Cases of pathological bone growth in Isanosaurus and Spinophorosaurus (Sauropoda): periosteal reaction and tumor-like conditions in dinosaurs

The long bone histology of sauropod dinosaurs appears uniform and conservative along the Sauropoda evolutionary tree. Fibrolamellar Complex (FLC), a scaffold of poorly and highly organized bone tissues, is the main 3D architecture building up the cortex of these bones. However, we report some Radial Fibrolamellar bone (RFB) in the outer cortex of the humeri of a young adult Isanosaurus (Nam Phong Formation, Upper Triassic of Thailand) and an adult Spinophorosaurus (‘Argiles de l’Irhazer’, Lower/Middle Jurassic of Niger). RFB is regarded as a fast-growing bone tissue and has been documented only in a few dinosaurian taxa including one titanosaur, but never among basal sauropods. Its outermost position within the cortex raises questions because such a rapidly apposited bone tissue would rather be expected in the inner cortex (corresponding to an early juvenile ontogenetic stage). Bone pathologies have been reviewed in order to determine whether these RFB occurrences are linked to a pathological condition. The RFB observed here is typical of periosteal reactions. Its ‘hair-on-end’ or sunburst appearance is probably related to sarcomas or metastasis around the coring sites than fracture/callus or bone infection, both of these last pathologies being completely absent from the sections and the sampled bone. Comparisons with human medicine studies and other periosteal reactions in dinosaurs are consistent with this interpretation. Intriguingly, the RFB of Spinophorosaurus is buried in the outer cortex, followed by a recovery of a ‘normal’ FLC. Spinophorosaurus thus survived its pathology and represents the first survival case of periosteal reaction-related event within Sauropoda.