Morphology of muscle attachment sites in the modern human hand does not reflect muscle architecture

Enthesis Size and Hand Preference: Asymmetry in Humans Contrasts With Symmetry in Nonhuman Primates

OBJECTIVES

Humans display species-wide right-hand preference across tasks, but this pattern has not been observed at comparable levels in nonhuman primates, suggesting the behavior arose after the panin-hominin split. Muscle attachment sites (entheses) are used to infer soft tissue anatomy and reconstruct behaviors within skeletal populations, but whether entheseal size asymmetry can reflect hand preference remains unclear. If entheseal asymmetry is linked to hand preference, we expect to see greater asymmetry in human hands, where hand preference is more pronounced, compared to nonhuman primates. We tested for bilateral asymmetry in the size of the opponens pollicis muscle flange using a sample of humans and catarrhine primates to determine if enthesis development can be a reliable indicator of hand preference.

MATERIALS AND METHODS

We assess the asymmetry of the opponens pollicis enthesis between paired (left/right) first metacarpals using distance-based heat maps generated from three-dimensional models of Homo sapiens (n = 85 individuals), Macaca fascicularis (n = 58 individuals), Gorilla spp. (n = 8 individuals), and Hylobates lar (n = 44 individuals). Metacarpals were cropped to isolate the metacarpal shaft and capture the majority of the enthesis while eliminating variation from the metacarpal ends.

RESULTS

We found right-directional asymmetry for humans; no significant differences are observed for Hylobates, Macaca, and Gorilla.

CONCLUSION

The opponens pollicis enthesis shows right/left hand bias in humans. The lack of significant asymmetry in nonhuman primates suggests entheseal development in these species does not reflect the same level of hand preference observed in humans. Nonhuman primates can serve as a baseline for studying enthesis asymmetry based on the size of the opponens pollicis enthesis.

Humanlike manual activities in Australopithecus

The evolution of the human hand is a topic of great interest in paleoanthropology. As the hand can be involved in a vast array of activities, knowledge regarding how it was used by early hominins can yield crucial information on the factors driving biocultural evolution. Previous research on early hominin hands focused on the overall bone shape. However, while such approaches can inform on mechanical abilities and the evolved efficiency of manipulation, they cannot be used as a definite proxy for individual habitual activity. Accordingly, it is crucial to examine bone structures more responsive to lifetime biomechanical loading, such as muscle attachment sites or internal bone architecture. In this study, we investigate the manual entheseal patterns of Australopithecus afarensis, Australopithecus africanus, and Australopithecus sediba through the application of the validated entheses-based reconstruction of activity method. Using a comparative sample of later Homo and three great ape genera, we analyze the muscle attachment site proportions on the thumb, fifth ray, and third intermediate phalanx to gain insight into the habitual hand use of Australopithecus. We use a novel statistical procedure to account for the effects of interspecies variation in overall size and ray proportions. Our results highlight the importance of certain muscles of the first and fifth digits for humanlike hand use. In humans, these muscles are required for variable in-hand manipulation and are activated during stone-tool production. The entheses of A. sediba suggest muscle activation patterns consistent with a similar suite of habitual manual activities as in later Homo. In contrast, A. africanus and A. afarensis display a mosaic entheseal pattern that combines indications of both humanlike and apelike manipulation. Overall, these findings provide new evidence that some australopith species were already habitually engaging in humanlike manipulation, even if their manual dexterity was likely not as high as in later Homo.

Cortical bone architecture of hominid intermediate phalanges reveals functional signals of locomotion and manipulation

OBJECTIVES

Reconstruction of fossil hominin manual behaviors often relies on comparative analyses of extant hominid hands to understand the relationship between hand use and skeletal morphology. In this context, the intermediate phalanges remain understudied. Thus, here we investigate cortical bone morphology of the intermediate phalanges of extant hominids and compare it to the cortical structure of the proximal phalanges, to investigate the relationship between cortical bone structure and inferred loading during manual behaviors.

MATERIALS AND METHODS

Using micro-CT data, we analyze cortical bone structure of the intermediate phalangeal shaft of digits 2-5 in Pongo pygmaeus (n = 6 individuals), Gorilla gorilla (n = 22), Pan spp. (n = 23), and Homo sapiens (n = 23). The R package morphomap is used to study cortical bone distribution, cortical thickness and cross-sectional properties within and across taxa.

RESULTS

Non-human great apes generally have thick cortical bone on the palmar shaft, with Pongo only having thick cortex on the peaks of the flexor sheath ridges, while African apes have thick cortex along the entire flexor sheath ridge and proximal to the trochlea. Humans are distinct in having thicker dorsal shaft cortex as well as thick cortex at the disto-palmar region of the shaft.

DISCUSSION

Variation in cortical bone distribution and properties of the intermediate phalanges is consistent with differences in locomotor and manipulative behaviors in extant great apes. Comparisons between the intermediate and proximal phalanges reveals similar patterns of cortical bone distribution within each taxon but with potentially greater load experienced by the proximal phalanges, even in knuckle-walking African apes. This study provides a comparative context for the reconstruction of habitual hand use in fossil hominins and hominids.

The deep trabecular structure of first metacarpals in extant hominids

OBJECTIVES

Recent studies have associated subarticular trabecular bone distribution in the extant hominid first metacarpal (Mc1) with observed thumb use, to infer fossil hominin thumb use. Here, we analyze the entire Mc1 to test for interspecific differences in: (1) the absolute volume of trabecular volume fraction, (2) the distribution of the deeper trabecular network, and (3) the distribution of trabeculae in the medullary cavity, especially beneath the Mc1 disto-radial flange.

MATERIALS AND METHODS

Trabecular bone was imaged using micro-computed tomography in a sample of Homo sapiens (n = 11), Pan paniscus (n = 10), Pan troglodytes (n = 11), Gorilla gorilla (n = 10) and Pongo sp., (n = 7). Using Canonical Holistic Morphometric Analysis (cHMA), we tested for interspecific differences in the trabecular bone volume fraction (BV/TV) and its relative distribution (rBV/TV) throughout the Mc1, including within the head, medullary cavity, and base.

RESULTS

P. paniscus had the highest, and H. sapiens the lowest, BV/TV relative to other species. rBV/TV distribution statistically distinguished the radial concentrations and lack of medullary trabecular bone in the H. sapiens Mc1 from all other hominids. H. sapiens and, to a lesser extent, G. gorilla also had a significantly higher trabecular volume beneath the disto-radial flange relative to other hominids.

DISCUSSION

These results are consistent with differences in observed thumb use in these species and may also reflect systemic differences in bone volume fraction. The trabecular bone extension into the medullary cavity and concentrations beneath the disto-radial flange may represent crucial biomechanical signals that will aid in the inference of fossil hominin thumb use.

The Prrx1eGFP Mouse Labels the Periosteum During Development and a Subpopulation of Osteogenic Periosteal Cells in the Adult

The identity of the cells that form the periosteum during development is controversial with current dogma suggesting these are derived from a Sox9-positive progenitor. Herein, we characterize a newly created Prrx1eGFP reporter transgenic mouse line during limb formation and postnatally. Interestingly, in the embryo Prrx1eGFP-labeled cells become restricted around the Sox9-positive cartilage anlage without themselves becoming Sox9-positive. In the adult, the Prrx1eGFP transgene live labels a subpopulation of cells within the periosteum that are enriched at specific sites, and this population is diminished in aged mice. The green fluorescent protein (GFP)-labeled subpopulation can be isolated using fluorescence-activated cell sorting (FACS) and represents approximately 8% of all isolated periosteal cells. The GFP-labeled subpopulation is significantly more osteogenic than unlabeled, GFP-negative periosteal cells. In addition, the osteogenic and chondrogenic capacity of periosteal cells in vitro can be extended with the addition of fibroblast growth factor (FGF) to the expansion media. We provide evidence to suggest that osteoblasts contributing to cortical bone formation in the embryo originate from Prrx1eGFP-positive cells within the perichondrium, which possibly piggyback on invading vascular cells and secrete new bone matrix. In summary, the Prrx1eGFP mouse is a powerful tool to visualize and isolate periosteal cells and to quantify their properties in the embryo and adult. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

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.

Muscle attachment sites and behavioral reconstruction: An experimental test of muscle-bone structural response to habitual activity

OBJECTIVE

Behavioral reconstruction from muscle attachment sites (entheses) is a common practice in anthropology. However, experimental evidence provides mixed support for the assumed association between enthesis size and shape with changes in habitual activity. In this study, a laboratory mouse model was used to experimentally test whether activity level and type alters muscle architecture and the underlying bone cross-sectional geometry of entheses in order to assess the underlying assumption that behavioral changes lead to quantifiable differences in both muscle and enthesis morphology.

MATERIALS AND METHODS

Female wild-type mice were separated into one control group and two experimentally increased activity groups (running, climbing) over an 11-week study period. At the start of the experiment, half of the mice were 4 weeks and half were 7 weeks of age. The postmortem deltoideus and biceps brachii muscles were measured for potential force production (physiological cross-sectional area) and potential muscle excursion (fiber length). Bone cross-sectional geometry variables were measured from microCT scans of the humerus and radius at the enthesis and non-enthesis regions of interest across activity groups.

RESULTS

Activity level and type altered potential force production and potential muscle excursion of both muscles in the younger cohort. We observed differences in cortical bone geometry in both the humerus enthesis and radius non-enthesis region driven exclusively among the younger wheel-running mice.

DISCUSSION

These results indicate that in addition to muscle architectural changes, bone structural properties at the enthesis do show an adaptive response to increased activity, such as running but only during earlier development. However, further research is required in order to apply these findings to the reconstruction of living behavior from anthropological specimens.

Homo naledi pollical metacarpal shaft morphology is distinctive and intermediate between that of australopiths and other members of the genus Homo

Homo naledi fossils from the Rising Star cave system provide important insights into the diversity of hand morphology within the genus Homo. Notably, the pollical (thumb) metacarpal (Mc1) displays an unusual suite of characteristics including a median longitudinal crest, a narrow proximal base, and broad flaring intrinsic muscle flanges. The present study evaluates the affinities of H. naledi Mc1 morphology via 3D geometric morphometric analysis of shaft shape using a broader comparative sample (n = 337) of fossil hominins, recent humans, apes, and cercopithecoid monkeys than in prior work. Results confirm that the H. naledi Mc1 is distinctive from most other hominins in being narrow at the proximal end but surmounted by flaring muscle flanges distally. Only StW 418 (Australopithecus cf. africanus) is similar in these aspects of shape. The gracile proximal shaft is most similar to cercopithecoids, Pan, Pongo, Australopithecus afarensis, and Australopithecus sediba, suggesting that H. naledi retains the condition primitive for the genus Homo. In contrast, Neandertal Mc1s are characterized by wide proximal bases and shafts, pinched midshafts, and broad distal flanges, while those of recent humans generally have straight shafts, less robust muscle flanges, and wide proximal shafts/bases. Although uncertainties remain regarding character polarity, the morphology of the H. naledi thumb might be interpreted as a retained intermediate state in a transformation series between the overall gracility of the shaft and the robust shafts of later hominins. Such a model suggests that the addition of broad medial and lateral muscle flanges to a primitively slender shaft was the first modification in transforming the Mc1 into the overall more robust structure exhibited by other Homo taxa including Neandertals and recent Homo sapiens in whose shared lineage the bases and proximal shafts became expanded, possibly as an adaptation to the repeated recruitment of powerful intrinsic pollical muscles.

Evaluating bony predictors of bite force across the order Carnivora

In carnivorans, bite force is a critical and ecologically informative variable that has been correlated with multiple morphological, behavioral, and environmental attributes. Whereas in vivo measures of biting performance are difficult to obtain in many taxa-and impossible in extinct species-numerous osteological proxies exist for estimating masticatory muscle size and force. These proxies include both volumetric approximations of muscle dimensions and direct measurements of muscular attachment sites. In this study, we compare three cranial osteological techniques for estimating muscle size (including 2D-photographic and 3D-surface data approaches) against dissection-derived muscle weights and physiological cross-sectional area (PCSA) within the jaw adductor musculature of 40 carnivoran taxa spanning eight families, four orders of magnitude in body size, and the full dietary spectrum of the order. Our results indicate that 3D-approaches provide more accurate estimates of muscle size than do surfaces measured from 2D-lateral photographs. However, estimates of a muscle's maximum cross-sectional area are more closely correlated with muscle mass and PCSA than any estimates derived from muscle attachment areas. These findings highlight the importance of accounting for muscle thickness in osteological estimations of the masticatory musculature; as muscles become volumetrically larger, their larger cross-sectional area does not appear to be associated with a proportional increase in the attachment site area. Though volumetric approaches approximate muscle dimensions well across the order as a whole, caution should be exercised when applying any single method as a predictor across diverse phylogenies.

New horizons in reconstructing past human behavior: Introducing the "Tübingen University Validated Entheses-based Reconstruction of Activity" method

An accurate reconstruction of habitual activities in past populations and extinct hominin species is a paramount goal of paleoanthropological research, as it can elucidate the evolution of human behavior and the relationship between culture and biology. Variation in muscle attachment (entheseal) morphology has been considered an indicator of habitual activity, and many attempts have been made to use it for this purpose. However, its interpretation remains equivocal due to methodological shortcomings and a paucity of supportive experimental data. Through a series of studies, we have introduced a novel and precise methodology that focuses on reconstructing muscle synergies based on three-dimensional and multivariate analyses among entheses. This approach was validated using uniquely documented anthropological samples, experimental animal studies, histological observations, and geometric morphometrics. Here, we detail, synthesize, and critically discuss the findings of these studies, which overall point to the great potential of entheses in elucidating aspects of past human behavior.

Human evolution: Thumbs up for efficiency

Modelling a muscle key to flexing the thumb, a new study suggests that the powerful opposability that characterises the dextrous human hand evolved in some of our fossil relatives about two million years ago - a time when tool use became more important.

Insertion sites in manual proximal phalanges of African apes and modern humans

OBJECTIVES

The primary aim of this study was to describe the insertion sites of the ligaments holding the flexor digitorum profundus and superficialis muscles (flexor ridges) in proximal phalanges 2-5 of African apes and modern humans. To interpret differences in flexor ridge size based on general behavioral differences among taxa.

MATERIALS AND METHODS

We analyzed 3D models of manual proximal phalanges 2-5 from 29 gorillas (Gorilla beringei and Gorilla gorilla), 30 chimpanzees (Pan troglodytes) and 36 recent modern humans. Flexor ridges (mm²) were compared within and across genera.

RESULTS

Gorillas and chimpanzees had larger flexor ridges for phalanges 2-4 than humans and this difference subsists when controlling for body size. Each genus had a unique insertion size pattern across the digits, with the most heterogeneous pattern found in chimpanzees, followed by humans, and lastly gorillas. These patterns corresponded strongly to the differences in the size of the phalanges within each genus, except for phalanx 5 in humans, which had a larger flexor ridge than expected.

DISCUSSION

When comparing these genera, the flexor ridges signal differences between taxa that use their hands for manipulation and locomotion (gorillas and chimpanzees) and taxa that use them exclusively for manipulation (humans). This functional signal was also apparent in the PP5 of humans, whose larger FR may be indicating the high recruitment of this digit during forceful precision grip characteristic of humans.

Biocultural evidence of precise manual activities in an Early Holocene individual of the high-altitude Peruvian Andes

OBJECTIVES

Cuncaicha, a rockshelter site in the southern Peruvian Andes, has yielded archaeological evidence for human occupation at high elevation (4,480 masl) during the Terminal Pleistocene (12,500-11,200 cal BP), Early Holocene (9,500-9,000 cal BP), and later periods. One of the excavated human burials (Feature 15-06), corresponding to a middle-aged female dated to ~8,500 cal BP, exhibits skeletal osteoarthritic lesions previously proposed to reflect habitual loading and specialized crafting labor. Three small tools found in association with this burial are hypothesized to be associated with precise manual dexterity.

MATERIALS AND METHODS

Here, we tested this functional hypothesis through the application of a novel multivariate methodology for the three-dimensional analysis of muscle attachment surfaces (entheses). This original approach has been recently validated on both lifelong-documented anthropological samples as well as experimental studies in nonhuman laboratory samples. Additionally, we analyzed the three-dimensional entheseal shape and resulting moment arms for muscle opponens pollicis.

RESULTS

Results show that Cuncaicha individual 15-06 shows a distinctive entheseal pattern associated with habitual precision grasping via thumb-index finger coordination, which is shared exclusively with documented long-term precision workers from recent historical collections. The separate geometric morphometric analysis revealed that the individual's opponens pollicis enthesis presents a highly projecting morphology, which was found to strongly correlate with long joint moment arms (a fundamental component of force-producing capacity), closely resembling the form of Paleolithic hunter-gatherers from diverse geo-chronological contexts of Eurasia and North Africa.

DISCUSSION

Overall, our findings provide the first biocultural evidence to confirm that the lifestyle of some of the earliest Andean inhabitants relied on habitual and forceful precision grasping tasks.

The study of the lower limb entheses in the Neanderthal sample from El Sidrón (Asturias, Spain): How much musculoskeletal variability did Neanderthals accumulate?

Entheses have rarely been systematically studied in the field of human evolution. However, the investigation of their morphological variability (e.g., robusticity) could provide new insight into their evolutionary significance in the European Neanderthal populations. The aim of this work is to study the entheses and joint features of the lower limbs of El Sidrón Neanderthals (Spain; 49 ka), using standardized scoring methods developed on modern samples. Paleobiology, growth, and development of both juveniles and adults from El Sidrón are studied and compared with those of Krapina Neanderthals (Croatia, 130 ka) and extant humans. The morphological patterns of the gluteus maximus and vastus intermedius entheses in El Sidrón, Krapina, and modern humans differ from one another. Both Neanderthal groups show a definite enthesis design for the gluteus maximus, with little intrapopulation variability with respect to modern humans, who are characterized by a wider range of morphological variability. The gluteus maximus enthesis in the El Sidrón sample shows the osseous features of fibrous entheses, as in modern humans, whereas the Krapina sample shows the aspects of fibrocartilaginous ones. The morphology and anatomical pattern of this enthesis has already been established during growth in all three human groups. One of two and three of five adult femurs from El Sidrón and from Krapina, respectively, show the imprint of the vastus intermedius, which is absent among juveniles from those Neanderthal samples and in modern samples. The scant intrapopulation and the high interpopulation variability in the two Neanderthal samples is likely due to a long-term history of small, isolated populations with high levels of inbreeding, who also lived in different ecological conditions. The comparison of different anatomical entheseal patterns (fibrous vs. fibrocartilaginous) in the Neanderthals and modern humans provides additional elements in the discussion of their functional and genetic origin.

A microarchitectural assessment of the gluteal tuberosity suggests two possible patterns in entheseal changes

OBJECTIVES

Macroscopic entheseal forms show two main features: predominant signs of bony formation or resorption. To understand the development of these forms, we investigated microarchitectural differences between the macroscopic proliferative and resorptive forms of the gluteus maximus enthesis.

MATERIALS AND METHODS

The macromorphological analysis of entheseal changes (EC) was based on the Villotte, visual scoring system for fibrous entheses. Gluteal tuberosity specimens of different stages of Villote's system were harvested from 16 adult males derived from an archaeological context and scanned using microcomputed tomography.

RESULTS

The microarchitectural analyzes of cortical bone demonstrated a trend of higher porosity in the resorptive compared to the proliferative phase in Stage B, whereas a 30% porosity reduction was detected in the resorptive compared to proliferative phase of Stage C. In terms of the trabecular bone between the resorptive and proliferative entheseal phases, there was a trend of increased connectivity density, whereas the structural model index decreased in B and increased in C. The assessment of the entire specimen showed an increase in porosity from the proliferative to the resorptive phase in the Stage B, in contrast to a decrease in the Stage C.

DISCUSSION

The results suggest that from an initial flat entheses, two directions of EC development are possible: (a) a bony prominence may form and, subsequently, it is subjected to trabecularization of the cortical bone inside the prominence, such cortical trabecularization can lead to visible porosity on the cortical external surface; (b) the cortical bone defect may develop with the regular underlying cortical bone.

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.

Experimental evidence that physical activity affects the multivariate associations among muscle attachments (entheses)

The morphology of entheses (muscle/tendon attachment sites) on bones is routinely used in paleontological and bioarcheological studies to infer the physical activity patterns of ancient vertebrate species including hominins. However, such inferences have often been disputed owing to limitations of the quantitative methods commonly employed and a lack of experimental evidence demonstrating direct effects of physical activity on entheseal morphology. Recently, we introduced a new and improved method of quantifying and analyzing entheseal morphology that involves repeatable three-dimensional measurements combined with multivariate statistics focused on associations among multiple entheses. Here, to assess the validity of our method for investigating variation in entheseal morphology related to physical activity patterns, we analyzed femora of growing turkeys that were experimentally exercised for 10 weeks on either an inclined or declined treadmill or served as controls (N=15 individuals, 5 per group). Our multivariate approach identified certain patterns involving three different entheses (associated with the gluteus primus, medial gastrocnemius, vastus medialis and adductor magnus muscles) that clearly differentiated controls from runners. Importantly, these differences were not observable when comparing groups within each of the three entheseal structures separately. Body mass was not correlated with the resulting multivariate patterns. These results provide the first experimental evidence that variation in physical activity patterns has a direct influence on entheseal morphology. Moreover, our findings highlight the promise of our newly developed quantitative methods for analyzing the morphology of entheses to reconstruct the behavior of extinct vertebrate species based on their skeletal remains.

Experimental proof that multivariate patterns among muscle attachments (entheses) can reflect repetitive muscle use

Reconstructions of habitual activity in past populations and extinct human groups is a primary goal of paleoanthropological research. Muscle attachment scars (entheses) are widely considered as indicators of habitual activity and many attempts have been made to use them for this purpose. However, their interpretation remains equivocal due to methodological limitations and a paucity of empirical data supporting an interaction between systematic muscle forces and entheseal morphology. We have recently addressed the first issue with precise three-dimensional measuring protocols and rigorous multivariate analysis focusing on the patterns among different entheses rather than comparing each entheseal structure separately. In a previous study, the resulting entheseal correlations reflected synergistic muscle groups that separated individuals according to their lifelong occupational activities. Here we address the second issue by applying this methodology to existing micro-computed tomography data from rats that have undergone muscle stimulation under experimental conditions. In contrast to previous animal studies, we relied on blind analytical procedures across two research institutions and controlled for most factors of interindividual variability. Results demonstrated that the multivariate associations among different entheseal surfaces can directly reflect repetitive muscle recruitment and provide essential information on muscle use.

Lower rotational inertia and larger leg muscles indicate more rapid turns in tyrannosaurids than in other large theropods

Synopsis

Tyrannosaurid dinosaurs had large preserved leg muscle attachments and low rotational inertia relative to their body mass, indicating that they could turn more quickly than other large theropods.

Methods

To compare turning capability in theropods, we regressed agility estimates against body mass, incorporating superellipse-based modeled mass, centers of mass, and rotational inertia (mass moment of inertia). Muscle force relative to body mass is a direct correlate of agility in humans, and torque gives potential angular acceleration. Agility scores therefore include rotational inertia values divided by proxies for (1) muscle force (ilium area and estimates of m. caudofemoralis longus cross-section), and (2) musculoskeletal torque. Phylogenetic ANCOVA (phylANCOVA) allow assessment of differences in agility between tyrannosaurids and non-tyrannosaurid theropods (accounting for both ontogeny and phylogeny). We applied conditional error probabilities a(p) to stringently test the null hypothesis of equal agility.

Results

Tyrannosaurids consistently have agility index magnitudes twice those of allosauroids and some other theropods of equivalent mass, turning the body with both legs planted or pivoting over a stance leg. PhylANCOVA demonstrates definitively greater agilities in tyrannosaurids, and phylogeny explains nearly all covariance. Mass property results are consistent with those of other studies based on skeletal mounts, and between different figure-based methods (our main mathematical slicing procedures, lofted 3D computer models, and simplified graphical double integration).

Implications

The capacity for relatively rapid turns in tyrannosaurids is ecologically intriguing in light of their monopolization of large (>400 kg), toothed dinosaurian predator niches in their habitats.

Cross-sectional properties of the humeral diaphysis of Paranthropus boisei: Implications for upper limb function

A ∼1.52 Ma adult upper limb skeleton of Paranthropus boisei (KNM-ER 47000) recovered from the Koobi Fora Formation, Kenya (FwJj14E, Area 1A) includes most of the distal half of a right humerus (designated KNM-ER 47000B). Natural transverse fractures through the diaphysis of KNM-ER 470000B provide unobstructed views of cortical bone at two sections typically used for analyzing cross-sectional properties of hominids (i.e., 35% and 50% of humerus length from the distal end). Here we assess cross-sectional properties of KNM-ER 47000B and two other P. boisei humeri (OH 80-10, KNM-ER 739). Cross-sectional properties for P. boisei associated with bending/torsional strength (section moduli) and relative cortical thickness (%CA; percent cortical area) are compared to those reported for nonhuman hominids, AL 288-1 (Australopithecus afarensis), and multiple species of fossil and modern Homo. Polar section moduli (Z_p) are assessed relative to a mechanically relevant measure of body size (i.e., the product of mass [M] and humerus length [HL]). At both diaphyseal sections, P. boisei exhibits %CA that is high among extant hominids (both human and nonhuman) and similar to that observed among specimens of Pleistocene Homo. High values for Z_p relative to size (M × HL) indicate that P. boisei had humeral bending strength greater than that of modern humans and Neanderthals and similar to that of great apes, A. afarensis, and Homo habilis. Such high humeral strength is consistent with other skeletal features of P. boisei (reviewed here) that suggest routine use of powerful upper limbs for arboreal climbing.

Evidence for precision grasping in Neandertal daily activities

Neandertal manual activities, as previously reconstructed from their robust hand skeletons, are thought to involve systematic power grasping rather than precise hand movements. However, this interpretation is at odds with increasing archeological evidence for sophisticated cultural behavior. We reevaluate the manipulative behaviors of Neandertals and early modern humans using a historical reference sample with extensive genealogical and lifelong occupational documentation, in combination with a new and precise three-dimensional multivariate analysis of hand muscle attachments. Results show that Neandertal muscle marking patterns overlap exclusively with documented lifelong precision workers, reflecting systematic precision grasping consistent with the use of their associated cultural remains. Our findings challenge the established interpretation of Neandertal behavior and establish a solid link between biological and cultural remains in the fossil record.

New Neandertal wrist bones from El Sidrón, Spain (1994-2009)

Twenty-nine carpal bones of Homo neanderthalensis have been recovered from the site of El Sidrón (Asturias, Spain) during excavations between 1994 and 2009, alongside ∼2500 other Neandertal skeletal elements dated to ∼49,000 years ago. All bones of the wrist are represented, including adult scaphoids (n = 6), lunates (n = 2), triquetra (n = 4), pisiforms (n = 2), trapezia (n = 2), trapezoids (n = 5), capitates (n = 5), and hamates (n = 2), as well as one fragmentary and possibly juvenile scaphoid. Several of these carpals appear to belong to the complete right wrist of a single individual. Here we provide qualitative and quantitative morphological descriptions of these carpals, within a comparative context of other European and Near Eastern Neandertals, early and recent Homo sapiens, and other fossil hominins, including Homo antecessor, Homo naledi, and australopiths. Overall, the El Sidrón carpals show characteristics that typically distinguish Neandertals from H. sapiens, such as a relatively flat first metacarpal facet on the trapezium and a more laterally oriented second metacarpal facet on the capitate. However, there are some distinctive features of the El Sidrón carpals compared with most other Neandertals. For example, the tubercle of the trapezium is small with limited projection, while the scaphoid tubercle and hamate hamulus are among the largest seen in other Neandertals. Furthermore, three of the six adult scaphoids show a distinctive os-centrale portion, while another is a bipartite scaphoid with a truncated tubercle. The high frequency of rare carpal morphologies supports other evidence of a close genetic relationship among the Neandertals found at El Sidrón.