Unusual Endosteally Formed Bone Tissue in a Patagonian Basal Sauropodomorph Dinosaur

Abstract

Mussaurus patagonicus (Dinosauria: Sauropodomorpha) is a basal sauropodomorph from the Late Triassic of southern Argentina that is known from a large number of individuals, including juveniles, subadults, and adults. Here, we report on the occurrence of an unusual bone tissue in an individual of M. patagonicus. The rather atypical bone tissue is located within the femoral medullary cavity and also occurs within several erosion cavities of the midinner part of the cortex. This tissue is well vascularized and is composed of a matrix that consists of abundant and densely packed osteocyte lacunae. Although some features of this tissue resembles avian medullary bone, the histological features are distinctive and share more features with the pathological, reactive bone produced in extant birds in response to a retrovirus-induced disease (avian osteopetrosis). Here, we also discuss and provide histological features to effectively differentiate endosteally formed medullary bone from pathological avian osteopetrosis.

Novel insight into the origin of the growth dynamics of sauropod dinosaurs

“…According to previous interpretations [46], the large amount of Sharpey’s fibers is possibly related to the insertion of the femorotibialis muscles [63–69]. Furthermore, besides the presence of a higher amount of Sharpey’s fibers, the abrupt change in the vascularization pattern also suggests an area of muscle attachment [69].…”

“…Although, as previously mentioned, an osseous pathology has been reported in Mussaurus MLP 61-III-20-22, this condition is clearly temporally unrelated to the high amount of woven fibered bone tissue observed in most of the cortical bone. The unusual bone tissue described in this specimen is endosteally formed and it is located within the medullary cavity and in the perimedullary region [46], which suggests that it occurred late during ontogeny. This contrasts with the occurrence of the periosteally formed woven fibered bone tissue in the perimedullary region, which indicates that this primary tissue began to be deposited from early stages of ontogeny onwards.…”

“…Sharpey’s fibers, penetrating the cortical bone at straight or slightly oblique angles are especially abundant in the outer cortex at the anterolateral region of the cross section. A pathological tissue occurs in the perimedullary region of the sample and this has been previously described [46]. …”

Medullary bone-like tissue in the mandibular symphyses of a pterosaur suggests non-reproductive significance

“…Alternatively, the dedicated reproductive function of medullary bone-like tissues could have evolved later on the dinosaurian lineage in which case these tissues may have common, homologous origin with diverging functions and physiological regulatory systems. A common evolutionary origin may gain some support by the findings of Cerda and Pol 71 who revealed medullary-bone like tissue in the basal sauropodomorph Mussaurus patagonicus, albeit in a recent study Cerda et al 72 reconsidered it as a pathologic condition possibly originating from avian osteopetrosis. Although close-up images of the lacunar features were not provided, the described fine-scale histological characteristics based on which they argued for this tissue to be distinct from medullary bone were its high density of large osteocyte lacunae and the lamellar coating of its vascular canals 72 .…”

“…A common evolutionary origin may gain some support by the findings of Cerda and Pol 71 who revealed medullary-bone like tissue in the basal sauropodomorph Mussaurus patagonicus, albeit in a recent study Cerda et al 72 reconsidered it as a pathologic condition possibly originating from avian osteopetrosis. Although close-up images of the lacunar features were not provided, the described fine-scale histological characteristics based on which they argued for this tissue to be distinct from medullary bone were its high density of large osteocyte lacunae and the lamellar coating of its vascular canals 72 . These features, however, match the characteristics of the apparently non-pathologic medullary bone-like tissues in the Bakonydraco symphyses.…”