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In the shell-rich, laminated clays of the Phaeinum Subzone (Athleta Zone, upper Callovian, Middle Jurassic) of the Peterborough Member of the Oxford Clay Formation, large numbers of statoliths and otoliths have been recovered. This apparent mass mortality is associated with the Christian Malford Lagerstätte in which there is exceptional, soft-bodied preservation of coleoid fossils. Statoliths are the aragonitic ‘stones' that are found in the fluid-filled cavities (or statocysts) within the cartilaginous head of all modern and probably many fossil coleoids. Jurassic statoliths are largely undescribed and there are no known genera or species available to aid their classification. Otoliths, which may be of somewhat similar appearance, are the aragonitic stato-acoustic organs of bony (teleost) fish. These are more familiar to micropaleontologists and have a better known, though limited, fossil record. The abundance of statoliths in the Phaeinum Subzone at Christian Malford may indicate a mass mortality of squid that extends over some 3 m of strata and, therefore, a considerable interval of time. This has been tentatively interpreted as a record of a breeding area (and subsequent death) of squid-like cephalopods over an extended period of time rather than a small number of catastrophic events.
Early study of what is now called the Opohonga Limestone in west-central Utah indicated that it was part of a massive formation of Carboniferous age. Later studies identified fossils from the formation's older carbonates that were known to be Early Ordovician. A significant conodont fauna from a section on Gardison Ridge permits an even more precise definition of the formation which includes correlation with the classic North American Ibexian sequence of western Utah. While an important part of the earliest Ibexian sequence is missing on Gardison Ridge, the Opohonga Limestone there contains conodont elements representing seven zones and subzones of the type Ibexian and can be firmly correlated with conodont zones of the upper House and most of the Fillmore Formations of the Pogonip Group in the Ibex area. This correlation also confirms that the Sauk III–IV sequence boundary occurs in the lower part of the formation. The oldest conodont faunas of the Ibexian Series were not recovered from the lowest beds of the Opohonga Limestone, and it is possible that missing earliest Ibexian species in the oldest Opohonga beds may indicate that the poorly known underlying Ajax Dolomite, considered to be upper Cambrian, may actually include part of the basal Ordovician Ibexian Series.
Degree of bioturbation, Thalassinoides isp. morphology, and diameters were compared across the Cretaceous–Paleogene (K–Pg) boundary interval at three localities along the New Jersey coastal plain. Within this regionally extensive ichnoassemblage, mean burrow diameters decrease abruptly by 26–29% (n = 1767) at the base of the Main Fossiliferous Layer (MFL) or laterally equivalent horizons. The base of the MFL has been previously interpreted as the K–Pg boundary based on the last occurrence of Cretaceous marine reptiles, birds, and ammonites, as well as iridium anomalies and associated shocked quartz. Along with the mean, the maximum and minimum burrow diameters exhibit a negative shift, which indicates that the changes are the result of a directional reduction in diameter, rather than an artifact of decreased variance. As a proxy for the size of the tracemaker, a change in burrow diameter indicates a decrease in thalassinid crustacean body size. We interpret this shift as dwarfing within the endobenthic community as detrital food sources became scarce following the mass extinction. Despite the difference in size, there is no change in framework geometry. Ichnofabric indices generally increase up-section at each site across the K–Pg chronostratigraphic boundary, indicating a regional reduction in sedimentation rate, which is supported by a gradual increase in glauconite maturity. Overall, the ichnological evidence at these localities suggests that a prolonged period of negative feedback followed a short-term positive endobenthic response across the K–Pg boundary.
Seagrass meadows are a key component of marine ecosystems that play a variety of prominent geobiological roles in modern coastal environments. However, seagrass itself has low preservation potential, and consequently seagrass meadows are hard to identify in the rock record. In this study we combine observational taphonomic data from a modern sparse seagrass meadow with actualistic taphonomic experiments, in order to test whether taphonomic disparity (i.e., evenness in the distribution of taphonomic grades among multiple individuals) in the larger benthic foraminiferan Archaias angulatus has potential as a paleo-indicator for seagrass dominated communities. Our observational study demonstrates that sparse seagrass meadows possess a higher proportion of both pristine and highly altered tests than non-seagrass settings. Our taphonomic experiments, conducted over a six-month period, demonstrate a greater magnitude of bioerosion and diversity of bioerosion types in foraminifera deployed within sparse seagrass patches, than those deployed in patches without any seagrass cover. Although our experimental results in particular have high variability, these combined approaches provide a link between pattern (high taphonomic disparity) and process (higher rates of bioerosion) in developing the taphonomic signature of seagrass meadows. On the basis of these results we suggest several taphonomic criteria that could be used to identify seagrass meadows in the rock record. These criteria are potentially species-independent, and so may have greater utility as seagrass proxies than invertebrate indicator species that frequently have limited temporal or spatial distributions.
The mechanisms by which soft-bodied organisms were preserved in late Ediacaran deep-marine environments are revealed by petrographic and geochemical investigation of fossil-bearing surfaces from the Conception and St. John's groups (Newfoundland, Canada). Framboidal pyrite veneers are documented on fossil-bearing horizons at multiple localities. The pyrite is interpreted to have formed via microbial processes in the hours to weeks following burial of benthic communities. This finding extends the ‘death mask' model for Ediacaran soft-tissue preservation to deep-marine settings. Remineralization of pyrite to iron oxides and oxyhydroxides is recognized to result from recent oxidation by meteoric fluids in the shallow subsurface. Consideration of other global Ediacaran macrofossil occurrences reveals that pyrite is observed in association with Ediacaran macrofossils preserved in all four previously described styles of moldic preservation (Flinders-, Conception-, Fermeuse- and Nama-type). This suggests that replication of external morphology by framboidal pyrite was a widespread mechanism by which soft-bodied organisms and associated organic surfaces were preserved, in multiple facies and depositional environments, 580–541 million years ago. The extensive global burial of pyrite in medium- to coarse-grained clastics and carbonates is a previously unrecognized yet potentially significant geological sink of iron and sulfur, and may have contributed to rising atmospheric and marine oxygen concentrations across the late Ediacaran interval.