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The Narial Anatomy of Extinct and Extant Sloths (Xenarthra, Folivora): Osteological Anomalies in the Extant Two-Toed Sloth Choloepus

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Abstract

The skeletal anatomy of the anterior narial region in mammals is complex, comprised of several bony and cartilaginous elements. Because it includes many cartilaginous components, preservation of this area in extant and extinct specimens is often incomplete. This, along with its complexity, means that this region of the cranium is generally understudied, with the exception of humans and a few select mammal species. The present study began with a cranial specimen of the extant Hoffmann’s two-toed sloth, Choloepus hoffmanni (Xenarthra, Folivora, Megalonychidae), that possessed several unusual well-preserved narial anomalies. In order to determine whether such features are present in other extant sloths, and if so, how frequently, we examined two- and three-toed sloth specimens from a variety of museum collections. Additionally, we examined museum specimens of extinct sloths known to possess unusual osseous narial structures, with the goal of better understanding the anatomy of the anterior opening of the nasal cavity in all sloths, living and extinct. A diverse series of unusual narial elements were found, including an internasal bone, an ossified anterior nasal septum, an os nariale, an ossified processus lateralis ventralis of the nasal capsule, and an internarial bar of varying composition and completeness. All of these features are present in Choloepus, although they are preserved in museum skulls infrequently, but none were found to be present in the three-toed sloth, Bradypus. An internarial bar, anteriorly elongated ossified nasal septum, and ossified processus lateralis ventralis were observed in several species of extinct mylodontid and megatheriid sloths, and an os nariale was recorded in the extant vermilinguan anteaters Cyclopes and Tamandua. It is not known how much of the variation we document in these features is genuine and how much is due to preservation issues. Given such uncertainty, it is difficult to assess the phylogenetic or functional significance of these features. We believe, however, that this is an area of the skull in Pilosa (sloths and anteaters) that merits further study.

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modified from Bargo et al. (2006). Abbreviations: inb internarial bar, mx maxilla, n nasal, ns nasal septum, pm premaxilla, sp septal process of premaxilla. Scale bars equal 10 cm

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Acknowledgements

We thank Andy Biewener (Harvard University, Cambridge, MA) for donation of the original Choloepus hoffmanni specimen (UTCM 1912) used in this report. That skull was cleaned by the late great Bill Stanley (Department of Mammalogy, Field Museum of Natural History [FMNH], Chicago, IL), with special attention paid to preserving nasal structures. For access to additional specimens, we thank Bill Simpson, John Flynn, and Ken Angielcyzk (Department of Geology, FMNH); Adam Ferguson, Bill Stanley, Bruce Patterson (Department of Mammalogy, FMNH); Bernardino Mamani Quispe (Museo Nacional de Historia Natural, La Paz, Bolivia); Martin Ezcurra, Laura Cruz, and Alejandro Kramarz (Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina); and Susana Bargo and Sergio Vizcaíno (Museo de La Plata [MLP], La Plata, Argentina). We thank François Pujos (IANIGLA, CONICET, Mendoza, Argentina), Alberto Boscaini (Univ. of Buenos Aires & CONICET, Buenos Aires, Argentina), and Adam Ferguson (FMNH) for graciously providing specimen photos used in this work, and Susana Bargo (MLP) for permission to modify photos from her excellent work on the muzzle of sloths. We also thank Julia Morgan Scott for her ever-excellent work in preparing the drawings for this paper. Lastly, we thank the two anonymous reviewers who helped in improving the manuscript. The work of TJG and KMS was supported in part by the University of Tennessee at Chattanooga and the Bramblett gift fund. The work of JRW was supported by National Science Foundation Grant DEB 1654949, and the R.K. Mellon North American Mammal Research Institute.

TJG: I want to begin this personal acknowledgement by stating what an honor it is to participate in this festschrift honoring my colleague and co-author, Dr. John Wible. Let me also make clear that John was unaware he was contributing to his own festschrift, a fact for which I hope he will forgive me. After some discussion with the other co-editors of this volume, we decided that his participation in this paper, and hence, in this volume, was appropriate. Certainly this is the kind of paper for which he is justifiably well-known – one that describes in detail novel aspects of mammalian cranial anatomy, interpreting it in the light of John’s vast knowledge of the subject. I also want to affirm how appropriate it is that John is being honored with this festschrift. His work with the Journal of Mammalian Evolution has been transformative, and he is personally responsible for its success to a remarkable degree. For myself, it has been my privilege to work with John for many years. I have not always been the best collaborator, often weighed down by teaching and administrative duties, but his understanding and support have been critical to whatever research success I have achieved. I am grateful for all he has taught me, but moreso, for the years of friendship we have enjoyed!

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  1. Department of Biology, Geology, and Environmental Science (Department 2653), University of Tennessee At Chattanooga, 615 McCallie Avenue, Chattanooga, TN, 37403-2598, USA

    Timothy J. Gaudin & Kaitlyn M. Smith

  2. Section of Mammals, Carnegie Museum of Natural History, 5800 Baum Boulevard, Pittsburgh, PA, 15206-3706, USA

    John R. Wible

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  1. Timothy J. Gaudin
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Correspondence to Timothy J. Gaudin.

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Gaudin, T.J., Smith, K.M. & Wible, J.R. The Narial Anatomy of Extinct and Extant Sloths (Xenarthra, Folivora): Osteological Anomalies in the Extant Two-Toed Sloth Choloepus. J Mammal Evol 28, 1181–1192 (2021). https://doi.org/10.1007/s10914-021-09562-9

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