Osteogenesis, homology, and function of the intercostal plates in ornithischian dinosaurs (Tetrapoda, Sauropsida)

Intercostal plates are bony structures positioned lateral to the anterior dorsal ribs in some ornithischian dinosaurs. Some propose these plates are homologous, or functionally analogous, with the uncinate processes of extant avian dinosaurs that assist in breathing, while others suggest they served a defensive function. To elucidate their osteogenesis, homology, and function, a histological survey of intercostal plates from three taxa (Hypsilophodon, Talenkauen, and Thescelosaurus) was undertaken. This study reveals that osteogenesis of intercostal plates closely resembles that of secondary centers of ossification in endochondral bone, typically present in the epiphyses of mammalian long bones. In contrast, ossification of avian uncinate processes begins at a primary ossification center via the development of a bony collar around a cartilaginous model. Based on these data, intercostal plates and avian uncinate processes are likely not evolutionary homologs. Dense packets of obliquely oriented Sharpey's fibers within the parallel-fibered bone of somatically mature intercostal plates indicate these plates were positioned medial to at least a portion of the hypaxial musculature, which does not support their use as bony armor. Rather, we propose that intercostal plates performed some biomechanical function, either assisting in breathing in a way analogous to avian uncinate processes, or working together with the sternal ribs and sternal plates of these ornithischian taxa to provide increased rigidity to the anterior portion of the ribcage.