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Abstract
The hyoid apparatus reflects aspects of the form and function of feeding in living and extinct organisms and, despite the availability of information about this structure for Xenarthra, it remains little explored from an evolutionary perspective. Here we compare the morphology of the hyoid apparatus in xenarthrans, describing its general morphology and variation in each major clade and score these variations as phylogenetic characters, which were submitted to ancestral states reconstructions. The general hyoid morphology of Xenarthra consists of a v-bone (basihyal fused with the thyrohyals) and three paired bones (stylohyal, epihyal and ceratohyal), which are unfused in the majority of taxa. The clade-specific morphology observed here, allowed us to obtain additional synapomorphies for all major clades of Xenarthra (Cingulata, Pilosa, Folivora and Vermilingua), for Glyptodontididae, and for Nothrotheriidae. The fusion of hyoid elements are convergentelly achieved among the diphyletic extant tree sloths, some extinct ground sloths and glyptodontids. Despite the heavy influence of adaptive evolution related to feeding habits, the morphology of the hyoid apparatus proved to be a valuable source of phylogenetic information.
Acknowledgments
We would like acknowledge CAPES for the financial support. We also would like to acknowledge Prof. Castor Cartelle, Prof. Claudia Guimarães Costa and Leandro de Oliveira Marques for kindly granting access to the specimens available in the Museu de Ciência Naturais (PUC Minas). We would like to thank Rodrigo Parisi Dutra and Guilherme Garbino for suggestions in early versions of the manuscript. We also would like to thank the two reviewers for their valuable commentaries and suggestions, which helped to improve our manuscript.