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Various opinions on the position of the Middle/Upper Triassic boundary have been concisely reviewed. The lower boundary of the Carnian Stage is accepted at the base of the Trachyceras aon Zone. The recentmost data on biostratigraphy of boundary beds of the Ladinian and Carnian Stages of the Southern Alps, Himalayas, British Columbia, northeastern Asia, Svalbard, and Arctic Canada have been considered and critically analyzed. Using sections of British Columbia with mixed Boreal and Tethyan fauna of Ammonoidea, we compared biostratigraphic schemes of the Middle/Upper Triassic boundary interval of the Boreal and Tethyan regions. On the basis of correlations and paleontological data, the lower boundary of the Carnian Stage in Northeastern Asia is suggested to be drawn at the base of the ammonoid “Protrachyceras” omkutchanicum Zone and its equivalents in other Boreal regions.

Studies of the paleontology and stratigraphy of the Triassic strata from the Lena–Olenek interfluve area and Kotelny Island have important implications for improving the accuracy of interregional and global correlations, refining the Boreal standard and international stratigraphic scale for the Triassic System. The importance of this study is also underlined by the necessity of refining the stratigraphic basis for regional geological exploration in Arctic zone that now became the focus for building the country’s strategic resource base. Analysis of recent paleontological and stratigraphic data from key Triassic sections in the Laptev Sea coastal region provides new age constraints for the Triassic strata based on different faunal groups. The Triassic stratigraphic scheme for the region has been refined using new data on the paleontology, thickness variations, and boundaries of local stratigraphic subdivisions.

The first ichthyosaurian specimens discovered from the Upper Triassic of the Russian Arctic (Kotelny Island, New Siberian Islands) are described herein. They include the remains of large- to small-bodied ichthyosaurians originating from six stratigraphic levels spanning the lower Carnian to middle Norian. The material is mostly represented by isolated vertebrae and ribs, which are not possible to accurately diagnose, but also includes specimens comprising associated vertebrae and a fragmentary skeleton that preserves cranial remains (parabasisphenoid, fragmentary quadrate, partial mandible and hyoids). Based on vertebral and rib morphology, we identify the specimens as representatives of the following taxonomic groups: large-bodied shastasaurids, medium-sized indeterminate ichthyosaurians with a single rib facet in the presacral centra, and small euichthyosaurians with double rib facets present throughout the presacral vertebrae that likely represent toretocnemids and/or basal parvipelvians. In addition, the cranial and mandibular remains preserved in one of the specimens, ZIN PH 5/250, were studied using micro-computed tomography. Its mandible is highly similar to that of toretocnemids, whereas the parabasisphenoid demonstrates a peculiar combination of both plesiomorphic and derived character states, providing the first detailed data on this cranial element in a Late Triassic ichthyosaurian. Furthermore, the specimen also demonstrates a distinctive condition of rib articulation in the anteriormost presacral (cervical) vertebrae, which together with other features allows for the erection of a new taxon – Auroroborealia incognita gen. et sp. nov. Although the phylogenetic position of this taxon is uncertain due to its fragmentary nature, its anatomy, indicating toretocnemid or parvipelvian affinities, further supports the previously hypothesised sister-group relationships between these two clades. The morphology of the parabasisphenoid and vertebral column of the new taxon is discussed in broader contexts of the patterns of evolution of these skeletal regions in ichthyosaurs.