Kazakhstanosaurus efimovi Yakupova et Akhmedenov sp. nov. (Ichthyosauria, Undorosauridae), a new species from the Upper Jurassic deposits of the Middle Volga region

D.B. Yakupova ^a*, K.M. Akhmedenov ^b**

^aKazan Federal University, Kazan, 420008 Russia

^bM. Utemisov West Kazakhstan University, Uralsk, 090000 Kazakhstan

E-mail: ^*yakupova_j@mail.ru, ^**kazhmurat78@mail.ru

Received January 29, 2021

ORIGINAL ARTICLE

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DOI: 10.26907/2542-064X.2021.2.251-263

For citation: Yakupova D.B., Akhmedenov K.M. Kazakhstanosaurus efimovi Yakupova et Akhmedenov sp.  nov. (Ichthyosauria, Undorosauridae), a new species from the Upper Jurassic deposits of the Middle Volga region. Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2021, vol. 163, no. 2, pp. 251–263. doi: 10.26907/2542-064X.2021.2.251-263. (In Russian)

Abstract

The skeleton fragments of a new ichthyosaur species, Kazakhstanosaurus efimovi Yakupova et Akhmedenov sp. nov., from the Middle Volgian substage of the Upper Jurassic of the Middle Volga region (Ulyanovsk region, Gorodishche section) are described. The species is assigned to the genus  Kazakhstanosaurus V. Efimov Akhmedenov et Yakupova, 2019 (fam. Undorosauridae), which has been recently described from the same-age deposits of Kazakhstan. The new species is diagnosed by the following characters: the subquadrate shape of the trunk vertebrae; the structure of the shoulder girdle and forelimbs, which have a two-facet distal end of the humerus and a weak articulation of the first finger; an increased size of all parts of the skeleton; the costal facets positioned on the sides of the end of the ribs; the connection of the costal facets with a bone crest. The facets of the ribs are connected by a bony ridge. The discovery of Kazakhstanosaurus efimovi confirms the connection of the Russian Sea during the Late Jurassic with the warm waters of the Tethys Ocean. Probably, the progenitors of undorosaurids descended from the Triassic and Early Jurassic ichthyosaurs that lived in the tropical regions of the eastern part of the Tethys Ocean. In the Late Jurassic, these ichthyosaurs migrated to the territory of modern Kazakhstan and Eastern Europe, settled in the shallow Russian Sea, and gave rise to representatives of the families Undorosauridae and Platypterygius.

Keywords: Undorosauridae, Late Jurassic, Middle Volgian substage, Zaraiskites zarajskensis zone, Middle Volga region

Acknowledgments. We thank V.M. Efimov (Candidate of Geological and Mineralogical Sciences, Chairman of the Ulyanovsk Branch of the Russian Paleontological Society) for his invaluable assistance during the analysis of the material, as well as the reviewer for their helpful comments and advice that improved the manuscript.

Figure Captions

Fig. 1. The area where the skeletal fragments of Kazakhstanosaurus efimovi sp. nov. were found: а – location of the Gorodishchi section; b – stratigraphic breakdown of the upper part of the Gorodishchi section based on ammonites (according to [18]); the place of finding is marked with an asterisk.

Fig. 2. Reconstruction of the rib head structure: a – Kazakhstanosaurus shchuchkinensis (WKU no. 750); b – Kazakhstanosaurus efimovi sp. nov. Key: df – diapophyseal facet; pf – parapophyseal facet; bg – bone crest.

Fig. 3. Elements of the axial skeleton of the ichthyosaur Kazakhstanosaurus efimovi sp. nov.: 1–2 – atlas + epistropheus (GM KFU P 1724/1): 1 – lateral surface, 2 – anterior articulation surface; 3–4 – cervical vertebra (GM KFU P 1724/2): 3 – lateral surface; 4 – anterior articulation surface; 5–6 – anterior vertebra (GM KFU P 1724/5):  5 – front view; 6 – side view; 7–8 – sacral vertebra (GM KFU P 1724/13):  7 – front view; 8 – side view; 9–10 – caudal vertebra (GM KFU P 1724/10): 9 – front view; 10 – side view; 11–12 – sacral vertebra (GM KFU P 1724/7): 11 – front view; 12 – side view. Key: p – parapophysis; d – diapophysis.

Fig. 4. Shoulder girdle of the ichthyosaurus Kazakhstanosaurus efimovi sp. nov.: 1 – fragment of the left coracoid (GM KFU P 1724/12); 2 – fragment of the right coracoid (GM KFU P 1724/14) with a medial facet; anterior surface; 3–5 – humerus (GM KFU P 1724/15): 3 – view of the distal end; 4 – dorsal view; 5 – view of the proximal end; 6 – right scapula (GM KFU P 1724/13); 7–8 – radius bone (GM KFU P 1724/16): 7 – dorsal view; 8 – proximal view.

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