Meeting Abstract
Xenarthran (armadillos, anteaters, sloths) vertebral columns are, with few notable exceptions, characterized by xenarthrae, ancillary intervertebral articulations spanning the post-diaphragmatic thoracic and lumbar regions. In recent work, we have shown that xenarthrous vertebrae constitute a discrete region of high stiffness and controlled mobility in the nine-banded armadillo vertebral column. They are thus thought to equip xenarthrans with the axial stability necessary for fossoriality, the group’s hypothesized ancestral locomotor mode. Here we explore the origins of this morphology and its correlated vertebral regionalization through an investigation into the morphological development of the nine-banded armadillo (Dasypus novemcinctus) vertebral column. We performed 3D geometric morphometrics and clustering analyses on thoracolumbar vertebrae of sixteen fetal, post-natal, and adult armadillos, which span the course of vertebral development. We found that the adult regionalization scheme sets in late in development, with thoracolumbar vertebrae divided primarily according to xenarthrous identity, and secondarily according to thoracic or lumbar identity. The division of the thoracic region at the position of the diaphragmatic vertebra has been identified morphologically and developmentally in mice, and is reflective of Hox-mediated regionalization schemes across amniotes. Our results suggest that xenarthrae evolved within the prescribed Hox-expression boundaries common to amniotes, thus facilitating the region’s specialization towards fossoriality in the ancestral xenarthran.
