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Cranial symmetry in baleen whales (Cetacea, Mysticeti) and the occurrence of cranial asymmetry throughout cetacean evolution

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Abstract

Odontoceti and Mysticeti (toothed and baleen whales) originated from Eocene archaeocetes that had evolved from terrestrial artiodactyls. Cranial asymmetry is known in odontocetes that can hear ultrasound (>20,000 Hz) and has been linked to the split function of the nasal passage in breathing and vocalization. Recent results indicate that archaeocetes also had asymmetric crania. Their asymmetry has been linked to directional hearing in water, although hearing frequencies are still under debate. Mysticetes capable of low-frequency and infrasonic hearing (<20 Hz) are assumed to have symmetric crania. This study aims to resolve whether mysticete crania are indeed symmetric and whether mysticete cranial symmetry is plesiomorphic or secondary. Cranial shape was analyzed applying geometric morphometrics to three-dimensional (3D) cranial models of fossil and modern mysticetes, Eocene archaeocetes, modern artiodactyls, and modern odontocetes. Statistical tests include analysis of variance, principal components analysis, and discriminant function analysis. Results suggest that symmetric shape difference reflects general trends in cetacean evolution. Asymmetry includes significant fluctuating and directional asymmetry, the latter being very small. Mysticete crania are as symmetric as those of terrestrial artiodactyls and archaeocetes, without significant differences within Mysticeti. Odontocete crania are more asymmetric. These results indicate that (1) all mysticetes have symmetric crania, (2) archaeocete cranial asymmetry is not conspicuous in most of the skull but may yet be conspicuous in the rostrum, (3) directional cranial asymmetry is an odontocete specialization, and (4) directional cranial asymmetry is more likely related to echolocation than hearing.

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Acknowledgments

We thank Frieder Mayer, Nora Lange, and Christiane Funk (Museum für Naturkunde Berlin, Germany), Reinhard Ziegler (SMNS), Eberhard Frey (SMNK), Giovanni Bianucchi and Chiara Sorbini (MSNTUP), Carlo Sarti and Federico Fanti (MGB), Olivier Lambert and Anneliese Folie (IRSNB), Philip Gingerich (UM), Tom Démére (SDNHM), Samuel McLeod, Vanessa Rhue, Jim Dines and Dave Janiger (LACM), Patricia Holroyd and Mark Goodwin (UCMP), Paul Collins (SBMNH), Christian de Muizon (MNHN), Mark Uhen (George Mason University, Fairfax, USA), Lori Marino (Emory University, Atlanta, USA), Nicholas Pyenson and Dave Bohaska (USNM), Hans Thewissen (Northeastern Ohio Medical University, Rootstown, USA), John Graf, Michael Polcyn, and Louis Jacobs (Southern Methodist University, Dallas, USA), Matthew McCurry (Monash University, Clayton, Australia), and Erich Fitzgerald (Museum Victoria, Melbourne, Australia) for access to specimens and/or 3D data or photos. Felix Marx and Ewan Fordyce (University of Otago, Dunedin New Zealand), Mark Uhen (George Mason University, Fairfax, USA), and Jonathan Geisler (New York Institute of Technology, Old Westbury, USA) provided phylogeny files, and Neil Brocklehurst (Museum für Naturkunde Berlin, Germany) generated the phylogenetic tree used in our analysis. Christian Klingenberg (University of Manchester, Manchester, UK) offered helpful comments on morphometrics and an updated version of MorphoJ. Heinrich Mallison, Wolfram Fritzsch, Kersten Barth, Hannelore Hoch, and Roland Mühlethaler (Museum für Naturkunde Berlin, Germany) are thanked for access to equipment and help with photography and software. We also thank Mark Clementz (University of Wyoming, Laramie, USA) for discussion and language proofreading. Funding was provided by the Deutsche Forschungsgemeinschaft (DFG FA 889/2-1 and DFG HA 1776/13-1) and by a Feodor Lynen Return Fellowship of the Alexander von Humboldt Foundation (JMF).

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  1. Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115, Berlin, Germany

    Julia M. Fahlke & Oliver Hampe

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  1. Julia M. Fahlke
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Correspondence to Julia M. Fahlke.

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Communicated by: Sven Thatje

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Fahlke, J.M., Hampe, O. Cranial symmetry in baleen whales (Cetacea, Mysticeti) and the occurrence of cranial asymmetry throughout cetacean evolution. Sci Nat 102, 58 (2015). https://doi.org/10.1007/s00114-015-1309-0

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