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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.

Fetal and Early Post-Natal Mineralization of the Tympanic Bulla in Fin Whales May Reveal a Hitherto Undiscovered Evolutionary Trait