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.

Technical note: A volumetric method for measuring the longitudinal arch of human tracks and feet.

Fossil footprints (i.e., tracks) were believed to document arch anatomical evolution, although our recent work has shown that track arches record foot kinematics instead. Analyses of track arches can thereby inform the evolution of human locomotion, although quantifying this 3-D aspect of track morphology is difficult. Here, we present a volumetric method for measuring the arches of 3-D models of human tracks and feet, using both Autodesk Maya and Blender software. The method involves generation of a 3-D object that represents the space beneath the longitudinal arch, and measurement of that arch object's geometry and spatial orientation. We provide relevant tools and guidance for users to apply this technique to their own data. We present three case studies to demonstrate potential applications. These include, (1) measuring the arches of static and dynamic human feet, (2) comparing the arches of human tracks with the arches of the feet that made them, and (3) direct comparisons of human track and foot arch morphology throughout simulated track formation. The volumetric measurement tool proved robust for measuring 3-D models of human tracks and feet, in static and dynamic contexts. This tool enables researchers to quantitatively compare arches of fossil hominin tracks, in order to derive biomechanical interpretations from them, and/or offers a different approach for quantifying foot morphology in living humans.

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.

The dual function of prokinesis in the feeding and locomotor systems of parrots.

Prokinesis, a mode of avian cranial kinesis involving motion between the neurocranium and upper beak, has long been investigated in biomechanical analyses of avian feeding and drinking. However, the modern avian beak is also used for non-feeding functions. Here, we investigate the dual function of prokinesis in the feeding and locomotor systems of the rosy-faced lovebird (Agapornis roseicollis). Lovebirds and other parrots utilize their beak both during feeding and as a third limb during vertical climbing. Thus, we experimentally measured both force-generating potential and movement of the rosy-faced lovebird mandible and maxilla (via prokinetic flexion of the craniofacial hinge) during tripedal climbing and mandibular/maxillary adduction. We found that whereas the maxilla is primarily responsible for generating force during locomotion, the mandible is primarily responsible for generating force during forceful jaw adduction, hinting at a remarkable capacity to alter prokinetic function with differing neuromuscular control. The ability of the prokinetic apparatus to perform functions with competing optimality criteria via modulation of motor control illustrates the functional plasticity of the avian cranial kinesis and sheds new light on the adaptive significance of cranial mobility.

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.

Multi-Joint Analysis of Pose Viability Supports the Possibility of Salamander-Like Hindlimb Configurations in the Permian Tetrapod Eryops megacephalus.

Salamanders are often used as analogs for early tetrapods in paleontological reconstructions of locomotion. However, concerns have been raised about whether this comparison is justifiable, necessitating comparisons of a broader range of early tetrapods with salamanders. Here, we test whether the osteological morphology of the hindlimb in the early tetrapod (temnospondyl amphibian) Eryops megacephalus could have facilitated the sequence of limb configurations used by salamanders during terrestrial locomotion. To do so, we present a new method that enables the examination of full limb configurations rather than isolated joint poses. Based on this analysis, we conclude that E. megacephalus may indeed have been capable of salamander-like hindlimb kinematics. Our method facilitates the holistic visual comparison of limb configurations between taxa without reliance on the homology of coordinate system definitions, and can thus be applied to facilitate various comparisons between extinct and extant taxa, spanning the diversity of locomotion both past and present.

Advances and Challenges in Paleobiological Reconstructions of Joint Mobility.

Paleobiological reconstructions of joint mobility are an essential component of functional analyses of extinct animals. Over the past half-decade, the methods underlying mobility studies have advanced rapidly in three main areas: increasing complexity of virtual joint manipulation, formalizing pose viability criteria, and constructing more rigorous quantitative frameworks. Here we contextualize and review the recent history of this field, and call attention to remaining challenges and potential future directions. Additionally, we make available and describe a set of user-friendly scripts for the animation software Autodesk Maya. In doing so, we aim to make many of the latest approaches for virtual mobility reconstruction more easily accessible to other researchers, encouraging their broader adoption and collaborative improvement.

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.

A new role for joint mobility in reconstructing vertebrate locomotor evolution.

Reconstructions of movement in extinct animals are critical to our understanding of major transformations in vertebrate locomotor evolution. Estimates of joint range of motion (ROM) have long been used to exclude anatomically impossible joint poses from hypothesized gait cycles. Here we demonstrate how comparative ROM data can be harnessed in a different way to better constrain locomotor reconstructions. As a case study, we measured nearly 600,000 poses from the hindlimb joints of the Helmeted Guineafowl and American alligator, which represent an extant phylogenetic bracket for the archosaurian ancestor and its pseudosuchian (crocodilian line) and ornithodiran (bird line) descendants. We then used joint mobility mapping to search for a consistent relationship between full potential joint mobility and the subset of joint poses used during locomotion. We found that walking and running poses are predictably located within full mobility, revealing additional constraints for reconstructions of extinct archosaurs. The inferential framework that we develop here can be expanded to identify ROM-based constraints for other animals and, in turn, will help to unravel the history of vertebrate locomotor evolution.

A coordinate-system-independent method for comparing joint rotational mobilities.

Three-dimensional studies of range of motion currently plot joint poses in a 'Euler space' whose axes are angles measured in the joint's three rotational degrees of freedom. Researchers then compute the volume of a pose cloud to measure rotational mobility. However, pairs of poses that are equally different from one another in orientation are not always plotted equally far apart in Euler space. This distortion causes a single joint's mobility to change when measured based on different joint coordinate systems and precludes fair comparison among joints. Here, we present two alternative spaces inspired by a 16th century map projection - cosine-corrected and sine-corrected Euler spaces - that allow coordinate-system-independent comparison of joint rotational mobility. When tested with data from a bird hip joint, cosine-corrected Euler space demonstrated a 10-fold reduction in variation among mobilities measured from three joint coordinate systems. This new quantitative framework enables previously intractable, comparative studies of articular function.

Integrating XMALab and DeepLabCut for high-throughput XROMM.

Marker tracking is a major bottleneck in studies involving X-ray reconstruction of moving morphology (XROMM). Here, we tested whether DeepLabCut, a new deep learning package built for markerless tracking, could be applied to videoradiographic data to improve data processing throughput. Our novel workflow integrates XMALab, the existing XROMM marker tracking software, and DeepLabCut while retaining each program's utility. XMALab is used for generating training datasets, error correction and 3D reconstruction, whereas the majority of marker tracking is transferred to DeepLabCut for automatic batch processing. In the two case studies that involved an in vivo behavior, our workflow achieved a 6 to 13-fold increase in data throughput. In the third case study, which involved an acyclic, post-mortem manipulation, DeepLabCut struggled to generalize to the range of novel poses and did not surpass the throughput of XMALab alone. Deployed in the proper context, this new workflow facilitates large scale XROMM studies that were previously precluded by software constraints.

Contrast-enhanced XROMM reveals in vivo soft tissue interactions in the hip of Alligator mississippiensis.

Extant archosaurs exhibit highly divergent articular soft tissue anatomies between avian and crocodilian lineages. However, the general lack of understanding of the dynamic interactions among archosaur joint soft tissues has hampered further inferences about the function and evolution of these joints. Here we use contrast-enhanced computed tomography to generate 3D surface models of the pelvis, femora, and hip joint soft tissues in an extant archosaur, the American alligator. The hip joints were then animated using marker-based X-Ray Reconstruction of Moving Morphology (XROMM) to visualize soft tissue articulation during forward terrestrial locomotion. We found that the anatomical femoral head of the alligator travels beyond the cranial extent of the bony acetabulum and does not act as a central pivot, as has been suggested for some extinct archosaurs. Additionally, the fibrocartilaginous surfaces of the alligator's antitrochanter and femoral neck remain engaged during hip flexion and extension, similar to the articulation between homologous structures in birds. Moreover, the femoral insertion of the ligamentum capitis moves dorsoventrally against the membrane-bound portion of the medial acetabular wall, suggesting that the inner acetabular foramen constrains the excursion of this ligament as it undergoes cyclical stretching during the step cycle. Finally, the articular surface of the femoral cartilage model interpenetrates with those of the acetabular labrum and antitrochanter menisci; we interpret such interpenetration as evidence of compressive deformation of the labrum and of sliding movement of the menisci. Our data illustrate the utility of XROMM for studying in vivo articular soft tissue interactions. These results also allow us to propose functional hypotheses for crocodilian hip joint soft tissues, expanding our knowledge of vertebrate connective tissue biology and the role of joint soft tissues in locomotor behavior.

A Practical Guide to Measuring Ex vivo Joint Mobility Using XROMM.

X-Ray Reconstruction of Moving Morphology (XROMM), though traditionally used for studies of in vivo skeletal kinematics, can also be used to precisely and accurately measure ex vivo range of motion from cadaveric manipulations. The workflow for these studies is holistically similar to the in vivo XROMM workflow but presents several unique challenges. This paper aims to serve as a practical guide by walking through each step of the ex vivo XROMM process: how to acquire and prepare cadaveric specimens, how to manipulate specimens to collect X-ray data, and how to use these data to compute joint rotational mobility. Along the way, it offers recommendations for best practices and for avoiding common pitfalls to ensure a successful study.

Intra-oropharyngeal food transport and swallowing in white-spotted bamboo sharks.

Despite the importance of intraoral food transport and swallowing, relatively few studies have examined the biomechanics of these behaviors in non-tetrapods, which lack a muscular tongue. Studies show that elasmobranch and teleost fishes generate water currents as a 'hydrodynamic tongue' that presumably transports food towards and into the esophagus. However, it remains largely unknown how specific musculoskeletal motions during transport correspond to food motion. Previous studies of white-spotted bamboo sharks (Chiloscyllium plagiosum) hypothesized that motions of the hyoid, branchial arches and pectoral girdle, generate caudal motion of the food through the long oropharynx of modern sharks. To test these hypotheses, we measured food and cartilage motion with XROMM during intra-oropharyngeal transport and swallowing (N=3 individuals, 2-3 trials per individual). After entering the mouth, food does not move smoothly toward the esophagus, but rather moves in distinct steps with relatively little retrograde motion. Caudal food motion coincides with hyoid elevation and a closed mouth, supporting earlier studies showing that hyoid motion contributes to intra-oropharyngeal food transport by creating caudally directed water currents. Little correspondence between pectoral girdle and food motion was found, indicating minimal contribution of pectoral girdle motion. Transport speed was fast as food entered the mouth, slower and step-wise through the pharyngeal region and then fast again as it entered the esophagus. The food's static periods in the step-wise motion and its high velocity during swallowing could not be explained by hyoid or girdle motion, suggesting these sharks may also use the branchial arches for intra-oropharyngeal transport and swallowing.

Rolling of the jaw is essential for mammalian chewing and tribosphenic molar function.

Over the past two centuries, mammalian chewing and related anatomical features have been among the most discussed of all vertebrate evolutionary innovations1-3. Chief among these features are two characters: the dentary-only mandible, and the tribosphenic molar with its triangulated upper cusps and lower talonid basin3-5. The flexible mandibular joint and the unfused symphysis of ancestral mammals-in combination with transformations of the adductor musculature and palate-are thought to have permitted greater mobility of each lower jaw, or hemimandible6,7. Following the appearance of precise dental occlusion near the origin of the mammalian crown8,9, therians evolved a tribosphenic molar with a craggy topography that is presumed to have been used to catch, cut and crush food. Here we describe the ancestral tribosphenic therian chewing stroke, as conserved in the short-tailed opossum Monodelphis domestica: it is a simple symmetrical sequence of lower tooth-row eversion and inversion during jaw opening and closing, respectively, enacted by hemimandibular long-axis rotation. This sequence is coupled with an eversion-inversion rotational grinding stroke. We infer that the ancestral therian chewing stroke relied heavily on long-axis rotation, including symmetrical eversion and inversion (inherited from the first mammaliaforms) as well as a mortar-and-pestle rotational grinding stroke that was inherited from stem therians along with the tribosphenic molar. The yaw-dominated masticatory cycle of primates, ungulates and other bunodont therians is derived; it is necessitated by a secondarily fused jaw symphysis, and permitted by the reduction of high, interlocking cusps10-12. The development of an efficient masticatory system-culminating in the tribosphenic apparatus-allowed early mammals to begin the process of digestion by shearing and crushing food into small boli instead of swallowing larger pieces in the reptilian manner, which necessitates a long, slow and wholly chemical breakdown. The vast diversity of mammalian teeth has emerged from the basic tribosphenic groundplan13.

ROM mapping of ligamentous constraints on avian hip mobility: implications for extinct ornithodirans.

Studies of soft tissue effects on joint mobility in extant animals can help to constrain hypotheses about joint mobility in extinct animals. However, joint mobility must be considered in three dimensions simultaneously, and applications of mobility data to extinct taxa require both a phylogenetically informed reconstruction of articular morphology and justifications for why specific structures' effects on mobility are inferred to be similar. We manipulated cadaveric hip joints of common quail and recorded biplanar fluoroscopic videos to measure a 'ligamentous' range of motion (ROM), which was then compared to an 'osteological' ROM on a ROM map. Nearly 95% of the joint poses predicted to be possible at the hip based on osteological manipulation were rendered impossible by ligamentous constraints. Because the hip joint capsule reliably includes a ventral ligamentous thickening in extant diapsids, the hip abduction of extinct ornithodirans with an offset femoral head and thin articular cartilage was probably similarly constrained by ligaments as that of birds. Consequently, in the absence of extraordinary evidence to the contrary, our analysis casts doubt on the 'batlike' hip pose traditionally inferred for pterosaurs and basal maniraptorans, and underscores that reconstructions of joint mobility based on manipulations of bones alone can be misleading.