Cartilaginous and bony fish remains from the Eocene Tavda Formation, Kostanay Region, northern Kazakhstan: Paleoecological and paleoenvironmental significance

The recent (2014-2019) controlled excavations carried out in the Kostanay Region of northern Kazakhstan have provided new data that contribute to the knowledge of the Eocene marine fauna from the Turgay Strait. Herein, we present a report of the cartilaginous and bony fish remains collected during campaigns carried out at two western Asian Eocene localities. The host sediments were deposited in a large epicontinental marine basin throughout the Early Paleogene, when the Turgay Strait connected the Arctic waters to the North and the Peri-Tethys to the South. The bony fish material examined is poorly preserved and is referred in this study to a variety of teleost groups, including eels (Anguilliformes), codfishes (Gadiformes), mackerels (Scombriformes) and indeterminate percomorphs, whereas the chondrichthyans are represented by lamniform sharks. This report documents one of the easternmost occurrences of these groups of fishes in the northern hemisphere during the Paleogene, providing evidence of their presence in the Turgay Strait before its closure