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Modern valves of Lucina pensylvanica (Bivalvia: Lucinidae) were analyzed for 18O/16O ratios, drilling predation traces, biometric measurements, and taphonomic descriptors to explore seasonal variations in ecological and taphonomic processes within a death assemblage from Grand Bahama Island. The δ18O values at the shell margin ( = last growth episode) were used as a proxy for temperature at the time closest to the organism's death. Temperature estimates suggest that most individuals died in warmer months, whereas mortality appeared to have been lower during cooler seasons (<24 °C). Only drilled valves yielded the coldest temperatures at the shell margin (∼18.1–21.3 °C), whereas exclusively undrilled valves exhibited the hottest temperatures at the last growth episode (∼25.5–26.5 °C). Drilled valves were significantly smaller and exhibited higher taphonomic alteration than undrilled valves, pointing to postmortem transport and sorting. While seasonal variations in predation and growth rates may have affected the observed patterns to some extent, the results, at least partly, reflect hydrodynamic variations throughout the year, which may have induced seasonal variation in postmortem sorting of drilled and undrilled valves. This study illustrates that, by combining geochemical proxies and quantitative paleoecological data, more informed ecologic and taphonomic interpretations can be achieved for subfossil and fossil assemblages.
The association of vertebrate, invertebrate, plant, and ichnofossils from two Upper Cretaceous lacustrine environments in West Texas permits interpretation of the paleolimnology of part of the Javelina Formation. The relatively rich fossil assemblages permit studies of paleoenvironments and food web complexities, as well as provide paleohydrological information. In Big Bend National Park, at least three juvenile Alamosaurus sauropods were preserved in situ in anoxic mud among shallow-water lacustrine charophytes. Alamosaur remains in the Moon Valley section are both in situ and transported. Mussels, gastropods, gar, and crayfish trace fossils largely define the paleoecology, as well as reveal details of specific biological relationships, especially at several intervals in Moon Valley. The modern larval-host reproduction system of mussels may have existed in the possible case of an ectoparasitic relationship in the co-occurrence of fossil Unio and Lepisosteus. Relatively fine time-frame resolution is present based on investigations of the mussel, Unio, and the presence of crayfish burrows. Unio sclerochronology indicates continuous inundation for at least seven years at two different stratigraphic intervals. Horizons of fossil crayfish burrows record both periods of nondeposition and the level of the water table at those times. Viviparus gastropods suggest subtleties of water movement and oxygen concentration. Studies of the dinosaur bones and the presence of probable footprint impressions suggest dinosaurs visited these types of water bodies.
We report on the first record of a therizinosaur from Alaska. This record consists of a single pes track from the lower part of the Upper Cretaceous Cantwell Formation in Denali National Park, Alaska, United States. This is the northernmost occurrence for this group of dinosaurs, and the presence of this animal in Alaska offers the first support of the proposed biogeographic model of faunal exchange during the Cretaceous for these unusual theropods.
The compositional fidelity of dead assemblages—the extent to which subfossil remains reflect the composition, structure, and scale of the original living community—is an indispensable component of studies assessing the quality of the fossil record and the effects of taphonomic biases on species composition and diversity. The aim of the present study is to evaluate (1) how faithfully abundances of subfossil freshwater mollusk shells deposited in dead assemblages correspond to their abundances in the local living assemblages, (2) how reliably diversity of living assemblages is captured by dead assemblages, (3) whether spatial variation in species composition in living assemblages is captured by dead assemblages in the southeastern Pampas, Argentina. Twenty sites corresponding to five lotic and five lentic sites from two geographic areas were analyzed in terms of taxonomic composition and diversity, and homogeneity of multivariate dispersions. We find that (1) living and dead assemblages occupy similar portions of multivariate space and do not significantly differ in composition within an area, (2) living and dead assemblages show similar richness and evenness, and (3) between-environment differences in among-site variation in composition (beta diversity) captured by living assemblages are preserved by dead assemblages. Thus, the results highlight the potential these environments have to enhance the preservation of mollusks and result in fossil assemblages that are suitable for paleoecological and paleoenvironmental studies of freshwater ecosystems.
Arpylorus antiquus, erected by Calandra in 1964, was isolated from upper Silurian sedimentary rocks from the Mechiguig 1 borehole in southern Tunisia, with other palynomorphs. The folded vesicle and the quadrangular form of the aperture break down into platelike fragments, resembling the tabulation of dinoflagellates. The presence of these elements has been used to interpret A. antiquus as a dinoflagellate cyst. The morphology and affinity of A. antiquus is reinterpreted herein based on investigation of larger sets of samples, including material from the type locality, together with material of Algeria, Saudi Arabia, and Brazil. More complete specimens than those previously described have been observed using gentle laboratory techniques, showing a large development of a fine membrane at the periphery of vesicles. This element was destroyed using classical palynological treatments, implying that the holotype is an incomplete specimen. The membrane at the periphery of vesicles and dorsoventral differentiation of these vesicles suggest that A. antiquus is a part of a more complex biological structure. We suggest a possible relationship with eurypterids, arthropods related to phyllocarids, represented by abundant fragments in the assemblages. Arpylorus antiquus is possibly a structure of storage. The chemical composition of A. antiquus using a Fourier transform infrared FTIR microspectroscopy analysis, reveals a wall composed of biopolymer that is not consistent with dinosporin. We conclude that Arpylorus antiquus is definitively not a dinoflagellate cyst. Although dinoflagellates may have older Paleozoic or even Proterozoic ancestors as the biomarker record may suggest, the dinoflagellate tabulation evolved only in the early Mesozoic.
The Paleozoic spread of plants, beginning in the Ordovician and reaching a phase of accelerated diversity and dispersal in the Early Devonian, was a critical episode in Earth history. The Lower Devonian (Emsian) Campbellton Formation represents a subtropical basin comprising coastal and fluvial–lacustrine intermontane environments. Although the formation has been recognized for a rich fossil assemblage since the mid-19th century, this is the first attempt to correlate the tracheophyte fossil record with physical attributes of the environment as inferred from the sedimentology. Marginal lacustrine beds contain a parautochthonous collection of plants interpreted as dislodged from lakeside plant stands and buried by density flows. Shallow, oxygen-depleted lakes or ponds subjected to fluctuations in water level collected a more diverse assemblage from the surrounding flooded marsh. Plants within fluvial sandstones were transported by ephemeral flows, and typically display poorer preservation. Although some plants showed adaptations to drier environments, there is little evidence of the environmental partitioning identified at contemporaneous localities between rhyniaceans, basal euphyllophytes (trimerophytes), zosterophylls, and lycopsids, all of which are here found in nearly every identified setting. Plant occupation of basin margins or upland areas is suggested by the occurrence of plant-bearing strata within blocks transported cohesively by hyperconcentrated flows.