Observations and comments on the reliability of muscle reconstruction in fossil vertebrates

Cervical anatomy and its relation to foraging habits in aquatic birds (Aves: Neornithes: Neoaves)

Birds have extremely flexible necks, which help in their search for food. However, studies on the variation in bird cervical anatomy and its relationship with foraging are rare, despite the different habits presented between species. Here, we analyze the anatomy of the neck of aquatic birds and relate it to their foraging strategies. We dissected specimens representing four species of Charadriiformes, 11 species of Phaethoquornithes, and two specimens belonging to the outgroup Telluraves. We chose to emphasize Charadriiformes and Phaethoquornithes because they present several strategies that require cervical mobility and stability. We note that vertebral anatomy and dimensions vary, which affects the shape and size of the soft tissues attached throughout the neck. The synovial cartilage present in the articulatio intercorporalis represents an additional length in the neck, however, this is not longer than that observed in animals with intervertebral discs. Our analysis indicates that birds have a prevalence of dorsoventral movements in the middle of the neck and lateral and rotational movements near the base of the neck, while the region near the head presents a wide range of movement in all directions. Cervical ligaments and muscles throughout the neck provide stability in all segments, although the robustness of the soft tissues indicates that the most caudal portion of the neck is the most stable. The vertebral and soft tissue anatomy is consistent with the extensive mobility in pitching, yaw, and roll movements performed mainly by the head and first segment of the neck during the different foraging of the analyzed birds. Furthermore, the muscles closer to the skull are robust and allow the execution of a variety of habits to capture food in different species. The subsequent cervical segments present differences that explain their reduction in mobility, but they are equally stable.

Appendicular myology of Skorpiovenator bustingorryi: A first attempt to reconstruct pelvic and hindlimb musculature in an abelisaurid theropod

We present the pelvic and hindlimb musculature of the abelisaurid Skorpiovenator bustingorryi, constituting the most comprehensive muscle reconstruction to date in ceratosaur theropods. Using extant phylogenetic bracket method, we reconstructed 39 muscles that can commonly found in extant archosaurs. Through the identification of bone correlates, we recognized thigh and hindlimb muscles including knee extensors, m. iliofibularis, m. flexor tibialis externus, mm. caudofemorales, mm. puboischiofemorales, and crus muscles important in foot extension and flexion (e.g., m. tibialis anterior, mm. gastrocnemii). Also, autopodial intrinsic muscles were reconstructed whose function involve extension (m. extensor digiti 2-4), flexion (mm. flexor digitorum brevis superficialis), interdigital adduction (m. interosseus dorsalis) and abduction (m. interosseous plantaris, m. abductor 4). Abelisaurids like Skorpiovenator show a deep pre- and postacetabular blade of the ilia and enlarged cnemial crests, which would have helped increasing the moment arm of muscles related to hip flexion and hindlimb extension. Also, pedal muscles related to pronation were probably present but reduced (e.g., m. pronator profundus). Despite some gross differences in the autopodial morphology in extant outgroups (e.g., crocodilian metatarsus and avian tarsometatarsus), the present study allows us to hypothesize several pedal muscles in Skorpiovenator. These muscles would not be arranged in tendinous bundles as in Neornithes, but rather the condition would be similar to that of crocodilians with several layers formed by fleshy bellies on the plantar and dorsal aspects of the metatarsus. The musculature of Skorpiovenator is key for future studies concerning abelisaurid biomechanics, including the integration of functional morphology and ichnological data.

Quantifying the relationship between bone and soft tissue measures within the rhesus macaques of Cayo Santiago

OBJECTIVES

Interpretations of the primate and human fossil record often rely on the estimation of somatic dimensions from bony measures. Both somatic and skeletal variation have been used to assess how primates respond to environmental change. However, it is unclear how well skeletal variation matches and predicts soft tissue. Here, we empirically test the relationship between tissues by comparing somatic and skeletal measures using paired measures of pre- and post-mortem rhesus macaques from Cayo Santiago, Puerto Rico.

MATERIALS AND METHODS

Somatic measurements were matched with skeletal dimensions from 105 rhesus macaque individuals to investigate paired signals of variation (i.e., coefficients of variation, sexual dimorphism) and bivariate codependence (reduced major axis regression) in measures of: (1) limb length; (2) joint breadth; and (3) limb circumference. Predictive models for the estimation of soft tissue dimensions from skeletons were built from Ordinary Least Squares regressions.

RESULTS

Somatic and skeletal measurements showed statistically equivalent coefficients of variation and sexual dimorphism as well as high epiphyses-present ordinary least square (OLS) correlations in limb lengths (R2 >0.78, 0.82), joint breadths (R2 >0.74, 0.83) and, to a lesser extent, limb circumference (R2 >0.53, 0.68).

CONCLUSION

Skeletal measurements are good substitutions for somatic values based on population signals of variation. OLS regressions indicate that skeletal correlates are highly predictive of somatic dimensions. The protocols and regression equations established here provide a basis for reliable reconstruction of somatic dimension from catarrhine fossils and validate our ability to compare or combine results of studies based on population data of either hard or soft tissue proxies.

Anatomically grounded estimation of hindlimb muscle sizes in Archosauria

In vertebrates, active movement is driven by muscle forces acting on bones, either directly or through tendinous insertions. There has been much debate over how muscle size and force are reflected by the muscular attachment areas (AAs). Here we investigate the relationship between the physiological cross-sectional area (PCSA), a proxy for the force production of the muscle, and the AA of hindlimb muscles in Nile crocodiles and five bird species. The limbs were held in a fixed position whilst blunt dissection was carried out to isolate the individual muscles. AAs were digitised using a point digitiser, before the muscle was removed from the bone. Muscles were then further dissected and fibre architecture was measured, and PCSA calculated. The raw measures, as well as the ratio of PCSA to AA, were studied and compared for intra-observer error as well as intra- and interspecies differences. We found large variations in the ratio between AAs and PCSA both within and across species, but muscle fascicle lengths are conserved within individual species, whether this was Nile crocodiles or tinamou. Whilst a discriminant analysis was able to separate crocodylian and avian muscle data, the ratios for AA to cross-sectional area for all species and most muscles can be represented by a single equation. The remaining muscles have specific equations to represent their scaling, but equations often have a relatively high success at predicting the ratio of muscle AA to PCSA. We then digitised the muscle AAs of Coelophysis bauri, a dinosaur, to estimate the PCSAs and therefore maximal isometric muscle forces. The results are somewhat consistent with other methods for estimating force production, and suggest that, at least for some archosaurian muscles, that it is possible to use muscle AA to estimate muscle sizes. This method is complementary to other methods such as digital volumetric modelling.

The appendicular myology of Stegoceras validum (Ornithischia: Pachycephalosauridae) and implications for the head-butting hypothesis

In this study, we use an exceptional skeleton of the pachycephalosaur Stegoceras validum (UALVP 2) to inform a comprehensive appendicular muscle reconstruction of the animal, with the goal of better understanding the functional morphology of the pachycephalosaur postcranial skeleton. We find that S. validum possessed a conservative forelimb musculature, particularly in comparison to early saurischian bipeds. By contrast, the pelvic and hind limb musculature are more derived, reflecting peculiarities of the underlying skeletal anatomy. The iliotibialis, ischiocaudalis, and caudofemoralis muscles have enlarged attachment sites and the caudofemoralis has greater leverage owing to the distal displacement of the fourth trochanter along the femur. These larger muscles, in combination with the wide pelvis and stout hind limbs, produced a stronger, more stable pelvic structure that would have proved advantageous during hypothesized intraspecific head-butting contests. The pelvis may have been further stabilized by enlarged sacroiliac ligaments, which stemmed from the unique medial iliac flange of the pachycephalosaurs. Although the pubis of UALVP 2 is not preserved, the pubes of other pachycephalosaurs are highly reduced. The puboischiofemoralis musculature was likely also reduced accordingly, and compensated for by the aforementioned improved pelvic musculature.

Skull Sutures and Cranial Mechanics in the Permian Reptile Captorhinus aguti and the Evolution of the Temporal Region in Early Amniotes

While most early limbed vertebrates possessed a fully-roofed dermatocranium in their temporal skull region, temporal fenestrae and excavations evolved independently at least twice in the earliest amniotes, with several different variations in shape and position of the openings. Yet, the specific drivers behind this evolution have been only barely understood. It has been mostly explained by adaptations of the feeding apparatus as a response to new functional demands in the terrestrial realm, including a rearrangement of the jaw musculature as well as changes in strain distribution. Temporal fenestrae have been retained in most extant amniotes but have also been lost again, notably in turtles. However, even turtles do not represent an optimal analog for the condition in the ancestral amniote, highlighting the necessity to examine Paleozoic fossil material. Here, we describe in detail the sutures in the dermatocranium of the Permian reptile Captorhinus aguti (Amniota, Captorhinidae) to illustrate bone integrity in an early non-fenestrated amniote skull. We reconstruct the jaw adductor musculature and discuss its relation to intracranial articulations and bone flexibility within the temporal region. Lastly, we examine whether the reconstructed cranial mechanics in C. aguti could be treated as a model for the ancestor of fenestrated amniotes. We show that C. aguti likely exhibited a reduced loading in the areas at the intersection of jugal, squamosal, and postorbital, as well as at the contact between parietal and postorbital. We argue that these “weak” areas are prone for the development of temporal openings and may be treated as the possible precursors for infratemporal and supratemporal fenestrae in early amniotes. These findings provide a good basis for future studies on other non-fenestrated taxa close to the amniote base, for example diadectomorphs or other non-diapsid reptiles.

Foreflipper and hindflipper muscle reconstructions of Cryptoclidus eurymerus in comparison to functional analogues: introduction of a myological mechanism for flipper twisting

BACKGROUND

Plesiosaurs, diapsid crown-group Sauropterygia, inhabited the oceans from the Late Triassic to the Late Cretaceous. Their most exceptional characteristic are four hydrofoil-like flippers. The question whether plesiosaurs employed their four flippers in underwater flight, rowing flight, or rowing has not been settled yet. Plesiosaur locomotory muscles have been reconstructed in the past, but neither the pelvic muscles nor the distal fore- and hindflipper musculature have been reconstructed entirely.

METHODS

All plesiosaur locomotory muscles were reconstructed in order to find out whether it is possible to identify muscles that are necessary for underwater flight including those that enable flipper rotation and twisting. Flipper twisting has been proven by hydrodynamic studies to be necessary for efficient underwater flight. So, Cryptoclidus eurymerus fore- and hindflipper muscles and ligaments were reconstructed using the extant phylogenetic bracket (Testudines, Crocodylia, and Lepidosauria) and correlated with osteological features and checked for their functionality. Muscle functions were geometrically derived in relation to the glenoid and acetabulum position. Additionally, myology of functionally analogous Chelonioidea, Spheniscidae, Otariinae, and Cetacea is used to extract general myological adaptations of secondary aquatic tetrapods to inform the phylogenetically inferred muscle reconstructions.

RESULTS

A total of 52 plesiosaur fore- and hindflipper muscles were reconstructed. Amongst these are flipper depressors, elevators, retractors, protractors, and rotators. These muscles enable a fore- and hindflipper downstroke and upstroke, the two sequences that represent an underwater flight flipper beat cycle. Additionally, other muscles were capable of twisting fore- and hindflippers along their length axis during down- and upstroke accordingly. A combination of these muscles that actively aid in flipper twisting and intermetacarpal/intermetatarsal and metacarpodigital/metatarsodigital ligament systems, that passively engage the successive digits, could have accomplished fore-and hindflipper length axis twisting in plesiosaurs that is essential for underwater flight. Furthermore, five muscles that could possibly actively adjust the flipper profiles for efficient underwater flight were found, too.

Osteophytes: The product of convergent evolution

The very reasonable suggestion, that diarthrodial joint and juxta-discal (vertebral centra-marginal) bony overgrowths (referred to as osteophytes) have different etiologies, has eluded previous confirmation. The prevailing perspective is that diarthrodial osteophytes represent the product of compressive forces and that those on the margins of vertebral centra result from traction and therefore are enthesial in derivation. If diarthrodial joint osteophytes result from intrinsic pressures, any surface responses would require transcortical nutritional support, easily recognized by en face microscopic examination. This contrasts with enthesially derived growth, the surface of which is characterized by Sharpey's fiber insertions. These are recognized as inverted cones with a central protrusion on examination of related bone surfaces. We hypothesize that diarthrodial and disc-adjacent osteophytes have a different pathophysiology, distinguishable on the basis of microscopic surface appearance. We pursued microscopic examination of the surfaces of osteophytes present on diarthrodial joints (hip, knee, elbow, costovertebral) and vertebrae (cervical, thoracic, and lumbar) from the CAL Milano Cemetery Skeletal Collection for presence of transcortical channels and the inverted cones of Sharpey's fiber insertions. Examination of 22 diarthrodial joint osteophytes reveals the presence solely of transcortical channels, while examination of 35 vertebral centra marginal osteophytes reveals the presence only of inverted cones. Findings are independent of age, gender, joint affected, position in the spinal column and osteophyte "severity." It is now evidenced that all osteophytes are not created equal. Diarthrodial joint osteophytes are endochondrally derived; vertebral centra osteophytes, enthesial in derivation. Different pathophysiology at least partially explain the clinical character of these processes.

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.

Locomotory behaviour of early tetrapods from Blue Beach, Nova Scotia, revealed by novel microanatomical analysis

Evidence for terrestriality in early tetrapods is fundamentally contradictory. Fossil trackways attributed to early terrestrial tetrapods long predate the first body fossils from the Late Devonian. However, the Devonian body fossils demonstrate an obligatorily aquatic lifestyle. Complicating our understanding of the transition from water to land is a pronounced gap in the fossil record between the aquatic Devonian taxa and presumably terrestrial tetrapods from the later Early Carboniferous. Recent work suggests that an obligatorily aquatic habit persists much higher in the tetrapod tree than previously recognized. Here, we present independent microanatomical data of locomotor capability from the earliest Carboniferous of Blue Beach, Nova Scotia. The site preserves limb bones from taxa representative of Late Devonian to mid-Carboniferous faunas as well as a rich trackway record. Given that bone remodels in response to functional stresses including gravity and ground reaction forces, we analysed both the midshaft compactness profiles and trabecular anisotropy, the latter using a new whole bone approach. Our findings suggest that early tetrapods retained an aquatic lifestyle despite varied limb morphologies, prior to their emergence onto land. These results suggest that trackways attributed to early tetrapods be closely scrutinized for additional information regarding their creation conditions, and demand an expansion of sampling to better identify the first terrestrial tetrapods.

Maniraptoran pelvic musculature highlights evolutionary patterns in theropod locomotion on the line to birds

Locomotion is a fundamental aspect of palaeobiology and often investigated by comparing osteological structures and proportions. Previous studies document a stepwise accumulation of avian-like features in theropod dinosaurs that accelerates in the clade Maniraptora. However, the soft tissues that influenced the skeleton offer another perspective on locomotory adaptations. Examination of the pelvis for osteological correlates of hind limb and tail musculature allowed reconstruction of primary locomotory muscles across theropods and their closest extant relatives. Additionally, the areas of pelvic muscle origins were quantified to measure relative differences within and between taxa, to compare morphological features associated with cursoriality, and offer insight into the evolution of locomotor modules. Locomotory inferences based on myology often corroborate those based on osteology, although they occasionally conflict and indicate greater complexity than previously appreciated. Maniraptoran pelvic musculature underscores previous studies noting the multifaceted nature of cursoriality and suggests that a more punctuated step in caudal decoupling occurred at or near the base of Maniraptora.

Myological reconstruction of the pelvic girdle and hind limb of the giant titanosaurian sauropod dinosaur Dreadnoughtus schrani

Osteological correlates preserve more readily than their soft tissue counterparts in the fossil record; therefore, they can more often provide insight into the soft tissue anatomy of the organism. These insights can in turn elucidate the biology of these extinct organisms. In this study, we reconstruct the pelvic girdle and hind limb musculature of the giant titanosaurian sauropod Dreadnoughtus schrani based on observations of osteological correlates and Extant Phylogenetic Bracket comparisons. Recovered fossils of Dreadnoughtus exhibit remarkably well-preserved, well-developed, and extensive muscle scars. Furthermore, this taxon is significantly larger bodied than any titanosaurian for which a myological reconstruction has previously been performed, rendering this contribution highly informative for the group. All 20 of the muscles investigated in this study are sufficiently well supported to enable reconstruction of at least one division, including reconstruction of the M. ischiocaudalis for the first time in a sauropod dinosaur. In total, 34 osteological correlates were identified on the pelvic girdle and hind limb remains of Dreadnoughtus, allowing the reconstruction of 14 muscles on the basis of Level I or Level II inferences (i.e., not Level I' or Level II' inferences). Comparisons among titanosaurians suggest widespread myological variation, yet potential phylogenetic and other paleobiologic patterns are often obscured by fragmentary preservation, infrequent myological studies, and lack of consensus on the phylogenetic placement of many taxa. However, a ventrolateral accessory process is present on the preacetabular lobe of the ilium in all of the largest titanosauriforms that preserve this skeletal element, suggesting that the presence of this process (representing the origin of the M. puboischiofemoralis internus part II) may be associated with extreme body size. By identifying such myological patterns among titanosauriforms, we can begin to address specific evolutionary and biomechanical questions related to their skeletal anatomy, how they were capable of leaving wide-gauge trackways, and resulting locomotor attributes unique to this clade.

Wing Musculature Reconstruction in Extinct Flightless Auks (Pinguinus and Mancalla) Reveals Incomplete Convergence with Penguins (Spheniscidae) Due to Differing Ancestral States

Despite longstanding interest in convergent evolution, factors that result in deviations from fully convergent phenotypes remain poorly understood. In birds, the evolution of flightless wing-propelled diving has emerged as a classic example of convergence, having arisen in disparate lineages including penguins (Sphenisciformes) and auks (Pan-Alcidae, Charadriiformes). Nevertheless, little is known about the functional anatomy of the wings of flightless auks because all such taxa are extinct, and their morphology is almost exclusively represented by skeletal remains. Here, in order to re-evaluate the extent of evolutionary convergence among flightless wing-propelled divers, wing muscles and ligaments were reconstructed in two extinct flightless auks, representing independent transitions to flightlessness: Pinguinus impennis (a crown-group alcid), and Mancalla (a stem-group alcid). Extensive anatomical data were gathered from dissections of 12 species of extant charadriiforms and 4 aequornithine waterbirds including a penguin. The results suggest that the wings of both flightless auk taxa were characterized by an increased mechanical advantage of wing elevator/retractor muscles, and decreased mobility of distal wing joints, both of which are likely advantageous for wing-propelled diving and parallel similar functional specializations in penguins. However, the conformations of individual muscles and ligaments underlying these specializations differ markedly between penguins and flightless auks, instead resembling those in each respective group's close relatives. Thus, the wings of these flightless wing-propelled divers can be described as convergent as overall functional units, but are incompletely convergent at lower levels of anatomical organization-a result of retaining differing conditions from each group's respective volant ancestors. Detailed investigations such as this one may indicate that, even in the face of similar functional demands, courses of phenotypic evolution are dictated to an important degree by ancestral starting points.

Appendicular myological reconstruction of the forelimb of the giant titanosaurian sauropod dinosaur Dreadnoughtus schrani

Soft tissues are variably preserved in the fossil record with external tissues, such as skin and feathers, more frequently preserved than internal tissues (e.g. muscles). More commonly, soft tissues leave traces of their locations on bones and, for muscles, these clues can be used to reconstruct the musculature of extinct vertebrates, thereby enhancing our understanding of how these organisms moved and the evolution of their locomotor patterns. Herein we reconstruct the forelimb and shoulder girdle musculature of the giant titanosaurian sauropod Dreadnoughtus schrani based on observations of osteological correlates and dissections of taxa comprising the Extant Phylogenetic Bracket of non-avian dinosaurs (crocodilians and birds). Fossils of Dreadnoughtus exhibit remarkably well-preserved, well-developed, and extensive muscle scars. Furthermore, this taxon is significantly larger-bodied than any titanosaurian for which a myological reconstruction has previously been attempted, rendering this myological study highly informative for the clade. In total, 28 muscles were investigated in this study, for which 46 osteological correlates were identified; these osteological correlates allowed the reconstruction of 16 muscles on the basis of Level I or Level II inferences (i.e. not Level I' or Level II' inferences). Comparisons with other titanosaurians suggest widespread myological variation in the clade, although potential phylogenetic patterns are often obscured by fragmentary preservation, infrequent myological studies, and lack of consensus on the systematic position of many taxa. By identifying myological variations within the clade, we can begin to address specific evolutionary and biomechanical questions related to the locomotor evolution in these sauropods.

Elevated activity levels do not influence extrinsic fiber attachment morphology on the surface of muscle-attachment sites

Extrinsic fibers (EFs) are a type of penetrating collagenous fiber, closely related to the periodontal ligament, which help anchor soft tissue into bone. These fibers are associated with muscle attachment sites (entheses). Their size and grouping patterns are thought to be indicative of the loading history of the muscle. EFs are of particular significance in anthropology as potential tools for the reconstruction of behavior from skeletal remains and, specifically, entheses. In this study, we used a mouse model to experimentally test how activity level alters the morphology of EF insertion sites on the bone surface of a fibrocartilaginous enthesis, the biceps brachii insertion. Further, we adapted surface metrological techniques from studies of dental wear to perform automated, quantitative and non-destructive analysis of bone surface histology. Our results show that experimentally increased activity had no significant effect on the quantity or density of EF insertions at the enthesis, nor on the size of those insertions. Although EF presence does indicate muscle attachment, activity did not have an observable effect on EF morphology.

New Reconstruction of Cranial Musculature in Ornithischian Dinosaurs: Implications for Feeding Mechanisms and Buccal Anatomy

The charismatic and diverse ornithischian dinosaurs exhibited some of the most extreme examples of cranial anatomy, inspiring decades of investigation into their muscular anatomy. Current ornithischian jaw muscle reconstructions, although parsimonious, pose concerns of small adductor muscles and caudally displaced insertions relative to mandibular proportions. Here, craniomandibular material of ornithischian genera spanning all subclades is reexamined for osteological correlates indicative of intracranial and oral soft tissues. M. adductor mandibulae externus (mAME) has traditionally been reconstructed as solely inserting along the caudal margin of the coronoid process for jaw closure. Here, a new mAME reconstruction is proposed in derived ornithischians, with the superficial-most mAME layer reconstructed as a rostrolabial expansion of muscle, exiting the cranium rostroventrally beneath a unique, laterally flaring jugal and inserting along the lateral surface of the coronoid process and its rostrally extending, shelf-like labial dentary ridge (LDR). Through previous dental microwear and morphological studies, ceratopsians, hadrosaurids, and ankylosaurs are known to have implemented a major palinal feeding component in their jaw motions, unlike other primarily basal ornithischians. This rostral fan-like extension of muscle in these derived clades would create a greater mandibular support system and mechanical advantage along the labial margin of the jaw, cradling the entire mandible while lifting it up into occlusion and retracting it. In hadrosaurids and ankylosaurs, this rostrolabially expanding muscle also acts in medial rotation of the dentaries about their long axes. With these new reconstructions, the notion of a novel, unparsimonious "cheek" muscle is rejected, with further discussion of plausible buccal soft tissues. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 303:347-362, 2020. © 2018 American Association for Anatomy.

Three-dimensional mobility and muscle attachments in the pectoral limb of the Triassic cynodont Massetognathus pascuali (Romer, 1967)

The musculoskeletal configuration of the mammalian pectoral limb has been heralded as a key anatomical feature leading to the adaptive radiation of mammals, but limb function in the non-mammaliaform cynodont outgroup remains unresolved. Conflicting reconstructions of abducted and adducted posture are based on mutually incompatible interpretations of ambiguous osteology. We reconstruct the pectoral limb of the Triassic non-mammaliaform cynodont Massetognathus pascuali in three dimensions, by combining skeletal morphology from micro-computed tomography with muscle anatomy from an extended extant phylogenetic bracket. Conservative tests of maximum range of motion suggest a degree of girdle mobility, as well as substantial freedom at the shoulder and the elbow joints. The glenoid fossa supports a neutral pose in which the distal end of the humerus points 45° posterolaterally from the body wall, intermediate between classically 'sprawling' and 'parasagittal' limb postures. Massetognathus pascuali is reconstructed as having a near-mammalian complement of shoulder muscles, including an incipient rotator cuff (m. subscapularis, m. infraspinatus, m. supraspinatus, and m. teres minor). Based on close inspection of the morphology of the glenoid fossa, we hypothesize a posture-driven scenario for the evolution of the therian ball-and-socket shoulder joint. The musculoskeletal reconstruction presented here provides the anatomical scaffolding for more detailed examination of locomotor evolution in the precursors to mammals.

Young's Modulus and Load Complexity: Modeling Their Effects on Proximal Femur Strain

Finite element analysis (FEA) is a powerful tool for evaluating questions of functional morphology, but the application of FEA to extant or extinct creatures is a non-trivial task. Three categories of input data are needed to appropriately implement FEA: geometry, material properties, and boundary conditions. Geometric data are relatively easily obtained from imaging techniques, but often material properties and boundary conditions must be estimated. Here we conduct sensitivity analyses of the effect of the choice of Young's Modulus for elements representing trabecular bone and muscle loading complexity on the proximal femur using a finite element mesh of a modern human femur. We found that finite element meshes that used a Young's Modulus between 500 and 1,500 MPa best matched experimental strains. Loading scenarios that approximated the insertion sites of hip musculature produced strain patterns in the region of the greater trochanter that were different from scenarios that grouped muscle forces to the superior greater trochanter, with changes in strain values of 40% or more for 20% of elements. The femoral head, neck, and proximal shaft were less affected (e.g. approximately 50% of elements changed by 10% or less) by changes in the location of application of muscle forces. From our sensitivity analysis, we recommend the use of a Young's Modulus for the trabecular elements of 1,000 MPa for the proximal femur (range 500-1,500 MPa) and that the muscular loading complexity be dependent on whether or not strains in the greater trochanter are the focus of the analytical question. Anat Rec, 301:1189-1202, 2018. © 2018 Wiley Periodicals, Inc.

Joint histology in Alligator mississippiensis challenges the identification of synovial joints in fossil archosaurs and inferences of cranial kinesis

Archosaurs, like all vertebrates, have different types of joints that allow or restrict cranial kinesis, such as synovial joints and fibrous joints. In general, synovial joints are more kinetic than fibrous joints, because the former possess a fluid-filled cavity and articular cartilage that facilitate movement. Even though there is a considerable lack of data on the microstructure and the structure-function relationships in the joints of extant archosaurs, many functional inferences of cranial kinesis in fossil archosaurs have hinged on the assumption that elongated condylar joints are (i) synovial and/or (ii) kinetic. Cranial joint microstructure was investigated in an ontogenetic series of American alligators, Alligator mississippiensis All the presumably synovial, condylar joints found within the head of the American alligator (the jaw joint, otic joint and laterosphenoid-postorbital (LS-PO) joint) were studied by means of paraffin histology and undecalcified histology paired with micro-computed tomography data to better visualize three-dimensional morphology. Results show that among the three condylar joints of A. mississippiensis, the jaw joint was synovial as expected, but the otherwise immobile otic and LS-PO joints lacked a synovial cavity. Therefore, condylar morphology does not always imply the presence of a synovial articulation nor mobility. These findings reveal an undocumented diversity in the joint structure of alligators and show that crocodylians and birds build novel, kinetic cranial joints differently. This complicates accurate identification of synovial joints and functional inferences of cranial kinesis in fossil archosaurs and tetrapods in general.

Forelimb muscle and joint actions in Archosauria: insights from Crocodylus johnstoni (Pseudosuchia) and Mussaurus patagonicus (Sauropodomorpha)

Many of the major locomotor transitions during the evolution of Archosauria, the lineage including crocodiles and birds as well as extinct Dinosauria, were shifts from quadrupedalism to bipedalism (and vice versa). Those occurred within a continuum between more sprawling and erect modes of locomotion and involved drastic changes of limb anatomy and function in several lineages, including sauropodomorph dinosaurs. We present biomechanical computer models of two locomotor extremes within Archosauria in an analysis of joint ranges of motion and the moment arms of the major forelimb muscles in order to quantify biomechanical differences between more sprawling, pseudosuchian (represented the crocodile Crocodylus johnstoni) and more erect, dinosaurian (represented by the sauropodomorph Mussaurus patagonicus) modes of forelimb function. We compare these two locomotor extremes in terms of the reconstructed musculoskeletal anatomy, ranges of motion of the forelimb joints and the moment arm patterns of muscles across those ranges of joint motion. We reconstructed the three-dimensional paths of 30 muscles acting around the shoulder, elbow and wrist joints. We explicitly evaluate how forelimb joint mobility and muscle actions may have changed with postural and anatomical alterations from basal archosaurs to early sauropodomorphs. We thus evaluate in which ways forelimb posture was correlated with muscle leverage, and how such differences fit into a broader evolutionary context (i.e. transition from sprawling quadrupedalism to erect bipedalism and then shifting to graviportal quadrupedalism). Our analysis reveals major differences of muscle actions between the more sprawling and erect models at the shoulder joint. These differences are related not only to the articular surfaces but also to the orientation of the scapula, in which extension/flexion movements in Crocodylus (e.g. protraction of the humerus) correspond to elevation/depression in Mussaurus. Muscle action is highly influenced by limb posture, more so than morphology. Habitual quadrupedalism in Mussaurus is not supported by our analysis of joint range of motion, which indicates that glenohumeral protraction was severely restricted. Additionally, some active pronation of the manus may have been possible in Mussaurus, allowing semi-pronation by a rearranging of the whole antebrachium (not the radius against the ulna, as previously thought) via long-axis rotation at the elbow joint. However, the muscles acting around this joint to actively pronate it may have been too weak to drive or maintain such orientations as opposed to a neutral position in between pronation and supination. Regardless, the origin of quadrupedalism in Sauropoda is not only linked to manus pronation but also to multiple shifts of forelimb morphology, allowing greater flexion movements of the glenohumeral joint and a more columnar forelimb posture.

Locomotor activity influences muscle architecture and bone growth but not muscle attachment site morphology

The ability to make behavioural inferences from skeletal remains is critical to understanding the lifestyles and activities of past human populations and extinct animals. Muscle attachment site (enthesis) morphology has long been assumed to reflect muscle strength and activity during life, but little experimental evidence exists to directly link activity patterns with muscle development and the morphology of their attachments to the skeleton. We used a mouse model to experimentally test how the level and type of activity influences forelimb muscle architecture of spinodeltoideus, acromiodeltoideus, and superficial pectoralis, bone growth rate and gross morphology of their insertion sites. Over an 11-week period, we collected data on activity levels in one control group and two experimental activity groups (running, climbing) of female wild-type mice. Our results show that both activity type and level increased bone growth rates influenced muscle architecture, including differences in potential muscular excursion (fibre length) and potential force production (physiological cross-sectional area). However, despite significant influences on muscle architecture and bone development, activity had no observable effect on enthesis morphology. These results suggest that the gross morphology of entheses is less reliable than internal bone structure for making inferences about an individual's past behaviour.

Cranial myology and bite force performance of Erlikosaurus andrewsi: a novel approach for digital muscle reconstructions

The estimation of bite force and bite performance in fossil and extinct animals is a challenging subject in palaeontology and is highly dependent on the reconstruction of the cranial myology. Furthermore, the morphology and arrangement of the adductor muscles considerably affect feeding processes and mastication and thus also have important dietary and ecological ramifications. However, in the past, the reconstruction of the (cranial) muscles was restricted to the identification of muscle attachment sites or simplified computer models. This study presents a detailed reconstruction of the adductor musculature of the Cretaceous therizinosaur Erlikosaurus andrewsi based on a stepwise and iterative approach. The detailed, three-dimensional models of the individual muscles allow for more accurate measurements of the muscle properties (length, cross-section, attachment angle and volume), from which muscle and bite force estimates are calculated. Bite force estimations are found to be the lowest at the tip of the snout (43-65 N) and respectively higher at the first (59-88 N) and last tooth (90-134 N) position. Nevertheless, bite forces are comparatively low for E. andrewsi, both in actual numbers as well as in comparison with other theropod dinosaurs. The results further indicate that the low bite performance was mainly used for leaf-stripping and plant cropping, rather than active mastication or chewing processes. Muscle and thus bite force in E. andrewsi (and most likely all therizinosaurs) is considerably constrained by the cranial anatomy and declines in derived taxa of this clade. This trend is reflected in the changes of dietary preferences from carnivory to herbivory in therizinosaurs.

Does morphological convergence imply functional similarity? A test using the evolution of quadrupedalism in ornithischian dinosaurs

Convergent morphologies are thought to indicate functional similarity, arising because of a limited number of evolutionary or developmental pathways. Extant taxa displaying convergent morphologies are used as analogues to assess function in extinct taxa with similar characteristics. However, functional studies of extant taxa have shown that functional similarity can arise from differing morphologies, calling into question the paradigm that form and function are closely related. We test the hypothesis that convergent skeletal morphology indicates functional similarity in the fossil record using ornithischian dinosaurs. The rare transition from bipedality to quadrupedality occurred at least three times independently in this clade, resulting in a suite of convergent osteological characteristics. We use homology rather than analogy to provide an independent line of evidence about function, reconstructing soft tissues using the extant phylogenetic bracket and applying biomechanical concepts to produce qualitative assessments of muscle leverage. We also optimize character changes to investigate the sequence of character acquisition. Different lineages of quadrupedal ornithischian dinosaur stood and walked differently from each other, falsifying the hypothesis that osteological convergence indicates functional similarity. The acquisition of features correlated with quadrupedalism generally occurs in the same order in each clade, suggesting underlying developmental mechanisms that act as evolutionary constraints.

Appendicular myology of the hadrosaurian dinosaur Maiasaura peeblesorum from the Late Cretaceous (Campanian) of Montana

Musculature of the pectoral and pelvic appendages and girdles of adult and nestling Maiasaura peeblesorum (Dinosauria: Ornithischia: Hadrosauridae) from the Late Cretaceous of Montana is restored according to a phylogenetically based methodology. This methodology uses an explicit, independently derived phylogenetic hypothesis of the fossil taxon and related extant taxa to generate a series of inferences regarding the presence of a muscle, its number of components, and the origin(s) and insertion(s) of these components. Corroborative osteological evidence is sought on the fossil in the form of scars and processes that fulfill the criteria for muscular attachment according to generalisations based upon extant vertebrates. A total of 46 muscles are restored, although separate attachment sites for numerous muscles cannot be discerned on the fossils. Osteological evidence for several muscles can be found in nestlings of Maiasaura despite their skeletal immaturity. Results of the phylogenetically based approach and new hypotheses for homologies of deep dorsal thigh muscles suggest that it is more parsimonious to restore the femoral insertions of M. iliofemoralis on the greater trochanter and M. puboischiofemoralis internus on the anterior (lesser) trochanter, a reversal of the traditional interpretation. The often-cited osteological specialisations of birds for flight are not accompanied in all instances by profound myological transformations, and birds must be included in any attempt to restore the myology of extinct dinosaurs.

Pelvic and hindlimb myology of the basal Archosaur Poposaurus gracilis (Archosauria: Poposauroidea)

The discovery of a largely complete and well preserved specimen of Poposaurus gracilis has provided the opportunity to generate the first phylogenetically based reconstruction of pelvic and hindlimb musculature of an extinct nondinosaurian archosaur. As in dinosaurs, multiple lineages of basal archosaurs convergently evolved parasagittally erect limbs. However, in contrast to the laterally projecting acetabulum, or "buttress erect" hip morphology of ornithodirans, basal archosaurs evolved a very different, ventrally projecting acetabulum, or "pillar erect" hip. Reconstruction of the pelvic and hindlimb musculotendinous system in a bipedal suchian archosaur clarifies how the anatomical transformations associated with the evolution of bipedalism in basal archosaurs differed from that of bipedal dinosaurs and birds. This reconstruction is based on the direct examination of the osteology and myology of phylogenetically relevant extant taxa in conjunction with osteological correlates from the skeleton of P. gracilis. This data set includes a series of inferences (presence/absence of a structure, number of components, and origin/insertion sites) regarding 26 individual muscles or muscle groups, three pelvic ligaments, and two connective tissue structures in the pelvis, hindlimb, and pes of P. gracilis. These data provide a foundation for subsequent examination of variation in myological orientation and function based on pelvic and hindlimb morphology, across the basal archosaur lineage leading to extant crocodilians.

Enthesis bilateral asymmetry in humans and African apes

Entheses (skeletal muscle and tendon attachment sites) have often been used to infer handedness and activity variability among human populations. However, the specific roles that intensity vs. frequency of muscle contractions play in modifying entheses are not well understood and the assumption that entheses reflect muscle activity levels has been challenged. This study explores the effect of habitual muscular activity on enthesis morphology in humans and African apes by investigating bilateral asymmetry in the forelimbs and hindlimbs of these taxa. Humans have generally more developed entheses in the lower limb while African apes have generally more developed entheses in the forelimbs. All species studied have more asymmetric forelimbs than hindlimbs except humans that show more asymmetrical expression of bony spurs in the lower limbs than in the upper limbs. When comparing species, humans are always more asymmetric in ethesis development than apes for both the forelimbs and hindlimbs, which reflects the relatively greater asymmetry in limb use in humans and the more symmetric use in apes. Enthesis development may reflect cross-symmetry patterns in humans and, more subtly, a moderate handedness in apes during manipulative activities. This study suggests that enthesis morphology provides information on muscle activity levels, with greater development of entheses associated with more habitual or powerful muscle use. The general similarity of ape and human responses to muscle activity suggests that muscle activity influenced enthesis development in Plio-Pleistocene hominins and that interpretation of muscle markings in these fossils can provide data for functional inferences in these extinct species.

Homo floresiensis and the evolution of the hominin shoulder

The holotype of Homo floresiensis, diminutive hominins with tiny brains living until 12,000 years ago on the island of Flores, is a partial skeleton (LB1) that includes a partial clavicle (LB1/5) and a nearly complete right humerus (LB1/50). Although the humerus appears fairly modern in most regards, it is remarkable in displaying only 110 degrees of humeral torsion, well below modern human average values. Assuming a modern human shoulder configuration, such a low degree of humeral torsion would result in a lateral set to the elbow. Such an elbow joint would function more nearly in a frontal than in a sagittal plane, and this is certainly not what anyone would have predicted for a tool-making Pleistocene hominin. We argue that Homo floresiensis probably did not have a modern human shoulder configuration: the clavicle was relatively short, and we suggest that the scapula was more protracted, resulting in a glenoid fossa that faced anteriorly rather than laterally. A posteriorly directed humeral head was therefore appropriate for maintaining a normally functioning elbow joint. Similar morphology in the Homo erectus Nariokotome boy (KNM-WT 15000) suggests that this shoulder configuration may represent a transitional stage in pectoral girdle evolution in the human lineage.

Middle phalanx skeletal morphology in the hand: can it predict flexor tendon size and attachments?

Specific sites on the palmar diaphysis of the manual middle phalanges provide attachment for the flexor digitorum superficialis (FDS) tendon. It has been assumed in the literature that lateral palmar fossae on these bones reflect locations for these attachments and offer evidence for relative size of the flexor tendon. This assumption has led to predictions about relative FDS muscle force potential from sizes of fossae on fossil hominin middle phalanges. Inferences about locomotor capabilities of fossil hominins in turn have been drawn from the predicted force potential of the flexor muscle. The study reported here provides a critical first step in evaluating hypotheses about behavioral implications of middle phalangeal morphology in fossil hominins, by testing the hypothesis that the lateral fossae reflect the size of the FDS tendon and the location of the terminal FDS tendon attachments on the middle phalanx. The middle phalangeal region was dissected in 43 individuals from 16 primate genera, including humans. Qualitative observations were made of tendon attachment locations relative to the lateral fossae. Length measurements of the fossae were tested as predictors of FDS tendon cross-sectional area and of FDS attachment tendon lengths. Our results lead to the conclusion that the hypothesis must be rejected, and that future attention should focus on functional implications of the palmar median bar associated with the lateral fossae.

The effect of endurance exercise on the morphology of muscle attachment sites

The morphology of muscle attachment sites, or entheses, has long been assumed to directly reflect in vivo muscle activity. The purpose of this study is to examine whether variations in muscle activity that are within normal physiological limits are reflected in variations in external attachment site morphology. This study tests the hypothesis that increased muscle activity (magnitude, number and frequency of loading cycles) results in the hypertrophy of muscle attachment sites. The attachment sites of six limb muscles and one muscle of mastication (control) in mature female sheep were measured and compared in exercised (weighted treadmill running for 1 h per day for 90 days) and sedentary control animals. Attachment site surface morphology was assessed by quantifying the size (3D surface area) and complexity (fractal dimension parallel and perpendicular to soft tissue attachment) of the surfaces.

The results of this study demonstrate no effect of the exercise treatment used in this experiment on any measure of enthesis morphology. Potential explanations for the lack of exercise response include the mature age of the animals, inappropriate stimulus type for inducing morphological change, or failure to surpass a hypothetical threshold of load for inducing morphological change. However, further tests also demonstrate no relationship between muscle size and either attachment site size or complexity in sedentary control animals. The results of this study indicate that the attachment site morphological parameters measured in this study do not reflect muscle size or activity. In spite of decades of assumption otherwise, there appears to be no direct causal relationship between muscle size or activity and attachment site morphology, and reconstructions of behavior based on these features should be viewed with caution.

Quantitative microanatomy of jaw muscle attachment in extant diapsids

Muscular reconstructions in vertebrate paleontology have often relied heavily on the presence of "muscle scars" and similar osteological correlates of muscle attachment, a practice complicated by the fact that approximately half of tendinous muscle attachments to bone in extant vertebrates do not leave readily interpretable scars. Microanatomical and histological correlates of tendinous muscle attachment are much less ambiguous. This study examines the microanatomical correlates of muscle attachment for the mandibular adductors in six species of diapsids. Most prominent tendinous or aponeurotic muscle attachments display a high density of extrinsic fibers (similar to Sharpey's fibers). There is also some indication that the density of extrinsic fibers at an attachment may be directly related to the amount of stress exerted on that attachment. The presence of comparable densities of extrinsic fibers in fossil tissue constitutes strong and readily interpretable positive evidence for the presence of adjacent fibrous connective tissue in life. Microanatomy and histology provide reliable data about muscle attachments that cannot be gleaned from gross observation alone. These additional data, when coupled with existing muscular reconstruction techniques, may be essential to the resolution of ambiguous character states, and will provide more severe tests for long-standing hypotheses of musculature in extinct diapsids. Increasing the accuracy and precision of muscular reconstructions lends greater strength to any phylogenetic, paleobiological, or paleoecological inferences that draw upon these reconstructions as important lines of evidence.