Precocious Ossification of the Tympanoperiotic Bone in Fetal and Newborn Dolphins: An Evolutionary Adaptation to the Aquatic Environment?

Abstract

The present study, performed with a dual-energy X-ray (DXA) bone densitometer on a series of fetal and newborn striped and short-beaked common dolphins, shows that the bone density of the area of the tympanic bulla within the tympanoperiotic complex starts with 0.483 g cm 22 in 5-to 6-month-old specimens of striped (or common) dolphin fetuses and reaches 1.841 g cm 22 in newborn striped dolphins, with values consistently higher than in other parts of the skull or elsewhere in the skeleton. The same results apply to the common bottlenose dolphins, in which the area of the tympanic bulla has a density of 0.312 g cm 22 in 5-month-old specimens and becomes four times as much in newborns. Regardless of the areal bone density results correlated to the DXA-technique, comparisons with DXAbone density data in the literature referred to other mammals emphasize the presence of very high mineral deposition in the area of the tympanoperiotic bone in fetal and newborn dolphins and the most dense part of it belongs to the tympanic bulla. The early osseous maturation of the tympanic bulla area may be compared to what described in fin whales and may represent an unique ontogenetic and phylogenetic feature of cetaceans, possibly related to the development of essential acoustic sense and establishment of immediate post-natal mother-calf relationship.

Forebrain neuroanatomy of the neonatal and juvenile dolphin (T. truncatus and S. coeruloalba)

“…The rationale for comparing the species that we mainly describe in our study, i.e., the bottlenose dolphin, with specimens of the striped dolphin, is based on their common origin in the family Delphinidae and on the similarity of their morphology (see also Table 2 ). Furthermore, their developmental stages are comparable, as showed in a recent study ( Cozzi et al, 2015 ) in which embryos and fetuses of Sc , Dd , and Tt were analyzed for progressive very early deposition of calcium salts in the tympanoperiotic complex (a rather distinctive trait of marine Cetartiodactyla). The bottlenose dolphin Tt is approx.…”

A new tropical Oligocene dolphin from Montañita/Olón, Santa Elena, Ecuador

“…[ 33 , 59 ] One might expect comparable ontogenetic change in the size and shape of bones and the skull in fossil cetaceans, with implications for cladistic coding and phylogenetic placement. In living odontocetes, the periotic ossifies early in fetal development [ 60 ], and generally shows little change in size or shape after birth [ 61 ]. Further, this single element is feature-laden [ 21 ], with many characters available for cladistic analysis and/or identification to species level [ 61 ].…”

Skull ontogeny and modularity in two species ofLagenorhynchus: Morphological and ecological implications

“…In addition, Frainer et al () found that the sound generating structures and auditory sense of P. blainvillei might already be functional at birth. Thus, the observed trends suggest that the development of the external nares and the posterior portion of the premaxilla are common features among odontocetes, probably associated with the early development of breathing and sound production apparatuses, albeit they have a continuous postnatal maturation (Rauschmann et al, ; Cozzi et al, ).…”