The first Pelagiceti whale from the Eocene of Poland with its evolutionary and biogeographic significance

The Eocene radiation of cetaceans marks the evolutionary transition of whales from amphibious ancestors to fully aquatic forms within the clade Pelagiceti. Here we report the first fossil record of an Eocene whale from Poland, represented by a fragmentary posterior portion of a left dentary recovered from glauconitic sands of the Siemień Formation near Lubartów (eastern Poland). Biostratigraphic data based on calcareous nannoplankton and dinocyst assemblages place the fossil-bearing deposits near the Bartonian-Priabonian boundary (~38 Ma) within the epicontinental Central European Basin. The specimen preserves several closely spaced alveoli corresponding to double-rooted molars and a robust posterior dentary crest interpreted as a muscle attachment site, allowing referral to Pelagiceti indet. Comparative analysis of alveolar dimensions suggests that the animal was small-bodied, with an estimated total length of approximately 1.7-2.1 m. This size places it among the smallest known fully aquatic Eocene whales and indicates the presence of dolphin-sized pursuit predators in late middle Eocene marine ecosystems. The occurrence of such a small pelagic cetacean alongside large macropredatory basilosaurids supports the hypothesis that significant ecological and body-size differentiation within Pelagiceti had already developed by the Bartonian-Priabonian transition. Paleogeographically, the Polish locality occupied a central position within a shallow epicontinental seaway connecting the North Sea Basin with the eastern Tethyan realm. The new record therefore fills a key geographic gap between western and eastern European occurrences of Eocene whales and supports the existence of dispersal pathways across the Central European Basin during the early global expansion of fully aquatic cetaceans.