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Abstract Molecular clocks estimate that diatom microalgae, one of Earth’s foremost primary producers, originated near the Triassic–Jurassic boundary (200 Ma), which is close in age to the earliest, generally accepted diatom fossils of the genus Pyxidicula. During an extensive search for Jurassic diatoms from twenty-five sites worldwide, three sites yielded microfossils initially recognized as diatoms. After applying stringent safeguards and evaluation criteria, however, the fossils found at each of the three sites were rejected as new diatom records. This led us to systematically reexamine published evidence in support of Lower- and Middle-Jurassic Pyxidicula fossils. Although Pyxidicula resembles some extant radial centric diatoms and has character states that may have been similar to those of ancestral diatoms, we describe numerous sources of uncertainty regarding the reliability of these records. We conclude that the Lower Jurassic Pyxidicula fossils were most likely calcareous nannofossils, whereas the Middle Jurassic Pyxidicula species has been reassigned to the Lower Cretaceous and is likely a testate amoeba, not a diatom. Excluding the Pyxidicula fossils widens the gap between the estimated time of origin and the oldest abundant fossil diatom record to 75 million years. This study underscores the difficulties in discovering and validating ancient microfossils.

The end-Triassic Mass Extinction (ETME) is generally regarded as a consequence of the environmental changes associated with the emplacement of the Central Atlantic Magmatic Province (CAMP) and ranks among the ‘big five’ mass extinctions in Earth history. A notable feature of the ETME is a halt in marine carbonate deposition followed by the formation of unusual facies such as carbonate cement fans and oolites in the early aftermath of the event. The ETME time interval has been well studied over the last few decades, in contrast to a few minor extinction events that preceded it, among them the extinctions associated with the Norian-Rhaetian boundary (NRB). This study provides new insights into these extinction events with complete mid-Norian to Hettangian δ18Ocarb and δ13Ccarb record from a key section at Wadi Milaha (Ras Al Khaimah Emirate, United Arab Emirates). Ooids are important proxies for palaeoenvironmental reconstruction. The post ETME oolite horizon is documented providing morphological classification as well as a detailed modal analysis of rock components and different types of coated grains. Through a multi-technique approach, we argue for the stability of the carbon cycle across the NRB extinction event and the existence of a hiatus at the TJB (Triassic-Jurassic Boundary) in Wadi Milaha. Our new morphological classification of post-extinction ooids is compatible with a major role for seawater geochemistry with respect to sedimentological processes, by example in the peculiar way ooids diversify and alternate with other kinds of coated grains.

Abstract Calcium isotopes (δ44/40Ca) are particularly useful in palaeo‐environmental studies due to the key role of carbonate minerals in continental weathering and their formation in seawater. The calcium isotope ratio can provide hints on past changes in the calcium fluxes, environmental shifts, ecological factors and alternatively diagenesis of carbonate rocks. The investigation of the Late Triassic calcium isotope record offers a great opportunity to evaluate such factors in a time interval that witnessed important environmental and ecological turnovers, such as the first appearance of calcareous nannoplankton, ocean acidification and periods of elevated extinction rates. In this study, we present a δ44/40Ca data set from the upper Norian (Upper Triassic) through the lower Hettangian (Lower Jurassic) interval. The isotope records reveal two globally significant signals: a ∼ 0.20‰ decrease through the early Rhaetian (Upper Triassic) and a small, negative (∼0.14‰) excursion corresponding to the emplacement of the Central Atlantic Magmatic Province, at the end of the Triassic. The possible explanations for these signals are changes in the isotopic ratio of the continental calcium influx to the ocean due to the high chemical weathering rate of carbonates and possibly ocean acidification, respectively. The considerable (∼0.15–0.30‰) offset in δ44/40Ca between study areas is likely the combined result of local differences in lithology and early marine diagenesis. The major evolutionary step represented by the first occurrence of calcareous nannoplankton did not have at this time a determining role on the calcium isotopic signature of the marine carbonates.

Abstract During the earliest Triassic microbial mats flourished in the photic zones of marginal seas, generating widespread microbialites. It has been suggested that anoxic conditions in shallow marine environments, linked to the end‐Permian mass extinction, limited mat‐inhibiting metazoans allowing for this microbialite expansion. The presence of a diverse suite of proxies indicating oxygenated shallow sea‐water conditions (metazoan fossils, biomarkers and redox proxies) from microbialite successions have, however, challenged the inference of anoxic conditions. Here, the distribution and faunal composition of Griesbachian microbialites from China, Iran, Turkey, Armenia, Slovenia and Hungary are investigated to determine the factors that allowed microbialite‐forming microbial mats to flourish following the end‐Permian crisis. The results presented here show that Neotethyan microbial buildups record a unique faunal association due to the presence of keratose sponges, while the Palaeotethyan buildups have a higher proportion of molluscs and the foraminifera Earlandia. The distribution of the faunal components within the microbial fabrics suggests that, except for the keratose sponges and some microconchids, most of the metazoans were transported into the microbial framework via wave currents. The presence of both microbialites and metazoan associations were limited to oxygenated settings, suggesting that a factor other than anoxia resulted in a relaxation of ecological constraints following the mass extinction event. It is inferred that the end‐Permian mass extinction event decreased the diversity and abundance of metazoans to the point of significantly reducing competition, allowing photosynthesis‐based microbial mats to flourish in shallow water settings and resulting in the formation of widespread microbialites.

Structures similar to fossilized nonspicular demosponges have been reported in carbonates throughout the Phanerozoic and recently in rocks dating back to 890 Ma ago. Interpretation of these records is increasingly influential to our understanding of metazoans in multiple aspects, including their early evolution, the ecology in fossil reefs, and recovery after mass extinction events. Here, we propose six identification criteria of “Keratosa”-type nonspicular demosponge fossils based on the well-established taphonomical models and their biological characteristics. Besides, sponge fossils of this kind from the lowermost Triassic of Chanakhchi (Armenia) are described with a 3-D reconstruction to exemplify the application of these criteria in recognition of such organisms. Subsequently, the state-of-the-art understanding of the taxonomy and evolution of these fossil sponges, a previously poorly addressed topic, is summarized. The morphology of the Triassic Chanakhchi fossils indicates an affinity with verongimorphs, a group that may have evolved by Cambrian Age 3. Other than that, further efforts are encouraged to forge quantitative criteria based on the here proposed descriptive version and to explore the taxonomic diversity and evolutionary details of these fossil nonspicular demosponges.

The end-Permian mass extinction was the most severe biotic crisis in Earth’s history. In its direct aftermath, microbial communities were abundant on shallow-marine shelves around the Tethys. They colonized the space left vacant after the dramatic decline of skeletal metazoans. The presence of sponges and sponge microbial bioherms has largely gone unnoticed due to the sponges’ size and the cryptic method of preservation. In addition to sponge dominated facies recently described in South Armenia and Northwestern Iran, we describe here sponge-microbial bioherms cropping out in two well-known Permian-Triassic boundary localities: the Kuh-e Hambast section, south-east of Abadeh city and the more distal Shahreza section, near Isfahan. In both sections, the extinction horizon is located at the top of an upper Changhsingian ammonoid-rich nodular limestone, called Paratirolites limestone. At Kuh-e Hambast, the overlying decimetric thick shale deposit called “boundary clay,” the latest Permian in age, is conformably overlain by well-dated transgressive basal Triassic platy limestone containing four successive levels of decimeter to meter scale, elongated to form cup-shaped mounds made of branching columnar stromatolites. Sponge fibers from possibly keratose demosponge, are widely present in the lime mudstone matrix. At the Shahreza section, above the extinction level, the boundary clay is much thicker (3 m), with thin platy limestone intervals, and contains two main levels of decimeter to meter scale mounds of digitate microbialite crossing the Permian-Triassic boundary with similar sponge fibers. Three levels rich in thrombolite domes can be seen in the overlying 20 m platy limestone of earliest Triassic age. Sponge fibers and rare spicules are present in their micritic matrix. These sponge fibers and spicules which are abundant in the latest Permian post-extinction boundary clay, followed microbial buildups during the Griesbachian time.

Abstract Carbonate veins in the igneous basement and in the lithified sedimentary cover of the Cocos Ridge at International Ocean Discovery Program (IODP) Hole 344‐U1414A reveal the hydrologic system and fluid‐rock interactions. IODP Hole 344‐U1414A was drilled on the northern flank of the Cocos Ridge and is situated 1 km seaward from the Middle America Trench offshore Costa Rica. Isotopic and elemental compositions were analyzed to constrain the fluid source of the carbonate veins and to reveal the thermal history of Hole 344‐U1414A. The formation temperatures (oxygen isotope thermometer) of the carbonate veins in the lithified sedimentary rocks range from 70 to 92 °C and in the basalt from 32 to 82 °C. 87Sr/86Sr ratios of the veins in the altered Cocos Ridge basalt range between 0.707307 and 0.708729. The higher ratios are similar to seawater strontium ratios in the Neogene. 87Sr/86Sr ratios lower 0.7084 indicate exchange of Sr with the igneous host rock. The calcite veins hosted by the sedimentary rocks are showing more primitive 87Sr/86Sr ratios