The Allen Formation is the oldest stratigraphic unit of the Malargüe Group that crops out mainly in the North of Patagonia. Although most of the fossil vertebrates recovered from this unit (e.g., Lepisosteiformes, Chelidae, Dipnoii, Theropoda and Sauropoda dinosaurs) are typically continental elements, plesiosaur remains were also found in several localities. Here we report the first record of a marine turtle, collected from the lower levels of the Allen Formation, in the proximities of Cerro Bonaparte locality, in the Bajo de Santa Rosa area, Río Negro Province. The new material (MML PV 426) consists of a distal fragment of a radius that shows anatomical features shared by Dermochelys coriacea (Linnaeus) (e.g., absence of medular cavity and distal portion with a convex surface, probably for the articulation with the intermedium carpal element, and covered by vascular foramina). The vascular foramina indicate the presence of transphyseal vascular channels that vascularized the chondroepiphyses. In reptiles this feature is also reported in plesiosaurs. Nevertheless the anatomical configuration of the appendicular bone mentioned here did not fit with the pattern present in the propodial elements of plesiosaurs limbs, which are characterized by the presence of two heads in its terminal portion (capitulum, terminally, and tuberosity/trochanter, dorsally). The addition of marine turtles to the vertebrate fauna of the Allen Formation supports the hypothesis of a marine environment influence for this unit.Sesiones libres