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• Here we describe ∼ 563 Ma pseudofossils that mimic metazoan trace fossils. • Pseudofossils are identified as tectographs and their tectonic origin is confirmed. • Further criteria are raised to refine the early animal trace fossil record. Simple and horizontal trace fossils from the Ediacaran Period (635–538.8 Ma) compose some of the oldest evidence for the presence of metazoans in the Proterozoic fossil record. Unfortunately, various abiotic processes may produce similar structures on sedimentary beds. One of the oldest records of the Ediacaran biota from Western Gondwana is preserved in the Itajaí Basin (SE Brazil), a peripheral foreland basin related to the Dom Feliciano Belt. The fossil assemblage in the Itajaí Basin consists of Aspidella , Nimbia , microbially induced sedimentary structures (MISS), and problematic taxa, including filamentous body fossils. Moreover, foreland basins are characterized by thrust-related fold belts, a result from their compressive tectonic settings. Here we report a new occurrence of enigmatic repeated horizontal markings. These structures exhibit linear to looping markings that are strikingly similar to simple horizontal animal trace fossils. However, the repetitive, preferentially oriented morphologies, and direct association with clasts show that they are tectographs, a rare pseudofossil interpreted to be formed by tectonically-induced bed-to-bed shearing. Indeed, their preferential alignment (NW-SE), following the original tectonic stress field associated with the development of the Itajaí Basin, strongly supports this interpretation. Considering the tectographs from North Carolina slate belt and Kupferschiefer of Germany, the Itajaí Basin structures are the third occurrence worldwide of such pseudofossils and the first case for Ediacaran successions, exhibiting a clear relationship to the tectonic setting associated with Western Gondwana assembly. Hence, some criteria to evaluate putative trace fossils are proposed, and we suggest that caution is needed when dealing with sedimentary structures that are candidates to be produced by metazoan activity in the deep time. This cautionary tale is valuable for Precambrian and Paleozoic paleontologists and geologists with implications for future astrobiological studies. [ABSTRACT FROM AUTHOR]

Schematic representation of the development of the pore network during weathering. Macrocracks are shown in red, microcracks in blue, and the microporosity of the minerals is shown in grey (D µ = density of microcracks; DM = density of macrocracks; ΦMp = microporosity; ΦM = porosity of macrocracks; Φ µ = porosity of microcracks). [Display omitted] • Pores densities and apertures were determined by 14C-PMMA autoradiography and XRCT. • Cracks densities and cracks apertures both increase during weathering. • Total porosity mainly increases by increase of the aperture of pre-existing cracks. • Initial porosity of un-weathered rock controls development of pores during weathering. The development of porosity during rock weathering is a key process controlling nutrients release, water holding capacity available for plants and water flow. Here we used X-ray Computed Tomography (XRCT) and 14C PolyMethylMethAcrylate (PMMA) autoradiography to show how cracks are created and enlarged during initial weathering stages (saprock and saprolite) of granodiorite in southern Brazil (Viamão - RS). The physical evolution is characterized by imaging the pore network, using 14C-PMMA and XRCT methods. Combined with bulk porosity measurements, they highlight the increase in porosity with the degree of weathering (un-weathered rock Φ = 1.66 %, saprolite Φ = 11.7 %). This increase is related to the joint increase of the density of the cracks (un-weathered rock D = 0.28 mm−1, saprolite D = 0.94 mm−1) and of the average opening of the microcracks (un-weathered rock w = 2.4 µm, saprolite w = 3.9 µm) and macrocracks (un-weathered rock w = 176 µm, saprolite w = 400 µm). However, these average crack openings do not account for the variability of the openings that govern the flows, characterized here by specific distribution ranging from the submicrometre to the centimetre scale. The results highlight that the pore network of the un-weathered rock plays a key role in the initial stages or incipient weathering. The density and aperture and cracks increase following the subcritical cracking concept and new pores are formed by chemo-mechanical processes. The presence/absence of initial fractures in the regolith is certainly a key parameter controlling the weathering of different rock types (mafic vs felsic). [ABSTRACT FROM AUTHOR]

Chemiographic projections of the chemical compositions of the minerals obtained with the SEM EDS probe in the 4Si – M+ – R2+ ternary diagram. The trend lines depict the weathering sequences of the minerals. Chloritization of some biotite of the un-weathered rock due to post-magmatic alteration. Weathering of biotite into smectite in a first step in the saprock and into kaolinite in a second step in the saprolite. Weathering of feldspars into smectite in the saprock and then into kaolinite in the saprolite. The trend lines for feldspars and biotites are distinct due to different chemical compositions of the secondary minerals, magnesian for biotite weathering and aluminous for feldspars weathering. [Display omitted] • Weathering intensity measured by chemical losses and fine-sized particles formation. • Weathering of biotites by expansion form vermiculites and subsequently kaolinite. • Weathering of felspars in dissolution pits forms kaolinite. • Incipient weathering of biotite creates porosity allowing fluids percolation. • Total porosity increases during weathering of granodiorite in South Brazil. Weathering transforms fresh un-weathered rock into saprolite and soils, porous materials that may hold available water for plants and nutrients. The conjoint characterization of mineral weathering and development of porosity is helpful in understanding rock weathering and the development of soil. The objective of this study was to investigate mineralogical transformations and the increase in total porosity during granodiorite weathering in South Brazil by combining petrographical observations, mineralogical analyses, and porosity measurements. The studied granodiorite profile presented a saprock of ∼2 m thickness with spheroidal weathering, a ∼15 m thick saprolite of and soil of ∼2 m thickness. The intensity of weathering was estimated using chemical indices (CIA and WIS) and by the production of fines particles of silt and clay sizes. As weathering increased from saprock to saprolite and soils total porosity increased from 1.73 % in the un-weathered rock, between 3.57 and 10.5 % in the saprock and 11.02 % in the saprolite. The chemical losses were limited in saprock and saprolite (CIA = 68, Δ4Si = 29.31 %) and increased in the topsoil (CIA = 85.73, Δ4Si = 56.83 %), indicating a moderate weathering intensity. Petrographic observations by optical microscope and SEM, chemical composition using SEM-EDS and X-ray diffraction results showed principal mineral weathering reactions were the transformations of biotite into vermiculite (with biotite/vermiculite mixed layers as intermediate weathering stages) and weathering of plagioclases and potassic felspars into kaolinite. The multi-mineral composition of the saprolite indicated a progressive transformation of primary minerals with increasing weathering intensity. The results show that the opening of the porosity is likely to control the development of the weathering and should deserve more detailed characterisation. [ABSTRACT FROM AUTHOR]

Biomineralized and organic metazoan tubular skeletons are by far the most common in the fossil record. However, several groups of organisms are also able to agglutinate particles to construct more rigid structures. Here we present a novel type of agglutinated tube from the austral and endemic palaeobiota of the Malvinokaffric realm (Devonian, Brazil). This fossil is characterized by an agglutinated tube made of silt-sized particles forming an unusual flanged morphology that is not known from the fossil record. Besides being able to select specific particles, these organisms probably lived partially buried and were detritus/suspension feeders. Comparisons across different modern groups show that these fossils are strongly similar to tubes made by polychaetes, specifically from the family Maldanidae. If this interpretation is correct, then an early divergence of the Sedentaria clade may have occurred before the Devonian.