OBSERVATIONS ON THE MICROSTRUCTURE OF THE ROSTRUM OF MESOPLODON DENSIROSTRIS (MAMMALIA, CETACEA, ZIPHIIDAE) - THE HIGHEST DENSITY BONE KNOWN

Measurement of density, mineral content, and the microscopic observati on of thin sections, reveal a very steep specialization of bone struct ure in the rostrum of Mesoplodon densirostris. The density (2.6 g/cm(3 )) and mineralization (86.7%) of bone reach the highest levels found t o date in tetrapod skeletal tissues which have a collagenous matrix. H istologically, the bone in the rostrum of M. densirostris is of the de nse Haversian type. The high density and mineralization of the rostrum are apparently due to the peculiar characteristics of the secondary o steons, which are composed of a hypermineralized tissue with longitudi nal fibers. The results are discussed with reference to the knowledge available on bone structure and mineralization, and to the possible fu nctional consequences of the hyperdensity of the rostrum.