Comparative forelimb myology and muscular architecture of a juvenile Malayan tapir (Tapirus indicus)

A Quantitative Analysis of the Manus Musculature in Tapirs (Perissodactyla: Tapiridae)

The distal limb of many hooved mammals (ungulates) has become highly specialised, with tendonised muscles and elongate bones and ligaments. Several clades of ungulates retain fleshy, muscularised distal forelimbs; these include hippopotamuses, rhinoceroses, and tapirs. Of these species, tapirs (Tapiridae: Tapirus) represent the most plesiomorphic manus anatomy for its higher taxonomic group (Perissodactyla); the tetradactyl manus of tapirs is reminiscent of the earliest members of the lineages leading to modern horses (equids) and rhinocerotids. Within the tapir manus, osteological evidence indicates clear differences in load distribution, digit use during locomotion, and phylogenetic signal in the shape of certain bony elements. To date, no quantitative investigation has ever been performed to explore differences in the muscular anatomy of the tapir distal forelimb (manus). Here, we conducted a comparative muscle architecture quantification of the muscles which are intrinsic to the tapir manus, across all four extant species (Tapirus indicus, T. bairdii, T. pinchaque, T. terrestris). Despite limited sample sizes, we observed notable variation across the different species with regard to the force-generating potential of the muscles (based on physiological cross-sectional area, PCSA) and the shortening range of each muscle (based on fascicle length). High force-generating capacities were recovered for the interosseus muscles (preventing hyperextension) for the third and fourth digits, as may be expected for a mesaxonic manus such as that of Tapirus. Our results also indicate subtle differences in patterns of force-generating potential in the interosseus muscles between specimens housed in captivity and those from the wild, specifically living in upland rainforest and exhibiting ranging behaviour up and downhill on a regular basis. These data offer tantalising insights into the variation in the force-excursion relationship in the muscles of the ungulate manus, providing both qualitative and quantitative information for veterinarians, biologists, and palaeontologists investigating perissodactyl locomotor anatomy and evolution.

Looking back over the shoulder: New insights on the unique scapular anatomy of the tapir (Perissodactyla: Tapiridae)

The musculoskeletal anatomy of the shoulder of many ungulates has been inferred from veterinary model taxa, with uniformity in muscle arrangements and attachment sites often assumed. In this study, I investigated the muscular and osteological anatomy of tapirs and their relatives (Perissodactyla: Tapiroidea), using a combination of gross dissection and digital imaging (photography and laser surface scanning). Dissections of three modern tapir species revealed that the m. infraspinatus originates from both supraspinous and infraspinous fossae for all species, lying on both sides of the distal scapular spine. The epimysial border between the m. supraspinatus and m. infraspinatus origin sites are marked in all species by an ossified ridge, sometimes extending the length of the scapular spine. This "supraspinous ridge" is clearly visible on the scapular surface of both modern and extinct Tapirus scapulae; however, the ridge does not appear present in any non-Tapirus tapiroids examined (e.g., Helaletes, Nexuotapirus), nor in other perissodactyls or artiodactyls. Moreover, the ridge exhibits a clearly distinct morphology in Tapirus indicus compared to all other Tapirus species examined. Combined, these findings indicate that the presence and position of the "supraspinous ridge" may represent a robust phylogenetic character for reconstructing relationships within tapiroids. Unfortunately, any functional locomotor outcomes or benefits of the m. infraspinatus straddling the scapular spine remains elusive. This study represents a firm reminder for anatomists, veterinarians, and paleontologists to (where possible) look beyond veterinary model systems when inferring musculoskeletal form or function in non-model organisms.

Anatomy and phylogeny of a new small macraucheniid (Mammalia: Litopterna) from the Bahía Inglesa Formation (late Miocene), Atacama Region, Northern Chile

We describe a new macraucheniine macraucheniid, Micrauchenia saladensis gen. et sp. nov., from the late Miocene (Huayquerian SALMA). This is the first litoptern from Bahía Inglesa Formation, Chile. The specimen includes a partial mandible, cervical and thoracic vertebrae fragments, and portions of the forelimbs (a scapula fragment, an ulna-radius fragment, seven carpals, three metapodials, two proximal phalanges and four intermediate phalanges). The postcranial anatomy of Micrauchenia saladensis is consistent with terrestrial and cursorial locomotion, which suggests an allochthonous position of this specimen within the marine Bahía Inglesa Formation. The fusion of the ulna and radius and the presence of a radial aliform expansion align Micrauchenia with other macraucheniines, with which it shares these features. We interpret the fusion of the ulna and radius as a cursorial specialization and the aliform expansion as an adaptation for strong flexion movements and to resist higher transverse stresses during locomotion. In addition, Micrauchenia saladensis is the smallest member of the subfamily Macraucheniinae. To test the systematics and phylogenetics of this specimen, we expanded previous morphological matrices of macraucheniids by adding one dental and eight postcranial characters and scoring Micrauchenia saladensis. We performed maximum parsimony and Bayesian phylogenetic analyses, the latter applied for the first time to macraucheniid phylogeny. Our analyses confirm Micrauchenia saladensis as a member of the subfamily Macraucheniinae, although with uncertain affinities within this subfamily.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

Limb myology and muscle architecture of the Indian rhinoceros Rhinoceros unicornis and the white rhinoceros Ceratotherium simum (Mammalia: Rhinocerotidae)

Land mammals support and move their body using their musculoskeletal system. Their musculature usually presents varying adaptations with body mass or mode of locomotion. Rhinocerotidae is an interesting clade in this regard, as they are heavy animals potentially reaching three tons but are still capable of adopting a galloping gait. However, their musculature has been poorly studied. Here we report the dissection of both forelimb and hindlimb of one neonate and one adult each for two species of rhinoceroses, the Indian rhinoceros (Rhinoceros unicornis) and the white rhinoceros (Ceratotherium simum). We show that their muscular organisation is similar to that of their relatives, equids and tapirs, and that few evolutionary convergences with other heavy mammals (e.g. elephants and hippopotamuses) are present. Nevertheless, they show clear adaptations to their large body mass, such as more distal insertions for the protractor and adductor muscles of the limbs, giving them longer lever arms. The quantitative architecture of rhino muscles is again reminiscent of that of horses and tapirs, although contrary to horses, the forelimb is much stronger than the hindlimb, which is likely due to its great role in body mass support. Muscles involved mainly in counteracting gravity (e.g. serratus ventralis thoracis, infraspinatus, gastrocnemius, flexores digitorum) are usually highly pennate with short fascicles facilitating strong joint extension. Muscles involved in propulsion (e.g. gluteal muscles, gluteobiceps, quadriceps femoris) seem to represent a compromise between a high maximal isometric force and long fascicles, allowing a reasonably fast and wide working range. Neonates present higher normalized maximal isometric force than the adults for almost every muscle, except sometimes for the extensor and propulsor muscles, which presumably acquire their great force-generating capacity during the growth of the animal. Our study clarifies the way the muscles of animals of cursorial ancestry can adapt to support a greater body mass and calls for further investigations in other clades of large body mass.