Evolutionary patterns of cat-like carnivorans unveil drivers of the sabertooth morphology

The sabertooth morphology stands as a classic case of convergence, manifesting recurrently across various vertebrate groups, prominently within two carnivorans clades: felids and nimravids. Nonetheless, the evolutionary mechanisms driving these recurring phenotypes remain insufficiently understood, lacking a robust phylogenetic and spatiotemporal framework. We reconstruct the tempo and mode of craniomandibular evolution of Felidae and Nimravidae and evaluate the strength of the dichotomy between conical and saber-toothed species, as well as within saber-toothed morphotypes. To do so, we investigate morphological variation, convergence, phenotypic integration, and evolutionary rates, employing a comprehensive dataset of nearly 200 3D models encompassing mandibles and crania from both extinct and extant feline-like carnivorans, spanning their entire evolutionary timeline. Our results reject the hypothesis of a distinctive sabertooth morphology, revealing instead a continuous spectrum of feline-like phenotypes in both the cranium and mandible, with sporadic instances of unequivocal convergence. Disparity peaked at the end of the Miocene and is usually higher in clades containing taxa with extreme sabertoothed adaptations. We show that taxa with saberteeth exhibit a lower degree of craniomandibular integration, allowing to exhibit a greater range of phenotypes. Those same groups usually show a burst of morphological evolutionary rate at the beginning of their evolutionary history. Consequently, we propose that a reduced degree of integration coupled with rapid evolutionary rates emerge as key components in the development of a sabertooth morphology in multiple clades.

Linking shape conspicuous asymmetry with shape covariation patterns and performance in the insect head and mandibles

Bilateral symmetry is widespread across animals, yet, among bilaterians, many cases of conspicuous asymmetries evolved. This means that bilaterally homologous structures on the left and right side display divergent phenotypes. Evolution of such divergent phenotypes between otherwise similarly shaped structures can be thought to be favoured by modularity, but this has rarely been studied in the context of left-right differences. Here, we provide an empirical example, using geometric morphometrics, to assess patterns of asymmetry and covariation between landmark partitions in a grasshopper with conspicuously asymmetric mandibles. Our morphometric data confirms the presence of strictly directional conspicuous asymmetry in the mandibles, and surrounding structures. Covariance patterns and tests hint at a strong integration between mandibles despite their divergent morphologies, and variational modularity with the head capsule. While mandibles have been selected to achieve a key-and-lock morphology by having interlocking shapes, the developmental modularity required to achieve this seems to be overwritten by developmental and/or functional integration, allowing the precise matching required for feeding. The consequent conflicting covariation patterns are reminiscent of the palimpsest model. Finally, the degree of directional asymmetry appears to be under selection, although we find no relationship between bite force and mandible shape or asymmetry.

Sexual dimorphism in the enamel-dentine junction (EDJ) of permanent canines of European modern humans

OBJECTIVES

Dental anthropological investigations into sexual dimorphism have conventionally concentrated on evaluating the dimensions and configuration of the enamel cap of canines. However, the morphology of the crown dentine surface can be closely linked to that of the enamel surface. This link can facilitate examination of crown morphology even when the enamel surface is slightly worn. Here, we determine if the morphology of the enamel-dentine junction (EDJ) differs within (maxillary vs. mandibular) and between a sample of male (n = 26) and female (n = 21) contemporary human permanent canines from Europe.

METHODS

The morphological data of the EDJ were gathered employing a template comprising 96 landmarks and sliding semilandmarks. Subsequently, the data underwent analysis through form space principal component analysis following Procrustes registration, utilizing standard 3D geometric morphometric techniques.

RESULTS

Significant differences in the morphology of the EDJ were observed between the sexes, particularly concerning the overall shape of the crown, the symmetry of the mesial and distal edges, and the development of the distal accessory ridge.

CONCLUSIONS

Sex differences in the morphology of the EDJ could relate in part to retention of the canine-premolar honing complex in males. Our results indicate that analyses of the permanent canine EDJ may potentially provide a novel method for estimating the sex of adult and nonadult skeletons.

Cranial morphology of Balkan and West Asian livestock guardian dogs

Several large "shepherd" or livestock guardian dog (LGD) breeds were historically selectively bred to protect sheep and goat flocks in the Balkans, Anatolia, and the Caucasus regions. Although these breeds exhibit similar behavior, their morphology is different. Yet, the fine characterization of the phenotypic differences remains to be analyzed. The aim of this study is to characterize cranial morphology in the specific Balkan and West Asian LGD breeds. We use a 3D geometric morphometric in order to assess morphological differences regarding both shape and size between LGD breeds and compare this phenotypic diversity to close relative wild canids. Our results indicate that Balkan and Anatolian LGDs form a distinct cluster within a relatively large dog cranial size and shape diversity. Most LGDs display a cranial morphology that could be described as intermediate to the mastiff breeds and large herding dogs, except for the Romanian Mioritic shepherd which has a more brachycephalic cranium strongly resembling the bully-type dog cranial morphotype. Although often considered to represent an ancient type of dog, the Balkan-West Asian LGDs are clearly distinguishable from wolves, dingoes, and most other primitive and spitz-type dogs and this group displays a surprising cranial diversity.

Variation in human limb joint articular morphology

OBJECTIVES

Synovial joints in human limbs strike a balance between mobility, stability, and articular fit, yet little is known about how these conflicting demands pattern intraspecific variation in articular shape. In this study, we use geometric morphometrics to establish the apportionment and magnitude of morphological variance of the articular surfaces of the human shoulder, elbow, hip, and knee. We hypothesize that variances will be comparable between articulating surfaces within a joint and will be larger in joints with smaller ranges of motion, given their plurality of functional demands.

MATERIALS AND METHODS

Three-dimensional landmarks were taken on the articular surfaces of the glenohumeral, humeroulnar, acetabulofemoral, and tibiofemoral joints from CT scans of 200 skeletons from the University of Tennessee Donated Skeletal Collection (84 females, 116 males). Root mean-squared distances between articulations calculated from Procrustes shape coordinates were used to determine variance distributions.

RESULTS

We found no difference in variances for each articular surface between the sexes or between left and right articular surfaces. A high range of motion is associated with greater morphological variance; however, this pattern is largely driven by the concave articular surfaces of each joint, which consistently exhibit statistically greater variance than their convex counterparts.

DISCUSSION

The striking pattern of differential variance between articulating morphologies points to potential disparities in development between them. Consistently higher variance in concave surfaces may relate to chondral modeling theory for the formation of joints. Establishing intraspecific morphological variance patterns is a first step in understanding coordinated evolution among articular features.

Covariance in human limb joint articular morphology

OBJECTIVES

Limb synovial joints exhibit complex shapes that must accommodate often-antagonistic demands of function, mobility, and stability. These demands presumably dictate coordination among joint articular shapes, but the structure of morphological covariance within and among joints is unknown. This study analyzes the human shoulder, elbow, hip, and knee to determine how articular covariance is structured in relation to joint structure, accessory cartilage, and function.

MATERIALS AND METHODS

Surface models were created from the CT scans of 200 modern skeletons from the University of Tennessee Donated Skeletal Collection. Three-dimensional landmarks were collected on the shoulder, elbow, hip, and knee joints. Two-block partial least squares were conducted to determine associations between surfaces of conarticular shapes, functionally analogous articulations, and articulations belonging to the same bone.

RESULTS

Except for the components of the shoulder, all conarticular pairs exhibit covariance, though the strength of these relationships appears unrelated to the amount of accessory cartilage in the joint. Only the analogous articulations of the humerus and femur exhibit significant covariance, but it is unlikely that this pattern is due to function alone. Stronger covariance within the lower limb than the upper limb is consistent broader primate patterns of within-limb integration.

DISCUSSION

With the exception of the elbow, complementary joint function does not appear to promote strong covariance between articulations. Analogous humeral and femoral surfaces are also serially homologous, which may result in the articular associations observed between these bones. Broadly, these patterns highlight the indirect relationship between joint congruence and covariance.

Sexual dimorphism in dinosaurs

Studying fossils from a mass-mortality event reveals evidence for sexual dimorphism and, unusually, equal numbers of males and females in a herd of dinosaurs.

Femora from an exceptionally large population of coeval ornithomimosaurs yield evidence of sexual dimorphism in extinct theropod dinosaurs

Sexual dimorphism is challenging to detect among fossils due to a lack of statistical representativeness. The Angeac-Charente Lagerstätte (France) represents a remarkable 'snapshot' from a Berriasian (Early Cretaceous) ecosystem and offers a unique opportunity to study intraspecific variation among a herd of at least 61 coeval ornithomimosaurs. Herein, we investigated the hindlimb variation across the best-preserved specimens from the herd through 3D Geometric Morphometrics and Gaussian Mixture Modeling. Our results based on complete and fragmented femora evidenced a dimorphism characterized by variations in the shaft curvature and the distal epiphysis width. Since the same features vary between sexes among modern avian dinosaurs, crocodilians, and more distant amniotes, we attributed this bimodal variation to sexual dimorphism based on the extant phylogenetic bracketing approach. Documenting sexual dimorphism in fossil dinosaurs allows a better characterization and accounting of intraspecific variations, which is particularly relevant to address ongoing taxonomical and ecological questions relative to dinosaur evolution.

Armed to the teeth: The underestimated diversity in tooth shape in snakes and its relation to feeding behavior and diet

The structure, composition, and shape of teeth have been related to dietary specialization in many vertebrate species, but comparative studies on snakes' teeth are lacking. Yet, snakes have diverse dietary habits that may impact the shape of their teeth. We hypothesize that prey properties, such as hardness and shape, as well as feeding behavior, such as aquatic or arboreal predation, or holding vigorous prey, impose constraints on the evolution of tooth shape in snakes. We compared the morphology of the dentary teeth of 63 species that cover the phylogenetic and dietary diversity of snakes, using 3D geometric morphometrics and linear measurements. Our results show that prey hardness, foraging substrate, and the main feeding mechanical challenge are important drivers of tooth shape, size, and curvature. Overall, long, slender, curved teeth with a thin layer of hard tissue are observed in species that need to maintain a grip on their prey. Short, stout, less curved teeth are associated with species that undergo high or repeated loads. Our study demonstrates the diversity of tooth morphology in snakes and the need to investigate its underlying functional implications to better understand the evolution of teeth in vertebrates.

Shape variation in the limb long bones of modern elephants reveals adaptations to body mass and habitat

During evolution, several vertebrate lineages have shown trends towards an increase in mass. Such a trend is associated with physiological and musculoskeletal changes necessary to carry and move an increasingly heavy body. Due to their prominent role in the support and movement of the body, limb long bones are highly affected by these shifts in body mass. Elephants are the heaviest living terrestrial mammals, displaying unique features allowing them to withstand their massive weight, such as the columnarity of their limbs, and as such are crucial to understand the evolution towards high body mass in land mammals. In this study, we investigate the shape variation of the six limb long bones among the modern elephants, Elephas maximus and Loxodonta africana, to understand the effect of body mass and habitat on the external anatomy of the bones. To do so, we use three-dimensional geometric morphometrics (GMMs) and qualitative comparisons to describe the shape variation, at both the intraspecific and interspecific levels. Our results reveal that the two species share similar negative ontogenetic allometric patterns (i.e. becoming stouter with increased length) in their humerus and femur, but not in the other bones: the proximal epiphyses of the stylopod bones develop considerably during growth, while the distal epiphyses, which are involved in load distribution in the elbow and knee joints, are already massive in juveniles. We attribute this pattern to a weight-bearing adaptation already present in young specimens. Among adults of the same species, bone robustness increases with body mass, so that heavier specimens display stouter bones allowing for a better mechanical load distribution. While this robustness variation is significant for the humerus only, all the other bones appear to follow the same pattern. This is particularly visible in the ulna and tibia, but less so in the femur, which suggests that the forelimb and hindlimb adapted differently to high body mass support. Robustness analyses, while significant for the humerus only, suggest more robust long bones in Asian elephants than in African savanna elephants. More specifically, GMMs and qualitative comparisons indicate that three bones are clearly distinct when comparing the two species: in E. maximus the humerus, the ulna and the tibia display enlarged areas of muscular insertions for muscles involved in joint and limb stabilization, as well as in limb rotation. These results suggest a higher limb compliance in Asian elephants, associated with a higher dexterity, which could be linked to their habitat and foraging habits.

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia

The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly correlated with size and modulated by cranial shape during development. Still, adult sinus morphology distinguishes specimens at the family, genus and species level. We observe a clear distinction between Alligatoridae and Longirostres, a separation of different Crocodylus species and the subfossil Malagasy genus Voay, and a distinction between the Tomistoma and Gavialis lineages. Our approach is independent of molecular methods but concurs with the molecular topologies. Therefore, sinus characters could add significantly to morphological datasets, offering an alternative viewpoint to resolve problems in crocodylian relationships.

Geometric morphometric analysis of the bony labyrinth of the Sima de los Huesos hominins

The bony labyrinth contains phylogenetic information that can be used to determine interspecific differences between fossil hominins. The present study conducted a comparative 3D geometric morphometric analysis on the bony labyrinth of the Middle Pleistocene Sima de los Huesos (SH) hominins. The findings of this study corroborate previous multivariate analyses of the SH hominin bony labyrinth. The analysis of the semicircular canals revealed the SH hominin canal morphologies appear closer to those of the Neandertals than to those of Homo sapiens. This is attributable to a Neandertal-like ovoid anterior canal, and mediolaterally expanded, circular posterior canal. However, the SH hominins lack the increased torsion in the anterior canal and the inferior orientation of the lateral canal seen in Neandertals. The results of the cochlear analysis indicated that, although there is some overlap, there are notable differences between the SH hominins and the Neandertals. In particular, the SH hominin cochlea appears more constricted than in Neandertals in the first and second turns. A principal component analysis of the full bony labyrinth separated most SH hominins from the Neandertals, which largely clustered with modern humans. A covariance ratio analysis found a significant degree of modularity within the bony labyrinth of all three groups, with the SH hominins and Neandertals displaying the highest modularity. This modular signal in the bony labyrinth may be attributable to different selective pressures related to locomotion and audition. Overall, the results of this study confirm previous suggestions that the semicircular canals in the SH hominins are somewhat derived toward Neandertals, while their cochlea is largely primitive within the genus Homo.

Reassessment of the human mandible from Banyoles (Girona, Spain)

Since the discovery of a human mandible in 1887 near the present-day city of Banyoles, northeastern Spain, researchers have generally emphasized its archaic features, including the lack of chin structures, and suggested affinities with the Neandertals or European Middle Pleistocene (Chibanian) specimens. Uranium-series and electron spin resonance dating suggest the mandible dates to the Late Pleistocene (Tarantian), approximately ca. 45-66 ka. In this study, we reassessed the taxonomic affinities of the Banyoles mandible by comparing it to samples of Middle Pleistocene fossils from Africa and Europe, Neandertals, Early and Upper Paleolithic modern humans, and recent modern humans. We evaluated the frequencies and expressions of morphological features and performed a three-dimensional geometric morphometric analysis on a virtual reconstruction of Banyoles to capture overall mandibular shape. Our results revealed no derived Neandertal morphological features in Banyoles. While a principal component analysis based on Euclidean distances from the first two principal components clearly grouped Banyoles with both fossil and recent Homo sapiens individuals, an analysis of the Procrustes residuals demonstrated that Banyoles did not fit into any of the comparative groups. The lack of Neandertal features in Banyoles is surprising considering its Late Pleistocene age. A consideration of the Middle Pleistocene fossil record in Europe and southwest Asia suggests that Banyoles is unlikely to represent a late-surviving Middle Pleistocene population. The lack of chin structures also complicates an assignment to H. sapiens, although early fossil H. sapiens do show somewhat variable development of the chin structures. Thus, Banyoles represents a non-Neandertal Late Pleistocene European individual and highlights the continuing signal of diversity in the hominin fossil record. The present situation makes Banyoles a prime candidate for ancient DNA or proteomic analyses, which may shed additional light on its taxonomic affinities.

Evidence for an evo-devo-derived hypothesis on three-dimensional flower shape modularity in a tropical orchid clade

Covarying suites of phenotypic traits, or modules, are increasingly recognized to promote morphological evolution. However, information on how modularity influences flower diversity is rare and lacking for Orchidaceae. Here, we combine high-resolution X-ray computed tomography scanning with three-dimensional geometric morphometrics and phylogenetic comparative methods to test various hypotheses about three-dimensional patterns of flower evolutionary modularity in Malagasy Bulbophyllum orchids and examine rates and modes of module evolution. Based on the four evolutionary modules identified (i.e., sepals, lateral petals, labellum + column-foot, and column-part), our data support the hypothesis that both genetic-developmental and functional adaptive factors shaped evolutionary flower trait covariation in these tropical orchids. In line with "evo-devo" studies, we also find that the labellum evolved independently from the rest of the petal whorl. Finally, we show that modules evolved with different rates, and either in a neutral fashion (only column-part) or under selective constraints, as likely imposed by pollinators. Overall, this study supports current views that modular units can enhance the range and rate of morphological evolution.

Comparative morpho-functional analysis of the humerus and ulna in three Western European moles species of the genus Talpa, including the newly described T. aquitania

The forelimb is involved in many behaviours including locomotion. Notably, the humero-ulnar articulation, implicated in the elbow joint, is of particular importance for both mobility and stability. Functional constraints, induced in part by environmental plasticity, are thought to drive an important part of the bone shape as bone directly responds and remodels in response to both muscle and external forces. In this context, the study of subterranean moles is of particular interest. These moles occupy a hard and heavy medium in comparison with air or water, requiring a powerful body structure to shear and shift the soil. Their general morphology is therefore adapted to digging and to their subterranean lifestyle. The various morpho-functional patterns, which drive diverse abilities according to the environment, are likely targets of natural selection and it is, therefore, useful to understand the relationships between the bone shape and their function. Here, we quantify, through 3D geometric morphometric methods, the interspecific variability in the morphology of the ulna and humerus of three Talpa species, including the new species Talpa aquitania, to infer their potential consequence in species digging performance. We also quantify shape covariation and morphological integration between the humerus and the ulna to test whether these bones evolve as a uniform functional unit or as more or less independent modules. Our results show that interspecific anatomical differences in the humerus and ulna exist among the three species. Shape changes are mostly located at the level of joints and muscle attachments. As the species tend to live in allopatry and the fossorial lifestyle induces strong ecological constraints, interspecific variations could be explained by the properties of the environment in which they live, such as the compactness of the soil. Our results also show that the humerus and ulna are highly integrated. The covariation between the humerus and ulna in moles is dominated by variation in the attachment areas and particularly of the attachment areas of shoulder muscles concerning the humerus, which affect the mechanical force deployed during locomotion and digging. This study also highlights that in the new species, T. aquitania, variations in anatomical structure (general shape and joints) exist and are related to the locality of collect of the individuals.

Morphological correlates of distal fibular morphology with locomotion in great apes, humans, and Australopithecus afarensis

OBJECTIVES

Recent studies highlighted the importance of the fibula to further our understanding of locomotor adaptations in fossil hominins. In this study, we present a three-dimensional geometric morphometric (3D-GM) investigation of the distal fibula in extant hominids and Australopithecus afarensis with the aim of pointing out morphological correlations to arboreal behavior.

METHODS

Three-dimensional surface meshes of the distal fibula were obtained using computer tomography for 40 extant hominid specimens and laser scanner for five A. afarensis specimens. Distal fibula morphology was quantified positioning 11 fixed landmarks, 40 curve semilandmarks, and 20 surface landmarks on each specimen. A generalized Procrustes analysis (GPA) was carried out on all landmark coordinates followed by Procrustes ANOVA. Principal component analysis (PCA) was performed on the GPA-aligned shape coordinates. Kruskal-Wallis tests and Mann-Whitney test were performed on scores along PCs.

RESULTS

Great apes are characterized by a shorter subcutaneous triangular surface (STS), more downward facing fibulotalar articular facets, more anteriorly facing lateral malleolus and wider/deeper malleolar fossa than humans. Within great apes, orangutans are characterized by more medially facing fibulotalar articular facets. Australopithecus afarensis shows a unique distal fibular morphology with several traits that are generally associated more to arboreality and less to bipedalism such as a short STS, a more anteriorly facing, laterally pointing malleolus and deeper and larger malleolar fossa.

CONCLUSIONS

The distal fibula morphology is indicative of locomotor patterns within extant hominids. The 3D-GM method presented here can be successfully used to further our understanding of arboreal adaptations in fossil hominins.

Relationship between flightlessness and brain morphology among Rallidae

Studies have suggested that the brain morphology and flight ability of Aves are interrelated; however, such a relationship has not been thoroughly investigated. This study aimed to examine whether flight ability, volant or flightless, affects brain morphology (size and shape) in the Rallidae, which has independently evolved to adapt secondary flightlessness multiple times within a single taxonomic group. Brain endocasts were extracted from computed tomography images of the crania, measured by 3D geometric morphometrics, and were analyzed using principal component analysis. The results of phylogenetic ANCOVA showed that flightless rails have brain sizes and shapes that are significantly larger than and different from those of volant rails, even after considering the effects of body mass and brain size respectively. Flightless rails tended to have a wider telencephalon and more inferiorly positioned foramen magnum than volant rails. Although the brain is an organ that requires a large amount of metabolic energy, reduced selective pressure for a lower body weight may have allowed flightless rails to have larger brains. The evolution of flightlessness may have changed the position of the foramen magnum downward, which would have allowed the support of the heavier cranium. The larger brain may have facilitated the acquisition of cognitively advanced behavior, such as tool-using behavior, among rails.

How does bone microanatomy and musculature covary? An investigation in the forelimb of two species of martens (Martes foina, Martes martes)

The long bones and associated musculature play a prominent role in the support and movement of the body and are expected to reflect the associated mechanical demands. But in addition to the functional response to adaptive changes, the conjoined effects of phylogenetic, structural and developmental constraints also shape the animal's body. In order to minimise the effect of the aforementioned constraints and to reveal the biomechanical adaptations in the musculoskeletal system to locomotor mode, we here study the forelimb of two closely related martens: the arboreal pine marten (Martes martes) and the more terrestrial stone marten (Martes foina), focusing on their forelimb muscle anatomy and long bone microanatomy; and, especially, on their covariation. To do so, we quantified muscle data and bone microanatomical parameters and created 3D and 2D maps of the cortical thickness distribution for the three long bones of the forelimb. We then analysed the covariation of muscle and bone data, both qualitatively and quantitatively. Our results reveal that species-specific muscular adaptations are not clearly reflected in the microanatomy of the bones. Yet, we observe a global thickening of the bone cortex in the radius and ulna of the more arboreal pine marten, as well a stronger flexor muscle inserting on its elbow. We attribute these differences to variation in their locomotor modes. Analyses of our 2D maps revealed a shift of cortical thickness distribution pattern linked to ontogeny, rather than species-specific patterns. We found that although intraspecific variation is not negligible, species distinction was possible when taking muscular and bone microanatomical data into consideration. Results of our covariation analyses suggest that the muscle-bone correlation is linked to ontogeny rather than to muscular strength at zones of insertion. Indeed, if we find a correlation between cortical thickness distribution and the strength of some muscles in the humerus, that is not the case for the others and in the radius and ulna. Cortical thickness distribution appears rather linked to bone contact zones and ligament insertions in the radius and ulna, and to some extent in the humerus. We conclude that inference on muscle from bone microanatomy is possible only for certain muscles in the humerus.

Sexual dimorphism in an adaptive radiation: Does intersexual niche differentiation result in ecological character displacement?

Evolutionary radiations are one plausible explanation for the rich biodiversity on Earth. Adaptive radiations are the most studied form of evolutionary radiations, and ecological opportunity has been identified as one factor permitting them. Competition among individuals is supposedly highest in populations of conspecifics. Divergent modes of resource use might minimize trophic overlap, and thus intersexual competition, resulting in ecological character displacement between sexes. However, the role of intersexual differentiation in speciation processes is insufficiently studied. The few studies available suggest that intersexual niche differentiation exists in adaptive radiations, but their role within the radiation, and the extent of differentiation within the organism itself, remains largely unexplored. Here, we test the hypothesis that multiple morphological structures are affected by intersexual niche differentiation in "roundfin" Telmatherina, the first case where intersexual niche differentiation was demonstrated in an adaptive fish radiation. We show that sexes of two of the three morphospecies differ in several structural components of the head, all of these are likely adaptive. Sexual dimorphism is linked to the respective morphospecies-specific ecology and affects several axes of variation. Trait variation translates into different feeding modes, processing types, and habitat usages that add to interspecific variation in all three morphospecies. Intrasexual selection, that is, male-male competition, may contribute to variation in some of the traits, but appears unlikely in internal structures, which are invisible to other individuals. We conclude that intersexual variation adds to the adaptive diversity of roundfins and might play a key role in minimizing intersexual competition in emerging radiations.

Three-dimensional form and function of the nasal cavity and nasopharynx in humans and chimpanzees

The facial differences between recent Pan troglodytes and Homo sapiens can be used as a proxy for the reduction of facial prognathism that happened during evolutionary transition between Australopithecines and early Homo. The projecting nasal morphology of Homo has been considered both a passive consequence of anatomical reorganization related to brain and integrated craniofacial evolution as well as an adaptation related to air-conditioning during physiological and behavioral shifts in human evolution. Yet, previous research suggested impaired air-conditioning in Homo challenging respiratory adaptations based on computational fluid dynamics (CFD) and airflow simulations. Here we improved CFD model at the inflow region and also carried out three-dimensional (3D) geometric morphometrics to address the hypothesis of impaired air-conditioning in humans and species differences in airway shape. With the new CFD model we simulated pressure, velocity, and temperature changes in airflow of six adult humans and six chimpanzees and analyzed 164 semi-landmarks of 10 humans and 10 chimpanzees for 3D size and shape comparisons. Our finding shows significantly different internal 3D nasal airways. Also, species means of pressure, velocity, and temperature differed statistically significantly. However, form-related differences in temperature exchanges seem subtle and may question adaptive disadvantages. We rather support a hypothesis of craniofacial changes in the Australopithecus-Homo transition that are related to brain evolution and craniofacial integration with facial and nasal modifications that contribute to maintain respiratory adaptations related to air conditioning.

Evidence for selectively constrained 3D flower shape evolution in a Late Miocene clade of Malagasy Bulbophyllum orchids

Questions concerning the evolution of complex biological structures are central to the field of evolutionary biology. Yet, still little information is known about the modes and temporal dynamics of three-dimensional (3D) flower shape evolution across the history of clades. Here, we combined high-resolution X-ray computed tomography with 3D geometric morphometrics and phylogenetic comparative methods to test models of whole-flower shape evolution in the orchid family, using an early Late Miocene clade (c. 50 spp.) of Malagasy Bulbophyllum as model system. Based on landmark data of 38 species, our high-dimensional model fitting decisively rejects a purely neutral mode of evolution, suggesting instead that flower shapes evolved towards a primary adaptive optimum. Only a small number of recently evolved species/lineages attained alternative shape optima, resulting in an increased rate of phenotypic evolution. Our findings provide evidence of constrained 3D flower shape evolution in a small-sized clade of tropical orchids, resulting in low rates of phenotypic evolution and uncoupled trait-diversification rates. We hypothesise that this deep imprint of evolutionary constraint on highly complex floral structures might reflect long-term (directional and/or stabilizing) selection exerted by the group's main pollinators (flies).

Inside the head of snakes: influence of size, phylogeny, and sensory ecology on endocranium morphology

Environmental properties, and the behavioral habits of species impact sensory cues available for foraging, predator avoidance and inter/intraspecific communication. Consequently, relationships have been discovered between the sensory ecology and brain morphology in many groups of vertebrates. However, these types of studies have remained scare on snake. Here, we investigate the link between endocranial shape and the sensory-related ecology of snakes by comparing 36 species of snakes for which we gathered six sensory-ecology characteristics. We use µCT scanning and 3D geometric morphometrics to compare their endocranium in a phylogenetically informed context. Our results demonstrate that size is a major driver of endocranial shape, with smaller species tending to maximize endocranial volume using a more bulbous shape, while larger species share an elongate endocranial morphology. Phylogeny plays a secondary role with more derived snakes diverging the most in endocranial shape, compared to other species. The activity period influences the shape of the olfactory and optic tract, while the foraging habitat impacts the shape of the cerebellum and cranial nerve regions: structures involved in orientation, equilibrium, and sensory information. However, we found that endocranial morphology alone is not sufficient to predict the activity period of a species without prior knowledge of its phylogenetic relationship. Our results thus demonstrate the value of utilizing endocranial shape as complementary information to size and volume in neurobiological studies.

Early mandibular morphological differences in patients with FGFR2 and FGFR3-related syndromic craniosynostoses: A 3D comparative study

Syndromic craniosynostoses are defined by the premature fusion of one or more cranial and facial sutures, leading to skull vault deformation, and midfacial retrusion. More recently, mandibular shape modifications have been described in FGFR-related craniosynostoses, which represent almost 75% of the syndromic craniosynostoses. Here, further characterisation of the mandibular phenotype in FGFR-related craniosynostoses is provided in order to confirm mandibular shape modifications, as this could contribute to a better understanding of the involvement of the FGFR pathway in craniofacial development. The aim of our study was to analyse early mandibular morphology in a cohort of patients with FGFR2- (Crouzon and Apert) and FGFR3- (Muenke and Crouzonodermoskeletal) related syndromic craniosynostoses. We used a comparative geometric morphometric approach based on 3D imaging. Thirty-one anatomical landmarks and eleven curves with sliding semi-landmarks were defined to model the shape of the mandible. In total, 40 patients (12 with Crouzon, 12 with Apert, 12 with Muenke and 4 with Crouzonodermoskeletal syndromes) and 40 age and sex-matched controls were included (mean age: 13.7 months ±11.9). Mandibular shape differed significantly between controls and each patient group based on geometric morphometrics. Mandibular shape in FGFR2-craniosynostoses was characterized by open gonial angle, short ramus height, and high and prominent symphysis. Short ramus height appeared more pronounced in Apert than in Crouzon syndrome. Additionally, narrow inter-condylar and inter-gonial distances were observed in Crouzon syndrome. Mandibular shape in FGFR3-craniosynostoses was characterized by high and prominent symphysis and narrow inter-gonial distance. In addition, narrow condylar processes affected patients with Crouzonodermoskeletal syndrome. Statistical analysis of variance showed significant clustering of Apert and Crouzon, Crouzon and Muenke, and Apert and Muenke patients (p < 0.05). Our results confirm distinct mandibular shapes at early ages in FGFR2- (Crouzon and Apert syndromes) and FGFR3-related syndromic craniosynostoses (Muenke and Crouzonodermoskeletal syndromes) and reinforce the hypothesis of genotype-phenotype correspondence concerning mandibular morphology.

Phenotypic integration in feliform carnivores: Covariation patterns and disparity in hypercarnivores versus generalists

The skeleton is a complex arrangement of anatomical structures that covary to various degrees depending on both intrinsic and extrinsic factors. Among the Feliformia, many species are characterized by predator lifestyles providing a unique opportunity to investigate the impact of highly specialized hypercarnivorous diet on phenotypic integration and shape diversity. To do so, we compared the shape of the skull, mandible, humerus, and femur of species in relation to their feeding strategies (hypercarnivorous vs. generalist species) and prey preference (predators of small vs. large prey) using three-dimensional geometric morphometric techniques. Our results highlight different degrees of morphological integration in the Feliformia depending on the functional implication of the anatomical structure, with an overall higher covariation of structures in hypercarnivorous species. The skull and the forelimb are not integrated in generalist species, whereas they are integrated in hypercarnivores. These results can potentially be explained by the different feeding strategies of these species. Contrary to our expectations, hypercarnivores display a higher disparity for the skull than generalist species. This is probably due to the fact that a specialization toward high-meat diet could be achieved through various phenotypes. Finally, humeri and femora display shape variations depending on relative prey size preference. Large species feeding on large prey tend to have robust long bones due to higher biomechanical constraints.

Morphological evolution of the carnivoran sacrum

The sacrum is a key piece of the vertebrate skeleton, since it connects the caudal region with the presacral region of the vertebral column and the hind limbs through the pelvis. Therefore, understanding its form and function is of great relevance in vertebrate ecomorphology. However, it is striking that morphometric studies that quantify its morphological evolution in relation to function are scarce. The main goal of this study is to investigate the morphological evolution of the sacrum in relation to its function in the mammalian order Carnivora, using three-dimensional (3D) geometric morphometrics. Principal component analysis under a phylogenetic background indicated that changes in sacrum morphology are mainly focused on the joint areas where it articulates with other parts of the skeleton allowing resistance to stress at these joints caused by increasing muscle loadings. In addition, we demonstrated that sacrum morphology is related to both the length of the tail relativised to the length of the body, and the length of the body relativised to body mass. We conclude that the sacrum in carnivores has evolved in response to the locomotor requirements of the species analysed, but in locomotion, each family has followed alternative morphological solutions to address the same functional demands.

Craniofacial allometry in the OIM-/- mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a skeletal disorder characterized by the impaired synthesis of type I collagen (Col1). This study tests the hypothesis that the craniofacial phenotype of severe OI is linked to an overall reduction in body size. 3D landmark data were collected from µCT scans of adult OIM^-/- and wild-type (WT) mice and used to calculate centroid sizes (CS) and interlandmark distances (ILDs). To remove the effect of body size, ILDs were scaled against craniomandibular lengths and CS. Mann-Whitney U tests were used to compare CS and absolute/scaled ILDs between genotypes. OIM^-/- mice are smaller than their WT littermates in body mass, craniomandibular CS, and absolute ILDs including skull, basicranial, palatal, mandibular, and toothrow lengths. When linear distances are scaled to CS, OIM^-/- mice have a relatively short midface, short nasal bones, tall mandibular corpora, and long mandibular toothrows. Results underscore the importance of size and scaling in morphometric analyses. The deleterious effect of Col1 mutations on global skeletal dimensions combined with localized morphometric changes may underlie the facial phenotype seen in human patients with severe OI. Attempts to identify these localized changes should first account for the restricted growth and small body sizes present in individuals with OI.

Colonize, radiate, decline: Unraveling the dynamics of island community assembly with Fijian trap-jaw ants

The study of island community assembly has been fertile ground for developing and testing theoretical ideas in ecology and evolution. The ecoevolutionary trajectory of lineages after colonization has been a particular interest, as this is a key component of understanding community assembly. In this system, existing ideas, such as the taxon cycle, posit that lineages pass through a regular sequence of ecoevolutionary changes after colonization, with lineages shifting toward reduced dispersal ability, increased ecological specialization, and declines in abundance. However, these predictions have historically been difficult to test. Here, we integrate phylogenomics, population genomics, and X-ray microtomography/3D morphometrics, to test hypotheses for whether the ecomorphological diversity of trap-jaw ants (Strumigenys) in the Fijian archipelago is assembled primarily through colonization or postcolonization radiation, and whether species show ecological shifts toward niche specialization, toward upland habitats, and decline in abundance after colonization. We infer that most Fijian endemic Strumigenys evolved in situ from a single colonization and have diversified to fill a large fraction of global morphospace occupied by the genus. Within this adaptive radiation, lineages trend to different degrees toward high elevation, reduced dispersal ability, and demographic decline, and we find no evidence of repeated colonization that displaces the initial radiation. Overall these results are only partially consistent with taxon cycle and associated ideas, while highlighting the potential role of priority effects in assembling island communities.

Correlation of skull morphology and bite force in a bird-eating bat (Ia io; Vespertilionidae)

Background

Genetic and ecological factors influence morphology, and morphology is compatible with function. The morphology and bite performance of skulls of bats show a number of characteristic feeding adaptations. The great evening bat, Ia io (Thomas, 1902), eats both insects and birds (Thabah et al. J Mammal 88: 728-735, 2007), and as such, it is considered to represent a case of dietary niche expansion from insects to birds. How the skull morphology or bite force in I. io are related to the expanded diet (that is, birds) remains unknown. We used three-dimensional (3D) geometry of the skulls and measurements of bite force and diets from I. io and 13 other species of sympatric or closely related bat species to investigate the characteristics and the correlation of skull morphology and bite force to diets.

Results

Significant differences in skull morphology and bite force among species and diets were observed in this study. Similar to the carnivorous bats, bird-eaters (I. io) differed significantly from insectivorous bats; I. io had a larger skull size, taller crania, wider zygomatic arches, shorter but robust mandibles, and larger bite force than the insectivores. The skull morphology of bats was significantly associated with bite force whether controlling for phylogeny or not, but no significant correlations were found between diets and the skulls, or between diets and residual bite force, after controlling for phylogeny.

Conclusions

These results indicated that skull morphology was independent of diet, and phylogeny had a greater impact on skull morphology than diet in these species. The changes in skull size and morphology have led to variation in bite force, and finally different bat species feeding on different foods. In conclusion, I. io has a larger skull size, robust mandibles, shortened dentitions, longer coronoid processes, expanded angular processes, low condyles, and taller cranial sagittal crests, and wider zygomatic arches that provide this species with mechanical advantages; their greater bite force may help them use larger and hard-bodied birds as a dietary component.

Analyses of the neandertal patellae from El Sidrón (Asturias, Spain) with implications for the evolution of body form in Homo

The evolution of the body form in Homo and its potential morphological connection to the arrangement of different skeletal systems is of major interest in human evolution. Patella morphology as part of the knee is potentially influenced by body form. Here, we describe for the first time the patellae remains recovered at El Sidrón Neandertal site and analyze them in a comparative evolutionary framework. We aim to clarify whether morphometric features frequently observed in Neandertal and modern human patellae are retained from a primitive anatomical arrangement or whether they represent derived features (apomorphies). For this purpose, we combine analyses of discrete features, classic anthropological measurements, and 3D geometric morphometrics based on generalized Procrustes analysis, mean size and shape comparisons, and principal components analysis. We found a size increment of the patella in hominin evolution, with large species showing a larger patella. Modern humans and Neandertals exhibit overall larger patellae, with maximum values observed in the latter, likely as a consequence of their broader body shape. Also, some Neandertals display a thicker patella, which has been linked to larger quadriceps muscles. However, Neandertals retain a primitive morphology in their patellar articular surfaces, with similar-sized lateral and medial articular facets, leading to a more symmetrical internal face. This feature is inherited from a primitive Homo ancestor and suggests a different configuration of the knee in Neandertals. Conversely, Homo sapiens exhibits an autoapomorphic patellar anatomy with expanded lateral articular facets. We propose that these distinct configurations of the patella within Homo may be a consequence of different body forms rather than specific functional adaptations of the knee. Thus, the slender body form of modern humans may entail a medial reorientation of the tibial tuberosity (patellar ligament), allowing lateral surface expansion. These anatomical evolutionary variations may involve subtle secondary differences in bipedalism within Homo.

The effects of phylogeny, body size, and locomotor behavior on the three-dimensional shape of the pelvis in extant carnivorans

The mammalian pelvis is thought to exhibit adaptations to the functional demands of locomotor behaviors. Previous work in primates has identified form-function relationships between pelvic shape and locomotor behavior; few studies have documented such relationships in carnivorans, instead focusing on long bones. Most work on the functional morphology of the carnivoran pelvis, in particular, has used univariate measures, with only a few previous studies incorporating a three-dimensional (3D) analysis. Here we test the hypothesis that carnivoran taxa that are characterized by different locomotor modes also differ in 3D shape of the os coxae. Using 3D geometric morphometrics and phylogenetic comparative methods, we evaluate the phylogenetic, functional, and size-related effects on 3D pelvis shape in a sample of 33 species of carnivorans. Using surface models derived from laser scans, we collected a suite of landmarks (N = 24) and curve semilandmarks (N = 147). Principal component analysis on Procrustes coordinates demonstrates patterns of shape change in the ischiopubis and ilium likely related to allometry. Phylogenetic generalized least squares analysis on principal component scores demonstrates that phylogeny and body size have greater effects on pelvic shape than locomotor function. Our results corroborate recent research finding little evidence of locomotor specialization in the pelvis of carnivorans. More research on pelvic morphological integration and evolvability is necessary to understand the factors driving pelvic evolution in carnivorans.

Interspecific variation in the limb long bones among modern rhinoceroses-extent and drivers

Among amniotes, numerous lineages are subject to an evolutionary trend toward body mass and size increases. Large terrestrial species may face important constraints linked to weight bearing, and the limb segments are particularly affected by such constraints due to their role in body support and locomotion. Such groups showing important limb modifications related to high body mass have been called “graviportal.” Often considered graviportal, rhinoceroses are among the heaviest terrestrial mammals and are thus of particular interest to understand the limb modifications related to body mass and size increase. Here, we present a morphofunctional study of the shape variation of the limb long bones among the five living rhinos to understand how the shape may vary between these species in relation with body size, body mass and phylogeny. We used three dimensional geometric morphometrics and comparative analyses to quantify the shape variation. Our results indicate that the five species display important morphological differences depending on the considered bones. The humerus and the femur exhibit noticeable interspecific differences between African and Asiatic rhinos, associated with a significant effect of body mass. The radius and ulna are more strongly correlated with body mass. While the tibia exhibits shape variation both linked with phylogeny and body mass, the fibula displays the greatest intraspecific variation. We highlight three distinct morphotypes of bone shape, which appear in accordance with the phylogeny. The influence of body mass also appears unequally expressed on the different bones. Body mass increase among the five extant species is marked by an increase of the general robustness, more pronounced attachments for muscles and a development of medial parts of the bones. Our study underlines that the morphological features linked to body mass increase are not similar between rhinos and other heavy mammals such as elephants and hippos, suggesting that the weight bearing constraint can lead to different morphological responses.

Neurocranial shape variation among Tarascan populations: Evidence for varying degrees in artificially modified crania in pre-Hispanic West Mexico (1200-1400 AD)

INTRODUCTION

Along the Mesoamerican western margin, the Zacapu basin has yielded a large number of human remains demonstrating usage of artificial cranial modification (ACM). However, at the onset of the Middle Postclassic (1200-1400 AD) only few individuals still exhibit clear signs of ACM. Some authors have suggested that, rather than disappearing entirely, ACM may have become less visible anatomically, making it difficult to identify based on simple visual analyses. Here, we used 3D geometric morphometric methods to investigate the extent to which ACM persisted during the Postclassic in this region.

MATERIALS AND METHODS

We measured the 3D vault's shape changes in a sample of surface-scanned human crania: 55 individuals from the Postclassic Zacapu basin and a control group of 31 individuals from a Huichol Mexican Indian sample and a French medieval series from La Granède. We used a principal component analysis to explore the shape variation within the sample and employed the neighbor joining method to identify morphological groups. Finally, we quantified each individual's asymmetry.

RESULTS

We identified three groups displaying shape features diverging from those of the control group. The first group is characterized by marked fronto-obelionic ACM, whereas the other two show mild forms of ACM. The individuals in all three groups display moderate to high degrees of asymmetry compared to the control group.

DISCUSSION

The marked fronto-obelionic modification is clear evidence of a specific ACM technique. The two types of mild ACM most likely result from different techniques but their moderate degree of modification brings into question the intentions behind their production.

Does 3D Phenotyping Yield Substantial Insights in the Genetics of the Mouse Mandible Shape?

We describe the application of high-resolution 3D microcomputed tomography, together with 3D landmarks and geometric morphometrics, to validate and further improve previous quantitative genetic studies that reported QTL responsible for variation in the mandible shape of laboratory mice using a new backcross between C57BL/6J and A/J inbred strains. Despite the increasing availability of 3D imaging techniques, artificial flattening of the mandible by 2D imaging techniques seems at first an acceptable compromise for large-scale phenotyping protocols, thanks to an abundance of low-cost digital imaging systems such as microscopes or digital cameras. We evaluated the gain of information from considering explicitly this additional third dimension, and also from capturing variation on the bone surface where no precise anatomical landmark can be marked. Multivariate QTL mapping conducted with different landmark configurations (2D vs. 3D; manual vs. semilandmarks) broadly agreed with the findings of previous studies. Significantly more QTL (23) were identified and more precisely mapped when the mandible shape was captured with a large set of semilandmarks coupled with manual landmarks. It appears that finer phenotypic characterization of the mandibular shape with 3D landmarks, along with higher density genotyping, yields better insights into the genetic architecture of mandibular development. Most of the main variation is, nonetheless, preferentially embedded in the natural 2D plane of the hemi-mandible, reinforcing the results of earlier influential investigations.

Virtual ancestor reconstruction: Revealing the ancestor of modern humans and Neandertals

The timing and geographic origin of the common ancestor of modern humans and Neandertals remain controversial. A poor Pleistocene hominin fossil record and the evolutionary complexities introduced by dispersals and regionalisation of lineages have fuelled taxonomic uncertainty, while new ancient genomic data have raised completely new questions. Here, we use maximum likelihood and 3D geometric morphometric methods to predict possible morphologies of the last common ancestor of modern humans and Neandertals from a simplified, fully resolved phylogeny. We describe the fully rendered 3D shapes of the predicted ancestors of humans and Neandertals, and assess their similarity to individual fossils or populations of fossils of Pleistocene age. Our results support models of an Afro-European ancestral population in the Middle Pleistocene (Homo heidelbergensis sensu lato) and further predict an African origin for this ancestral population.

Do constraints associated with the locomotor habitat drive the evolution of forelimb shape? A case study in musteloid carnivorans

Convergence in morphology can result from evolutionary adaptations in species living in environments with similar selective pressures. Here, we investigate whether the shape of the forelimb long bones has converged in environments imposing similar functional constraints, using musteloid carnivores as a model. The limbs of quadrupeds are subjected to many factors that may influence their shape. They need to support body mass without collapsing or breaking, yet at the same time resist the stresses and strains induced by locomotion. This likely imposes strong constraints on their morphology. Our geometric morphometric analyses show that locomotion, body mass and phylogeny all influence the shape of the forelimb. Furthermore, we find a remarkable convergence between: (i) aquatic and semi-fossorial species, both displaying a robust forelimb, with a shape that improves stability and load transfer in response to the physical resistance imposed by the locomotor environment; and (ii) aquatic and arboreal/semi-arboreal species, with both groups displaying a broad capitulum. This augments the degree of pronation/supination, an important feature for climbing as well as grasping and manipulation ability, behaviors common to aquatic and arboreal species. In summary, our results highlight how musteloids with different locomotor ecologies show differences in the anatomy of their forelimb bones. Yet, functional demands for limb movement through dense media also result in convergence in forelimb long-bone shape between diverse groups, for example, otters and badgers.

The influence of feeding on the evolution of sensory signals: a comparative test of an evolutionary trade-off between masticatory and sensory functions of skulls in southern African horseshoe bats (Rhinolophidae)

The skulls of animals have to perform many functions. Optimization for one function may mean another function is less optimized, resulting in evolutionary trade-offs. Here, we investigate whether a trade-off exists between the masticatory and sensory functions of animal skulls using echolocating bats as model species. Several species of rhinolophid bats deviate from the allometric relationship between body size and echolocation frequency. Such deviation may be the result of selection for increased bite force, resulting in a decrease in snout length which could in turn lead to higher echolocation frequencies. If so, there should be a positive relationship between bite force and echolocation frequency. We investigated this relationship in several species of southern African rhinolophids using phylogenetically informed analyses of the allometry of their bite force and echolocation frequency and of the three-dimensional shape of their skulls. As predicted, echolocation frequency was positively correlated with bite force, suggesting that its evolution is influenced by a trade-off between the masticatory and sensory functions of the skull. In support of this, variation in skull shape was explained by both echolocation frequency (80%) and bite force (20%). Furthermore, it appears that selection has acted on the nasal capsules, which have a frequency-specific impedance matching function during vocalization. There was a negative correlation between echolocation frequency and capsule volume across species. Optimization of the masticatory function of the skull may have been achieved through changes in the shape of the mandible and associated musculature, elements not considered in this study.

New insights into the phenotypic covariance structure of the anthropoid cranium

In complex organisms, suites of non-random, highly intercorrelated phenotypic traits, organized according to their developmental history and forming semi-autonomous units (i.e. modules), have the potential to impose constraints on morphological diversification or to improve evolvability. Because of its structural, developmental and functional complexity, the cranium is arguably one of the best models for studying the interplay between developmental history and the need for various parts of a structure to specialize in different functions. This study evaluated the significance of two specific types of developmental imprints in the adult anthropoid cranium, those imposed by ossification pattern (i.e. ossification with and without a pre-existing cartilaginous phase) and those imposed by tissue origin (i.e. tissues derived principally from neural-crest vs. those derived from paraxial mesoderm). Specifically, this study tests the hypothesis that the face and the basicranium form two distinct modules with higher within-unit trait integration magnitudes compared with the cranium as a whole. Data on 12 anthropoid primate species were collected in the form of 20 three-dimensional landmarks digitized on cranial surface models that sample the basicranium as well as regions of functional importance during feeding [corrected]. The presence of a significant modularity imprint in the adult cranium was assessed using a between-region within-species comparison of multivariate correlations (RV coefficients) obtained with partial least-squares, using within-module within-species eigenvalue variance (EV), and using cluster analyses and non-metric multidimensional scaling. In addition to addressing the validity of the cranial modularity hypothesis in anthropoids, this study addressed methodological aspects of the interspecific comparison of morphological integration, namely the effect of sample size and the effect of landmark number on integration magnitudes. Two methodological findings that are of significance to research in morphological integration are that: (i) a smaller sample size increases integration magnitude, but preserves the pattern of variation of integration magnitudes from block to block within species; and that (ii) the number of landmarks per cranial block does not significantly impact block integration magnitude measured as EV. Results from the analyses testing for cranial modularity imprints in the adult anthropoid cranium show that some facial landmarks covary more strongly with basicranial landmarks than with other facial landmarks. Cluster methods, non-metric multidimensional scaling and, to an extent, RV results show that the rostral and the zygomatic landmarks covary more strongly with the basicranial landmarks than they do with the molar landmarks. However, the rostral-zygomatic-basicranial block, the molar block, the facial block, the basicranial block and the other analyzed cranial and facial blocks are not more integrated than the cranium. Thus, the morphological variation in the adult anthropoid cranium is not significantly constrained by at least two of the potential developmental sources of its covariance structure.

Morphological integration in the forelimb of musteloid carnivorans

The forelimb forms a functional unit that allows a variety of behaviours and needs to be mobile, yet at the same time stable. Both mobility and stability are controlled, amongst others, at the level of the elbow joint. This joint is composed of the humero-ulnar articulation, mainly involved during parasagittal movements; and the radio-ulnar articulation, mainly allowing rotation. In contrast, the humero-radial articulation allows both movements of flexion-extension and rotation. Here, we study the morphological integration between each bone of the forelimb at the level of the entire arm, as well as at the elbow joint, in musteloid carnivorans. To do so, we quantitatively test shape co-variation using surface 3D geometric morphometric data. Our results show that morphological integration is stronger for bones that form functional units. Different results are obtained depending on the level of investigation: for the entire arm, results show a greater degree of shape co-variation between long bones of the lower arm than between the humerus and either bone of the lower arm. Thus, at this level the functional unit of the lower arm is comprised of the radius and ulna, permitting rotational movements of the lower arm. At the level of the elbow, results display a stronger shape co-variation between bones allowing flexion and stability (humerus and ulna) than between bones allowing mobility (ulna and radius and humerus and radius). Thus, the critical functional unit appears to be the articulation between the humerus and ulna providing the stability of the joint.