Comparative study of notoungulate (Placentalia, Mammalia) bony labyrinths and new phylogenetically informative inner ear characters

Authors

  • Thomas E. Macrini,

    Corresponding author
    1. Department of Vertebrate Paleontology, Division of Paleontology, American Museum of Natural History, New York, NY, USA
    • Department of Biological Sciences, St Mary's University, San Antonio, TX, USA
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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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  • Xijun Ni,

    1. Department of Vertebrate Paleontology, Division of Paleontology, American Museum of Natural History, New York, NY, USA
    2. Key Laboratory of Vertebrate Evolution and Human Origin, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China
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  • Darin A. Croft,

    1. Department of Anatomy, Case Western University School of Medicine, Cleveland, OH, USA
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  • André R. Wyss

    1. Department of Earth Science, University of California, Santa Barbara, CA, USA
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Correspondence

Thomas E. Macrini, Department of Biological Sciences, One Camino Santa Maria, St Mary's University, San Antonio, TX 78228, USA. T: +210 431 4304; F: +210 431 4363; E: tmacrini@stmarytx.edu

Abstract

The phylogenetic relationships of notoungulates, an extinct group of predominantly South American herbivores, remain poorly resolved with respect to both other placental mammals and among one another. Most previous phylogenetic analyses of notoungulates have not included characters of the internal cranium, not least because few such features, including the bony labyrinth, have been described for members of the group. Here we describe the inner ears of the notoungulates Altitypotherium chucalensis (Mesotheriidae), Pachyrukhos moyani (Hegetotheriidae) and Cochilius sp. (Interatheriidae) based on reconstructions of bony labyrinths obtained from computed tomography imagery. Comparisons of the bony labyrinths of these taxa with the basally diverging notoungulate Notostylops murinus (Notostylopidae), an isolated petrosal from Itaboraí, Brazil, referred to Notoungulata, and six therian outgroups, yielded an inner ear character matrix of 25 potentially phylogenetically informative characters, 14 of them novel to this study. Two equivocally optimized character states potentially support a pairing of Mesotheriidae and Hegetotheriidae, whereas four others may be diagnostic of Notoungulata. Three additional characters are potentially informative for diagnosing more inclusive clades: one for crown Placentalia; another for a clade containing Kulbeckia, Zalambdalestes, and Placentalia; and a third for Eutheria (crown Placentalia plus stem taxa). Several other characters are apomorphic for at least one notoungulate in our study and are of potential interest for broader taxonomic sampling within Notoungulata to clarify currently enigmatic interrelationships. Measures of the semicircular canals were used to infer agility (e.g. capable of quick movements vs. lethargic movements) of these taxa. Agility scores calculated from these data generally corroborate interpretations based on postcranial remains of these or closely related species. We provide estimates of the low-frequency hearing limits in notoungulates based on the ratio of radii of the apical and basal turns of the cochlea. These limits range from 15 Hz in Notostylops to 149 Hz in Pachyrukhos, values comparable to the Asian elephant (Elephas maximus) and the California sea lion (Zalophus californianus) when hearing in air, respectively.

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