SHAW, Barbara J.*; RUEDAS, Luis A.; Portland State University; Portland State University: Evolution of Order Xenarthra modality distinct from all other mammals
The mammalian clade Xenarthra (formerly Order Xenarthra, split into 1) Order Pilosa including anteaters, ground and tree sloths, and 2) Order Cingulata including glyptodonts, pampatheres and armadillos) constitutes a bizarre and fascinating group of mammals. The evolutionary hypothesis that Xenarthra represent a group derived from a single common ancestor in the Neotropics is undisputed because of an abundance of unusual and exclusive characteristics when compared to other mammals. The fossil Ernanodon antelios, laid down during the Paleocene in China, has been attributed to Xenarthra. This is unexpected since there are no Old World xenarthrans; such a hypothesis therefore requires a high degree of explanation. Xenarthrans are remarkably diverse, ranging from fossorial to hanging arboreal modes of life. The biomechanics of this group are also bizarre. This is the only taxon of mammals that possess a unique structure on their vertebrae: a xenarthrous articulation (doubly articulated vertebrae, vs. remaining mammals’ single articulation). All Xenarthra—except glyptodonts—have xenarthrous articulations. All have a solid pelvis, with the transverse process of the caudal vertebrae fused to the ischium. For this project, twenty-three measurements from a total of 314 specimens representing 39 species were analyzed using Principal Component Analysis to examine the variability among families and within genera, and the results were then compared to 19 eutherian, 1 metatherian and 1 prototherian species of mammals, together with the fossil E. antelios. The results group all Xenarthra together while all other mammals, including E. antelios, are separately grouped. This lends support for excluding E. antelios as a xenarthran, and for the evolution of unique modality in Xenarthra.