The other half of the contributions to this special issue all provide new original data and focus on diverse aspects of the morphology and evolution of the Xenarthra. Néstor Toledo and co-workers in their paper “The concept of pedolateral pes revisited: the giant sloths Megatherium and Eremotherium (Xenarthra, Folivora, Megatheriinae) as a case study” focus on a specific peculiarity of fossil sloths, the habitually inverted pes already described in very early accounts of fossil sloth morphology (Toledo et al. 2017, this issue). Re-analyzing two of the truly gigantic forms, the team surprisingly found little evidence that suggests a strongly inverted pes. These results call for a thorough reappraisal of the available fossil material and could also benefit from relatively recent approaches like computer modelling. In a completely different approach, but also with a focus on fossil xenarthrans, Luciano Varela and colleagues study the paleo-biogeographical distribution of 15 extinct xenarthran taxa using species distribution models in their paper “Potential distribution of fossil xenarthrans in South America during the late Pleistocene: co-occurrence and provincialism” (Varela et al. 2017, this issue). Their new data provide a differentiated basis for inferences in terms of suitable and preferential areas of the species in light of climatic changes and accompanying changes of sea level. In the final contribution with a primary focus on fossil xenarthrans, Daniela Kalthoff and Jeremy Green study the “Feeding ecology in Oligocene mylodontoid sloths (Mammalia, Xenarthra) as revealed by orthodentine microwear analysis” (Kalthoff and Green 2017, this issue). The microwear analyses on the recovered molariforms of two of the earliest sloths carried out by these authors support the reconstruction of Deseadan environments as open habitats and indicate that the studied species likely were bulk feeders at ground level.
This special issue is rounded out by two contributions on extant xenarthrans. Both focus on the postcranial musculature. These soft tissues usually are not preserved in fossils and therefore insights into the muscle anatomy of extant sloths may provide novel insight not only for a better understanding of the modern forms, but also for some of the extinct species (but see the paper by Vizcaíno et al. 2017, this issue). Tim Gaudin and John Nyakatura take measurements of the epaxial musculature in a species of armadillo (Dasypus) and a sloth (Choloepus) and compare these data to the Virginia opossum which is often considered a “generalized mammal.” In their paper entitled “Epaxial musculature in armadillos, sloths, and opossums: functional significance and implications for the evolution of back muscles in the Xenarthra,” the authors suggest that the specific muscular arrangement and relative sizes of the epaxial muscles in the armadillo present an adaptation to effective digging (Gaudin and Nyakatura 2017, this issue). Furthermore, the epaxial muscle anatomy of the sloths is surprisingly similar to that of the armadillos and was probably inherited from digging ancestors. Last but not least, Rachel Olson and co-workers thoroughly study the “Architectural properties of sloth forelimb muscles (Pilosa: Bradypodidae)” (Olson et al. 2017, this issue). Surprisingly, the authors find that several properties of the forelimb muscles in these slow moving animals reflect a mechanical design indicative of fast rotational velocity instead of large joint torques. Nevertheless, many flexors are characterized by large moment arms that appear to compensate for overall reduced skeletal muscle mass.
We are sure that this special issue will represent a valuable contribution and valuable source of information for experienced xenarthrologists and those on their way to becoming one. Functioning as guest editors, we (MSB and JAN) would like to thank all contributors for their sustained efforts during the peer review phase of the articles published here. It was worth it! We would also like to express our appreciation and gratitude to John R. Wible, the editor-in-chief of the Journal of Mammalian Evolution, for his constant support and for giving this collection of themed papers a home.