Vertebral bending mechanics and xenarthrous morphology in the nine-banded armadillo (Dasypus novemcinctus)

Abstract

The vertebral column has evolved to accommodate the broad range of locomotor pressures found across vertebrate lineages. Xenarthran (armadillos, sloths and anteaters) vertebral columns are characterized by xenarthrous articulations, novel intervertebral articulations located in the posterior trunk that are hypothesized to stiffen the vertebral column to facilitate digging. To determine the degree to which xenarthrous articulations impact vertebral movement, we passively measured compliance and range of motion during ventroflexion, dorsiflexion and lateral bending across the thoracolumbar region of the nine-banded armadillo, Dasypus novemcinctus. Patterns of bending were compared with changes in vertebral morphology along the column to determine which morphological features best predict intervertebral joint mechanics. We found that compliance was lower in post-diaphragmatic, xenarthrous vertebrae relative to pre-xenarthrous vertebrae in both sagittal and lateral planes of bending. However, we also found that range of motion was higher in this region. These changes in mechanics are correlated with the transition from pre-xenarthrous to xenarthrous vertebrae, as well as with the transition from thoracic to lumbar vertebrae. Our results thus substantiate the hypothesis that xenarthrous articulations stiffen the vertebral column. Additionally, our data suggest that xenarthrous articulations, and their associated enlarged metapophyses, also act to increase the range of motion of the post-diaphragmatic region. We propose that xenarthrous articulations perform the dual role of stiffening the vertebral column and increasing mobility, resulting in passively stable vertebrae that are capable of substantial bending under appropriate loads.

Experimental determination of three-dimensional cervical joint mobility in the avian neck

“…Other researchers have found that long centra allow more dorsiflexion and sometimes lateroflexion [69–73]. Turkeys do not appear to follow either of these patterns: centrum length increases from C3 to C7, then plateaus (Fig.…”

“…6 and 10). Centrum width has been reported to restrict lateroflexion [18, 73, 74] while zygapophyseal width has been reported to restrict torsion [69]. Again, neither morphological trend matches the mobility trends in turkeys (Figs.…”

“…8c), but neither dorsoventral flexion nor lateroflexion results match this pattern. In mammals, increased height of centra has been suggested to restrict dorsoventral flexion [70, 73, 74]. The pattern is different in turkeys; centra gradually increase in height caudally (Fig.…”

Analysis of the spatio‐temporal parameters of gaits in Dasypus novemcinctus (Xenarthra: Dasypodidae)

“…According to Gray (), the tripod gait (i.e., three‐legged support of ground locomotion) for lateral sequences provides more stability when compared to diagonal sequences. This kind of gait would favour the undulation of the spine in many tetrapods (e.g., salamanders) (Gray, ), but although armadillos have developed the mechanical capacity of ventroflexing their columns (e.g., Tolypeutes Illiger, 1811), there is a significant lateral stiffness—that could be attributable to xenarthrae (Gaudin & Biewener, ; Oliver et al., ). Indeed, the dorsoventral degree of mobility in the mammalian vertebral column is directly related to locomotor function, which would reflect different patterns of flexion/extension of joints (Molnar, Pierce, & Hutchinson, ).…”

“…The presence of an armoured carapace, for example, as a feature for all cingulates, has been related to a relatively slow plantigrade locomotion in other mammals (Superina & Loughry, ), but no data have been provided to confirm this pattern in armadillos. Moreover, the presence of xenarthrous vertebrae adds a quite original peculiarity to this design, and as these elements provide the stiffening of the vertebral column they are also related to locomotion in the sense that the stability thus provided would allow these animals to bend (Oliver, Jones, Hautier, Loughry, & Pierce, ).…”