Fil: Talevi, Marianella. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina.Fil: Campos, Lisandro. Universidad Nacional de Río Negro. Facultad de Ciencias Naturales y Museo. División Paleontología Vertebrados. Buenos Aires, Argentina.Fil: Talevi, Marianella. Consejo Nacional de Investigaciones Científicas y Tecnológicas. Buenos Aires, Argentina.Fil: Fernández, Marta Susana. CONICET-División Paleontología Vertebrados, Museo de La Plata, Universidad Nacional de La Plata; Argentina.One of the most significant morphological modifications in numerous tetrapod lineages in their secondary adaptation to life in open marine environment is the transformation of the limb into fins. The loss of perichondral bone has been pointed out as the mechanism through which this transformation was achieved. Advanced ichthyosaurs, including ophthalmosaurids, are characterized by the zeugopodium and autopodium not clearly differentiated, and bones dorsoventrally flattened and nodular. In the case of distal limb elements, particularly phalanges, two main arrangements can be recognized in dorsal and ventral views: one is characterized by spaced and quite rounded elements, whereas in the other phalanges tightly packed arrangement is observed, showing almost straight articular surfaces which result in polygonal outlines. Previously only distal limb elements of non-ophthalmosaurids, were described. In this study, we describe and interpret the microstructure of distal limb elements of six specimens of ichthyosaur, five ophthalmosaurids and one non-ophthalmosaurid. Our result shows persistence of abundant cartilage in articular and non-articular surfaces (with exception of the dorsal and ventral surfaces) independently of ontogenetic stage and shape. The coat layer of calcified cartilage is thicker in juvenile than adult specimens and this could be related to the bone remodeling. It is probable that the persistence of significant amount of cartilage in the joint surfaces of the distal limb elements of ichthyosaurs would be linked to more evenly distribute forces through the limb, the increase in the number of articulations and the increase maneuverability during swimming.true-