Chondrichthyan fishes from the Middle Eocene Osinovaya Formation of Rostov Region, Russia

ABSTRACT

A new chondrichthyan assemblage comprising 27 genera is described from the Middle Eocene Osinovaya Formation of the Bystraya River Basin, Morozovsk District of Rostov Region, in the southern part of European Russia. The Morozovsk assemblage consists of 21 genera and 24 species of shark, 5 genera and species of batoid and a single genus and species of chimaeroid fish (Edaphodon sp.). A composition of the assemblage is characterised by the coexistence of Otodus sokolovi, O. auriculatus, Isurolamna bajarunasi, Macrorhizodus praecursor, Jaekelotodus trigonalis and Brachycarcharias lerichei and indicates the Lutetian age of the deposits (possibly middle – upper Lutetian because of the presence of O. sokolovi together with O. auriculatus). This corresponds to the previous dating of the Osinovaya Formation. Lamniformes (12 taxa) and Myliobatiformes (5 taxa) are dominated in assemblage between elasmobranchs, characterising the inner to middle neritic warm-water environments. The Morozovsk elasmobranch assemblage belongs to the European paleobiogeographic region and shows a great similarity with the previously known coeval assemblages from the Western Europe, as well as with Uzbek assemblages from the adjacent Crimean-Caucasian paleobiogeographic region. Scarce data on chimaeroid fishes show that only the genus Edaphodon is rarely present in shallow-water environments and only in the European paleobiogeographic region.

KEYWORDS:

• Chondrichthyes
• Elasmobranchii
• Holocephali
• Middle Eocene
• biostratigraphy
• palaeobiogeography

Acknowledgments

Former SSU geology student Dmitry I. Sagaev is thanked for his help during fieldwork in the Morozovsk area in 2010. Local unnamed young geologists assisted in collecting shark teeth at the Gru-1 and Obs-5 sections in different years. Dr Vadim V. Titov (Southern Federal University, Rostov-on-Don) kindly donated tooth of Otodus auriculatus (SSU 155/346) from the section Gru-1. Dr Galina N. Aleksandrova (Geological Institute of the RAS, Moscow) is thanked for the discussion of the Don River Paleogene stratigraphy. Dr Andrei M. Zakharevich, head of the Laboratory of Diagnostics of Nanomaterials and Structures at the Institute of Nanostructures and Biosystems (SSU, Saratov), is thanked for access granted to the SEM facility. Dr Zerina Johanson (NHMUK, London) is thanked for access to the chimaeroid collection under her care at the museum in 2007, which were used here as comparative material. We greatly thank Dr David J. Ward (NHMUK, London) for critical reading of the manuscript and correcting the English, as well as for the early scientific support of this project in 2011. In addition, we wish to thank Dr Gareth Dyke (Editor) and two anonymous reviewers for the reviews that improved the quality of this work. Finally, Jürgen Pollerspöck and the “Shark-references” web-project (Pollerspöck and Straube 2022) are thanked for permanent access to rare chondrichthyan publications.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The research received support from the Russian Science Foundation, grant 22-27-00134, https://rscf.ru/en/project/22-27-00134/.