Forelimb myology of armadillos (Xenarthra: Cingulata, Chlamyphoridae): Anatomical correlates with fossorial ability

Abstract

Descriptions of myology reflect adaptations of the post‐cranium and are essential for understanding the functional morphology of animal limbs. Armadillos (Cingulata) are the most species‐rich group of the basal superorder Xenarthra, which is evident by their various lifestyles (subterranean vs. terrestrial) and levels of fossoriality (fossorial vs. semi‐fossorial). While there have been several studies on limb bone proportions in numerous armadillos, limb myology has been reported for a limited number of species. Many of these descriptions need updating, and detailed quantitative muscle data are available only for nine‐banded armadillos. The main objective of this study is to assess the forelimb myology of the pichi (Zaedyus pichiy), screaming hairy (Chaetophractus vellerosus), large hairy (Chaetophractus villosus), and pink fairy (Chlamyphorus truncatus) armadillos with comparisons to previous observations to specify muscle traits that indicate scratch‐digging specializations in cingulates. Several myological features are variable among the species studied, including the origin of m. trapezius pars cervicalis, presence of a distinct m. rhomboideus profundus and m. omotransversarius, and number of heads present for m. triceps brachii and m. flexor digitorum profundus, all of which can be associated with variability in their respective habitats and functional habits. These traits are consistently observed in the members of the Euphractinae, whereas they are slightly divergent (i.e., reduced complexity) in the pink fairy armadillo despite a similar distribution of muscle mass in the limb retractors, elbow extensors, and carpal/digital flexors across species. The ecomorphology observed here among cingulates also confirms their recent reorganization into separate families and subfamilies.

Computational Modeling of Gluteus Medius Muscle Moment Arm in Caviomorph Rodents Reveals Ecomorphological Specializations

“…A larger osteological in-lever increases the distance between the MLoA and the CoR and hence will ultimately lead to an increase in all IMMAs. Thus, as is the case for the IMMAs, increasing in-lever length while keeping everything else constant results in a larger torque ( Zajac, 1992 ; Payne et al, 2006 ; Moore et al, 2013 ; Marshall et al, 2021 ). Importantly, a longer muscle moment arm will also result in a decline in angular velocity at the joint given a constant contraction speed ( Hutchinson et al, 2005 ; Channon et al, 2010 ).…”

“…Osteological in-levers are defined as the distance between the attachment site of a muscle and the CoR of the joint the muscle is acting on ( Hildebrand and Goslow, 1995 ). From a pragmatic standpoint, osteological in-levers can readily be analyzed using dry skeletal collection material and/or well-preserved fossil specimens not only in 2D (e.g., Vizcaino and Milne, 2002 ; Marshall et al, 2021 ) but also in 3D (e.g., Mielke et al, 2018 ; Wölfer et al, 2019 ), and without further knowledge of joint movements. The IMMA and the osteological in-lever are geometrically related.…”

Morphological and radiological study of the shoulder and elbow joints of the giant anteater (Myrmecophaga tridactyla)

“…These organisms have strong forelimbs with powerful flexion of the elbow joint, and during the terrestrial locomotion keep the elbow joint abducted and flexed, like other mammals of the superorder Xenarthra such as sloths of the genus Bradypus and Choloepus (Fujiwara et al, 2011; Mendel, 1985; Scheidt et al, 2022). Otherwise, the cingulates, the other order from superorder Xenarthra that includes armadillos, have also strong forelimbs adapted to fossorial behaviors of this species, with powerful flexion for forelimb retraction, although this species does not have a great flexion of elbow joint, in contrast with a strong extension of elbow joint necessary to scratch‐digging habits of this species (Marshall et al, 2020).…”

Myology of the pelvic limb of the brown‐throated three‐toed sloth (Bradypus variegatus)

“…Muscle fiber architecture (e.g., pennate vs. parallel fibers) was verified ex vivo by micro‐dissection. Muscles with fascicle angles <15° (relative to the axis of force production) were generally classified as parallel regardless of belly shape, whereas muscles with fascicle angles >15° were classified as either unipennate, bipennate, or multipennate depending on fiber orientation and the number of intramuscular divisions or tendinous inscriptions observed (Marshall et al, 2021b). Dissections were documented by photographs taken with an EOS Rebel T5i D‐SLR camera (Cannon, USA) and these images were used to create limb muscle maps and illustrations of the limb muscle topography.…”

“…The fundamental presence or absence of muscles and their relative mass distributions are informative about limb function (Ercoli et al, 2013; Fisher et al, 2008; Marshall et al, 2021b; Olson et al, 2016), especially that of sloth forelimbs (Diniz et al, 2018; Miller, 1935; Nyakatura & Fischer, 2011; Olson et al, 2018). Myological descriptions are also available for the pelvic limb of sloths, but it is much less studied in relation to limb function (Nyakatura, 2012).…”