Isotopic values of diet of Blastocerus dichotomus (marsh deer) in Paraná Basin, South America

► The marsh deer is of enormous archaeological importance in South America. ► This represents the first systemic work on its isotopic diet (δ13C collagen). ► This deer has a plant-based diet C3 and C4 plants secondarily. ► Values average is (δ13Cco) −21.09‰ ± 1.46; variation coefficient 6.92‰. ► Major isotopic intake of C4 plants is synchronous to the medieval warm pulse.