THE MORPHOLOGY AND HISTOLOGY OF THORACIC PLATES ON NEORNITHISCHIAN DINOSAURS

The presence of large, ‘plate-like' structures overlying the anterior dorsal ribs has been reported in several neornithischian taxa. Their preservation in presumed life position on a specimen of Thescelosaurus (NCSM 15728) has allowed a detailed examination of these structures. Each of the seven plates is longer dorsoventrally than anteroposteriorly and D-shaped in lateral view. Grossly, the anterior edge of each plate is in proximity, yet unattached, to the posterior surface of its respective rib. The body of the plate angles slightly posterolaterally, overlapping the lateral surface of the next rib and plate.

Histological sections of one plate show a circumferential layer of calcified cartilage of variable thickness, embedded within an amorphous layer apparently organic in origin. Internal to the organic layer is a region of spiculated, non-osteonal bone that encases numerous large, open cavities. Transverse sections taken from the mid-diaphysis region of the fibula show the presence of three widely spaced lines of arrested growth (LAGs) and the lack of an external fundamental system (EFS), indicating this specimen is a subadult. The subadult status of this specimen and the presence of calcified cartilage on the outer surfaces of these plates suggests they may be metaplastic (i.e., formed through the direct ossification of mature connective tissues into bone) and that ossification of these tissues began relatively late in ontogeny.

The mode of attachment between these plates and the endoskeleton is currently unknown, as muscle scars or other evidence of connective tissue attachment has not yet been identified either on the plates or on their respective ribs. This does not support the claims made by previous authors that these plates were functionally homologous to uncinate processes of avian dinosaurs, which serve as surfaces of attachment for muscles that assist respiration. The phylogenetic distribution of thoracic plates parallels that of ossified anterior sternal ribs among neornithischian taxa with the exception of Othnielosaurus (thoracic plates currently unknown) and Talenkauen (sternal region not preserved). We hypothesize that these structures may have evolved to facilitate a single physiological function; however, more functional work is needed to support this interpretation.