Abstract
The evolution of the cetacean skeleton followed a path that differentiated this group from other terrestrial mammals about 50 million years ago [1], and debate is still going on about the relationships between Cetacea and Artiodactyla [2], [3], [4]. Some skeletal traits of the basilosaurids (the more advanced forms of Archaeocetes), such as the expansion of the peribullary air sinuses, dental modification and vertebral size uniformity [5] are maintained and further emphasized also in contemporary odontocetes and mysticetes. Using Dual-Energy X-Ray Absorptiometry here we report that the deposition of bone mineral in fetal and newborn specimens of the fin whale Balaenoptera physalus is remarkably higher in the bulla tympanica than in the adjacent basal skull or in the rest of the skeleton. Ossification of the tympanic bulla in fetal Artiodactyla (bovine, hippopotamus) is minimal, becomes sensible after birth and then progresses during growth, contrarily to the precocious mineralization that we observed in fin whales. Given the importance of the ear bones for the precise identification of phylogenetic relationship in therian evolution [6], this feature may indicate a specific evolutionary trait of fin whales and possibly other cetacean species or families. Early mineralization of the tympanic bulla allows immediate sound conduction in the aquatic medium and consequently holds potential importance for mother-calf relationship and postnatal survival.
Highlights
The evolution of the cetacean skeleton followed a path that differentiated this group from other terrestrial mammals about 50 million years ago [1], and debate is still going on about the relationships between Cetacea and Artiodactyla [2,3,4]
Densitometric scans performed in our laboratory evidenced a specific and differential deposition of bone mineral matter in the bullae of two newborn fin whales (BMDs 2.722–2.825 g/cm2), but not in other parts of the temporal complex (BMD 0.729 g/cm2), adjacent skull bones, or other parts of the skeleton of the same animals
The precocious density of the bone walls of the tympanic bulla in early postnatal fin whales is due to progressive deposition of mineral matter during fetal development, as indicated by the high mineral content of the same part in a 6 month fetus (BMD 1.055 g/cm2; age of the fetus determined using a fetal growth formula based on the body weight [26])
Summary
The evolution of the cetacean skeleton followed a path that differentiated this group from other terrestrial mammals about 50 million years ago [1], and debate is still going on about the relationships between Cetacea and Artiodactyla [2,3,4]. Imaging evidenced peculiar features of cetacean bones, including absence of a medullary cavity in the humerus, radius and ulna substituted by an hour-glass shaped trabecular architecture [8,9]; modification of the mandible to increase sound-receiving properties [10]; presence of atypical gradients of density of the rostrum in deep-divers [11,12]. These morphological features are considerably different from the equivalent of terrestrial mammals, and constitute a peculiar acquisition of the order
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