First occurrences of Trionychidae (Testudines, Cryptodira) from the Miocene of Poland: Detailed cranial anatomy and biogeographic implications

Modern trionychids (Testudines, Cryptodira) have a pan-tropical distribution, but their range encompassed Central Europe during the Miocene. However, their true Neogene northernmost limit is uncertain because of limited Miocene exposures in the northern half of Europe, including Poland. Here, we address this problem, describing the first Polish remains from this family, coming from three new Middle Miocene vertebrate localities. The material includes isolated shell fragments and an incomplete skull, studied using computed tomography. Through a detailed review of the variation of cranial characters in extant and extinct trionychids, as well as considering the already existing occurrences of the family in the Miocene of Europe, we propose a tentative assignment for this last specimen to Trionyx cf. vindobonensis. This is somehow unexpected since the geographically closest diagnostic records of the family from the Early Miocene of Czechia belong to Rafetus. These new occurrences hint toward an environment still suitable for thermophilic organisms between the Mid-Miocene Climatic Optimum and Late Miocene "washhouse" climate in the northern part of Central Europe.