Changes In Cranial Morphology Research Articles

Ontogenetic variation in the cranium of Mixosaurus cornalianus, with implications for the evolution of ichthyosaurian cranial development

Relatively complete ontogenetic series are comparatively rare in the vertebrate fossil record. This can create biases in our understanding of morphology and evolution, since immaturity can represent a source of unrecognized intraspecific variation in both skeletal anatomy and ecology. In the extinct marine reptile clade Ichthyopterygia, ontogenetic series were widely studied only in some Jurassic genera, while the ontogeny of the oldest and most basal members of the clade is very poorly understood. Here, we investigate cranial ontogeny in Mixosaurus cornalianus, from the Middle Triassic Besano Formation of the Swiss and Italian Alps. This small-bodied taxon is represented by a wealth of material from multiple size classes, including fetal material. This allows us to assess ontogenetic changes in cranial morphology, and identify stages in the ontogenetic trajectory where divergence with more derived ichthyosaurs has occurred. Early ontogenetic stages of Mixosaurus show developmental patterns that are reminiscent of the presumed ancestral (early diverging sauropsid) condition. This is prominently visible in the late fetal stage in both the basioccipital, which shows morphology akin to basal tubera, and in the postorbital, which has a triradiate head. The ontogenetic trajectory of at least some of the cranial elements of Mixosaurus is therefore likely still very akin to the ancestral condition, even though the adult cranium diverges from the standard diapsid morphology.

Cranial Shape Changes in Nonsyndromic Unilateral Lambdoid Synostosis after Open Posterior Vault Remodeling.

The purpose of this study was to quantify change in cranial morphology in patients with nonsyndromic unilateral lambdoid craniosynostosis (ULC) from presentation (t0), after open posterior switch-cranioplasty (t1), and at 2-year follow-up (t2). Volumetric, linear, and angular analysis were performed on computed tomographic scans at the three time points and against normal control subjects. Significance was set at P < 0.05. Twenty-two patients were included. ULC cranial vault asymmetry index was higher than in control subjects before surgery (6.22 ± 3.55) but decreased after surgery (3.00 ± 2.53) to become comparable with the normal asymmetry range present in the controls. After surgery, both diagonals increased, but more on the fused side. In the 2 years after surgery, both diagonals in patients with ULC grew proportionately, but the fused diagonal remained slightly shorter than the patent side. Total cranial volume was higher in patients with ULC than in control subjects after surgery but became comparable at t2. Cranial base angulation improved by t2 but did not approach normal, and ear position remained unchanged. The facial twist was higher than in controls at t0 and t1 but was comparable at t2. Coronal asymmetry improved with surgery but remained undercorrected at t2, with the greatest residual asymmetry at opisthion. Open-switch cranioplasty normalizes cranial vault asymmetry index by increasing the fused cranial diagonal more than the patent side and is stable at 2 years. Skull base twist does not normalize, but facial twist approaches normal. Technique improvement should focus on residual coronal asymmetry present at opisthion. Therapeutic, IV.

A guide for the use of fNIRS in microcephaly associated to congenital Zika virus infection

Congenital Zika Syndrome (CZS) is characterized by changes in cranial morphology associated with heterogeneous neurological manifestations and cognitive and behavioral impairments. In this syndrome, longitudinal neuroimaging could help clinicians to predict developmental trajectories of children and tailor treatment plans accordingly. However, regularly acquiring magnetic resonance imaging (MRI) has several shortcomings besides cost, particularly those associated with childrens' clinical presentation as sensitivity to environmental stimuli. The indirect monitoring of local neural activity by non-invasive functional near-infrared spectroscopy (fNIRS) technique can be a useful alternative for longitudinally accessing the brain function in children with CZS. In order to provide a common framework for advancing longitudinal neuroimaging assessment, we propose a principled guideline for fNIRS acquisition and analyses in children with neurodevelopmental disorders. Based on our experience on collecting fNIRS data in children with CZS we emphasize the methodological challenges, such as clinical characteristics of the sample, desensitization, movement artifacts and environment control, as well as suggestions for tackling such challenges. Finally, metrics based on fNIRS can be associated with established clinical metrics, thereby opening possibilities for exploring this tool as a long-term predictor when assessing the effectiveness of treatments aimed at children with severe neurodevelopmental disorders.

One of these wolves is not like the other: morphological effects and conservation implications of captivity in Mexican wolves

AbstractMaintaining animal populations in captivity can be a useful way to conserve species, especially those that are extinct in the wild or endangered in their native habitats. These animals could be reintroduced into the wild from captivity if conditions improve. However, captive populations of endangered species are often established with few wild‐caught individuals and are maintained in novel habitats, conditions which may make these populations prone to morphological divergence. Changes in cranial morphology, in particular, may limit reintroduction success because these morphological changes may alter cranial functionality related to capturing and consuming prey. To assess whether unintentional morphological changes occur in captivity, we examined the crania and mandibles of 275 specimens of wild, captive and reintroduced Mexican wolvesCanis lupus baileyi. We detected significant differences in skull size and shape among these populations. Captive specimens displayed the greatest overall shape variation and were significantly differentiated from the wild population in regions of the skull that are essential to cranial functionality. The reintroduced population displayed the narrowest trait distribution, with cranial morphology that grouped consistently with the captive morphotype. The small number of individuals of the founding population along with the increased variation in cranial morphology among captive Mexican wolves suggests that morphological changes in captivity may be related to genetic changes such as genetic drift or relaxed selection. This study helps to define the changes that may occur in captivity, the potential mechanisms behind these shifts, and the ways in which morphological variation in captive populations may impact conservation initiatives.

Cranial Morphology of the Flat-Headed Bat Myotis planiceps (Chiroptera: Vespertilionidae) in the Context of American Myotis

Myotis planiceps is an insectivorous species with a distinctive flat-headed morphology among Myotis species. Its distribution is restricted to the northeastern Mexico highlands, and uses trees of Yucca carnerosana to roost. The particular ecological and morphological characteristics of this species make it a likely resource-use specialist. We evaluated the morphological specialization of M. planiceps as compared with thirteen American congeners belonging to different dietary categories, using a geometric morphometric protocol and multivariate statistical analyses. We estimated morphological differences across dietary categories, including M. planiceps as a separate group, and evaluated the effect of cranial size and diet on variations in cranial morphology. We found a significant differentiation of this species in all the cranial shape characters, with soft insectivores as the dietary group closest to M. planiceps. Additionally, diet explained similar percentages of shape variance in cranial and mandibular characters, and the highest interaction between diet and cranial size was recorded in the braincase. In general, the smallest sizes and thinner skulls were observed in soft insectivores. Our findings do not support a significant relationship between skull shape characters, described by the first principal component, and hardness of food items, but we found a significant association with the mandible shape. No phylogenetic structure was observed in the residual variance of these models. We found trends of change in cranial morphology associated with specialized habitats, the mandibular characters being more contrasting in the fishing bat M. vivesi, and the braincase morphology being more contrasting in M. planiceps, a species that lives in restricted habitats and probably feeds on soft insects.

Cranial morphology of captive mammals: a meta-analysis

BackgroundCaptive facilities such as zoos are uniquely instrumental in conservation efforts. To fulfill their potential as bastions for conservation, zoos must preserve captive populations as appropriate proxies for their wild conspecifics; doing so will help to promote successful reintroduction efforts. Morphological changes within captive populations may be detrimental to the fitness of individual animals because these changes can influence functionality; thus, it is imperative to understand the breadth and depth of morphological changes occurring in captive populations. Here, we conduct a meta-analysis of scientific literature reporting comparisons of cranial measures between captive and wild populations of mammals. We investigate the pervasiveness of cranial differences and whether cranial morphological changes are associated with ecological covariates specific to individual species, such as trophic level, dietary breadth, and home range size.ResultsCranial measures of skull length, skull width, and the ratio of skull length-to-width differed significantly between many captive and wild populations of mammals reported in the literature. Roughly half of captive populations differed from wild populations in at least one cranial measure, although the degree of changes varied. Carnivorous species with a limited dietary breadth displayed the most consistent changes associated with skull widening. Species with a more generalized diet displayed less morphological changes in captivity.ConclusionsWild and captive populations of mammals differed in cranial morphology, but the nature and magnitude of their cranial differences varied considerably across taxa. Although changes in cranial morphology occur in captivity, specific changes cannot be generalized for all captive mammal populations. The nature of cranial changes in captivity may be specific to particular taxonomic groups; thus, it may be possible to establish expectations across smaller taxonomic units, or even disparate groups that utilize their cranial morphology in a similar way. Given that morphological changes occurring in captive environments like zoos have the potential to limit reintroduction success, our results call for a critical evaluation of current captive husbandry practices to prevent unnecessary morphological changes.

Tracing craniovascular traits in artificially modified pre‐Columbian crania from Cuba, the largest Caribbean archipelago

AbstractThe impressions left by the vascular system on the surface of the skull, like those of the middle meningeal vessels, the dural venous sinuses, and the emissary veins, are a unique source of vascular and anatomical evidence when working with osteological collections, as is the case with bioarchaeological, forensic, and paleoanthropological investigations. Craniovascular features have been investigated in multiple modern human populations; however, no information is available on their analysis in artificially modified pre‐Hispanic Caribbean skulls. To shed light on this matter, this report provides a reference for the prevalence, characterization, and distribution of craniovascular traits using a collection of fronto‐occipital tabular oblique artificially shaped pre‐Columbian crania from Cuba (the largest archipelago in the Caribbean) as a case study. Results suggest that changes produced by intentional head‐shaping may have affected the pattern and direction of endocranial vascular structures, whereas emissary foramina placed in less‐affected areas appear to be less influenced by changes in cranial morphology. Future work will investigate the consequences of artificial modification on the cranial and endocranial vascular systems and compare them with unmodified pre‐Columbian specimens. Also, the effects of head shaping in endocranial thermoregulation should be explored in the future. Providing new knowledge about the prevalence and variation of the craniovascular traits may be useful not only in research based on osteoarcheological collections but also in medicine.

Sex-specific ontogenetic patterns of cranial morphology, theoretical bite force, and underlying jaw musculature in fishers and American martens.

The carnivoran cranium undergoes tremendous growth in size and development of shape to process prey as adults and, importantly, these ontogenetic processes can also differ between the sexes. How these ontogenetic changes in morphology actually relate to the underlying jaw musculature and overall bite performance has rarely been investigated. In this study, I examined sex-specific ontogenetic changes in cranial morphology, jaw adductor muscles, and theoretical bite force between subadults and adults in the fisher (Pekania pennanti) and American marten (Martes americana). I found evidence that cranial size alone does not completely explain ontogenetic increases in bite forces as found in other mammalian species. Instead, cranial shape development also drives ontogenetic increases in relative bite force by broadening the zygomatic arches and enlargement of the sagittal crest, both of which enable relatively larger jaw adductor muscles to attach. In contrast, examination of sexual dimorphism within each age-class revealed that cranial shape dimorphism did not translate to dimorphism in either size-corrected bite forces or size-corrected physiological cross-sectional area of the jaw adductor muscles. These results reveal that morphological size and shape variation can have different influences on bite performance depending on the level of intraspecific variation that is examined (i.e. ontogenetic versus sexual dimorphism).

From Phenotype to Genotype And Back Again

Traditionally, anthropologists study evolutionary change throughmorphological analysis of fossils and comparative primate data. For the analysis of the genotypephenotype continuum, the current emphasis on genes is misplaced because genes don’t make structure. Developmental processes make structure through the activity of cells that use instructions specified by genes. A critical mechanism underlying any phenotypic trait is the genetically guided change in developmental events that produce the trait. But even when a developmental mechanism is identified, the links between genetically guided instructions and phenotypic outcome are lengthy, complicated, flexible, and sensitive to physical forces of functioning organs. We use the study of craniofacial phenotypes of craniosynostosis (premature closure of sutures) to demonstrate how patterns produced by the covariation of cranial traits cannot always reveal mechanism. Next we turn toencephalization, a critical feature of human evolution that covaries with cranial phenotypes, and show how experimental approaches can be used to analyze mechanism underlying this well-documented pattern in human evolution. With the realization that no single line of evidence can explain the dramatic changes in cranial morphology that characterize human evolution come fundamental changes in the way we conduct anthropological inquiry - collaborative efforts from scientists with diverse expertise will continue to push the field forward.

Sclerostin Antibody-Induced Changes in Bone Mass Are Site Specific in Developing Crania.

Sclerostin antibody (Scl-Ab) is an anabolic bone agent that has been shown to increase bone mass in clinical trials of adult diseases of low bone mass, such as osteoporosis and osteogenesis imperfecta (OI). Its use to decrease bone fragility in pediatric OI has shown efficacy in several growing mouse models, suggesting translational potential to pediatric disorders of low bone mass. However, the effects of pharmacologic inhibition of sclerostin during periods of rapid growth and development have not yet been described with respect to the cranium, where lifelong deficiency of functioning sclerostin leads to patterns of excessive bone growth, cranial compression, and facial palsy. In the present study, we undertook dimensional and volumetric measurements in the skulls of growing Brtl/+ OI mice treated with Scl-Ab to examine whether therapy-induced phenotypic changes were similar to those observed clinically in patients with sclerosteosis or Van Buchem disorder. Mice treated between 3 and 14 weeks of age with high doses of Scl-Ab show significant calvarial thickening capable of rescuing OI-induced deficiencies in skull thickness. Other changes in cranial morphology, such as lengths and distances between anatomic landmarks, intracranial volume, and suture interdigitation, showed minimal effects of Scl-Ab when compared with growth-induced differences over the treatment duration. Treatment-induced narrowing of foramina was limited to sites of vascular but not neural passage, suggesting patterns of local regulation. Together, these findings reveal a site specificity of Scl-Ab action in the calvaria with no measurable cranial nerve impingement or brainstem compression. This differentiation from the observed outcomes of lifelong sclerostin deficiency complements reports of Scl-Ab treatment efficacy at other skeletal sites with the prospect of minimal cranial secondary complications. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

Variability in the Evolution of Ornithischian Cranial Morphology

Phylogenetic relationships within Dinosauria have been recently called into question, including shifts among major taxonomic groups such as Ornithischia. As phylogenetic hypotheses change, so do hypotheses of morphological character evolution. Ornithischian dinosaurs exhibit a wide variety of cranial morphologies. They range from generalized ancestral morphologies, to derived and varied skull shapes with frills, thickened domes, or complex dental batteries associated with complex jaw joints. Here, I describe the evolution of cranial morphology within Ornithischia using an updated phylogenetic analysis and ancestral character state reconstruction methods to map changes in cranial characters through time. With an updated phylogenetic analysis of Ornithischia, I used both parsimony and maximum likelihood methods for ancestral state reconstructions to determine the changes in cranial characters. The ancestral reconstructions also allowed for a broader view of cranial morphological change through time. These estimates demonstrated that basal ornithischians, including those at the base of major clades (e.g. Thyreophora, Ornithopoda), maintained skull morphologies not unlike other basal dinosaurs outside of Ornithischia. The thyreophoran and neornithischian skulls show similar changes in cranial character evolution: relative reduction of the quadratojugal, general lengthening of the preorbital skull, including modifications to the premaxilla and nasal, an initial reduction in the size, placement, and shape of the antorbital fenestra, and incorporation of the palpebral in the supraorbital margin. While slight changes in the postorbital skull of thyreophorans were noted, the morphology within Neornithischia was markedly different. In addition to well‐known modifications of the parietal and squamosal in marginocephalians, other noted changes include movement of cranial foramina in the occipital and basal regions of the skull. This is in contrast to most ornithopods, whose skull bases and occipital regions vary relatively little from basal neornithischians. Derived ornithopods (e.g., hadrosaurs) demonstrate elaborate preorbital modifications, particularly involving the premaxilla and nasal. The dentition and mandible demonstrate modifications throughout the ornithischian tree. However, dramatic changes are noted among derived ornithopods with the development of massive dental batteries and laterally offset, handle‐shaped coronoid processes. This occurs to a lesser degree in ceratopsians as well. These changes in cranial morphology, while present in all ornithischian clades, varies in terms of rates of change. Within Thyreophora, a group that persisted for nearly 150 million years, the general changes within the skull and mandible are relatively small when compared to ornithopods. Ceratopsians and ornithopods demonstrate the most dramatic changes in overall cranial morphology, but ornithopods show the most remarkable alterations in mandibular anatomy through time. With these results, the analysis presented here may be used to develop new hypotheses for the selective pressures behind the varied evolution of the ornithischian skull.Support or Funding InformationUniversity of Iowa Max and Lorraine Littlefield Fund and T. Anne Cleary International Dissertation Research FellowshipThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Cranial biomechanics in basal urodeles: the Siberian salamander (Salamandrella keyserlingii) and its evolutionary and developmental implications

Developmental changes in salamander skulls, before and after metamorphosis, affect the feeding capabilities of these animals. How changes in cranial morphology and tissue properties affect the function of the skull are key to decipher the early evolutionary history of the crown-group of salamanders. Here, 3D cranial biomechanics of the adult Salamandrella keyserlingii were analyzed under different tissue properties and ossification sequences of the cranial skeleton. This helped unravel that: (a) Mechanical properties of tissues (as bone, cartilage or connective tissue) imply a consensus between the stiffness required to perform a function versus the fixation (and displacement) required with the surrounding skeletal elements. (b) Changes on the ossification pattern, producing fontanelles as a result of bone loss or failure to ossify, represent a trend toward simplification potentially helping to distribute stress through the skull, but may also imply a major destabilization of the skull. (c) Bone loss may be originated due to biomechanical optimization and potential reduction of developmental costs. (d) Hynobiids are excellent models for biomechanical reconstruction of extinct early urodeles.

Phylogenetic analysis of landmark data and the morphological evolution of cranial shape and diets in species of Myotis (Chiroptera: Vespertilionidae)

Species in genus Myotis exhibit a pattern of cranial variation associated with insectivorous, facultative piscivorous, and truly piscivorous diets, which has not been studied in a phylogenetic context. Variation in landmark configurations of five cranial structures in 22 Myotis species was analyzed with phylogenetic methods to infer evolution of shape. Our goals were to detect changes in cranial morphology and to correlate these with concerted changes among diets. A reference phylogeny was estimated using a combined data matrix with previously available Cyt-b and RAG2 sequences and our five configurations of landmarks. We included the insectivorous Kerivoula papillosa Temminck, 1840, Noctilio leporinus Linnaeus, 1758 (piscivorous), and N. albiventris Desmarest, 1818 (insectivorous) as out-groups. The optimization of five landmark configurations on the combined phylogeny shows no evidence of convergent shape changes among species with similar piscivorous diets. Our findings document that facultative piscivory does not imply the same particular morphotype. In four cranial features, there is small shape change between estimated ancestral shapes and seven observed descendant shapes for the piscivorous species. Only the mandible shows major changes from insectivorous ancestors to facultative piscivorous or piscivorous Myotis.

Morphological and dietary responses of chipmunks to a century of climate change.

Predicting how individual taxa will respond to climatic change is challenging, in part because the impacts of environmental conditions can vary markedly, even among closely related species. Studies of chipmunks (Tamias spp.) in Yosemite National Park provide an important opportunity to explore the reasons for this variation in response. While the alpine chipmunk (T.alpinus) has undergone a significant elevational range contraction over the past century, the congeneric and partially sympatric lodgepole chipmunk (T.speciosus) has not experienced an elevational range shift during this period. As a first step toward identifying the factors underlying this difference in response, we examined evidence for dietary changes and changes in cranial morphology in these species over the past century. Stable isotope analyses of fur samples from modern and historical museum specimens of these species collected at the same localities indicated that signatures of dietary change were more pronounced in T.alpinus, although diet breadth did not differ consistently between the study species. Morphometric analyses of crania from these specimens revealed significant changes in cranial shape for T.alpinus, with less pronounced changes in shape for T.speciosus; evidence of selection on skull morphology was detected for T.alpinus, but not for T.speciosus. These results are consistent with growing evidence that T.alpinus is generally more responsive to environmental change than T.speciosus, but emphasize the complex and often geographically variable nature of such responses. Accordingly, future studies that make use of the taxonomically and spatially integrative approach employed here may prove particularly informative regarding relationships between environmental conditions, range changes, and patterns of phenotypic variation.

Pachycephalosaurs (Dinosauria: Ornithischia) from the Upper Cretaceous (upper Campanian) of New Mexico: A reassessment of Stegoceras novomexicanum

Pachycephalosaurs, a group of ornithischian dinosaurs with distinctive cranial ornamentation and skull domes, underwent dramatic changes in cranial morphology during ontogeny. This has caused debate about whether some specimens belong to juveniles or adults, which impacts studies of pachycephalosaur phylogeny and evolution. One such debate concerns a small skull roof specimen from the Campanian (Upper Cretaceous) of New Mexico, NMMNH P-33898, which was originally described as an indeterminate juvenile but later regarded as a mature adult and erected as the holotype of a new small-bodied species, Stegoceras novomexicanum. We restudied NMMNH P-33898 using computed tomography scanning, morphometric and phylogenetic analyses, and comparisons to growth series of other pachycephalosaurs (Stegoceras validum, Pachycephalosaurus wyomingensis). We conclude that two purported paratype specimens of Stegoceras novomexicanum cannot be referred to the same taxon as the holotype, that the holotype and controversial paratypes all belong to immature specimens and not aberrant small-bodied adults, but that current evidence cannot clearly determine whether NMMNH P-33898 is a juvenile belonging to its own diagnostic species (S. novomexicanum) or is a juvenile of Stegoceras validum, Sphaerotholus goodwini, or another known taxon. We review the pachycephalosaur record of New Mexico and demonstrate that pachycephalosaurs were important components of dinosaur faunas in the southern part of Western North America during the ∼15 million years before the end-Cretaceous extinction, just as they were in roughly contemporaneous northern localities.

An Examination of the Differential Effects of the Modern Epidemiological Transition on Cranial Morphology in the United States and Portugal.

This research examines the pattern of secular change in the cranial morphology of two populations experiencing the epidemiological transition associated with decreased mortality rates in children, followed by declines in infant mortality and subsequent increases in adult longevity. The two samples examined in this study come from US and Portuguese individuals. The epidemiological transition occurred at different times in the United States and Portugal, with Portugal entering into the transition later than the United States. The results of the study show that the US and Portuguese samples experienced significant changes in cranial morphology during the approximately 150 years under study. In all of the samples the cranial base morphology changes significantly over time. However, the pattern of change in the US and Portuguese samples varies in the other regions of the crania. The US samples exhibit significant changes associated with the posterior cranial fossa, which experiences the greatest growth during the fetal period and the first year of life. Conversely, in the Portuguese samples the region of the cranium that shows the greatest change is in the face and lateral cranial base, which experiences the greatest growth from three to nine years. This differential pattern may reflect differences in changing mortality patterns in the two countries. During the period under study the United States had already proceeded through the early stages of the epidemiological transition, and improvements in the juvenile mortality and juvenile growth had occurred previously. Subsequently, the United States experienced significant declines in infant mortality, and the regions of the crania that exhibit the greatest changes occur in area with maximum growth velocity under one year. However, Portugal entered into the epidemiological transition later than the United States and therefore the greatest changes in growth occurred during the juvenile period, which is reflected in the adult morphology in this group. This study demonstrates the utility of variation in growth patterns in different cranial regions to document changes in the demographic parameters in two different populations.

The Remarkable Change in Euro-American Cranial Shape and Size.

Secular changes in stature, weight, or other components of the body that can be obtained from historical records have been extensively studied. Cranial change has been central to anthropology for more than a century, but the focus has normally been on change measured in centuries or millennia. Cranial change measured in decades, normally considered to result from plastic response to the environment, has been less studied. This article reports on change in cranial vault dimensions in white Americans. Variables were glabello-occipital length (GOL), basion-bregma height (BBH), basion-nasion length (BNL), maximum cranial breadth (XCB), and biauricular breadth (AUB). Cranial size was calculated as the geometric mean of these variables, and shape dimensions were calculated as described by Darroch and Mosimann ( 1985 ). Cranial module and cranial capacity were also calculated. Samples consisted of 1,112 males and 668 females complete for those variables. Samples were organized into 10-year birth cohorts, with birth years ranging from 1820 to 1990. One-way ANOVA was used to test for variation among cohorts. The pattern of secular change was examined graphically and was compared with quality-of-life and environmental indicators, including stature, infant mortality, calories per person, and relative number of immigrants. All variables showed significant secular change, but BBH, XCB, and BNL responded most strongly. Over the past 170 years, crania became relatively higher, narrower, and larger with longer cranial bases. Both sexes changed, but female change was less pronounced than male change. The cranial variables tracked secular changes in stature, most prominently BNL. The highest correlation between a cranial variable and quality-of-life indicator was BBH and infant mortality. We are not able to identify specific causes of secular changes in cranial morphology. However, given that modern Americans have introduced themselves into a novel environment never before experienced by human populations, we consider it unlikely that it is pure plasticity. In addition to possible plastic responses, it is likely that selection, acting through the dramatic changes in infant mortality, is also involved.

Rate of evolutionary change in cranial morphology of the marsupial genus Monodelphis is constrained by the availability of additive genetic variation.

We tested the hypothesis that the rate of marsupial cranial evolution is dependent on the distribution of genetic variation in multivariate space. To do so, we carried out a genetic analysis of cranial morphological variation in laboratory strains of Monodelphis domestica and used estimates of genetic covariation to analyse the morphological diversification of the Monodelphis brevicaudata species group. We found that within-species genetic variation is concentrated in only a few axes of the morphospace and that this strong genetic covariation influenced the rate of morphological diversification of the brevicaudata group, with between-species divergence occurring fastest when occurring along the genetic line of least resistance. Accounting for the geometric distribution of genetic variation also increased our ability to detect the selective regimen underlying species diversification, with several instances of selection only being detected when genetic covariances were taken into account. Therefore, this work directly links patterns of genetic covariation among traits to macroevolutionary patterns of morphological divergence. Our findings also suggest that the limited distribution of Monodelphis species in morphospace is the result of a complex interplay between the limited dimensionality of available genetic variation and strong stabilizing selection along two major axes of genetic variation.

Exploring the Relative Importance of Spatial and Environmental Variation on the Craniometrics of the Modern Portuguese

Previous research on the causes of craniometric variation within and among human populations has invoked both genetic and environmental explanations. Recent studies of modern populations in the United States and Portugal, among other populations, suggest that changes in environmental conditions have resulted in significant changes in cranial morphology. While similar changes in cranial morphology have been observed in genetically diverse populations, these populations do not appear to be converging on a common form. This study seeks to understand the role that population history and environmental variation play in explaining craniometric variation in the modern Portuguese. Using three-dimensional craniometric data collected from an identified skeletal collection from Lisbon, Portugal, this research examines the relationship between cranial morphology and place of birth. Two hypotheses are tested with regard to craniometric variation. The first asks whether phenotypic variation is related to geographic distance in the sample and therefore accurately represents genetic relationships. The second explores whether the significant secular changes observed in the sample obscure the population history reconstructed using craniometric data. The results demonstrate a significant relationship between geography and cranial morphology in the modern Portuguese sample. The clinal distribution to the craniometric variation is similar to what has been reported in some studies of molecular DNA from the Portuguese. The results show that cranial morphology reflects both the geography and genetic history of the sample and, furthermore, that secular changes in the population neither obscure nor erase this pattern. This study illustrates that even when secular changes have a significant impact on cranial morphology, they do not impede researchers’ ability to reconstruct genetic relationships using craniometric data.

Exploring the Relative Importance of Spatial and Environmental Variation on the Craniometrics of the Modern Portuguese

Previous research on the causes of craniometric variation within and among human populations has invoked both genetic and environmental explanations. Recent studies of modern populations in the United States and Portugal, among other populations, suggest that changes in environmental conditions have resulted in significant changes in cranial morphology. While similar changes in cranial morphology have been observed in genetically diverse populations, these populations do not appear to be converging on a common form. This study seeks to understand the role that population history and environmental variation play in explaining craniometric variation in the modern Portuguese. Using three-dimensional craniometric data collected from an identified skeletal collection from Lisbon, Portugal, this research examines the relationship between cranial morphology and place of birth. Two hypotheses are tested with regard to craniometric variation. The first asks whether phenotypic variation is related to geographic distance in the sample and therefore accurately represents genetic relationships. The second explores whether the significant secular changes observed in the sample obscure the population history reconstructed using craniometric data. The results demonstrate a significant relationship between geography and cranial morphology in the modern Portuguese sample. The clinal distribution to the craniometric variation is similar to what has been reported in some studies of molecular DNA from the Portuguese. The results show that cranial morphology reflects both the geography and genetic history of the sample and, furthermore, that secular changes in the population neither obscure nor erase this pattern. This study illustrates that even when secular changes have a significant impact on cranial morphology, they do not impede researchers' ability to reconstruct genetic relationships using craniometric data.