Comparison of cranial ontogenetic trajectories among great apes and humans

Skull shape variation in extant pangolins (Pholidota: Manidae): allometric patterns and systematic implications

Pangolins are among the most endangered groups of mammals, comprising eight extant species delineated into three genera. Despite several studies dedicated to their skeletal anatomy, the potential taxonomic insight from cranial morphological variation in extant Pholidota is yet to be assessed with modern geometric morphometric methods. We present the first comprehensive study on the cranial morphology of extant pangolins and discuss its implications for the taxonomy and evolution of the group. We performed landmark-based morphometric analyses on 241 museum specimens to describe the variation in skull shape in seven of the eight extant species. Our analyses revealed genus- and species-level morphological discrimination, with Asian species (Manis spp.) being grouped together, whereas African pangolins present distinct skull shapes between small (Phataginus spp.) and large (Smutsia spp.) species. Analyses of allometry also identified a set of traits whose allometric trajectories distinguish Asian from African specimens. Finally, we uncovered intraspecific variation in skull shape in white-bellied pangolins (Phataginus tricuspis) that partly corroborates recent DNA-based differentiation among biogeographically distinct populations. Overall, our results shed light on the morphological diversity of the skull of these enigmatic myrmecophagous mammals and confirm the genus-level classification and cryptic diversity within the white-bellied pangolin revealed by molecular phylogenetics.

Integration and the Developmental Genetics of Allometry

Allometry refers to the ways in which organismal shape is associated with size. It is a special case of integration, or the tendency for traits to covary, in that variation in size is ubiquitous and evolutionarily important. Allometric variation is so commonly observed that it is routinely removed from morphometric analyses or invoked as an explanation for evolutionary change. In this case, familiarity is mistaken for understanding because rarely do we know the mechanisms by which shape correlates with size or understand their significance. As with other forms of integration, allometric variation is generated by variation in developmental processes that affect multiple traits, resulting in patterns of covariation. Given this perspective, we can dissect the genetic and developmental determinants of allometric variation. Our work on the developmental and genetic basis for allometric variation in craniofacial shape in mice and humans has revealed that allometric variation is highly polygenic. Different measures of size are associated with distinct but overlapping patterns of allometric variation. These patterns converge in part on a common genetic basis. Finally, environmental modulation of size often generates variation along allometric trajectories, but the timing of genetic and environmental perturbations can produce deviations from allometric patterns when traits are differentially sensitive over developmental time. These results question the validity of viewing allometry as a singular phenomenon distinct from morphological integration more generally.

Measuring the magnitude of morphological integration: The effect of differences in morphometric representations and the inclusion of size

The magnitude of morphological integration is a major aspect of multivariate evolution, providing a simple measure of the intensity of association between morphological traits. Studies concerned with morphological integration usually translate phenotypes into morphometric representations to quantify how different morphological elements covary. Geometric and classic morphometric representations translate biological form in different ways, raising the question if magnitudes of morphological integration estimates obtained from different morphometric representations are compatible. Here we sought to answer this question using the relative eigenvalue variance of the covariance matrix obtained for both geometric and classical representations of empirical and simulated datasets. We quantified the magnitude of morphological integration for both shape and form and compared results between representations. Furthermore, we compared integration values between shape and form to evaluate the effect of the inclusion or not of size on the quantification of the magnitude of morphological integration. Results show that the choice of morphological representation has significant impact on the integration magnitude estimate, either for shape or form. Despite this, ordination of the integration values within representations is relatively the same, allowing for similar conclusions to be reached using different methods. However, the inclusion of size in the dataset significantly changes the estimates of magnitude of morphological integration, hindering the comparison of this statistic obtained from different spaces. Morphometricians should be aware of these differences and must consider how biological hypothesis translate into predictions about integration in each particular choice of representation.

A geometric morphometric approach to the analysis of skull shape in Triassic dicynodonts (Therapsida, Anomodontia) from South America

Dicynodont therapsids were a major component of the Permo-Triassic terrestrial ecosystems across Pangea and have been regarded as specialized herbivores. In South America, the group was represented by several taxa of the clade Kannemeyeriiformes spanning from the Middle to the Late Triassic. In order to evaluate if cranial differences among taxa are potentially related to differences in feeding function, we performed a geometric morphometric analysis on 28 South American dicynodont crania. We digitized 19 cranial landmarks and conducted generalized Procrustes analysis, principal component analysis (PCA), principal component analysis between groups (bg-PCA), and a branch weighted squared-change parsimony approach. Phylogenetic inertia was not a significant driver of cranial shape evolution in the group, whereas PCA and bg-PCA support that major morphological shape differences are concentrated in the preorbital region (relative length of the snout and width of the caniniform process), in the position of quadrate condyle in relation to the caniniform process, and in the increase in the intertemporal surface area. In this context, tusked Dinodontosaurus, "Kannemeyeria," and Vinceria have relatively smaller adductor attachment areas and input moment arm than younger taxa lacking tusks, such as Ischigualastia, Stahleckeria, and Jachaleria. Differences in cranial morphology in later dicynodonts reflect modifications in feeding mechanics, probably due to changes in food resources (vegetation) in their habitats toward the end of the Triassic.

Quantifying shape and ecology in avian pedal claws: The relationship between the bony core and keratinous sheath

Terrestrial tetrapods use their claws to interact with their environments in a plethora of ways. Birds in particular have developed a diversity of claw shapes since they are often not bound to terrestrial locomotion and have heterogeneous body masses ranging several orders of magnitude. Numerous previous studies have hypothesized a connection between pedal claw shape and ecological mode in birds, yet have generated conflicting results, spanning from clear ecological groupings based on claw shape to a complete overlap of ecological modes. The majority of these studies have relied on traditional morphometric arc measurements of keratinous sheaths and have variably accounted for likely confounding factors such as body mass and phylogenetic relatedness. To better address the hypothesized relationship between ecology and claw shape in birds, we collected 580 radiographs allowing visualization of the bony core and keratinous sheath shape in 21 avian orders. Geometric morphometrics was used to quantify bony core and keratinous sheath shape and was compared to results using traditional arc measurements. Neither approach significantly separates bird claws into coarse ecological categories after integrating body size and phylogenetic relatedness; however, some separation between ecological groups is evident and we find a gradual shift from the claw shape of ground-dwelling birds to those of predatory birds. Further, the bony claw core and keratinous sheath are significantly correlated, and the degree of functional integration does not differ across ecological groups. Therefore, it is likely possible to compare fossil bony cores with extant keratinous sheaths after applying corrections. Finally, traditional metrics and geometric morphometric shape are significantly, yet loosely correlated. Based on these results, future workers are encouraged to use geometric morphometric approaches to study claw geometry and account for confounding factors such as body size, phylogeny, and individual variation prior to predicting ecology in fossil taxa.

Shape variation in the facial part of the cranium in macaques and African papionins using geometric morphometrics

Macaques are one of the most successful nonhuman primates, and morphological distinctions from their close relatives, African papionins, are easily detected by the naked eye. Nevertheless, evolutionary allometry often accounts for a large amount of the total variation and potentially hides and precludes the detection of morphological distinctions that exist between macaques and African papionins, thus distorting their phyletic comparison. Geometric morpgometric analyses were performed using landmark coordinates in cranial samples from macaques (N =  135) and African papionins (N =  152) to examine the variation in their facial shape. A common allometric trend was confirmed to represent a moderately long face in macaques as being small-to-moderate-bodied papionins. Macaques possessed many features that were distinct from those of African papionins, while they simultaneously showed a large intrageneric variation in every feature, which precluded the separation of some groups of macaques from African papionins. This study confirmed that a moderately smooth sagittal profile is present in non-Sulawesi macaques. It also confirmed that a well-developed anteorbital drop is distinct in Mandrillus and Theropithecus, but it showed that Papio resembles macaques regarding this feature. This finding showed that apparently equivalent features which can be detected by the naked eye were probably formed by different combinations of the principal patterns. It should be noted that the differences detected here between macaques and African papionins are revealed after appropriate adjustments are made to eliminate the allometric effects over the shape features. While landmark data sets still need to be customized for specific studies, the information provided by this article is expected to help such customization and to improve future phyletic evaluation of the fossil papionins.

Peramorphosis, an evolutionary developmental mechanism in neotropical bat skull diversity

BACKGROUND

The neotropical leaf-nosed bats (Chiroptera, Phyllostomidae) are an ecologically diverse group of mammals with distinctive morphological adaptations associated with specialized modes of feeding. The dramatic skull shape changes between related species result from changes in the craniofacial development process, which brings into focus the nature of the underlying evolutionary developmental processes.

RESULTS

In this study, we use three-dimensional geometric morphometrics to describe, quantify, and compare morphological modifications unfolding during evolution and development of phyllostomid bats. We examine how changes in development of the cranium may contribute to the evolution of the bat craniofacial skeleton. Comparisons of ontogenetic trajectories to evolutionary trajectories reveal two separate evolutionary developmental growth processes contributing to modifications in skull morphogenesis: acceleration and hypermorphosis.

CONCLUSION

These findings are consistent with a role for peramorphosis, a form of heterochrony, in the evolution of bat dietary specialists.

Allometry and advancing age significantly structure craniofacial variation in adult female baboons

Primate craniofacial growth is traditionally assumed to cease upon maturation or at least be negligible, whereas bony remodeling is typically associated with advanced adult age and, in particular, tooth loss. Therefore, size and shape of the craniofacial skeleton of young and middle-aged adults should be stable. However, research on both modern and historic human samples suggests that portions of the CFS exhibit age-related changes in mature individuals, both related to and independent of tooth loss. These results demonstrate that the age-category 'adult' is heterogeneous, containing individuals demonstrating post-maturational age-related variation, but the topic remains understudied outside of humans and in the cranial vault and base. Our research quantifies variation in a sample of captive adult female baboons (n = 97) in an effort to understand how advancing age alters the mature CFS. Craniometric landmarks and sliding semilandmarks were collected from computed tomography (CT) scans of adult baboons aged 7-32 years old. To determine whether craniofacial morphology is sensitive to aging mechanisms and whether any such effects are differentially distributed throughout the cranium, geometric morphometric techniques were employed to compare the shapes of various cranial regions among individuals of increasing age. Unexpectedly, the biggest form differences were observed between young and middle-aged adults, rather than between adults with full dentitions and those with some degree of tooth loss. Shape variation was greatest in masticatory and nuchal musculature attachment areas. Our results indicate that the craniofacial skeleton changes form during adulthood in baboons, raising interesting questions about the molecular and biological mechanisms governing these changes.

Facial aging trajectories: A common shape pattern in male and female faces is disrupted after menopause

OBJECTIVES

Despite variation in lifestyle and environment, first signs of human facial aging show between the ages of 20-30 years. It is a cumulative process of changes in the skin, soft tissue, and skeleton of the face. As quantifications of facial aging in living humans are still scarce, we set out to study age-related changes in three-dimensional facial shape using geometric morphometrics.

MATERIALS AND METHODS

We collected surface scans of 88 human faces (aged 26-90 years) from the coastal town Split (Croatia) and neighboring islands. Based on a geometric morphometric analysis of 585 measurement points (landmarks and semilandmarks), we modeled sex-specific trajectories of average facial aging.

RESULTS

Age-related facial shape change was similar in both sexes until around age 50, at which time the female aging trajectory turned sharply. The overall magnitude of facial shape change (aging rate) was higher in women than men, especially in early postmenopause. Aging was generally associated with a flatter face, sagged soft tissue ("broken" jawline), deeper nasolabial folds, smaller visible areas of the eyes, thinner lips, and longer nose and ears. In postmenopausal women, facial aging was best predicted by the years since last menstruation and mainly attributable to bone resorption in the mandible.

DISCUSSION

With high spatial and temporal resolution, we were able to extract a shared facial aging pattern in women and men, and its divergence after menopause. This fully quantitative three-dimensional analysis of human facial aging may not only find applications in forensic and ancient human facial reconstructions, but shall include lifestyle and endocrinological measures, and also reach out to studies of social perception.

Ontogeny of the human maxilla: a study of intra-population variability combining surface bone histology and geometric morphometrics

Bone modeling is the process by which bone grows in size and models its shape via the cellular activities of the osteoblasts and osteoclasts that respectively form and remove bone. The patterns of expression of these two activities, visible on bone surfaces, are poorly understood during facial ontogeny in Homo sapiens; this is due mainly to small sample sizes and a lack of quantitative data. Furthermore, how microscopic activities are related to the development of morphological features, like the uniquely human-canine fossa, has been rarely explored. We developed novel techniques for quantifying and visualizing variability in bone modeling patterns and applied these methods to the human maxilla to better understand its development at the micro- and macroscopic levels. We used a cross-sectional ontogenetic series of 47 skulls of known calendar age, ranging from birth to 12 years, from a population of European ancestry. Surface histology was employed to record and quantify formation and resorption on the maxilla, and digital maps representing each individual's bone modeling patterns were created. Semilandmark geometric morphometric (GM) methods and multivariate statistics were used to analyze facial growth. Our results demonstrate that surface histology and GM methods give complementary results, and can be used as an integrative approach in ontogenetic studies. The bone modeling patterns specific to our sample are expressed early in ontogeny, and fairly constant through time. Bone resorption varies in the size of its fields, but not in location. Consequently, absence of bone resorption in extinct species with small sample sizes should be interpreted with caution. At the macroscopic level, maxillary growth is predominant in the top half of the bone where bone formation is mostly present. Our results suggest that maxillary growth in humans is highly constrained from early stages in ontogeny, and morphological changes are likely driven by changes in osteoblastic and osteoclastic rates of expression rather than differences in the bone modeling patterns (i.e. changes in location of formation and resorption). Finally, the results of the micro- and macroscopic analyses suggest that the development of the canine fossa results from a combination of bone resorption and bone growth in the surrounding region.

A mathematical landmark-based method for measuring worn molars in hominoid systematics

Worn teeth pose a major limitation to researchers in the fields of extinct and extant hominoid systematics because they lack clearly identifiable anatomical landmarks needed to take measurements on the crown enamel surface and are typically discarded from a study. This is particularly detrimental when sample sizes for some groups are already characteristically low, if there is an imbalance between samples representing populations, sexes or dietary strategies, or if the worn teeth in question are type specimens of fossil species or other key specimens. This study proposes a methodology based predominantly on mathematically-derived landmarks for measuring size and shape features of molars, irrespective of wear. With 110 specimens of lower second molars from five species of extant hominoids (Pan troglodytes, P. paniscus, Gorilla gorilla, G. beringei, Homo sapiens), n ≥ 20 per species, n ≥ 10 per subspecies, good species separation in morphospace is achieved in a principal components analysis. Classification accuracy in a discriminant function analysis is 96.4% at the species level and 88.2% at the subspecies level (92.7% and 79.1%, respectively, on cross-validation). The classification accuracy compares favorably to that achieved by anatomically-derived measurements based on published research (94% and 84% at the species and subspecies level respectively; 91% and 76% on cross-validation). The mathematical landmarking methodology is rapid and uncomplicated. The results support the use of mathematical landmarks to enable the inclusion of worn molar teeth in dental studies so as to maximize sample sizes and restore balance between populations and/or sexes in hominoid systematic studies.

Variation of 3D outer and inner crown morphology in modern human mandibular premolars

Objectives

This study explores the outer and inner crown of lower third and fourth premolars (P₃, P₄) by analyzing the morphological variation among diverse modern human groups.

Materials and Methods

We studied three‐dimensional models of the outer enamel surface and the enamel–dentine junction (EDJ) from μCT datasets of 77 recent humans using both an assessment of seven nonmetric traits and a standard geometric morphometric (GM) analysis. For the latter, the dental crown was represented by four landmarks (dentine horns and fossae), 20 semilandmarks along the EDJ marginal ridge, and pseudolandmarks along the crown and cervical outlines.

Results

Certain discrete traits showed significantly different regional frequencies and sexual dimorphism. The GM analyses of both P₃s and P₄s showed extensive overlap in shape variation of the various populations (classification accuracy 15–69%). The first principal components explained about 40% of shape variance with a correlation between 0.59 and 0.87 of the features of P₃s and P₄s. Shape covariation between P₃s and P₄s expressed concordance of high and narrow or low and broad crowns.

Conclusions

Due to marked intragroup and intergroup variation in GM analyses of lower premolars, discrete traits such as the number of lingual cusps and mesiolingual groove expression provide better geographic separation of modern human populations. The greater variability of the lingual region suggests a dominance of functional constraints over geographic provenience or sex. Additional information about functionally relevant aspects of the crown surface and odontogenetic data are needed to unravel the factors underlying dental morphology in modern humans.

Heterochronic Shifts Mediate Ecomorphological Convergence in Skull Shape of Microcephalic Sea Snakes

Morphological variation among the viviparous sea snakes (Hydrophiinae), a clade of fully aquatic elapid snakes, includes an extreme “microcephalic” ecomorph that has a very small head atop a narrow forebody, while the hind body is much thicker (up to three times the forebody girth). Previous research has demonstrated that this morphology has evolved at least nine times as a consequence of dietary specialization on burrowing eels, and has also examined morphological changes to the vertebral column underlying this body shape. The question addressed in this study is what happens to the skull during this extreme evolutionary change? Here we use X-ray micro-computed tomography and geometric morphometric methods to characterize cranial shape variation in 30 species of sea snakes. We investigate ontogenetic and evolutionary patterns of cranial shape diversity to understand whether cranial shape is predicted by dietary specialization, and examine whether cranial shape of microcephalic species may be a result of heterochronic processes. We show that the diminutive cranial size of microcephalic species has a convergent shape that is correlated with trophic specialization to burrowing prey. Furthermore, their cranial shape is predictable for their size and very similar to that of juvenile individuals of closely related but non-microcephalic sea snakes. Our findings suggest that heterochronic changes (resulting in pedomorphosis) have driven cranial shape convergence in response to dietary specializations in sea snakes.

Testing the evolutionary constraints of metamorphosis: The ontogeny of head shape in Triturus newts

In vertebrates with complex, biphasic, life cycles, larvae have a distinct morphology and ecological preferences compared to metamorphosed juveniles and adults. In amphibians, abrupt and rapid metamorphic changes transform aquatic larvae to terrestrial juveniles. The main aim of this study is to test whether, relative to larval stages, metamorphosis (1) resets the pattern of variation between ontogenetic stages and species, (2) constrains intraspecific morphological variability, and (3) similar to the "hour-glass" model reduces morphological disparity. We explore postembryonic ontogenetic trajectories of head shape (from hatching to completed metamorphosis) of two well-defined, morphologically distinct Triturus newts species and their F1 hybrids. Variation in head shape is quantified and compared on two levels: dynamic (across ontogenetic stages) and static (at a particular stage). Our results show that the ontogenetic trajectories diverge early during development and continue to diverge throughout larval stages and metamorphosis. The high within-group variance and the largest disparity level (between-group variance) characterize the metamorphosed stage. Hence, our results indicate that metamorphosis does not canalize head shape variation generated during larval development and that metamorphosed phenotype is not more constrained relative to larval ones. Therefore, metamorphosis cannot be regarded as a developmental constraint, at least not for salamander head shape.

Cranial and endocranial diversity in extant and fossil atelids (Platyrrhini: Atelidae): A geometric morphometric study

OBJECTIVES

Platyrrhines constitute a diverse clade, with the modern Atelidae exhibiting the most variation in cranial and endocast morphology. The processes responsible for this diversification are not well understood. Here, we present a geometric morphometric study describing variation in cranial and endocranial shape of 14 species of Alouatta, Ateles, Brachyteles, and Lagothrix and two extinct taxa, Cartelles and Caipora.

METHODS

We examined cranial and endocranial shape variation among species using images reconstructed from CT scans and geometric morphometric techniques based on three-dimensional landmarks and semilandmarks. Principal components analyses were used to explore variation, including the Procrustes shape coordinates, summing the logarithm of the Centroid Size, the common allometric component, and residual shape components.

RESULTS

Differences in endocranial shape are related to a relative increase or decrease in the volume of the neocortex region with respect to brainstem and cerebellum regions. The relative position of the brainstem varies from a posterior position in Alouatta to a more ventral position in Ateles. The shape of both the cranium and endocast of Caipora is within the observed variation of Brachyteles. Cartelles occupies the most differentiated position relative to the extant taxa, especially in regards to its endocranial shape.

CONCLUSIONS

The pattern of variation in the extant species in endocranial shape is similar to the variation observed in previous cranial studies, with Alouatta as an outlier. The similarities between Caipora and Brachyteles were unexpected and intriguing given the frugivorous adaptations inferred from the fossil's dentition. Our study shows the importance of considering both extant and fossil species when studying diversification of complex traits.

Investigating the effect of endocranial volume on cranial shape in platyrrhines and the relevance of this relationship to interpretations of the fossil record

OBJECTIVES

Fossils have been linked to Alouatta based on shared cranial morphology and small brain size. However, the relationship between endocranial volume and cranial shape is unclear; it is possible that any platyrrhine with a small brain may exhibit "Alouatta-like" features due to being "de-encephalized." We test two hypotheses: (a) there are aspects of cranial shape related to encephalization common to all platyrrhines; (b) it is these cranial traits that unite the small-brained "Alouatta-like" fossils.

MATERIALS AND METHODS

Three-dimensional cranial shape and endocranial volume (ECV) were measured on 350+ extant platyrrhine crania, Cartelles, Paralouatta, and Antillothrix. Encephalization quotient (EQ) was calculated using regressions of ECV on cranial centroid size. Multivariate regressions were performed using the shape coordinates and EQ and shape changes associated with EQ were visualized. Cranial shape was predicted for a hypothetical primate with an EQ matching the fossils and this shape was compared to the Alouatta mean.

RESULTS

There is a significant proportion of cranial shape variation explained by EQ in some taxa. The aspects of shape that are correlated with EQ are shared by several taxa and some have parallel regression vectors, but there is no overall pattern of shape change common to all platyrrhines. However, all taxa look more similar to Alouatta when their EQ is decreased, particularly Pithecia.

DISCUSSION

Given that a decrease in encephalization can cause a more Alouatta-like cranial shape in many extant platyrrhines, it should not be automatically assumed that Alouatta-like cranial traits in a small-brained fossil are evidence of a phylogenetic link to the alouattin clade.

Craniofacial skeletal response to encephalization: How do we know what we think we know?

Dramatic changes in cranial capacity have characterized human evolution. Important evolutionary hypotheses, such as the spatial packing hypothesis, assert that increases in relative brain size (encephalization) have caused alterations to the modern human skull, resulting in a suite of traits unique among extant primates, including a domed cranial vault, highly flexed cranial base, and retracted facial skeleton. Most prior studies have used fossil or comparative primate data to establish correlations between brain size and cranial form, but the mechanistic basis for how changes in brain size impact the overall shape of the skull resulting in these cranial traits remains obscure and has only rarely been investigated critically. We argue that understanding how changes in human skull morphology could have resulted from increased encephalization requires the direct testing of hypotheses relating to interaction of embryonic development of the bones of the skull and the brain. Fossil and comparative primate data have thoroughly described the patterns of association between brain size and skull morphology. Here we suggest complementing such existing datasets with experiments focused on mechanisms responsible for producing the observed patterns to more thoroughly understand the role of encephalization in shaping the modern human skull.

Covariation of the endocranium and splanchnocranium during great ape ontogeny

That great ape endocranial shape development persists into adolescence indicates that the splanchnocranium succeeds brain growth in driving endocranial development. However, the extent of this splanchnocranial influence is unknown. We applied two-block partial least squares analyses of Procrustes shape variables on an ontogenetic series of great ape crania to explore the covariation of the endocranium (the internal braincase) and splanchnocranium (face, or viscerocranium). We hypothesized that a transition between brain growth and splanchnocranial development in the establishment of final endocranial form would be manifest as a change in the pattern of shape covariation between early and adolescent ontogeny. Our results revealed a strong pattern of covariation between endocranium and splanchnocranium, indicating that chimpanzees, gorillas, and orangutans share a common tempo and mode of morphological integration from the eruption of the deciduous dentition onwards to adulthood: a reflection of elongating endocranial shape and continuing splanchnocranial prognathism. Within this overarching pattern, we noted that species variation exists in magnitude and direction, and that the covariation between the splanchnocranium and endocranium is somewhat weaker in early infancy compared to successive age groups. When correcting our covariation analyses for allometry, we found that an ontogenetic signal remains, signifying that allometric variation alone is insufficient to account for all endocranial-splanchnocranial developmental integration. Finally, we assessed the influence of the cranial base, which acts as the interface between the face and endocranium, on the shape of the vault using thin-plate spline warping. We found that not all splanchnocranial shape changes during development are tightly integrated with endocranial shape. This suggests that while the developmental expansion of the brain is the main driver of endocranial shape during early ontogeny, endocranial development from infancy onwards is moulded by the splanchnocranium in conjunction with the neurocranium.

Why the long face? Static allometry in the sexually dimorphic phenotypes of Neotropical electric fishes

The evolution of sexually dimorphic traits is thought to have marked effects on underlying patterns of static allometry. These traits can negatively affect organismal survivability by creating trade-offs between trait size and performance. Here we use three-dimensional geometric morphometrics to study the static allometry of two species of sexually dimorphic electric fishes (Apteronotus rostratus and Compsaraia samueli) in which mature males grow elongate jaws used in agonistic male–male interactions. We also estimate jaw-closing performance between the sexes of both species to track changes in kinematic transmission associated with the development of sexual weaponry. We find significantly different patterns of static allometry between the sexes of both species, with males exhibiting more positive allometric slopes relative to females. We also find a negative relationship between skull shape and mandibular kinematic transmission in C. samueli, suggesting a trade-off where males with longer faces exhibit lower mechanical advantages, suggesting weaker jaw leverage. In contrast, males and females of A. rostratus exhibit no difference between sexes in mechanical advantage associated with facial elongation.

Morphological trends in arcade shape and size in Middle Pleistocene Homo

OBJECTIVES

Middle Pleistocene fossil hominins, often summarized as Homo heidelbergensis sensu lato, are difficult to interpret due to a fragmentary fossil record and ambiguous combinations of primitive and derived characters. Here, we focus on one aspect of facial shape and analyze shape variation of the dental arcades of these fossils together with other Homo individuals.

MATERIALS AND METHODS

Three-dimensional landmark data were collected on computed tomographic scans and surface scans of Middle Pleistocene fossil hominins (n = 8), Homo erectus s.l. (n = 4), Homo antecessor (n = 1), Homo neanderthalensis (n = 13), recent (n = 52) and fossil (n = 19) Homo sapiens. To increase sample size, we used multiple multivariate regression to reconstruct complementary arches for isolated mandibles, and explored size and shape differences among maxillary arcades.

RESULTS

The shape of the dental arcade in H. erectus s.l. and H. antecessor differs markedly from both Neanderthals and H. sapiens. The latter two show subtle but consistent differences in arcade length and width. Shape variation among Middle Pleistocene fossil hominins does not exceed the amount of variation of other species, but includes individuals with more primitive and more derived morphology, all more similar to Neanderthals and H. sapiens than to H. erectus s.l.

DISCUSSION

Although our results cannot reject the hypothesis that the Middle Pleistocene fossil hominins belong to a single species, their shape variation comprises a more primitive morph that represents a likely candidate for the shape of the last common ancestor of Neanderthals and H. sapiens, and a more derived morph resembling Neanderthals. The arcade shape difference between Neanderthals and H. sapiens might be related to different ways to withstand mechanical stress.

Ontogenetic changes to muscle architectural properties within the jaw-adductor musculature of Macaca fascicularis

OBJECTIVES

Changes to soft- and hard-tissue components of the masticatory complex during development can impact functional performance by altering muscle excursion potential, maximum muscle forces, and the efficiency of force transfer to specific bitepoints. Within Macaca fascicularis, older individuals exploit larger, more mechanically resistant food items and more frequently utilize wide-gape jaw postures. We therefore predict that key architectural and biomechanical variables will scale during ontogeny to maximize bite force and gape potential within older, larger-bodied individuals.

MATERIALS AND METHODS

We analyzed 26 specimens of M. fascicularis, representing a full developmental spectrum. The temporalis, superficial masseter, and deep masseter were dissected to determine muscle mass, fiber length, and physiologic cross-sectional area (PCSA). Lever-arm lengths were also measured for each muscle, alongside the height of the temporomandibular joint (TMJ) and basicranial length. These variables were scaled against two biomechanical variables (jaw length and condyle-molar length) to determine relative developmental changes within these parameters.

RESULTS

During ontogeny, muscle mass, fiber length, and PCSA scaled with positive allometry relative to jaw length and condyle-molar length within all muscles. TMJ height also scaled with positive allometry, while muscle lever arms scaled with isometry relative to jaw length and with positive allometry (temporalis) or isometry (superficial and deep masseter) relative to condyle-molar length.

CONCLUSION

Larger individuals demonstrate adaptations during development towards maximizing gape potential and bite force potential at both an anterior and posterior bitepoint. These data provide anatomical evidence to support field observations of dietary and behavioral differences between juvenile and adult M. fascicularis.

Phylogeography of a widespread sub-Saharan murid rodent Aethomys chrysophilus: the role of geographic barriers and paleoclimate in the Zambezian bioregion

Murid rodents of the genus Aethomys are one of the most common rodents in drier habitats in sub-Saharan Africa. Among them, the red veld rat Aethomys chrysophilus is the most widespread species with the core distribution located in the Zambezian bioregion. In this study, we describe phylogeographic structure of the species and estimate its age from a time-calibrated phylogeny of the genus. Seven parapatric clades were identified in the mitochondrial cytochrome b phylogeny, where some of the distributions of these clades have been separated by previously described biogeographical divides (Zambezi-Kafue river system, Rukwa Rift and the Eastern Arc Mountains). One internal clade corresponded to populations previously described as a distinct species, Aethomys ineptus. The whole A. chrysophilus complex was estimated to be 1.3 (0.5–2.4) Mya old, with A. ineptus originating 0.7 (0.1–1.4) Mya before present. The internal position of A. ineptus was also recovered in phylogenetic reconstruction based on two nuclear genes and thus it is not a consequence of mitochondrial introgression. In addition, we analyzed skull form variation across the species’ distributional range and found no significant difference between A. ineptus and the rest of A. chrysophilus complex.

Allometric shell growth in infaunal burrowing bivalves: examples of the archiheterodonts Claibornicardia paleopatagonica (Ihering, 1903) and Crassatella kokeni Ihering, 1899

We present two cases of study of ontogenetic allometry in outlines of bivalves using longitudinal data, a rarity among fossils, based on the preserved post-larval record of shells. The examples are two infaunal burrowing bivalves of the southern South America, Claibornicardia paleopatagonica (Archiheterodonta: Carditidae) (early Paleocene) and Crassatella kokeni (Archiheterodonta: Crassatellidae) (late Oligocene-late Miocene). Outline analyses were conducted using a geometric morphometric approach (Elliptic Fourier Analysis), obtaining successive outlines from shells' growth lines, which were used to reconstruct ontogenetic trajectories. In both taxa, ontogenetic changes are characterized by the presence of positive allometry in the extension of posterior end, resulting in elongated adult shells. This particular allometric growth is known in others infaunal burrowing bivalves (Claibornicardia alticostata and some Spissatella species) and the resulting adult morphology is present in representatives of several groups (e.g., Carditidae, Crassatellidae, Veneridae, Trigoniidae). Taxonomic, ecological and evolutionary implications of this allometric growth pattern are discussed.

Multimethod Approach to the Early Postnatal Growth of the Mandible in Mice from a Zone of Robertsonian Polymorphism

The western European house mouse (Mus musculus domesticus) shows high karyotypic diversity owing to Robertsonian translocations. Morphometric studies conducted with adult mice suggest that karyotype evolution due to these chromosomal reorganizations entails variation in the form and the patterns of morphological covariation of the mandible. However, information is much scarcer regarding the effect of these rearrangements on the growth pattern of the mouse mandible over early postnatal ontogeny. Here we compare mandible growth from the second to the eighth week of postnatal life between two ontogenetic series of mice from wild populations, with the standard karyotype and with Robertsonian translocations respectively, reared under the same conditions. A multi-method approach is used, including bone histology analyses of mandible surfaces and cross-sections, as well as geometric morphometric analyses of mandible form. The mandibles of both standard and Robertsonian mice display growth acceleration around weaning, anteroposterior direction of bone maturation, a predominance of bone deposition fields over ontogeny, and relatively greater expansion of the posterior mandible region correlated with the ontogenetic increase in mandible size. Nevertheless, differences exist between the two mouse groups regarding the timing of histological maturation of the mandible, the localization of certain bone remodeling fields, the temporospatial patterns of morphological variation, and the organization into two main modules. The dissimilarities in the process of mandible growth between the two groups of mice become more evident around sexual maturity, and could arise from alterations that Robertsonian translocations may exert on genes involved in the bone remodeling mechanism. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

Using geometric morphometric visualizations of directional selection gradients to investigate morphological differentiation

Researchers studying extant and extinct taxa are often interested in identifying the evolutionary processes that have lead to the morphological differences among the taxa. Ideally, one could distinguish the influences of neutral evolutionary processes (genetic drift, mutation) from natural selection, and in situations for which selection is implicated, identify the targets of selection. The directional selection gradient is an effective tool for investigating evolutionary process, because it can relate form (size and shape) differences between taxa to the variation and covariation found within taxa. However, although most modern morphometric analyses use the tools of geometric morphometrics (GM) to analyze landmark data, to date, selection gradients have mainly been calculated from linear measurements. To address this methodological gap, here we present a GM approach for visualizing and comparing between-taxon selection gradients with each other, associated difference vectors, and "selection" gradients from neutral simulations. To exemplify our approach, we use a dataset of 347 three-dimensional landmarks and semilandmarks recorded on the crania of 260 primate specimens (112 humans, 67 common chimpanzees, 36 bonobos, 45 gorillas). Results on this example dataset show how incorporating geometric information can provide important insights into the evolution of the human braincase, and serve to demonstrate the utility of our approach for understanding morphological evolution.

Phylo-Allometric Analyses Showcase the Interplay between Life-History Patterns and Phenotypic Convergence in Cleaner Wrasses

Phenotypic convergence is a macroevolutionary pattern that need not be consistent across life history. Ontogenetic transitions in dietary specialization clearly illustrate the dynamics of ecological selection as organisms grow. The extent of phenotypic convergence among taxa that share a similar ecological niche may therefore vary ontogenetically. Because ontogenetic processes have been shown to evolve, phylogenetic comparative methods can be useful in examining how the scaling of traits relates to ecology. Cleaning, a behavior in which taxa consume ectoparasites off clientele, is well represented among wrasses (Labridae). Nearly three-fourths of labrids that clean do so predominately as juveniles, transitioning away as adults. We examine the scaling patterns of 33 labrid species to understand how life-history patterns of cleaning relate to ontogenetic patterns of phenotypic convergence. We find that as juveniles, cleaners exhibit convergence in body and cranial traits that enhance ectoparasitivory. We then find that taxa that transition away from cleaning exhibit ontogenetic trajectories that are distinct from those of other wrasses. Obligate and facultative species that continue to clean over ontogeny, however, maintain characteristics that are conducive to cleaning. Collectively, we find that life-history patterns of cleaning behavior are concordant with ontogenetic patterns in phenotype in wrasses.

Head shape variation in cerambycid saproxylic beetles as a function of host plant selection

Saproxylic insects depend on deadwood for larval development, and a certain degree of specialization may be involved in their choice of host plants and/or wood in a particular stage of degradation. The plant species chosen for oviposition in turn act as an environmental pressure on the head morphology of larvae and it is expected that head shape plasticity varies directly with the number of woody plant species used for larval development in each insect species. We analyzed head shape variation in saproxylic beetles with respect to host plant species, maximum time of larval emergence and season of the year when insects colonized branches. Generalist species in the use of host plants showed significant variation in head shape and size. Time of emergence and season did not appear to affect head shape, although season was a determinant factor of abundance and possibly head size variation.

The tympanic region of Otaria byronia (Otariidae, Carnivora) - morphology, ontogeny, age classes and dimorphism

Here we describe and explore for the first time the ontogeny and sexual dimorphism of the auditory region of Otaria byronia. We studied the tympanic region of skulls of 237 specimens of different ages and sexes. Geometric morphometric methods were used to analyze the tympanic bulla. In addition, 3D reconstructions of the tympanic bulla were performed using computed tomography analysis scans and a serial wearing technique. We provide a description of the external and internal morphology of the tympanic bulla in both sexes and across different stages (bioclasses). The average shape of the bulla in O. byronia has a subtriangular contour, with variations between sexes and ages. Each stage (bioclasses I, II, and III) is characterized by the respective mean shape of the tympanic bulla and designated as a morphoclass (1, 2, and 3). In all cases, the ectotympanic shows greater surface area than the endotympanic, as in other otariids, in contrast to Phocidae. During ontogeny, the relative size of the ectotympanic increases, growing in all directions and covering the endotympanic. This pattern is seen to the greatest extent in adult males, in which the ectotympanic forms an extremely well-developed apophysis jugulare. No differences in internal morphology of the tympanic cavity were recorded between ages and sexes. The bulla does not increase in thickness in successive age classes; in fact, the walls are extremely thin in the adult stages, despite the extensive development of its processes. This pattern is opposite that observed in Phocidae. In morphoclass 3, adult males older than 7 years undergo hypermorphic change that results in a peramorphic condition when compared to adult females. These changes probably follow the same pattern shown by the rest of the skull and contribute to the marked sexual dimorphism of the species.

The evolution of genital shape variation in female cetaceans

Male genital diversification is likely the result of sexual selection. Female genital diversification may also result from sexual selection, although it is less well studied and understood. Female genitalia are complex among whales, dolphins, and porpoises, especially compared to other vertebrates. The evolutionary factors affecting the diversity of vaginal complexity could include ontogeny, allometry, phylogeny, sexual selection, and natural selection. We quantified shape variation in female genitalia using 2D geometric morphometric analysis, and validated the application of this method to study soft tissues. We explored patterns of variation in the shape of the cervix and vagina of 24 cetacean species (n = 61 specimens), and found that genital shape varies primarily in the relative vaginal length and overall aspect ratio of the reproductive tract. Extensive genital shape variation was partly explained by ontogenetic changes and evolutionary allometry among sexually mature cetaceans, whereas phylogenetic signal, relative testis size, and neonate size were not significantly associated with genital shape. Female genital shape is diverse and evolves rapidly even among closely related species, consistent with predictions of sexual selection models and with findings in invertebrate and vertebrate taxa. Future research exploring genital shape variation in 3D will offer new insights into evolutionary mechanisms because internal vaginal structures are variable and can form complex spirals.

Body size and allometric variation in facial shape in children

OBJECTIVES

Morphological integration, or the tendency for covariation, is commonly seen in complex traits such as the human face. The effects of growth on shape, or allometry, represent a ubiquitous but poorly understood axis of integration. We address the question of to what extent age and measures of size converge on a single pattern of allometry for human facial shape.

METHODS

Our study is based on two large cross-sectional cohorts of children, one from Tanzania and the other from the United States (N = 7,173). We employ 3D facial imaging and geometric morphometrics to relate facial shape to age and anthropometric measures.

RESULTS

The two populations differ significantly in facial shape, but the magnitude of this difference is small relative to the variation within each group. Allometric variation for facial shape is similar in both populations, representing a small but significant proportion of total variation in facial shape. Different measures of size are associated with overlapping but statistically distinct aspects of shape variation. Only half of the size-related variation in facial shape can be explained by the first principal component of four size measures and age while the remainder associates distinctly with individual measures.

CONCLUSIONS

Allometric variation in the human face is complex and should not be regarded as a singular effect. This finding has important implications for how size is treated in studies of human facial shape and for the developmental basis for allometric variation more generally.