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The dynamic physical interval where postmortem alteration of biological remains takes place is widely known as the taphonomically active zone (TAZ). In benthic systems, the TAZ is conventionally considered to be delimited by an upper boundary at the sediment-water interface and a lower boundary corresponding roughly to the deepest sediment layer influenced by bioturbation. However, this definition was developed in the context of marine or continental environments inhabited by benthic fauna and disregards the modifications that pelagic remains undergo while sinking through the water column. Indeed, long before the skeletal remains of planktonic organisms reach the sediment-water interface, they may suffer significant taphonomic damage, primarily due to dissolution. The magnitude of dissolution depends on the composition of the skeletal remains, seawater properties, and the nature and intensity of biological processes in the water column. In open ocean environments, siliceous remains (e.g., diatoms, radiolarians) suffer enhanced dissolution in the upper water column, where seawater is undersaturated in silica, whereas pelagic carbonate remains (e.g., foraminifers, coccolithophores) experience higher dissolution below the lysocline (the depth where there is a sharp increase in dissolution rate) until they reach the carbonate compensation depth (CCD), where dissolution is complete. Therefore, we argue that the TAZ concept for pelagic organisms should be extended to include the water column through which they settle after death. Furthermore, the extent of taphonomic damage of pelagic microfossils can be used as a potential proxy for past changes in seawater chemistry and circulation related to oceanographic conditions.
The Carnegie Quarry at Dinosaur National Monument was excavated in the Quarry Sandstone bed of the Brushy Basin Member of the Morrison Formation (Jurassic). The Quarry Sandstone is a discontinuous, lenticular, heterolithic unit that rests upon drab, greenish-gray claystones and is capped by reddish-brown mudstones. The underlying drab claystones are interpreted as being deposited in an overbank interfluvial wetland or lacustrine environment. By tracing the sandstone and studying detailed measured stratigraphic sections along its 1.8-km outcrop belt, this interval can be shown to be a complex of intergrading sandstone bodies and mudstone units. The Quarry Sandstone interval is reinterpreted as being deposited within a crevasse splay complex. The lowest part of this deposit is a laterally continuous, tabular unit composed of polymictic sandstone and granule conglomerate that directly overlies lacustrine claystones. This unit contains re-sedimented carbonate clasts from the surrounding Brushy Basin, displays a gradational to sharp basal contact, and is interpreted as the initial crevasse splay deposit. Overlying the tabular basal unit are numerous, laterally continuous, upward-coarsening sandstone bodies interpreted as distributary mouth bars. The interpreted distributary mouth bar deposits are cut by lenticular, trough cross-bedded, upward fining sandstone units exhibiting erosional bases. These sandstone lenses are interpreted as distributary channels within the crevasse splay. They, in turn, are overlain by discontinuous sandstone bodies that interfinger with, and are overlain by reddish brown and variegated, rooted mudstones containing calcic paleosols. The Carnegie dinosaur quarry was excavated in one of the main distributary channels of this splay complex.
Lotosaurus adentus is an unusual sail-backed, edentulous poposauroid pseudosuchian primarily known from a single, nearly monospecific bonebed discovered and excavated in the 1970s in the Middle-Upper Triassic Badong Formation of Sangzhi County, Hunan Province, South China. Renewed interest in this unique taxon prompted exposure of an additional 90 m2 of the bonebed. Almost 1000 new L. adentus bones, 28% of which were articulated, were discovered during this excavation. The bones lack evidence of tooth marks, trample marks, or insect modification, and display minimal weathering. The site is reinterpreted as a pedogenically modified floodplain pond (and overlying fluvial channel) within a warm, semi-arid sub-tropical region (paleolatitude ∼ 34°N), contrasting with previous interpretations of the locality as a tidal flat deposit. The occurrence of mudcracks, conchostrachan fossils, and vertic paleosol development with calcium carbonate accumulations in both overlying and underlying facies indicates periodic aridity and ephemeral conditions. The bonebed is characterized by partial disarticulation and minor transport, and is confined to a thin, < 30 cm-thick interval. Considered together, these features are most consistent with a mass mortality event, possibly drought related considering the sedimentological context, followed by minor transport during a rapid burial event.
U-Pb detrital zircon geochronology and Lu-Hf isotope analysis were utilized to reassess the provenance and age of the deposit, and suggest that L. adentus was likely Ladinian or possibly even Carnian in age, rather than Anisian as previously reported. Paleocurrent data, detrital zircon age spectra, and Lu-Hf isotopes indicate that fluvial sediments were partially derived from sources in the North China craton and Qinling-Dabieshan tectonic belt, implying that faunal interchange between the North and South China blocks was possible by this time.