Low elbow mobility indicates unique forelimb posture and function in a giant extinct marsupial

Relationship between joint shape and function as revealed through ex vivo XROMM

Skeletal joint morphology and mobility underlie movement, behavior and ecology in vertebrates. Joints can be categorized by their shape and articulation type, but such schemes might be unreliable for inferring function across the full diversity of vertebrates. We test hypothesized relationships between joint form and function by collecting marker-based ex vivo, cadaveric XROMM data on the shoulder and elbow joints of the tegu lizard (Salvator merianae) and Virginia opossum (Didelphis virginiana), which between them contain articulations historically classified as ball-and-socket, hemi-sellar, hinge and condylar joints. We measured 3D rotational and translational mobility at each joint and compared our experimental results against predictions based on articular morphology. Contrary to our predictions, the opossum ball-and-socket shoulder joint was less mobile - it had a smaller 3D range of motion envelope - than the tegu hemi-sellar shoulder joint and even the tegu condylar elbow joint, challenging the notion that ball-and-socket joints provide an inherent mobility advantage. However, the ball-and-socket opossum shoulder also had a less complex mobility envelope, with fewer interactions between degrees of freedom, allowing it to transition between poses more easily. Matching osteological predictions, the hinge elbow of the opossum was the least mobile. All joints exhibited coupling between rotational and translational degrees of freedom, further emphasizing the need to incorporate translational motion and soft tissue constraints for accurately modeling joint mobility. Our findings underscore the complexity of form-function relationships in vertebrate skeletal joints, and demonstrate that joint morphology alone, in the absence of soft tissues, does not provide a complete picture of joint mobility.

Late acquisition of erect hindlimb posture and function in the forerunners of therian mammals

The evolutionary transition from early synapsids to therian mammals involved profound reorganization in locomotor anatomy and function, centered around a shift from "sprawled" to "erect" limb postures. When and how this functional shift was accomplished has remained difficult to decipher from the fossil record alone. Through biomechanical modeling of hindlimb force-generating performance in eight exemplar fossil synapsids, we demonstrate that the erect locomotor regime typifying modern therians did not evolve until just before crown Theria. Modeling also identifies a transient phase of increased performance in therapsids and early cynodonts, before crown mammals. Further, quantifying the global actions of major hip muscle groups indicates a protracted juxtaposition of functional redeployment and conservatism, highlighting the intricate interplay between anatomical reorganization and function across postural transitions. We infer a complex history of synapsid locomotor evolution and suggest that major evolutionary transitions between contrasting locomotor behaviors may follow highly nonlinear trajectories.

Articular surface interactions distinguish dinosaurian locomotor joint poses

Our knowledge of vertebrate functional evolution depends on inferences about joint function in extinct taxa. Without rigorous criteria for evaluating joint articulation, however, such analyses risk misleading reconstructions of vertebrate animal motion. Here we propose an approach for synthesizing raycast-based measurements of 3-D articular overlap, symmetry, and congruence into a quantitative "articulation score" for any non-interpenetrating six-degree-of-freedom joint configuration. We apply our methodology to bicondylar hindlimb joints of two extant dinosaurs (guineafowl, emu) and, through comparison with in vivo kinematics, find that locomotor joint poses consistently have high articulation scores. We then exploit this relationship to constrain reconstruction of a pedal walking stride cycle for the extinct dinosaur Deinonychus antirrhopus, demonstrating the utility of our approach. As joint articulation is investigated in more living animals, the framework we establish here can be expanded to accommodate additional joints and clades, facilitating improved understanding of vertebrate animal motion and its evolution.

Elbow dimensions in quadrupedal mammals driven by lubrication regime

Synovial joints, such as the elbow, experience different lubrication regimes, ranging from fluid film to boundary lubrication, depending on locomotion conditions. We explore the relationship between the elbow lubrication regime and the size of quadrupedal mammals. We use allometry to analyze the dimensions, contact stress, and sliding speed of the elbow in 110 quadrupedal mammals. Our results reveal that the average diameter and width of the distal humerus are scaled [Formula: see text], which allowed us to estimate a consistent contact pressure and sliding speed across mammals. This consistency likely promotes fluid film lubrication regardless of body mass. Further, the ratio between the diameter and width is about 0.5 for all analyzed taxa, which is a good compromise between loading capacity and size. Our study deepens our understanding of synovial joints and their adaptations, with implications for the development of treatments, prostheses, and bioinspired joint designs.

Restudy of shoulder motion in the theropod dinosaur Mononykus olecranus (Alvarezsauridae)

Background

Range of motion in the forelimb of the Upper Cretaceous theropod dinosaur Mononykus olecranus, a member of the family Alvarezsauridae, has previously been investigated. However, the method used to investigate range of motion at the shoulder in M. olecranus did not follow the standardized procedure used in subsequent studies. The latter procedure yields more reliable results, and its standardization provides that its results are directly comparable to the results of similar studies in other species. I therefore reinvestigated the range of motion at the shoulder in M. olecranus, using the latter procedure.

Methods

Casts of the left scapula and coracoid of M. olecranus were posed on a horizontal surface, supported from beneath with modeling clay, with the medial surface of the scapula facing toward the horizontal surface. A cast of the left humerus was posed at the limits of motion through the transverse and parasagittal planes. Photos of the poses in orthal views were superimposed and used to measure range of motion, which was measured as the angle between lines drawn down the long axis of the humerus in each position.

Results

Through the transverse plane, the humerus of M. olecranus could be elevated to a subhorizontal position and depressed to a subvertical position. It could move through the parasagittal plane from a subvertical position at full protraction to a position above the horizontal at full retraction. These results correct the previous mischaracterization of shoulder motion in M. olecranus as restricted to a small arc with the arms held in a permanent sprawl. The range of humeral motion in M. olecranus is much greater than that found by the previous method and allowed the animal to tuck its arms in at the sides, in addition to allowing them to sprawl so as to orient the palm downward. The wide range of humeral motion allowed M. olecranus to forage for insects by employing hook-and-pull digging at surfaces with a wider range of orientations than the previous study showed to be possible.

The functional significance of aberrant cervical counts in sloths: insights from automated exhaustive analysis of cervical range of motion

Besides manatees, the suspensory extant 'tree sloths' are the only mammals that deviate from a cervical count (CC) of seven vertebrae. They do so in opposite directions in the two living genera (increased versus decreased CC). Aberrant CCs seemingly reflect neck mobility in both genera, suggesting adaptive significance for their head position during suspensory locomotion and especially increased ability for neck torsion in three-toed sloths. We test two hypotheses in a comparative evolutionary framework by assessing three-dimensional intervertebral range of motion (ROM) based on exhaustive automated detection of bone collisions and joint disarticulation while accounting for interacting rotations of roll, yaw and pitch. First, we hypothesize that the increase of CC also increases overall neck mobility compared with mammals with a regular CC, and vice versa. Second, we hypothesize that the anatomy of the intervertebral articulations determines mobility of the neck. The assessment revealed that CC plays only a secondary role in defining ROM since summed torsion (roll) capacity was primarily determined by vertebral anatomy. Our results thus suggest limited neck rotational adaptive significance of the CC aberration in sloths. Further, the study demonstrates the suitability of our automated approach for the comparative assessment of osteological ROM in vertebral series.

A comparative approach for characterizing the relationship among morphology, range-of-motion and locomotor behaviour in the primate shoulder

Shoulder shape directly impacts forelimb function by contributing to glenohumeral (GH) range-of-motion (ROM). However, identifying traits that contribute most to ROM and visualizing how they do so remains challenging, ultimately limiting our ability to reconstruct function and behaviour in fossil species. To address these limitations, we developed an in silico proximity-driven model to simulate and visualize three-dimensional (3D) GH rotations in living primate species with diverse locomotor profiles, identify those shapes that are most predictive of ROM using geometric morphometrics, and apply subsequent insights to interpret function and behaviour in the fossil hominin Australopithecus sediba. We found that ROM metrics that incorporated 3D rotations best discriminated locomotor groups, and the magnitude of ROM (mobility) was decoupled from the anatomical location of ROM (e.g. high abduction versus low abduction). Morphological traits that enhanced mobility were decoupled from those that enabled overhead positions, and all non-human apes possessed the latter but not necessarily the former. Model simulation in A. sediba predicted high mobility and a ROM centred at lower abduction levels than in living apes but higher than in modern humans. Together these results identify novel form-to-function relationships in the shoulder and enhance visualization tools to reconstruct past function and behaviour.

Description of the Pliocene marsupial Ambulator keanei gen. nov. (Marsupialia: Diprotodontidae) from inland Australia and its locomotory adaptations

Diprotodontids were the largest marsupials to exist and an integral part of Australian terrestrial ecosystems until the last members of the group became extinct approximately 40 000 years ago. Despite the frequency with which diprotodontid remains are encountered, key aspects of their morphology, systematics, ecology and evolutionary history remain poorly understood. Here we describe new skeletal remains of the Pliocene taxon Zygomaturus keanei from northern South Australia. This is only the third partial skeleton of a late Cenozoic diprotodontid described in the last century, and the first displaying soft tissue structures associated with footpad impressions. Whereas it is difficult to distinguish Z. keanei and the type species of the genus, Z. trilobus, on dental grounds, the marked cranial and postcranial differences suggest that Z. keanei warrants genus-level distinction. Accordingly, we place it in the monotypic Ambulator gen. nov. We, also recognize the late Miocene Z. gilli as a nomen dubium. Features of the forelimb, manus and pes reveal that Ambulator keanei was more graviportal with greater adaptation to quadrupedal walking than earlier diprotodontids. These adaptations may have been driven by a need to travel longer distances to obtain resources as open habitats expanded in the late Pliocene of inland Australia.

Modern three-dimensional digital methods for studying locomotor biomechanics in tetrapods

Here, we review the modern interface of three-dimensional (3D) empirical (e.g. motion capture) and theoretical (e.g. modelling and simulation) approaches to the study of terrestrial locomotion using appendages in tetrapod vertebrates. These tools span a spectrum from more empirical approaches such as XROMM, to potentially more intermediate approaches such as finite element analysis, to more theoretical approaches such as dynamic musculoskeletal simulations or conceptual models. These methods have much in common beyond the importance of 3D digital technologies, and are powerfully synergistic when integrated, opening a wide range of hypotheses that can be tested. We discuss the pitfalls and challenges of these 3D methods, leading to consideration of the problems and potential in their current and future usage. The tools (hardware and software) and approaches (e.g. methods for using hardware and software) in the 3D analysis of tetrapod locomotion have matured to the point where now we can use this integration to answer questions we could never have tackled 20 years ago, and apply insights gleaned from them to other fields.

Joint mobility as a bridge between form and function

Joints enable nearly all vertebrate animal motion, from feeding to locomotion. However, despite well over a century of arthrological research, we still understand very little about how the structure of joints relates to the kinematics they exhibit in life. This Commentary discusses the value of joint mobility as a lens through which to study articular form and function. By independently exploring form-mobility and mobility-function relationships and integrating the insights gained, we can develop a deep understanding of the strength and causality of articular form-function relationships. In turn, we will better illuminate the basics of 'how joints work' and be well positioned to tackle comparative investigations of the diverse repertoire of vertebrate animal motion.

Spherical frame projections for visualising joint range of motion, and a complementary method to capture mobility data

Quantifying joint range of motion (RoM), the reachable poses at a joint, has many applications in research and clinical care. Joint RoM measurements can be used to investigate the link between form and function in extant and extinct animals, to diagnose musculoskeletal disorders and injuries or monitor rehabilitation progress. However, it is difficult to visually demonstrate how the rotations of the joint axes interact to produce joint positions. Here, we introduce the spherical frame projection (SFP), which is a novel 3D visualisation technique, paired with a complementary data collection approach. SFP visualisations are intuitive to interpret in relation to the joint anatomy because they 'trace' the motion of the coordinate system of the distal bone at a joint relative to the proximal bone. Furthermore, SFP visualisations incorporate the interactions of degrees of freedom, which is imperative to capture the full joint RoM. For the collection of such joint RoM data, we designed a rig using conventional motion capture systems, including live audio-visual feedback on torques and sampled poses. Thus, we propose that our visualisation and data collection approach can be adapted for wide use in the study of joint function.

A toolbox for the retrodeformation and muscle reconstruction of fossil specimens in Blender

Accurate muscle reconstructions can offer new information on the anatomy of fossil organisms and are also important for biomechanical analysis (multibody dynamics and finite-element analysis (FEA)). For the sake of simplicity, muscles are often modelled as point-to-point strands or frustra (cut-off cones) in biomechanical models. However, there are cases in which it is useful to model the muscle morphology in three dimensions, to better examine the effects of muscle shape and size. This is especially important for fossil analyses, where muscle force is estimated from the reconstructed muscle morphology (rather than based on data collected in vivo). The two main aims of this paper are as follows. First, we created a new interactive tool in the free open access software Blender to enable interactive three-dimensional modelling of muscles. This approach can be applied to both palaeontological and human biomechanics research to generate muscle force magnitudes and lines of action for FEA. Second, we provide a guide on how to use existing Blender tools to reconstruct distorted or incomplete specimens. This guide is aimed at palaeontologists but can also be used by anatomists working with damaged specimens or to test functional implication of hypothetical morphologies.

A toolbox for the retrodeformation and muscle reconstruction of fossil specimens in Blender

Accurate muscle reconstructions can offer new information on the anatomy of fossil organisms and are also important for biomechanical analysis (multibody dynamics and finite-element analysis (FEA)). For the sake of simplicity, muscles are often modelled as point-to-point strands or frustra (cut-off cones) in biomechanical models. However, there are cases in which it is useful to model the muscle morphology in three dimensions, to better examine the effects of muscle shape and size. This is especially important for fossil analyses, where muscle force is estimated from the reconstructed muscle morphology (rather than based on data collected in vivo). The two main aims of this paper are as follows. First, we created a new interactive tool in the free open access software Blender to enable interactive three-dimensional modelling of muscles. This approach can be applied to both palaeontological and human biomechanics research to generate muscle force magnitudes and lines of action for FEA. Second, we provide a guide on how to use existing Blender tools to reconstruct distorted or incomplete specimens. This guide is aimed at palaeontologists but can also be used by anatomists working with damaged specimens or to test functional implication of hypothetical morphologies.

A toolbox for the retrodeformation and muscle reconstruction of fossil specimens in Blender

Accurate muscle reconstructions can offer new information on the anatomy of fossil organisms and are also important for biomechanical analysis (multibody dynamics and finite-element analysis (FEA)). For the sake of simplicity, muscles are often modelled as point-to-point strands or frustra (cut-off cones) in biomechanical models. However, there are cases in which it is useful to model the muscle morphology in three dimensions, to better examine the effects of muscle shape and size. This is especially important for fossil analyses, where muscle force is estimated from the reconstructed muscle morphology (rather than based on data collected in vivo). The two main aims of this paper are as follows. First, we created a new interactive tool in the free open access software Blender to enable interactive three-dimensional modelling of muscles. This approach can be applied to both palaeontological and human biomechanics research to generate muscle force magnitudes and lines of action for FEA. Second, we provide a guide on how to use existing Blender tools to reconstruct distorted or incomplete specimens. This guide is aimed at palaeontologists but can also be used by anatomists working with damaged specimens or to test functional implication of hypothetical morphologies.

Reconstructing Articular Cartilage in the Australopithecus afarensis Hip Joint and the Need for Modeling Six Degrees of Freedom

The postcranial skeleton of Australopithecus afarensis (AL 288-1) exhibits clear adaptations for bipedality, although there is some debate as to the efficiency and frequency of such upright movement. Some researchers argue that AL 288-1 walked with an erect limb like modern humans do, whilst others advocate for a "bent-hip bent-knee" (BHBK) gait, although in recent years the general consensus favors erect bipedalism. To date, no quantitative method has addressed the articulation of the AL 288-1 hip joint, nor its range of motion (ROM) with consideration for joint spacing, used as a proxy for the thickness of the articular cartilage present within the joint spacing which can affect how a joint moves. Here, we employed ROM mapping methods to estimate the joint spacing of AL 288-1's hip joint in comparison to a modern human and chimpanzee. Nine simulations assessed different joint spacing and tested the range of joint congruency (i.e., ranging from a closely packed socket to loosely packed). We further evaluated the sphericity of the femoral head and whether three rotational degrees of freedom (DOFs) sufficiently captures the full ROM or if translational DOFs must be included. With both setups, we found that the AL 288-1 hip was unlikely to be highly congruent (as it is in modern humans) because this would severely restrict hip rotational movement and would severely limit the capability for both bipedality and even arboreal locomotion. Rather, the hip was more cartilaginous than it is in the modern humans, permitting the hip to rotate into positions necessitated by both terrestrial and arboreal movements. Rotational-only simulations found that AL 288-1 was unable to extend the hip like modern humans, forcing the specimen to employ a BHBK style of walking, thus contradicting 40+ years of previous research into the locomotory capabilities of AL 288-1. Therefore, we advocate that differences in the sphericity of the AL 288-1 femoral head with that of a modern human necessitates all six DOFs to be included in which AL 288-1 could osteologically extend the hip to facilitate a human-like gait.

A therian mammal with sprawling kinematics? Gait and 3D forelimb X-ray motion analysis in tamanduas

Therian mammals are known to move their forelimbs in a parasagittal plane, retracting the mobilised scapula during stance phase. Non-cursorial therian mammals often abduct the elbow out of the shoulder-hip parasagittal plane. This is especially prominent in Tamandua (Xenarthra), which suggests they employ aspects of sprawling (e.g., lizard-like-) locomotion. Here, we test if tamanduas use sprawling forelimb kinematics, i.e., a largely immobile scapula with pronounced lateral spine bending and long-axis rotation of the humerus. We analyse high speed videos and use X-ray motion analysis of tamanduas walking and balancing on branches of varying inclinations and provide a quantitative characterization of gaits and forelimb kinematics. Tamanduas displayed lateral sequence lateral-couplets gaits on flat ground and horizontal branches, but increased diagonality on steeper in- and declines, resulting in lateral sequence diagonal-couplets gaits. This result provides further evidence for high diagonality in arboreal species, likely maximising stability in arboreal environments. Further, the results reveal a mosaic of sprawling and parasagittal kinematic characteristics. The abducted elbow results from a constantly internally rotated scapula about its long axis and a retracted humerus. Scapula retraction contributes considerably to stride length. However, lateral rotation in the pectoral region of the spine (range: 21°) is higher than reported for other therian mammals. Instead, it is similar to skinks and alligators, indicating an aspect generally associated with sprawling locomotion is characteristic for forelimb kinematics of tamanduas. Our study contributes to a growing body of evidence of highly variable non-cursorial therian mammal locomotor kinematics.

A proposed standard for quantifying 3-D hindlimb joint poses in living and extinct archosaurs

The last common ancestor of birds and crocodylians plus all of its descendants (clade Archosauria) dominated terrestrial Mesozoic ecosystems, giving rise to disparate body plans, sizes, and modes of locomotion. As in the fields of vertebrate morphology and paleontology more generally, studies of archosaur skeletal structure have come to depend on tools for acquiring, measuring, and exploring three‐dimensional (3‐D) digital models. Such models, in turn, form the basis for many analyses of musculoskeletal function. A set of shared conventions for describing 3‐D pose (joint or limb configuration) and 3‐D kinematics (change in pose through time) is essential for fostering comparison of posture/movement among such varied species, as well as for maximizing communication among scientists. Following researchers in human biomechanics, we propose a standard methodological approach for measuring the relative position and orientation of the major segments of the archosaur pelvis and hindlimb in 3‐D. We describe the construction of anatomical and joint coordinate systems using the extant guineafowl and alligator as examples. Our new standards are then applied to three extinct taxa sampled from the wider range of morphological, postural, and kinematic variation that has arisen across >250 million years of archosaur evolution. These proposed conventions, and the founding principles upon which they are based, can also serve as starting points for measuring poses between elements within a hindlimb segment, for establishing coordinate systems in the forelimb and axial skeleton, or for applying our archosaurian system more broadly to different vertebrate clades.

Paleobiological reconstructions of articular function require all six degrees of freedom

Paleobiologists typically exclude impossible joint poses from reconstructions of extinct animals by estimating the rotational range of motion (ROM) of fossil joints. However, this ubiquitous practice carries the assumption that osteological estimates of ROM consistently overestimate true joint mobility. Because studies founded on ROM-based exclusion have contributed substantially to our understanding of functional and locomotor evolution, it is critical that this assumption be tested. Here, we evaluate whether ROM-based exclusion is, as currently implemented, a reliable strategy. We measured the true mobilities of five intact cadaveric joints using marker-based X-ray Reconstruction of Moving Morphology and compared them to virtual osteological estimates of ROM made allowing (a) only all three rotational, (b) all three rotational and one translational, and (c) all three rotational and all three translational degrees of freedom. We found that allowing combinations of motions in all six degrees of freedom is necessary to ensure that true mobility is always successfully captured. In other words, failing to include joint translations in ROM analyses results in the erroneous exclusion of many joint poses that are possible in life. We therefore suggest that the functional and evolutionary conclusions of existing paleobiological reconstructions may be weakened or even overturned when all six degrees of freedom are considered. We offer an expanded methodological framework for virtual ROM estimation including joint translations and outline recommendations for future ROM-based exclusion studies.

Validation of an Echidna Forelimb Musculoskeletal Model Using XROMM and diceCT

In evolutionary biomechanics, musculoskeletal computer models of extant and extinct taxa are often used to estimate joint range of motion (ROM) and muscle moment arms (MMAs), two parameters which form the basis of functional inferences. However, relatively few experimental studies have been performed to validate model outputs. Previously, we built a model of the short-beaked echidna (Tachyglossus aculeatus) forelimb using a traditional modelling workflow, and in this study we evaluate its behaviour and outputs using experimental data. The echidna is an unusual animal representing an edge-case for model validation: it uses a unique form of sprawling locomotion, and possesses a suite of derived anatomical features, in addition to other features reminiscent of extinct early relatives of mammals. Here we use diffusible iodine-based contrast-enhanced computed tomography (diceCT) alongside digital and traditional dissection to evaluate muscle attachments, modelled muscle paths, and the effects of model alterations on the MMA outputs. We use X-ray Reconstruction of Moving Morphology (XROMM) to compare ex vivo joint ROM to model estimates based on osteological limits predicted via single-axis rotation, and to calculate experimental MMAs from implanted muscles using a novel geometric method. We also add additional levels of model detail, in the form of muscle architecture, to evaluate how muscle torque might alter the inferences made from MMAs alone, as is typical in evolutionary studies. Our study identifies several key findings that can be applied to future models. 1) A light-touch approach to model building can generate reasonably accurate muscle paths, and small alterations in attachment site seem to have minimal effects on model output. 2) Simultaneous movement through multiple degrees of freedom, including rotations and translation at joints, are necessary to ensure full joint ROM is captured; however, single-axis ROM can provide a reasonable approximation of mobility depending on the modelling objectives. 3) Our geometric method of calculating MMAs is consistent with model-predicted MMAs calculated via partial velocity, and is a potentially useful tool for others to create and validate musculoskeletal models. 4) Inclusion of muscle architecture data can change some functional inferences, but in many cases reinforced conclusions based on MMA alone.

A multimethod approach to the differentiation of enthesis bone microstructure based on soft tissue type

Whereas there is a wealth of research studying the nature of various soft tissues that attach to bone, comparatively little research focuses on the bone's microscopic properties in the area where these tissues attach. Using scanning electron microscopy to generate a dataset of 1600 images of soft tissue attachment sites, an image classification program with novel convolutional neural network architecture can categorize images of attachment areas by soft tissue type based on observed patterns in microstructure morphology. Using stained histological thin section and liquid crystal cross-polarized microscopy, it is determined that soft tissue type can be quantitatively determined from the microstructure. The primary diagnostic characters are the orientation of collagen fibers and heterogeneity of collagen density throughout the attachment area thickness. These determinations are made across broad taxonomic sampling and multiple skeletal elements.