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Concretions are the most characteristic mode of fossil occurrence in the Upper Cretaceous Western Interior of the United States. An in-depth analysis of a single concretion from the upper Campanian Pierre Shale, South Dakota, drawing upon sedimentology, paleontology, shell preservation, degree of encrustation, and geochemistry allows us to determine a time frame for the accumulation and burial of the organisms and the process of cementation and diagenesis of the concretion. The concretion is very fossiliferous and dominated by mollusks. Large ammonites are commonly broken up with pieces missing from the adapical end of the body chamber. This breakage pattern is widely interpreted as evidence of lethal damage, implying introduction into the burial site via predation. In contrast, smaller ammonites are nearly complete and may have died due to smothering in resuspended sediment produced by bottom currents. The concretion is rich in cephalopod jaws, which mostly appear as isolated occurrences, usually deformed, with the calcite covering (aptychus) missing. The preservation of jaws suggests that the organic debris did not remain in the taphonomically active zone for more than a few years. The concretion, thus, represents a time-averaged deposit of organisms derived from the local community. In contrast, host sediments contain fewer fossils, most of which are crushed. Oxygen and carbon isotopic composition of samples in the concretion and the host sediments reveals a two-stage diagenetic history of the concretion. First, cementation probably occurred at shallow burial depths in early diagenesis in association with the decomposition of organic matter and the oxidation of methane. Second, alteration of the shelly material and the formation of calcite crystals filling the empty chambers of ammonites probably occurred during later diagenesis in contact with meteoric water.
During the early Holocene, rising waters of the Caribbean Sea flooded the Enriquillo Valley of southwestern Dominican Republic. A fringing coral reef developed and flourished along the margins of the Enriquillo Seaway for several millennia, until the seaway became restricted due to a combination of slowed sea-level rise, tectonic uplift, and increased sedimentation. Following changes in salinity and death of the coral reef by about 5 ka, meter-scale bioherms composed of the tubes of opportunistic, aggregating serpulid worms and associated with carbonate tufa that precipitated from the ancient lake or declining seaway waters formed along the steep walls of the valley over a period of 1,000 years or more. Paleoenvironmental conditions likely conducive to formation of these unusual bioherms include: non-normal marine salinity, warm and restricted waters, periods of stable water level, high Ca2 and CO32− influx, and moderate wave action. Although apparently rare elsewhere in the rock record, occurrences of serpulid-tufa bioherms provide useful constraints on paleoenvironmental settings. In previous literature, variations in terminology used to describe both tufa and serpulid-tufa bioherms have hindered intersite comparisons. Herein, serpulid-tufa bioherm structures are described at macro-, meso-, and microscales, and the varied bioherm macromorphologies are classified as individual, clustered, terraced, and patch types. These form descriptions are compared with previously published classifications of similar structures; the goal is to facilitate development of a comprehensive and universal classification of serpulid-tufa bioherms to further understand their formation and paleoenvironmental significance.
The Early Cretaceous Jehol biota of China, preserved in lacustrine deposits, yields abundant well-preserved invertebrate and vertebrate fossils. However, the exact mode and nature of its extraordinary preservation remains unclear. Here we investigated the preservation of fossil insects from lacustrine successions at 12 Lower Cretaceous localities of China, and found that many insects in Jehol biota were preserved by pyrite (later pseudomorphed by iron oxide). These pyritized fossils were observed solely within lacustrine deposits with volcanogenic sediments. The pyritization of Jehol insects has been facilitated by the moderate sedimentation rate, anoxic bottom environment, abundant dissolved iron and sulfur compounds from frequent volcanic eruptions, and sediments low in organic carbon. In addition, the pyritization process has been evidently aided by microbial films, which may have been widespread in the Jehol paleolakes. Our result provides the first record of widespread pyritization of insects from the freshwater Jehol biota, and shows that pyritization is an important preservational pathway in lacustrine successions.
Early Paleozoic calcareous algae are potentially useful for stratigraphic correlation but remain underutilized, likely due to presence of graptolites, conodonts, brachiopods, and other fossils that are commonly used in high-resolution biostratigraphy. This study focuses on the siphonous green algae within a 2-to-24-m-thick B interval of the Red River Formation, North Dakota, where the abundance of green algae suggests an important paleoenvironmental control; the algae also had a major role in carbonate production during that narrow stratigraphic interval. The bryopsidalean genus Dimorphosiphon Høeg is abundant in algal wacke-packstone facies interpreted as shallow subtidal deposits. One hundred and twenty-two individual Dimorphosiphon thalli were identified and studied in detail in randomly oriented thin sections; measurements indicate that Williston Basin specimens belong to the species D. talbotorum Boyd, previously reported exclusively from the Bighorn Mountains, Wyoming. Dimorphosiphon is found in Upper Ordovician low-latitude, warm-water shelf carbonates of Kazakhstania, Baltica, and Laurentia; commonly, it is a major component of sediment. Several species of Dimorphosiphon appeared simultaneously in different and remote parts of the Paleotethys and Iapetus Oceans, suggesting a geologically instantaneous dispersal of the genus. Dimorphosiphon talbotorum, the focus of this study, has only been reported from western North America where it occurs within strata corresponding to the upper Katian Aphelognathus divergens Subzone of the Aphelognathus ordovicicus conodont Zone. Given its abundance, ease of identification, and short stratigraphic range, D. talbotorum potentially is very useful for regional correlation, facies, and paleobiogeographic studies of Upper Ordovician Richmondian shallow-marine strata of western North America.
We performed a series of neoichnological experiments with elephants to investigate the relationship between the various factors involved in controlling megafaunal footprint formation. Our ultimate goal was to provide a means to calculate original sedimentary properties of fossil-footprint–bearing siliciclastic rocks, especially those containing sauropod dinosaur tracks. Previous semiquantitative and model-based research identified multiple variables that influence footprint creation and preservation, but no rigorous, empirically based models have been constructed. We conducted track-making trials with experimental sediments and one adult female African elephant (Loxodonta africana) and one adult female Asian elephant (Elephas maximus) in a zoo setting. Data collected included track dimensions, sediment particle size distribution, sediment bulk density (ρb), volumetric water content of the sediment (θv), and trackmaker walking velocity (v) and weight. We performed multiple regression analysis with a backward elimination technique to obtain the following relationship:
where Vn is track volume normalized by track length, measured in cm2, θv is in percent, ρb is measured in g/cm3, and v is measured in m/s.
We demonstrate the utility of this equation by calculating the original moisture content of sauropod-track–bearing siltstone and sandstone beds in the Upper Jurassic Morrison Formation. Original water content values are extremely useful for paleoenvironmental and paleohydrological interpretations of sediments and paleosols. Furthermore, paleoclimate studies can benefit greatly from original soil moisture values calculated from megafaunal footprints associated with paleosols.
Late Paleozoic fusuline foraminifera are thought to have hosted photosymbionts, as do modern larger foraminifera, but the ancient host-symbiont relationship has never been demonstrated conclusively. Among modern larger foraminifera, deeper-dwelling species exhibit large surface-to-volume ratios in order to maximize the amount of sunlight that can be captured for use by photosymbionts. Shallower-dwelling species exhibit smaller surface-to-volume ratios in order to limit incoming sunlight, especially ultraviolet radiation. If modern symbiont-bearing foraminifera are appropriate analogues for fusulines, then deeper-dwelling fusulines ought to exhibit larger surface-to-volume ratios than shallower-dwelling ones. This prediction was tested by analyzing fusuline shells from the Virgilian (Upper Pennsylvanian) Oread, Lecompton and Deer Creek cyclothems in Kansas. Specimens from deeper-water “middle” limestones exhibit significantly larger surface-to-volume ratios than those from regressive “upper” limestones, and specimens with the smallest surface-to-volume ratios occur in shoaling deposits at or near the tops of regressive limestones. Shell shape does not vary predictably with depth of habitat. Rather, changes in surface-to-volume ratio were accomplished mainly by changes in size, with larger shells always characterized by smaller ratios. The observed trend is significantly nonrandom with respect to depth of habitat (p = 0.012). The trend is not likely the result of hydrodynamic adaptation, postmortem size sorting or size decrease along a bottom oxygen gradient. It most likely reflects geometric optimization for photosymbiosis.