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Ichthyosaurs from the Upper Triassic (Carnian–Norian) of the New Siberian Islands, Russian Arctic, and their implications for the evolution of the ichthyosaurian basicranium and vertebral column

Published online by Cambridge University Press:  13 December 2021

Nikolay G. ZVERKOV Open the ORCID record for Nikolay G. ZVERKOV [Opens in a new window] ,
Dmitry V. GRIGORIEV ,
Andrzej S. WOLNIEWICZ ,
Alexey G. KONSTANTINOV  and
Evgeny S. SOBOLEV
Nikolay G. ZVERKOV*
Affiliation:
Borissiak Paleontological Institute of the Russian Academy of Sciences, 123 Profsoyuznaya Street, Moscow 117997, Russia. Geological Institute of the Russian Academy of Sciences, 7 Pyzhevsky Lane, Moscow 119017, Russia.
Dmitry V. GRIGORIEV
Affiliation:
Faculty of Biology, St Petersburg State University, 7/9 Universitetskaya Embankment, St Petersburg 199034, Russia. Zoological Institute of the Russian Academy of Sciences, Universitetskaya Embankment 1, St Petersburg 199034, Russia.
Andrzej S. WOLNIEWICZ
Affiliation:
School of Resources and Environmental Engineering, Hefei University of Technology, 193 Tunxi Rd, Hefei 230009, Anhui, China. Institute of Paleobiology, Polish Academy of Sciences, 51/55 Twarda, Warsaw 00-818, Poland.
Alexey G. KONSTANTINOV
Affiliation:
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3 Academician Koptug Avenue, Novosibirsk 630090, Russia.
Evgeny S. SOBOLEV
Affiliation:
Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, 3 Academician Koptug Avenue, Novosibirsk 630090, Russia.
*
*Corresponding author. Email: zverkovnik@mail.ru
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Abstract

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.

Keywords

EuichthyosauriaIchthyosauriaLate TriassicNorianparabasisphenoidParvipelviaToretocnemidae
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Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 113 , Issue 1 , March 2022 , pp. 51 - 74
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Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of The Royal Society of Edinburgh

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