| Summary: | The Antarctic Paleogene marine fossil record has been the key to reconstructing the evolution of the Weddellian Sea and final dismemberment of Southern Gondwana. In this context, Eocene marine vertebrates from Seymour (Marambio) Island have provided valuable information. We present the first Eocene record of marine reptiles from the southern Atlantic Coast of South America. This corresponds to several postcranial turtle remains represented by a proximal end of the right humerus, three caudal and one thoracic vertebrae, a fragment of the left pubis, and ten ossicles of the dorsal carapace, coming from the Leticia Formation (late-mid Eocene) at Cabo Tiburones, Tierra del Fuego, Argentina. These materials show several features such as the size and general morphology of the humerus and vertebrae, and the presence of relatively small, irregular, smooth, and unkeeled ossicles, which allow us to assign them to Dermochelyidae indet. Dermochelyids are a cosmopolitan group of cryptodiran turtles, registered from the late Cretaceous up to the recent, with some physiological-biological peculiarities (e.g., endothermy and an exclusive jellyfish-based diet) and characterized by the presence of an osseous carapace formed by ossicles. The new finding from the Leticia Formation is an addition to the scarce and extremely fragmentary record of Eocene dermochelyids from the southern seas like those from the La Meseta and Submeseta formations (Antarctica) and the Waihao and Burnside formations (New Zealand). This new information allows us to discuss the presence of these turtles in such high latitudes in the past and its implication in the evolution of the Weddellian fauna. Facultad de Ciencias Naturales y Museo Consejo Nacional de Investigaciones Científicas y Técnicas
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