Some structural and developmental characteristics of cetacean (odontocete) radii. A study of adaptive osteogenesis

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Abstract
Odontocete radii are short, flattened, longer than the humeri, and lacking in open medullary cavities. Their cross‐section is essentially a semi‐streamlined wedge. End‐on articulation at the elbow is a synchondrosis. In the streamlined, amuscular antebrachium, the radius sustains loadings predominantly in the mediallateral transverse plane as flipper movements (at the shoulder) counter body inertia and water resistance in controlling body attitude.This study treats secondary marine adaptation as reflected in radial development, growth and mechanical organization. Early osteogenesis is similar to that we reported for cetacean humeri, and parallels conditions in the manatee. Later differential growth is analyzed in extenso. Geometry of growth is visualized radiographically in the relationship of endochondral to periosteal bone in the absence of medullary cavitation. Furthermore, the neonatal outline is distinguishable within the porous structure of (only) beluga radii, so that pre‐ and postnatal phases may be differentiated. Radiographs and models demonstrate that as elongation occurs, circumferential increments to growth cartilages are eccentric, i.e., minimal anteriorly, moderate medially and laterally and maximal posteriorly. By this method, the centers of epiphyseal cartilage plates migrate posteriad with time. Periosteal depositions follow the same pattern and are deepest posteriorly. Eccentric growth of the ulna is in the opposite sense. Additionally, proximal growth cartilages of radii tilt posteriad as the bone elongates. Some alterations in growth occur at birth and later. Eccentric growth is an adaptation by which anterior and posterior margins, respectively, of radius and ulna move apart as the bones elongate, and by which functional cross‐sections are maintained in antebrachial bones, singly and as a pair.Bone is concentrated on medial and lateral sides of the radius. This feature, and porosity of the central region, varies with species. Radii, like humeri, thus mimic an engineered box‐beam, and trabecular orientation resembles trajectorial distribution in a loaded beam.Radii have higher absolute densities than do humeri, although both bones have the same range of mineralization as typical mammalian bones. The beluga radius is half again as dense as that of the pilot whale, and thus is intermediate in density between those of most odontocetes and that of the hypothyroid, pachyostotic manatee. The possible evolution of odontocete bones from a more dense dense form, as well as the phylogenetic implications of density changes, are discussed. It is suggested that some of the characteristics of growth and structure, including an apparent periodicity in structural density, may be related to temperature and vascular supply in the high‐area, low‐volume flipper in colder waters.