Comparative anatomy of the bony labyrinth of extant and extinct porpoises (Cetacea: Phocoenidae)

Authors

  • Rachel A. Racicot,

    Corresponding author
    1. The Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA, USA
    2. Smithsonian Institution, Washington, DC, USA
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  • William Gearty,

    1. Department of Geological Sciences, Stanford University, Stanford, CA, USA
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  • Naoki Kohno,

    1. Department of Geology and Paleontology, Division of Biotic Evolution, National Museum of Nature and Science, Tokyo, Japan
    2. Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan
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  • John J. Flynn

    1. Department of Vertebrate Paleontology, Division of Paleontology, American Museum of Natural History, New York, NY, USA
    2. Richard Gilder Graduate School, American Museum of Natural History, New York, NY, USA
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Abstract

The inner ear anatomy of cetaceans, now more readily accessible by means of nondestructive high-resolution X-ray computed tomographic (CT) scanning, provides a window into their acoustic abilities and ecological preferences. Inner ear labyrinths also may be a source for additional morphological characters for phylogenetic analyses. In this study, we explore digital endocasts of the inner ear labyrinths of representative species of extinct and extant porpoises (Mammalia: Cetacea: Phocoenidae), a clade of some of the smallest odontocete cetaceans, which produce some of the highest-frequency clicks for biosonar and communication. Metrics used to infer hearing ranges based on cochlear morphology indicate that all taxa considered could hear high-frequency sounds, thus the group had already acquired high-frequency hearing capabilities by the Miocene (9–11 Mya) at the latest. Vestibular morphology indicates that extant species with pelagic preferences have similarly low semicircular canal deviations from 90°, values indicating more sensitivity to head rotations. Species with near-shore preferences have higher canal deviation values, indicating less sensitivity to head rotations. Extending these analyses to the extinct species, we demonstrate a good match between those predicted to have coastal (such as Semirostrum cerutti) preferences and high canal deviation values. We establish new body length relationships based on correlations with inner ear labyrinth volume, which can be further explored among other aquatic mammals to infer body size of specimens consisting of fragmentary material.

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