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Continental siltstones of the Mesoproterozoic Copper Harbor Formation, Michigan contain macroscopic structures of a size and morphological complexity commonly associated with fossils of eukaryotic macroorganisms. A biogenic origin for these structures would significantly augment the Proterozoic continental fossil record, which is currently poor, and also add to a growing body of sedimentological and geochemical data that, albeit indirectly, indicates the presence of life in continental settings early in Earth's history. These three-dimensional structures occur abundantly within a single cm-scale siltstone bed. Along this bedding plane, these structures are generally circular-to-ovoid, range up to several centimeters in diameter, and most specimens possess a transecting lenticular element. Structures exhibit sharp, well-rounded external margins and, in contrast to the surrounding aluminosilicate-rich matrix, are calcitic in composition. Surrounding sedimentary laminae are deflected by and cross cut the structures. A fossiliferous origin is considered but rejected and an authigenic concretionary origin is favored based on these characters. However, a concretionary origin does not exclude the possibility of a biogenic precursor that served as a locus for early diagenetic calcite precipitation. This study highlights the need for careful analysis of morphological, mineralogical, distributional, and sedimentological characteristics when considering the origin of enigmatic structures; morphological complexity alone is an insufficient criterion for assignment of biogenicity. The unusual morphology of these concretions augments known concretion morphologies generally comparable to unusual fossil forms, and draws into question the biogenicity of similarly cryptic Proterozoic structures including, in particular, those of the 2.0 Ga Francevillian B Formation of Gabon.
The Trout Valley Formation of Emsian–Eifelian age in northern Maine hosts evidence of early vascular plant communities interpreted as having colonized wetland, estuarine environments. To date, collections made under permits issued by the Baxter State Park Commission originate from outcrop along Trout Brook, its tributaries, and poorly exposed lithologies inside the woodlands. Construction of a new roadway in the Scientific Forest Management Area, oriented parallel to Trout Brook, unearthed pavement exposures in which new fossil assemblages were found. These were collected under a salvage and conservation effort.
Plant megafossils, as well as new macroinvertebrates and ichnofossils, were recovered from sandy siltstone and siltstone lithofacies along a west-to-east transect along Wadleigh Mountain Road. Autochthonous plants fossils include erect axes of cf. Psilophyton forbesii and vertical rooting structures of unknown affinity. Parautochthonous and allochthonous plants include Pertica quadrifaria, Taeniocrada dubia, three species of Psilophyton, and cf. Kaulangiophyton sp., all of which are known from the area. A newly discovered shell ground of a monotypic assemblage of Modiomorpha concentrica is accompanied by trace fossils assigned to Skolithos, Planolites, and Spirophyton. The invertebrates are described to have inhabited shallow, nearshore estuarine to open marine conditions, which are unreported in the area. The co-occurrence of Modiomorpha, Spirophyton, and allochthonous axes of Pertica and Psilophyton is used as additional evidence to support a brackish water, estuarine depositional environment for the Trout Valley Formation.
Diverse faunas at the Gaddis site near Terlingua, Texas are present at four stratigraphic levels along a hill in the Upper Cretaceous Aguja Formation providing a rich fossil record of the transition from an aquatic environment to dry land while recording exceptional events such as predation and wildfires. Given evidence for minimal transport the microvertebrates, macrovertebrates, coprolites, and plants may be reliably utilized for paleoecological analysis. Paleoenvironments shift upsection from nearshore marine, through tidal channel, to swamp, and, finally, to well-vegetated dry land. The faunas change from a basal layer of oysters with shark teeth to a microvertebrate assemblage just above logs with Teredolites borings and pristine leaves. Above the microvertebrates are diverse macrovertebrates including numerous dinosaurs associated with leaves, logs, and scrambling vines. The macrovertebrate layer is a rich assemblage of herbivorous and carnivorous dinosaurs, crocodiles, turtles, and rare pterosaurs. Several skeletal elements exhibit signs of predation that include punctures and gouges that precisely match crocodile and theropod teeth also recovered at the site. Abundant plant material that includes charred vines and logs is likely evidence of wildfires having ranged across the area. The preponderance of young dinosaur remains may support a wildfire scenario. An exposed surface with 24 coprolites, two hadrosaur vertebrae, logs, and in situ stumps caps the section. The larger coprolites are likely dinosaurian. Most contain plant materials while one contains a bone fragments suggesting the presence of herbivorous and carnivorous dinosaurs on the same surface, an occurrence that echoes herbivore/carnivore interactions in the underlying bonebed.
Laminoid, weakly fused sponge spicule networks are intercalated between convex stromatolitic laminae in an upper Cambrian (Furongian) reef. Sponges are small, millimeter to centimeter-sized lithistids that encrusted automicritic laminae and in turn were encrusted by microbial biofilms, eventually leading to columnar, crudely laminated ‘stromatolites'. Weakly fused desmas, now replaced with fine, blocky calcite, possess an arcuate geometry along the medial to distal ray axes. Minimal decay and separation of sponge tissue from these spicules produced arcuate, filament-like cavities that obscure the former presence of spicule networks when viewed in cross-section. Further deterioration produced peloidal networks and voids. These observations have important implications when reconstructing middle to late Cambrian reef-building communities. Until recently, these periods were assumed to be virtually devoid of calcified metazoan reefal components. An increasing number of recent studies, however, are demonstrating that metazoan reef-builders were more prevalent. This paper adds a unique element to the nature of framebuilding by demonstrating that lithistid sponge-microbial reef-building communities constructed laminated ‘stromatolites,' and thus may have had a flourishing ecology within late Cambrian microbial buildups.