Abstract
Abstract Cetaceans are obligate aquatic mammals derived from terrestrial artiodactyls. The defining characteristic of cetaceans is a thick and dense lip (pachyosteosclerotic involucrum) of an ear bone (the tympanic). This unique feature is absent in modern terrestrial artiodactyls and is suggested to be important in underwater hearing. Here, we investigate the mineralogical and biochemical properties of the involucrum, as these may hold clues to the aquatic adaptations of cetaceans. We compared bioapatites (enamel, dentine, cementum, and skeletal bone) of cetaceans with those of terrestrial artiodactyls and pachyosteosclerotic ribs of manatees (Sirenia). We investigated organic, carbonate, and mineral composition as well as crystal size and crystallinity index. In all studied variables, bioapatites of the cetacean involucrum were intermediate in composition and structure between those of tooth enamel on the one hand and those of dentine, cementum, and skeletal bone on the other. We also studied the amino acid composition of the cetacean involucrum relative to that of other skeletal bone. The central involucrum had low glycine and hydroxyproline concentrations but high concentrations of nonessential amino acids, unlike most bone samples but similar to the tympanic of hippos and the (pachyosteosclerotic) ribs of manatees. These amino acid results are evidence of rapid bone development. We hypothesize that the mineralogical and amino acid composition of cetacean bullae differs from that of other bone because of (1) functional modifications for underwater sound reception and (2) structural adaptations related to rapid ossification.
Highlights
Ancestral cetaceans evolved from land mammals of the order Artiodactyla approximately 50 million years ago
Amino acid compositions for all dense bone types were similar and, unexpectedly, distinct from those of other skeletal bones or those of tympanics sampled from terrestrial artiodactyls
We hypothesize that differences in structural composition between the involucrum of cetacean bullae and other bones of artiodactyls are related to functional modifications for underwater sound conduction as well as structural constraints inherent to rapid ossification during prenatal development (Cozzi et al 2012)
Summary
Ancestral cetaceans (whales, dolphins, and porpoises) evolved from land mammals of the order Artiodactyla approximately 50 million years ago. This transition is well documented in the fossil record and indicates many skeletal modifications to accommodate an aquatic lifestyle (reviewed in Thewissen et al 2009). The cetacean bulla developed an enlarged medial lip, the involucrum, which is present in raoellids but not in other artiodactyls (Thewissen et al 2007). Studies of material properties of the involucrum have shown differences from other mammalian bone (Carter and Hayes 1976; Currey 1999; Ketten 2000; de Buffrenil et al 2004; Thewissen et al 2007; Buckley et al 2012), but there are no detailed comparisons of mineral and amino acid composition among artiodactyls or in cetaceans
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