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The Placerias Quarry of northeastern Arizona is one of the most diverse Triassic vertebrate fossil localities in the world. Several long-standing hypotheses are tested about depositional environment, paleoecology, and thanatocoenotic influences. Sedimentological features indicate a low-energy depositional environment. Bones are associated mostly with mudstones and a layer that contains numerous carbonate nodules. Thin-sections and field characteristics reflect that this nodule zone is pedogenic in origin. Taphonomic data show that the bones were not transported into the site by fluvial processes. The frequency of modification features is minimal and there is little evidence of postmortem alteration of the bones from prolonged exposure, trampling, or predation.
Element survivorship curves were generated for the five most abundant taxa in the quarry. Curves for left and right elements tracked each other closely, but no pattern of preservation of cranial and postcranial elements was evident either within or among taxa. No clear signal was provided by either size or hydrodynamic potential of the elements. Because the same elements were not preserved preferentially among the best represented taxa, there is no systematic bias of preservation or removal by predators, though it cannot be eliminated, and taphonomic evidence provides no reason to suspect stream transport as a preservational bias. It is proposed as a general guide for assessing the relative completeness of preservation of an assemblage that the convexity of the element survivorship curve is proportional to the representation of the original death assemblage in the fossil assemblage. Thus, it provides increasing confidence in the assessment of the numbers of individuals actually represented.
Sedimentologic and taphonomic data indicate that the Placerias Quarry was a site of bone accumulation in a soil with a high water table. The causes of death for the animals remain largely unknown, but recently proposed climatic models for the Chinle Formation are consistent with the hypothesis that seasonal droughts may have caused mortality events like this one.
The substrate-controlled Glossifungites ichnofacies is a firmground suite of trace fossils that commonly demarcates erosional discontinuities in sedimentary successions. The Glossifungites ichnofacies typically is characterized by biogenic structures that are vertical to subvertical, sharp-walled, unlined and commonly passively infilled. The structures hitherto have been restricted to permanent or semi-permanent domiciles, and predominantly to vertical to subvertical burrows of suspension-feeding organisms.
Several cores of the late Albian (Lower Cretaceous) Viking Formation from the Hamilton Lake Field of Alberta, Canada contain an atypical Glossifungites ichnofacies dominated by the foraging, probing and deposit-feeding/dwelling structures of firmground Thalassinoides, Rhizocorallium, and Zoophycos. The Thalassinoides are passively infilled with coarse grains of sand and chert granules. Rhizocorallium displays active stages of infill, although the final, horizontal U-shaped tube is passively filled. The Zoophycos, however, demonstrates active infill manifest by chert granules and coarse sand distributed in the spreite, and this is a departure from the normal expression of the Glossifungites ichnofacies.
The firmground structures subtend from the regional stratigraphic discontinuity BD4, excavated into black, silt- and sand-poor offshore and shelf mudstones. BD4 is interpreted to reflect a transgressively modified sequence boundary. Most Zoophycos, Thalassinoides, and Rhizocorallium penetrate no deeper than 3–4 cm below the discontinuity into the underlying mudstones. Where BD4 overlies a sandier substrate, firmground Zoophycos are not present within the suite.
The facies directly overlying BD4 consist of fining-upward, pebble-, granule-, and very coarse-grained sand-bearing, thoroughly burrowed muddy sandstones to sandy mudstones, typically 5–25 cm thick. This basal transgressive lag contains Teichichnus, Planolites, Palaeophycus, Diplocraterion, Chondrites and rare Helminthopsis, and is the source of the coarse-grained material incorporated into the fill of the firmground ichnogenera. The granule-bearing sandy mudstones overlying the transgressive lag contain abundant Teichichnus, Planolites, Helminthopsis, Anconichnus, Chondrites, and Terebellina, and reflect rapid deepening to proximal offshore conditions. Progressive deepening is recorded in the accumulation of distal offshore and shelf mudstones of the late Albian Westgate Formation.
Although excavation of BD4 occurred above fairweather wave base, its colonization occurred under much lower energy conditions. Within distal softground settings, ichnogenera capable of deeply penetrating muddy substrates are largely restricted to Zoophycos, Thalassinoides, Rhizocorallium, and Chondrites. These ichnogenera constitute forms capable of being excavated, albeit shallowly, within a firm
Coral-rich strata of the Leitha Limestone of Badenian age (Middle Miocene) at the type-locality in Grosshoeflein (Burgenland province, Austria) were re-interpreted from an actualistic viewpoint in comparison with Recent Bahamian and Arabian Gulf coral communities. The investigated outcrop (quarry “Fenk”) is interpreted as a sequence of framebuilding coral carpets and non-framebuilding biostromal coral communities, alternating with bryozoan and corallinacean calcarenites and bivalve beds. No indication for the presence of a reef was found. In total, a sequence of 10 coral intervals was observed, which can be grouped into 2 coral carpet types (high carpet framework, low bushy framework), four non-framework communities, and one bivalve/coral community. Within this sequence, no unequivocal indication for sea-level changes at the time of deposition was apparent. The different coral facies are interpreted as having been deposited within roughly the same water depth as Recent coral systems observed in the Bahamas (Andros, Exuma Cays) and the Arabian Gulf (Dubai, UAE). According to the model presented, the facies merely represent different lateral positions within the same spatially highly complex environment. A distribution map of benthic biota in the Arabian Gulf (Jebel Ali, Dubai, UAE) was used to illustrate the likely lateral distribution of the Miocene facies.
Of More than 4000 specimens of the naticid gastropods Euspira heros (Say) and Neverita duplicata (Say) from southern New Jersey, the lower Pleistocene of North Carolina, upper Miocene of Maryland, and the lower Miocene of Delaware, subsamples with complete and incomplete boreholes (n = 613) were compared for borehole-site stereotypy, prey size-selectivity, prey profitability, and prey effectiveness. In confamilial encounters, adaptation of naticid predators is evidenced by a shift in borehole-site stereotypy on the body whorl toward the umbilicus during the last 18 my, particularly for N. duplicata. Inferentially, an umbilical drilling position enabled the base of the predator's foot to occlude the prey aperture and prevent egress of the dangerous prey's foot, thereby reducing the prey's escape potential. The tradeoff was that the umbilical site required the predator to drill through a thicker shell location.
Prey effectiveness, the ratio of incomplete boreholes to total attempts, was initially low (0.03) for both species in the lower Miocene, but increased appreciably from the Pleistocene to the Recent for N. duplicata (0.32). Such increase in successful prey escape indirectly may reflect prey adaptation since the Miocene. Cost/benefit curves, i.e., log of the ratio of apertural lip thickness /internal shell volume regressed on whorl diameter (WD), have significantly greater negative slopes for Miocene versus Recent conspecifics. Lower cost/benefit ratios for successive size classes of modern naticids suggest that confamilial prey have become increasingly profitable molluscan options as they increased in size, despite increased risk of fatality to the predator. Decreasing naticid prey size-selectivity, as evidenced by lower regression correlation coefficients since the Miocene, reflects increasing mismatches between predator and prey size. Outcomes of size mismatches in predatory encounters between E. heros and N. duplicata were not predictable necessarily given potential differences in species agressiveness and foot size. This unpredictability fueled coevolution between these cannibalistic moonsnails and their dangerous intraspecific and interspecific prey.
The first detailed reconstruction of the continental paleoclimate evolution of the Northwest German Tertiary (Late Oligocene to Pliocene) is presented. The paleoclimate data are derived from the paleobotanical record using the coexistence approach, a method recently introduced that employs climatic requirements of the Nearest Living Relatives of a fossil flora. Twenty six megafloras (fruits and seeds, leaves, woods) from the Tertiary succession of the Lower Rhine Basin and neighboring areas are analyzed with respect to ten meteorological parameters. Additionally, two sample sets from Late Miocene to Early Pliocene sediments comprising 396 palynofloras are analyzed by the same method providing a higher temporal resolution. The temperature curves show a comparatively cooler phase in the Late Oligocene, a warm interval the Middle Miocene, and a cooling starting at 14 Ma. The cooling trend persisted until Late Pliocene with a few higher frequency temperature variations observed. From the beginning of Late Miocene to the present, the seasonality increases and climate appears to have been less stable. As indicated by the precipitation data, a Cfa climate with wet summers persisted in NW Germany from Late Oligocene to Late Pliocene.
The results obtained are well in accordance with regional and global isotope curves derived from the marine record, and allow for a refined correlation of the Tertiary succession in the Lower Rhine Basin with the international standard. It is shown that the reconstructed data are largely consistent with the continental climate record for the Northern Hemisphere, as reported by various authors. Discrepancies with previous reconstructions are discussed in detail.
Small spicular, columnar, and blade-shaped stromatolites are common features of hot spring and geyser systems in the Taupo Volcanic Zone of North Island, New Zealand. These organosedimentary structures can grow in many settings, where water temperature and pH vary widely. Although most common in neutral and alkaline waters, stromatolites also are forming in several acid-sulfate spring and geyser systems in the Waiotapu geothermal area and at Lake Rotokawa. Growth of these stromatolites is partly mediated by a biota that is dominated by fungi and locally, diatoms.
When stromatolites from acidic thermal waters are compared with those from neutral and alkali waters, significant differences in their biota and mineralogy are evident. The biota in stromatolites from the neutral and alkali waters is dominated by prokaryotic bacteria (including cyanobacteria), whereas stromatolites from acidic waters are dominated by eukaryotic fungi and to a lesser extent, diatoms. Stromatolites in the neutral and alkali thermal waters are formed almost entirely of opaline silica, with calcite laminae present in a few localities. Although stromatolites in the acidic systems also are composed mainly of opaline silica, they contain substantial amounts of kaolinite and, locally, sulfur and/or jarosite. In ancient thermal deposits it may be possible to distinguish stromatolites that grew in acidic waters from those that formed in neutral and alkali systems by considering their preserved biota and mineralogy.
The taphonomy of a newly discovered petrified forest in the Two Medicine Formation, south of Choteau, Montana, provides new constraints on the pre-thrusting position of the Boulder batholith and Elkhorn Mountains Volcanics. The petrified forest consists of more than 200 charcoalified, subaligned trees encased in an ash flow tuff and an immediately overlying bentonite. The ash flow tuff has been dated at 80.002± 0.114Ma (39Ar/40Ar on plagioclase). The ash flow and bentonite are interpreted to record a single catastrophic eruptive event that toppled and entombed the forest. The mean azimuth of the prostrate trunks, which are interpreted as the direction of travel of the ash flow, is 037° (2σ = 35°).
Based upon age, mineralogy, and the absence of other contemporaneous volcanic centers, it is concluded that the Late Cretaceous Elkhorn Mountains Volcanics were the most likely source of the tuff and bentonite that encase the trees. The mean azimuth of the tree trunks is consistent with palinspastic restorations of the northern Rocky Mountain fold-thrust belt that place the Elkhorn Mountains Volcanics and their plutonic equivalent, the Boulder batholith, ∼65–110 km northwest of their present position during Campanian time. This restored position is ∼150–200 km from the petrified forest, making the ash flow one of the farthest-traveled documented flows with enough power to topple mature trees.
Coral samples recovered from the Falmouth Formation in north Jamaica, an emergent Pleistocene (Sangamonian) fringing reef complex, contain a wide range of boring morphologies belonging to the ichnogenera Entobia (7 ( 1?) ichnospecies), Gastrochaenolites (2 ichnospecies), Maeandropolydora and Trypanites. Sampling from four distinct shallow-water reef biofacies permitted quantification of spatial variations in boring community composition and degree of substrate infestation. Coral samples from back-reef/lagoon facies (mostly coral rubble) are characterized by diverse assemblages of borers, dominated by both sponge and “worm” borings, but with bivalve borings locally important. By contrast, shallow fore-reef facies are dominated by sponge borings, with “worm” borings locally important and bivalve borings rare or absent. Highest rates of substrate infestation (expressed as a % of framework removed by borers) occur within lagoon/back-reef samples. Although facies constraints (especially the absence of deeper water facies) are likely to have exerted an influence on these data, broadly similar borer assemblages have been reported from both modern and other Cenozoic reef systems. The data provide a further link in what is, at present, the very poorly documented history of bioerosion in fossil reef systems.