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

Abstract

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 ossiWcation in endochondral bone, typically present in the epiphyses of mammalian long bones. In contrast, ossiWcation of avian uncinate processes begins at a primary ossiWcation 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 Wbers within the parallel-Wbered 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.

A new ornithopod (Dinosauria, Ornithischia) from the Upper Cretaceous of Antarctica and its palaeobiogeographical implications

“…Furthermore, as indicated above, the large uncinate process in Talenkauen (Novas et al, 2002) and Macrogryphosaurus (Calvo et al, 2007) could be also related with improved pulmonar dynamics, as indicated by previous authors (Boyd et al, 2011).…”

“…The plates in the ribcage of Talenkauen (Novas et al, 2004) and Macrogryphosaurus (Calvo et al, 2007) could be related with the anchoring of well developed intercostal muscles (Novas et al, 2004). A third hypothesis proposes that these plates contribute to the stiffness of the ribcage (Boyd, Cleland, & Novas, 2011).…”

Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs

“…More recently, a nearly identical structure has been identified in the European nodosaurid Hungarosaurus (O si & Mak adi 2009). The only other sheet-like structure associated with the dorsal ribs in dinosaurs are the intercostal plates found in some basal ornithopods like Thescelosaurus, and these structures are located well away from the vertebrae and do not taper into narrow points (Boyd et al 2011). Molnar & Frey (1987) interpreted the sheet-like ossification in Minmi as an ossified aponeurosis, an interpretation that we uphold here.…”

Mass Spectrometry and Antibody-Based Characterization of Blood Vessels from Brachylophosaurus canadensis

“…To demonstrate minimum alteration of bone microstructure, which may favor preservation of endogenous organic content, cortical bone fragments were embedded in Silmar resin, cut, and ground following previous protocols 42 for histological analyses. Light images were taken on a Zeiss Axioskop 2, and cross-polarized images were taken on a Zeiss Axioskop 40.…”