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

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.

Ontogenetic variation in the cranium of Mixosaurus cornalianus, with implications for the evolution of ichthyosaurian cranial development

“…It has elongated facets for the angular ventrally and the splenial anteriorly on its lateral surface. This morphology is consistent with many other ichthyosaurs (Kear, 2005 ; Marek et al, 2015 ; McGowan, 1973 ; Miedema & Maxwell, 2022 ; Zverkov et al, 2021 ).…”

“…Basal tubera are ubiquitously present in early diverging sauropsids (Ford & Benson, 2018 ; Gardner et al, 2010 ). Moreover, the overall more triangular outline and broad cultriform process is an ancestral character,in Euichthyosauria the basisphenoid is more quadrangular in outline and the cultriform process is narrow (Zverkov et al, 2021 ). Interestingly, the fetal specimen does show a slight constriction of the posterior parasphenoid as well as a more quadrangular basisphenoid, giving it an outline that is relatively more akin to the euichthyosaurian condition than the adult morphology.…”

Heads or tails first? Evolution of fetal orientation in ichthyosaurs, with a scrutiny of the prevailing hypothesis

“…The orientation of the fetuses was inferred using the methodology outlined in Klein et al [ 9 ]. Ichthyosaur vertebral centra have rib facets on the anterior side of the lateral surfaces [ 87 , 88 ], and in Triassic ichthyosaurs, these become differentiated early enough in development to be useful in orienting fetuses relative to the mother. Therefore, if an articulated vertebral column is preserved in lateral view, the relative position of the rib facets indicates the anteroposterior orientation of the fetus.…”