Functional Morphology of the Forelimb of the Nine-Banded Armadillo (Dasypus novemcinctus): Comparative Perspectives on the Myology of Dasypodidae

Abstract

A b s t r a c t T h e n i n e -b a n d e d a r m a d i l l o , D a s y p u s novemcinctus, is a member of the family Dasypodidae, which contains all extant species of armadillos and represents the most diverse group of xenarthran mammals by their speciation, form, and range of scratch-digging ability. This study aims to identify muscle traits that reflect specialization for fossorial habit by observing forelimb structure in D. novemcinctus and comparing it among armadillos using available myological data. A number of informative traits were observed in D. novemcinctus and among Dasypodidae, including the absence of m. rhomboideus profundus, the variable presence of a m. articularis humeri and m. coracobrachialis, two heads of m. triceps brachii with scapular origin, and a lack of muscle mass devoted to antebrachial supination. Muscle mass and myosin heavy chain (MHC) isoform content were also quantified from our forelimb dissections. New osteological indices are additionally calculated and reported for D. novemcinctus. Collectively, the findings emphasize muscle mass and power output for limb retraction and specialization of the distal limb for sustained purchase of soil by strong pronation and carpal/digital flexion. Moreover, the myological traits assessed here provide a valuable resource for interpretation of muscle architecture specializations among digging mammals and future reassessment of armadillo phylogeny.

Osteology and Radiological Anatomy of the Thoracic Limbs of Temminck's Ground Pangolin (Smutsia temminckii)

“…The hand of these species is capable of a wide range of specialized motions as well as movement of the individual digits (Bishop, ; McGrew and Marchant, ). Although the first digit is reduced in domestic carnivores (Liebich et al, ), it is completely absent in highly‐fossorial animals, such as the aardvark (Thewissen and Badoux, ), the meerkat (van Staden, ) and the nine‐banded armadillo (Olson et al, ). The absence of the first digit (and reduction of the first metacarpal bone) in these animals may contribute to a more efficient digging action (van Staden, ).…”

Mechanical similarity across ontogeny of digging muscles in an Australian marsupial (Isoodon fusciventer)

“…Fiber length (FL) represents the number of sarcomeres in a series (Michilsens, Vereecke, D'Août, & Aerts, ), and consequently longer fibers have faster shortening velocity (Azizi, Brainerd, & Roberts, ; Kikuchi, ; Lieber & Fridén, ). In digging animals such as badgers ( Taxidea taxus ; Moore et al, ), groundhogs ( Marmota monax ; Rupert, Rose, Organ, & Butcher, ), and armadillos ( Dasypus novemcinctus ; Olson et al, ), longer fibers in muscles such as the latissimus dorsi, pectoralis, triceps brachii, and teres major provide rapid fiber contraction during the power stoke of scratch‐digging. The force a muscle produces is influenced by the arrangement of the muscle fibers (number of sarcomeres in parallel; Lieber & Ward, ; Rupert et al, ).…”

“…As PCSA is a representation of maximum isometric force production for individual muscles, the PCSA values of muscles in a limb can be used to interpret biological functions in correlation with skeletal morphology (Myatt et al, ; Thorpe, Crompton, GÜnther, Ker, & McNeill, ). Scratch‐digging mammals have relatively large PCSA values for muscles used as main movers in the power stroke, such as the triceps brachii long head, which reflect a greater number of fibers per unit volume of muscles; this allows them to produce a greater force for scratch‐digging (Moore et al, ; Olson et al, ; Rose et al, ; Rupert et al, ).…”

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

“…Xenarthrans are hypothesized to have evolved as ancestrally fossorial mammals (Simpson, 1931). Several traits characterizing this major mammalian lineage are used as evidence for this hypothesis, including the limb morphology of modern and fossil xenarthrans, the lack of color vision in living xenarthrans, and the xenarthrous articulation (Emerling and Springer, 2015;Frechkop, 1949;Jenkins, 1970;Nyakatura and Fischer, 2011;Olson et al, 2016;Vizcaíno and Milne, 2002). Xenarthrous articulations, or xenarthrae, are ancillary intervertebral articulations found across the posterior thoracic and lumbar vertebrae, spanning the post-diaphragmatic region of xenarthrans (e.g.…”