Functional morphology of indrid lumbar vertebrae

Abstract

Indrids are primarily vertical clingers and leapers but also engage to various extents in climbing, suspensory postures, bimanual movement, bipedal hopping, and quadrupedalism. It has been demonstrated that these behaviors are well reflected in the appendicular anatomy of these primates, while indrid vertebral anatomy has received relatively little attention. In this morphometric study, biomechanically relevant aspects of the lumbar vertebrae of Indri indri, Propithecus diadema, and Propithecus verreauxi were compared to those of Varecia variegata, a large-bodied predominantly pronograde and quadrupedal lemur. Results indicate that, compared to Varecia, the indrids have relatively shorter lumbar vertebral bodies, shorter lumbar regions, more dorsally projecting lumbar spinous processes, and more dorsally positioned lumbar transverse process tips. In addition, indrid lumbar spinous and transverse processes are oriented differently than those of Varecia. Overall, indrid lumbar vertebral morphology converges with that of hominoids, atelines, and possibly lorids, suggesting a lumbar adaptation to upright or "antipronograde" postures that require a reduction in spinal flexibility. The dorsally projecting spinous processes may be related to back muscle extensor power during leaping and/or the maintenance of upright postures. By contrast, the lumbar vertebrae of Varecia resemble those of primates and other mammals that have habitually pronograde postures and emphasize spinal flexibility in the sagittal plane during locomotion.

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

“…The xenarthrous articulation itself stiffens and imparts mobility to the region through increased articular surface area. Adding to this, the metapophysis provides more room for ligament and muscle attachment and increases mechanical advantage, thereby facilitating the powered movement necessary for flexion under xenarthry (Buchholtz and Schur, 2004;Granatosky et al, 2014;Pierce et al, 2011;Shapiro, 1995). Our results demonstrate that the post-diaphragmatic region of D. novemcinctus is characterized by decreased compliance and increased range of motion, two traits that appear to be especially conducive to a semi-fossorial lifestyle.…”

“…6C, MH). Caudal to T8, metapophyses increase in height to become massive processes with the capacity for increased ligament and muscle insertion, thus enabling a higher range of motion (Buchholtz and Schur, 2004;Granatosky et al, 2014;Pierce et al, 2011;Shapiro, 1995). As evidenced by the involvement of PC2 in models predicting both compliance and range of motion, lumbar morphology is also correlated with higher range of motion (Table 5).…”

“…Nonetheless, our data do implicate xenarthrous articulations and post-diaphragmatic zygapophyses with an increase in range of motion, as PC1, which was heavily weighted in xenarthrous and zygapophyseal measures, was found to be predictive of range of motion in all three bending directions (Tables 4, 5). It appears that, rather than acting as bony stops, xenarthrous articulations may help to amplify the observed effects of the curved, protruding postdiaphragmatic zygapophyses in mammals, which permit and secure flexion by increasing the surface area upon which vertebrae can slide without disarticulating, thereby enhancing range of motion (Filler, 2007;Jenkins, 1974;Pierce et al, 2011;Russo, 2010;Shapiro, 1995). In functioning in this way, xenarthrous articulations may extend beyond a purely stabilizing role and increase range of motion by providing greater surface area for movement.…”

“…Boszczyk et al, 2001;Buchholtz, 2001;Cullinane and Bertram, 2000;Filler, 2007;Gál, 1993a,b;Gaudin and Biewener, 1992;Hebrank et al, 1990;Hildebrand, 1959;Jones and German, 2014;Jones and Pierce, 2016;Molnar et al, 2014;Pierce et al, 2011;Rockwell et al, 1938;Schilling, 2011;Shapiro, 1995;Slijper, 1946;Ward and Mehta, 2014). Vertebral evolution is facilitated by regionalization, the Hox-mediated division of the column into morphological and functional units (Head and Polly, 2015;Schilling, 2011;Wellik, 2007).…”

“…In mammals, the thoracic and lumbar regions are foremost implicated in locomotion, prompting investigations into the functional link between thoracolumbar morphology and mechanical properties of intervertebral joints (e.g. Boszczyk et al, 2001;Gál, 1993a,b;Gaudin and Biewener, 1992;Granatosky et al, 2014;Jones, 2016;Long et al, 1997;Nyakatura and Fischer, 2010;Pierce et al, 2011;Russo, 2010;Shapiro, 1995Shapiro, , 2007Slijper, 1946). These studies illustrate the morphological and mechanical diversity of thoracolumbar vertebrae within and between mammals, and demonstrate that morphological predictors of mechanics can vary according to size, taxon and mode of locomotion.…”