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Geyserite is a type of terrestrial siliceous hot spring deposit (sinter) formed subaerially in proximal vent areas, with near‐neutral pH, alkali chloride discharge fluids characterized by initial high temperatures (~73°C to up to 100°C) that fluctuate rapidly in relation to dynamic hydrology, seasonality, wind, and other environmental parameters. We analyzed sinters at the Claudia paleogeothermal field from the Late Jurassic (~150 Ma) Deseado Massif geological province, Argentinean Patagonia. The geyserite samples—with spicular to columnar to nodular morphologies—contain abundant microfossils in monotypic assemblages that occur in three diagenetic states of preservation. The best‐preserved microfossils consist of vesicle‐like structures with radial heteropolar symmetry (~35 μm average diameter), circular apertures, smooth walls lacking ornamentation, and disk‐ or beret‐like shapes. Comparisons with extant, morphologically similar organisms suggest an affinity with the testate amoebae of the Arcella hemisphaerica–Arcella rotundata complex and Centropyxis aculeata strain discoides. These species occur in active geothermal pools between 22°C and 45°C, inconsistent with the temperature of formation of modern geyserites. We propose that the testate amoebae may have colonized the geyserite during cooler phases in between spring‐vent eruptive cycles to prey on biofilms. Silica precipitation through intermittent bathing and splashing of fluctuating thermal fluid discharge could have led to their entrapment and fossilization. Petrographic analysis supports cyclicity in paleovent water eruptions and later diagenesis that transformed the opal into quartz. Spatially patchy degradation and modification of the silicified microorganisms resulted in variable preservation quality of the microfossils. This contribution illustrates the importance of microscale analysis to locate early silicification and identify high‐quality preservation of fossil remains in siliceous hot spring deposits, which are important in early life studies on Earth and potentially Mars. [ABSTRACT FROM AUTHOR]

We present the anatomical study of axes of the corystosperm Rhexoxylon piatnitzkyi and the interactions of this plant with wood-boring beetles. We used a CT scanner and thin sections to reconstruct the boring pattern inside the plant axes, as well as the tissues affected by the beetle. The galleries were abundant in the branch of R. piatnitzkyi, including both longitudinal and transverse borings, and less frequent in the trunks. The galleries are bored preferentially in the secondary phloem and in the pith. The position of the galleries as well as the cell and tissue composition of the frass indicates that the borer was mainly a corticophagous insect. The sample also presents evidence of white rot by lignivorous fungi, and fragments of tracheids can be recognised in the frass that fills the galleries, indicating that the borer had some capacity to feed on decayed wood. The anatomical features of the secondary xylem and phloem suggest that the plant deployed defence mechanisms against the fungal decay and/or the boring beetle. This is the first evidence of insect galleries in axes of corystosperms, a dominant group of plants during the Triassic in Gondwana, and the first possible record of cerambycid borings. [ABSTRACT FROM AUTHOR]