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Epaxial Musculature in Armadillos, Sloths, and Opossums: Functional Significance and Implications for the Evolution of Back Muscles in the Xenarthra

Abstract

To investigate the evolution of xenarthran epaxial muscles, fresh specimens of the North American Common long-nosed armadillo Dasypus novemcinctus and of a marsupial, the Virginia opossum Didelphis virginiana, were dissected. Data from one fixed specimen of a two-toed sloth Choloepus didactylus were also used for comparison, because it is a xenarthran exhibiting a highly derived locomotor mode. The opossum was used to represent a more generalized mammalian condition. Each of the three mammalian epaxial muscle groups, the iliocostalis, longissimus dorsi, and transversospinalis, was removed and its mass was determined. All data were corrected for body mass and length. Unpaired, one-tailed t-tests showed the average mass of the iliocostalis and transversospinalis of Dasypus to be significantly larger than the mass of the same muscles in Didelphis, whereas the average mass of the longissimus dorsi was not statistically different between the two species. In agreement with pronounced lateral bending and de-emphasized dorso-ventral flexion and extension, Choloepus also had a relatively large iliocostalis and small longissimus. Our limited data suggest that this condition was inherited from non-arboreal and probably digging early xenarthrans. We believe the relatively larger iliocostalis and transversospinalis muscles in Dasypus can be attributed to the need to provide vertical stabilization of the trunk and resist lateral reaction forces generated by digging. Thus, for Xenarthra it represents a synapomorphy linked to adaptations for fossoriality.

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Notes

  1. Coues (1872:101) description of the muscles in this region differs from the terminology used here, and is somewhat confusing to us. He recognized a fused erector spinae comprised of longissimus dorsi and “sacro-lumbalis” muscles. The latter, presumably the posterior part of iliocostalis, has an “upward [=cranial?] prolongation” called “musculus accessorius,” and is purportedly blended with the quadratus lumborum, which is a hypaxial muscle, internal to the mm. transversus abdominus, external and internal obliques in humans (Pick and Howden 1977; Clemente 1987) and other mammals (Evans and Christiansen 1979; Homberger and Walker 2004). We follow Slijper (1946: table 3), who recognized a separate “iliocostalis lumborum” in the related opossum Metacheirus, which is attached but not fused to the “pars ilio-lumbalis of M. longissimus dorsi.” We saw no evidence of a fusion between the iliocostalis and quadratus lumborum.

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Acknowledgements

This project began as an undergraduate research project by Ms. Laurie Fortin (student of TJG, then a Visiting Professor at the College of the Holy Cross) in 1993-4. However, the manuscript has morphed considerably in the ensuing 23-plus years, and it was no longer possible to contact Ms. Fortin to obtain her permission to be included as an author. Nevertheless, she should be credited for doing the original work of dissection, measurement, and statistical calculations. We also wish to thank the Department of Biology at the College of the Holy Cross for furnishing laboratory facilities and funding for this project. The armadillo and opossum specimens dissected were purchased from Ray Singleton & Co. of Arcadia, Fla. We thank Bruce Patterson and the late Bill Stanley of the Department of Mammalogy at the Field Museum of Natural History, Chicago, for access to specimens of various fossorial mammals examined in connection with this report. Mary Morton graciously provided us access to her lyopholyzer. We thank Andy Biewener, David Carrier, Virginia Naples, Ann Pabst, Laura Panko, John Wible, guest editor Susanna Bargo, reviewer Marcos Ercoli, and a second, anonymous reviewer for critical comments on earlier drafts of this manuscript, and Susan Berman, Mary Morton, and Laura Panko for encouragement and suggestions.

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Correspondence to Timothy J. Gaudin.

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Gaudin, T.J., Nyakatura, J.A. Epaxial Musculature in Armadillos, Sloths, and Opossums: Functional Significance and Implications for the Evolution of Back Muscles in the Xenarthra. J Mammal Evol 25, 565–572 (2018). https://doi.org/10.1007/s10914-017-9402-0

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Keywords

  • Xenarthra
  • Choloepus
  • Dasypus
  • Didelphis
  • Epaxial muscle
  • Fossoriality