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Functional cranial morphology of cetaceans

Cranial asymmetry is known in odontocetes that can hear ultrasound and has been linked to the split function of the nasal passage in breathing and vocalization.  Mysticete crania are as symmetric as those of terrestrial artiodactyls and archaeocetes, without significant differences within Mysticeti.

In order to understand the evolution and functional morphology of cranial asymmetry in whales in relation to the frequencies they hear or heard, 3D surface models of mysticete, odontocete, archaeocete, and terrestrial artiodactyl crania were generated using photogrammetry, CT scanning, and laser scanning, and cranial asymmetry was quantitatively analyzed using 3D geometric morphometrics. The relationship of cranial and cochlear shape with hearing frequencies was investigated by correlating cranial shape data with cochlear shape data derived from CT scans, and with known hearing frequencies as well as cochlear and cranial measurements. Finally, the suitability of photogrammetry and CT scanning for 3D surface reconstruction of paleontological specimens was tested.
Results of the asymmetry analysis are published in Fahlke & Hampe (2015) and indicate that archaeocete cranial asymmetry is not significant when the cranial shape is analyzed in 3D and the rostrum is taken out of the equation, that there was no conspicuous directional cranial asymmetry in mysticetes at any time during the evolution of this clade, and that directional cranial asymmetry in cetaceans is restricted to odontocetes. Therefore, the original working hypothesis (cranial symmetry in extant mysticetes is secondary, asymmetry was reduced during mysticete evolution) is not supported.
Results of the comparison of digitization methods for paleontological application are published in Fahlke & Autenrieth (2016) who generally recommend manual photogrammetry for external surface reconstruction, because it has a high degree of reproducibility and is the most efficient and least costly method of those tested. Their technical provide valuable advice for the application of 3D digitization techniques in paleontology.
Results of the correlation of cranial and cochlear shape are published in Ritsche et al. (under review) who find that asymmetric and symmetric components of cranial shape correlate with cochlear shape and hearing frequencies in that the symmetric, broad mysticete cranial shape is linked to lower hearing frequencies and the asymmetric, “telescoped” odontocete cranial shape to higher hearing frequencies.

Partners

Helmholtz-Zentrum Berlin

Bundesanstalt für Materialforschung und -prüfung Berlin

Duration

01.01.2013 – 16.08.2016

Funding

Deutsche Forschungsgemeinschaft - DFG