Intratendinous ossification is widespread in dinosaurs (including birds). Although intratendinous ossification in living birds is well understood, the physiological process of tendon metaplasia and associated histological variability in Dinosauria are not. Therefore, ossified tendons were histologically sampled across extinct dinosaurian clades. Tendons of living birds and alligators were also sampled. Despite various anatomical locations and large differences in body size, ossified tendons were found to possess uniform microstructure even in specimens that do not normally experience intratendinous ossification (such as Spinostropheus and Camarasaurus). The ossified tendons of non-avian dinosaurs are largely indistinguishable from skeletal bone with respect to microanatomical features. However, ossified tendons in birds lack periosteal bone and associated fibrolamellar structures associated with ornithischian dinosaur tendons. Variation in periosteal bone occurs along the length of individual tendons. Ossified tendons from marginocephalians are unique in that they have large quantities of anisotropic fibrolamellar bone, while those from pachycephalosaurids have radial vascularity.