Intensive sampling of the lower portion of the Thaynes and Moenkopi Groups (Lower Triassic) at separate localities within the Confusion Range, Pahvant Range, Mineral Mountains, Star Range, Kanarraville, Cedar City, Torrey and San Rafael Swell areas (mainly central and southern Utah, USA) leads to the recognition of a new key regional Smithian ammonoid succession. The new biostratigraphical sequence, which is more precise than the long-recognized Meekoceras gracilitatis and Anasibirites kingianus Zones, comprises twelve subdivisions, thus resulting in a sequence with much higher resolution that can be correlated not only with other western USA sites, but also with major worldwide localities as well. Middle and late Smithian faunas contain many taxa with wide geographic distribution, thus enabling long-distance correlation with faunal successions from other regions (e.g., British Columbia, Canadian Arctic, South China, Spiti and Oman). New assemblages from the lowermost beds are the least diversified and poorest preserved; they represent the earliest early/middle Smithian ammonoid faunas reported from the western North American basin. They highlight (a) the sudden Smithian advancement of the marine transgression within this epicontinental sea, (b) that this event is diachronous, and (c) that the paleotopography of the basin most likely was highly irregular. The newly obtained ammonoid succession also allows us to date and follow the transgression from the northern and central part of the basin to the southwesternmost and southeasternmost parts, which were reached during the late Smithian (Anasibirites kingianus beds). In addition, we briefly discuss the now-limited previous regional biozonation in the light of these new results. One new genus (Minersvillites) and nine new species (Kashmirites utahensis, Kashmirites confusionensis, Kashmirites stepheni, ?Xiaoqiaoceras americanum, Minersvillites farai, Inyoites beaverensis, Meekoceras olivieri, Meekoceras millardense, Vercherites undulatus) are also described.

A new marine fossil assemblage from the Early Triassic shows unexpected phyletic diversity and functional complexity.

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International audienceData indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation, hydrodynamics, bathymetry and grain supply. This study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions. Due to the presence of numerous metazoans associated with microbialites throughout the Smithian increase inaccommodation and Spathian decrease in accommodation, the commonly assumed anachronistic character of the Early Triassic microbialites and thetraditional view of prolonged deleterious conditions during the Early Triassic time interval is questioned

Size reduction in the aftermath of the Permian-Triassic mass extinction event has repeatedly been described for various marine organisms, including gastropods (the Lilliput effect). A Smithian gastropod assemblage from Utah, USA, reveals numerous large-sized specimens of different genera as high as 70 mm, the largest ever reported from the Early Triassic. Other gastropods reported from Serbia and Italy are also as large as 35 mm. Size frequency distributions of the studied assemblages indicate that they were not unusually small when compared with later Mesozoic and modern faunas. The occurrence of large-sized gastropods less than 2 Ma after the Permian-Triassic mass extinction refutes the Lilliput hypothesis in this clade, at least for the last ~75% of the Early Triassic.