Cancellous bone and theropod dinosaur locomotion. Part II-a new approach to inferring posture and locomotor biomechanics in extinct tetrapod vertebrates

Reconstruction of the pelvic girdle and hindlimb musculature of the early tetanurans Piatnitzkysauridae (Theropoda, Megalosauroidea)

Piatnitzkysauridae were Jurassic theropods that represented the earliest diverging branch of Megalosauroidea, being one of the earliest lineages to have evolved moderate body size. This clade's typical body size and some unusual anatomical features raise questions about locomotor function and specializations to aid in body support; and other palaeobiological issues. Biomechanical models and simulations can illuminate how extinct animals may have moved, but require anatomical data as inputs. With a phylogenetic context, osteological evidence, and neontological data on anatomy, it is possible to infer the musculature of extinct taxa. Here, we reconstructed the hindlimb musculature of Piatnitzkysauridae (Condorraptor, Marshosaurus, and Piatnitzkysaurus). We chose this clade for future usage in biomechanics, for comparisons with myological reconstructions of other theropods, and for the resulting evolutionary implications of our reconstructions; differential preservation affects these inferences, so we discuss these issues as well. We considered 32 muscles in total: for Piatnitzkysaurus, the attachments of 29 muscles could be inferred based on the osteological correlates; meanwhile, in Condorraptor and Marshosaurus, we respectively inferred 21 and 12 muscles. We found great anatomical similarity within Piatnitzkysauridae, but differences such as the origin of M. ambiens and size of M. caudofemoralis brevis are present. Similarities were evident with Aves, such as the division of the M. iliofemoralis externus and M. iliotrochantericus caudalis and a broad depression for the M. gastrocnemius pars medialis origin on the cnemial crest. Nevertheless, we infer plesiomorphic features such as the origins of M. puboischiofemoralis internus 1 around the "cuppedicus" fossa and M. ischiotrochantericus medially on the ischium. As the first attempt to reconstruct muscles in early tetanurans, our study allows a more complete understanding of myological evolution in theropod pelvic appendages.

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.

Locomotor and postural diversity among reptiles viewed through the prism of femoral microanatomy: Palaeobiological implications for some Permian and Mesozoic taxa

The water-to-land transition by the first tetrapod vertebrates represents a key stage in their evolution. Selection pressures exerted by this new environment on animals led to the emergence of new locomotor and postural strategies that favoured access to different ecological niches and contributed to their evolutionary success. Today, amniotes show great locomotor and postural diversity, particularly among Reptilia, whose extant representatives include parasagittally locomoting erect and crouched bipeds (birds), sub-parasagittal 'semi-erect' quadrupeds (crocodylians) and sprawling quadrupeds (squamates and turtles). But the different steps leading to such diversity remain enigmatic and the type of locomotion adopted by many extinct species raises questions. This is notably the case of certain Triassic taxa such as Euparkeria and Marasuchus. The exploration of the bone microanatomy in reptiles could help to overcome these uncertainties. Indeed, this locomotor and postural diversity is accompanied by great microanatomical disparity. On land, the bones of the appendicular skeleton support the weight of the body and are subject to multiple constraints that partly shape their external and internal morphology. Here we show how microanatomical parameters measured in cross-section, such as bone compactness or the position of the medullocortical transition, can be related to locomotion. We hypothesised that this could be due to variations in cortical thickness. Using statistical methods that take phylogeny into account (phylogenetic flexible discriminant analyses), we develop different models of locomotion from a sample of femur cross-sections from 51 reptile species. We use these models to infer locomotion and posture in 7 extinct reptile taxa for which they remain debated or not fully clear. Our models produced reliable inferences for taxa that preceded and followed the quadruped/biped and sprawling/erect transitions, notably within the Captorhinidae and Dinosauria. For taxa contemporary with these transitions, such as Terrestrisuchus and Marasuchus, the inferences are more questionable. We use linear models to investigate the effect of body mass and functional ecology on our inference models. We show that body mass seems to significantly impact our model predictions in most cases, unlike the functional ecology. Finally, we illustrate how taphonomic processes can impact certain microanatomical parameters, especially the eccentricity of the section, while addressing some other potential limitations of our methods. Our study provides insight into the evolution of enigmatic locomotion in various early reptiles. Our models and methods could be used by palaeontologists to infer the locomotion and posture in other extinct reptile taxa, especially when considered in combination with other lines of evidence.

Quantitative biomechanical assessment of locomotor capabilities of the stem archosaur Euparkeria capensis

Birds and crocodylians are the only remaining members of Archosauria (ruling reptiles) and they exhibit major differences in posture and gait, which are polar opposites in terms of locomotor strategies. Their broader lineages (Avemetatarsalia and Pseudosuchia) evolved a multitude of locomotor modes in the Triassic and Jurassic periods, including several occurrences of bipedalism. The exact timings and frequencies of bipedal origins within archosaurs, and thus their ancestral capabilities, are contentious. It is often suggested that archosaurs ancestrally exhibited some form of bipedalism. Euparkeria capensis is a central taxon for the investigation of locomotion in archosaurs due to its phylogenetic position and intermediate skeletal morphology, and is argued to be representative of facultative bipedalism in this group. However, no studies to date have biomechanically tested if bipedality was feasible in Eupakeria. Here, we use musculoskeletal models and static simulations in its hindlimb to test the influences of body posture and muscle parameter estimation methods on locomotor potential. Our analyses show that the resulting negative pitching moments around the centre of mass were prohibitive to sustainable bipedality. We conclude that it is unlikely that Euparkeria was facultatively bipedal, and was probably quadrupedal, rendering the inference of ancestral bipedal abilities in Archosauria unlikely.

The efficacy of computed tomography scanning versus surface scanning in 3D finite element analysis

Finite element analysis (FEA) is a commonly used application in biomechanical studies of both extant and fossil taxa to assess stress and strain in solid structures such as bone. FEA can be performed on 3D structures that are generated using various methods, including computed tomography (CT) scans and surface scans. While previous palaeobiological studies have used both CT scanned models and surface scanned models, little research has evaluated to what degree FE results may vary when CT scans and surface scans of the same object are compared. Surface scans do not preserve the internal geometries of 3D structures, which are typically preserved in CT scans. Here, we created 3D models from CT scans and surface scans of the same specimens (crania and mandibles of a Nile crocodile, a green sea turtle, and a monitor lizard) and performed FEA under identical loading parameters. It was found that once surface scanned models are solidified, they output stress and strain distributions and model deformations comparable to their CT scanned counterparts, though differing by notable stress and strain magnitudes in some cases, depending on morphology of the specimen and the degree of reconstruction applied. Despite similarities in overall mechanical behaviour, surface scanned models can differ in exterior shape compared to CT scanned models due to inaccuracies that can occur during scanning and reconstruction, resulting in local differences in stress distribution. Solid-fill surface scanned models generally output lower stresses compared to CT scanned models due to their compact interiors, which must be accounted for in studies that use both types of scans.

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.

Computational modelling of muscle fibre operating ranges in the hindlimb of a small ground bird (Eudromia elegans), with implications for modelling locomotion in extinct species

The arrangement and physiology of muscle fibres can strongly influence musculoskeletal function and whole-organismal performance. However, experimental investigation of muscle function during in vivo activity is typically limited to relatively few muscles in a given system. Computational models and simulations of the musculoskeletal system can partly overcome these limitations, by exploring the dynamics of muscles, tendons and other tissues in a robust and quantitative fashion. Here, a high-fidelity, 26-degree-of-freedom musculoskeletal model was developed of the hindlimb of a small ground bird, the elegant-crested tinamou (Eudromia elegans, ~550 g), including all the major muscles of the limb (36 actuators per leg). The model was integrated with biplanar fluoroscopy (XROMM) and forceplate data for walking and running, where dynamic optimization was used to estimate muscle excitations and fibre length changes throughout both gaits. Following this, a series of static simulations over the total range of physiological limb postures were performed, to circumscribe the bounds of possible variation in fibre length. During gait, fibre lengths for all muscles remained between 0.5 to 1.21 times optimal fibre length, but operated mostly on the ascending limb and plateau of the active force-length curve, a result that parallels previous experimental findings for birds, humans and other species. However, the ranges of fibre length varied considerably among individual muscles, especially when considered across the total possible range of joint excursion. Net length change of muscle-tendon units was mostly less than optimal fibre length, sometimes markedly so, suggesting that approaches that use muscle-tendon length change to estimate optimal fibre length in extinct species are likely underestimating this important parameter for many muscles. The results of this study clarify and broaden understanding of muscle function in extant animals, and can help refine approaches used to study extinct species.

3D hindlimb joint mobility of the stem-archosaur Euparkeria capensis with implications for postural evolution within Archosauria

Triassic archosaurs and stem-archosaurs show a remarkable disparity in their ankle and pelvis morphologies. However, the implications of these different morphologies for specific functions are still poorly understood. Here, we present the first quantitative analysis into the locomotor abilities of a stem-archosaur applying 3D modelling techniques. μCT scans of multiple specimens of Euparkeria capensis enabled the reconstruction and three-dimensional articulation of the hindlimb. The joint mobility of the hindlimb was quantified in 3D to address previous qualitative hypotheses regarding the stance of Euparkeria. Our range of motion analysis implies the potential for an erect posture, consistent with the hip morphology, allowing the femur to be fully adducted to position the feet beneath the body. A fully sprawling pose appears unlikely but a wide range of hip abduction remained feasible-the hip appears quite mobile. The oblique mesotarsal ankle joint in Euparkeria implies, however, a more abducted hindlimb. This is consistent with a mosaic of ancestral and derived osteological characters in the hindlimb, and might suggest a moderately adducted posture for Euparkeria. Our results support a single origin of a pillar-erect hip morphology, ancestral to Eucrocopoda that preceded later development of a hinge-like ankle joint and a more erect hindlimb posture.

Differing trabecular bone architecture in dinosaurs and mammals contribute to stiffness and limits on bone strain

The largest dinosaurs were enormous animals whose body mass placed massive gravitational loads on their skeleton. Previous studies investigated dinosaurian bone strength and biomechanics, but the relationships between dinosaurian trabecular bone architecture and mechanical behavior has not been studied. In this study, trabecular bone samples from the distal femur and proximal tibia of dinosaurs ranging in body mass from 23-8,000 kg were investigated. The trabecular architecture was quantified from micro-computed tomography scans and allometric scaling relationships were used to determine how the trabecular bone architectural indices changed with body mass. Trabecular bone mechanical behavior was investigated by finite element modeling. It was found that dinosaurian trabecular bone volume fraction is positively correlated with body mass similar to what is observed for extant mammalian species, while trabecular spacing, number, and connectivity density in dinosaurs is negatively correlated with body mass, exhibiting opposite behavior from extant mammals. Furthermore, it was found that trabecular bone apparent modulus is positively correlated with body mass in dinosaurian species, while no correlation was observed for mammalian species. Additionally, trabecular bone tensile and compressive principal strains were not correlated with body mass in mammalian or dinosaurian species. Trabecular bone apparent modulus was positively correlated with trabecular spacing in mammals and positively correlated with connectivity density in dinosaurs, but these differential architectural effects on trabecular bone apparent modulus limit average trabecular bone tissue strains to below 3,000 microstrain for estimated high levels of physiological loading in both mammals and dinosaurs.

The use of extruded finite-element models as a novel alternative to tomography-based models: a case study using early mammal jaws

Finite-element (FE) analysis has been used in palaeobiology to assess the mechanical performance of the jaw. It uses two types of models: tomography-based three-dimensional (3D) models (very accurate, not always accessible) and two-dimensional (2D) models (quick and easy to build, good for broad-scale studies, cannot obtain absolute stress and strain values). Here, we introduce extruded FE models, which provide fairly accurate mechanical performance results, while remaining low-cost, quick and easy to build. These are simplified 3D models built from lateral outlines of a relatively flat jaw and extruded to its average width. There are two types: extruded (flat mediolaterally) and enhanced extruded (accounts for width differences in the ascending ramus). Here, we compare mechanical performance values resulting from four types of FE models (i.e. tomography-based 3D, extruded, enhanced extruded and 2D) in Morganucodon and Kuehneotherium. In terms of absolute values, both types of extruded model perform well in comparison to the tomography-based 3D models, but enhanced extruded models perform better. In terms of overall patterns, all models produce similar results. Extruded FE models constitute a viable alternative to the use of tomography-based 3D models, particularly in relatively flat bones.

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.

Cancellous bone and theropod dinosaur locomotion. Part I-an examination of cancellous bone architecture in the hindlimb bones of theropods

This paper is the first of a three-part series that investigates the architecture of cancellous ('spongy') bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and has previously been used to infer locomotor biomechanics in extinct tetrapod vertebrates, especially primates. Despite great promise, cancellous bone architecture has remained little utilized for investigating locomotion in many other extinct vertebrate groups, such as dinosaurs. Documentation and quantification of architectural patterns across a whole bone, and across multiple bones, can provide much information on cancellous bone architectural patterns and variation across species. Additionally, this also lends itself to analysis of the musculoskeletal biomechanical factors involved in a direct, mechanistic fashion. On this premise, computed tomographic and image analysis techniques were used to describe and analyse the three-dimensional architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs for the first time. A comprehensive survey across many extant and extinct species is produced, identifying several patterns of similarity and contrast between groups. For instance, more stemward non-avian theropods (e.g. ceratosaurs and tyrannosaurids) exhibit cancellous bone architectures more comparable to that present in humans, whereas species more closely related to birds (e.g. paravians) exhibit architectural patterns bearing greater similarity to those of extant birds. Many of the observed patterns may be linked to particular aspects of locomotor biomechanics, such as the degree of hip or knee flexion during stance and gait. A further important observation is the abundance of markedly oblique trabeculae in the diaphyses of the femur and tibia of birds, which in large species produces spiralling patterns along the endosteal surface. Not only do these observations provide new insight into theropod anatomy and behaviour, they also provide the foundation for mechanistic testing of locomotor hypotheses via musculoskeletal biomechanical modelling.

Cancellous bone and theropod dinosaur locomotion. Part III-Inferring posture and locomotor biomechanics in extinct theropods, and its evolution on the line to birds

This paper is the last of a three-part series that investigates the architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is highly sensitive to its prevailing mechanical environment, and may therefore help further understanding of locomotor biomechanics in extinct tetrapod vertebrates such as dinosaurs. Here in Part III, the biomechanical modelling approach derived previously was applied to two species of extinct, non-avian theropods, Daspletosaurus torosus and Troodon formosus. Observed cancellous bone architectural patterns were linked with quasi-static, three-dimensional musculoskeletal and finite element models of the hindlimb of both species, and used to derive characteristic postures that best aligned continuum-level principal stresses with cancellous bone fabric. The posture identified for Daspletosaurus was largely upright, with a subvertical femoral orientation, whilst that identified for Troodon was more crouched, but not to the degree observed in extant birds. In addition to providing new insight on posture and limb articulation, this study also tested previous hypotheses of limb bone loading mechanics and muscular control strategies in non-avian theropods, and how these aspects evolved on the line to birds. The results support the hypothesis that an upright femoral posture is correlated with bending-dominant bone loading and abduction-based muscular support of the hip, whereas a crouched femoral posture is correlated with torsion-dominant bone loading and long-axis rotation-based muscular support. Moreover, the results of this study also support the inference that hindlimb posture, bone loading mechanics and muscular support strategies evolved in a gradual fashion along the line to extant birds.