of variable energy. This combination of ichnologic features is suggestive of multiphase depositional histories. Initial stages are represented by intense invertebrate feeding and pelletizing large quantities of seafloor sediment. Glauconitization of the pellets in relatively quiet-water environments represents intermediate stages. Burrowing of the units rich in relict glauconitic pellets in high-energy settings represents the final episode at each site. Findings demonstrate a major shift in benthic paleoecology in response to a changing paleoenvironment. The glauconitic pellets were sufficiently resistant to survive processes associated with a shallowing marine environment, as they were incorporated into primary sedimentary structures (ripplemarks and crossbeds) and biogenic structures (burrows). The combined ichnologic, mineralogic and sedimentologic observations offer insight for depositional interpretation.
The late Permian mass extinction was the most severe biotic crisis of the Phanerozoic, with associated environmental changes that included the expansion of hypoxic and anoxic conditions in shallow shelf settings. It has been hypothesized that wave aeration promoted oxygen transport to the seafloor providing a ‘habitable zone' in the shallowest marine environments that allowed the survival and rapid recovery of benthic invertebrates during the Early Triassic. We test this hypothesis by studying the rock and fossil records of the Lower Triassic Servino Formation, Italy. We also provide the first δ13Ccarb isotope curve, and present new occurrence data of stratigraphically important fossils (i.e., cf. Tirolites cassianus), to improve the stratigraphic framework of the Servino Formation. The low-diversity fossil assemblages of the Servino Formation have similar compositions to other western Paleotethyan localities. Facies analysis demonstrates that benthic invertebrates were restricted to wave-aerated settings, supporting the proposed ‘habitable zone' hypothesis. However, there is no evidence for rapid recovery in the ‘habitable zone' prior to the Spathian, which may indicate additional environmental stresses. In the lower Spathian Myophoria Beds Member, an increase in taxonomic and functional richness, the appearance of stenohaline, erect taxa, significant turnover, and increased heterogeneity in the composition of benthic assemblages indicate significant benthic recovery, which is attributed to reduced environmental stress. Prior to the late Spathian “Upper Member”, bioturbation is poorly developed and restricted to only a few thin horizons, but in the “Upper Member” the intensity of bioturbation and proportion of bioturbated rock increase. This change can be attributed to climatic cooling and a related decrease in environmental stress. This upper Spathian recovery pulse can now be traced across the western Paleotethys, in both nearshore and deep offshore (below wave base) settings.