Tympanohyal bone in toothed whales and the formation of the tympano-periotic complex (Mammalia: Cetacea)



A tympanohyal bone is reported in dolphins for the first time. The exceptional occurrence of this element in the bottlenose dolphin (Tursiops truncatus) and its location in a furrow of the tympanic can be taken as a vestige of an ancestral conformation indicating that, in cetacean ancestry, uncoupling of the periotic from the mastoid must have taken place laterally and dorsally to attachment of the hyoid arch and the stylomastoid foramen. There is a good correspondence between morphology and topographical relations of structures surrounding the facial canal in toothed whales and terres-trial mammals (especially perissodactyls and artiodactyls).

During early cetacean evolution, the tympanic had to undergo strong modification because of its functional correlation with the periotic. In precetaceans, the tympanic was probably loosely attached to neighboring skull bones, while at the same time it was suspended from the periotic via the tympanohyal. The earliest known cetaceans obviously lost this indirect osseous suspension but retained the peripheral attachments of the tympanic. In advanced archeocetes, two of these attachments are maintained but have shifted onto the periotic. In modern dolphins, the tympanic is in firm osseous contact exclusively with the periotic (tympano-periotic complex). Both elements are isolated from the skull acoustically and form a separate mechanical unit specialized for high-frequency underwater sound perception.