Structure and growth pattern of the bizarre hemispheric prominence on the rostrum of the fossil beaked whale Globicetus hiberus (Mammalia, Cetacea, Ziphiidae)

Abstract

The rostrum of most ziphiids (beaked whales) displays bizarre swollen regions, accompanied with extreme hypermineralisation and an alteration of the collagenous mesh of the bone. The functional significance of this specialization remains obscure. With the voluminous and dense hemispheric excrescence protruding from the premaxillae, the recently described fossil ziphiid Globicetus hiberus is the most spectacular case. This study describes the histological structure and interprets the growth pattern of this unique feature. Histologically, the prominence in Globicetus is made up of an atypical fibro-lamellar complex displaying an irregular laminar organization and extreme compactness (osteosclerosis). Its development is suggested to have resulted from a protraction of periosteal accretion over the premaxillae, long after the end of somatic growth. Complex shifts in the geometry of this tissue are likely to have occurred during its accretion and no indication of Haversian remodeling could be found. X-ray diffraction and Raman spectroscopy indicate that the bone matrix in the premaxillary prominence of Globicetus closely resembles that of the rostrum of the extant beaked whale Mesoplodon densirostris: apatite crystals are of common size and strongly oriented, but the collagenous meshwork within bone matrix seems to be extremely sparse. These morphological and structural data are discussed in the light of functional interpretations proposed for the highly unusual and diverse ziphiid rostrum. J. Morphol. 277:1292-1308, 2016. © 2016 Wiley Periodicals, Inc.

New beaked whales from the late Miocene of Peru and evidence for convergent evolution in stem and crown Ziphiidae (Cetacea, Odontoceti)

“…The rostral portion of the premaxilla anterior to the premaxillary sac fossa exhibits some degree of thickening, particularly in its medial region, where the bone is ca 10 mm thick. A similar but more pronounced thickening is present in all members of the Messapicetus clade; the pachyosteosclerotic condition of the premaxilla is particularly developed in Globicetus and Tusciziphius , both showing a huge premaxillary prominence on the rostrum ( Bianucci et al, 2013 ; Dumont et al, 2016 ). In part of the members of the Messapicetus clade, this premaxillary thickening is followed posteriorly by an excavation of the premaxillae generating a dorsal fossa named prenarial basin; such a feature is absent in Chimuziphius .…”

“…Observed to evolve independently in the CZ and the MC, ziphiid pachyosteosclerosis is a clear case of convergent evolution, involving different bones (vomer vs. premaxillae). Interestingly, histological studies revealed strikingly different degrees of remodeling in the different bones of different taxa ( de Buffrénil & Lambert, 2011 ; Lambert, de Buffrénil & de Muizon, 2011 ; Dumont et al, 2016 ); for example the inner organization of the pachyosteosclerotic premaxillae of Aporotus recurvirostris (a species of the MC) is entirely different from all the rostral bones of Mesoplodon densirostris ( Blainville, 1817 ) (a species of the CZ), suggesting that a roughly similar process may have evolved independently in several lineages in response to common selective pressures, possibly linked to the shift to deep waters. Several functional explanations have been provided for the thick and dense rostral bones of ziphiids: as an help for deep diving (ballast) ( de Buffrénil et al, 2000 ), as a structure facilitating sound transmission ( Zioupos et al, 1997 ; Cranford, Krysl & Hildebrand, 2008 ), or as a structure strengthening the rostrum during intraspecific fights between adult males ( Heyning, 1984 ; McLeod, 2002 ; Lambert, Bianucci & Post, 2010 , Lambert, de Buffrénil & de Muizon, 2011 ; de Buffrénil & Lambert, 2011 ).…”

Macrobiomineralogy: Insights and Enigmas in Giant Whale Bones and Perspectives for Bioinspired Materials Science

“…Fossilized specimens are also of significant scientific relevance for better understanding comparative studies. Recently, it was reported 73 that bone in the premaxillary prominence of the fossilized ziphiid Globicetus hiberus is similar to the bone found in the rostrum of the extant beaked whale M. densirostris. A sparse collagenous meshwork was reported within the bone matrix; however, hydroxyapatite crystals appear to have the same size with clear orientation in both species.…”

“…Dumont and co-workers 73 reported that beaked whales possess both extremely light skeletons (relative mass: 2.2− 2.5% in Mesoplodon species) but also the most highly mineralized, densest, compact, and rigid osseous tissues (e.g., the maxillae, premaxillae, vomer, palatine, mesethmoid) known. Thus, the rostrum of Mesoplodon densirostris contain the highest density (BMD) 69 varying from 3.25 to 4.48 g/cm 2 .…”