Functional implications of dicynodont cranial suture morphology

Cranial suture morphology of Lystrosaurus and the generalized dicynodont Oudenodon was investigated to determine the strain environment during mastication, which in turn may indicate a difference in cranial function between the two taxa. Finite element (FE) analysis indicated that less strain accumulated in the cranium of Lystrosaurus during orthal bite simulations than in Oudenodon. Despite the overall difference in strain magnitude, moderate to high FE‐predicted strain accumulated in similar areas of the cranium of both taxa. The suture morphology in these cranial regions of Lystrosaurus and Oudenodon was investigated further by examination of histological sections and supplemented by observations of serial sections and computed tomography (CT) scans. The predominant type of strain from selected blocks of finite elements that contain sutures was determined, enabling comparison of suture morphology to strain type. Drawing from strain‐suture correlations established in extant taxa, the observed patterns of sutural morphology for both dicynodonts were used to deduce cranial function. The moderate to high compressive and tensile strain experienced by the infraorbital bar, zygomatic arch, and postorbital bar of Oudenodon and Lystrosaurus may have been decreased by small adjustive movements at the scarf sutures in those regions. Disparities in cranial suture morphology between the two taxa may reflect differences in cranial function. For instance, the tongue and groove morphology of the postorbital‐parietal suture in Oudenodon could have withstood the higher FE‐predicted tensile strain in the posterior skull roof. The scarf premaxilla‐nasal suture of Lystrosaurus provided an additional region of sutural mobility in the anterior surface of the snout, suggesting that Lystrosaurus may have employed a different biting regime than Oudenodon. The morphology of several sutures sampled in this study correlated with the FE‐predicted strain, although other cranial functional hypotheses remain to be tested. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Relationship of cranial robusticity to cranial form,geography and climate in Homo sapiens

Variation in cranial robusticity among modern human populations is widely acknowledged but not well‐understood. While the use of “robust” cranial traits in hominin systematics and phylogeny suggests that these characters are strongly heritable, this hypothesis has not been tested. Alternatively, cranial robusticity may be a response to differences in diet/mastication or it may be an adaptation to cold, harsh environments. This study quantifies the distribution of cranial robusticity in 14 geographically widespread human populations, and correlates this variation with climatic variables, neutral genetic distances, cranial size, and cranial shape. With the exception of the occipital torus region, all traits were positively correlated with each other, suggesting that they should not be treated as individual characters. While males are more robust than females within each of the populations, among the independent variables (cranial shape, size, climate, and neutral genetic distances), only shape is significantly correlated with inter‐population differences in robusticity. Two‐block partial least‐squares analysis was used to explore the relationship between cranial shape (captured by three‐dimensional landmark data) and robusticity across individuals. Weak support was found for the hypothesis that robusticity was related to mastication as the shape associated with greater robusticity was similar to that described for groups that ate harder‐to‐process diets. Specifically, crania with more prognathic faces, expanded glabellar and occipital regions, and (slightly) longer skulls were more robust than those with rounder vaults and more orthognathic faces. However, groups with more mechanically demanding diets (hunter‐gatherers) were not always more robust than groups practicing some form of agriculture. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

The evolutionary improbability of ‘generalism’ in nature,with special reference to insects

In addition to feeding, many vertebrates use their skulls for other functions that are highly relevant to fitness. One such function is roost excavation by the bat Lophostoma silvicolum. Males of this species use their canines to create cavities inside active termite nests, which are significantly harder than the prey they eat. Here we investigate whether the skull of L. silvicolum is specialized for roost excavation relative to the ecologically similar species Tonatia saurophila and Micronycteris hirsuta, which do not excavate roosts. We conducted a finite element analysis that simulated roost excavating and feeding behaviours. These analyses were informed by our observations of feeding and roost‐excavating behaviours, bite force, and dissections of the cranial musculature of the three bat species. During the simulation of roost excavation (bilateral canine biting), our data indicate that most regions of the skull of L. silvicolum exhibit less stress than those of T. saurophila and M. hirsuta; however, the latter exhibited the lowest peak stress at the zygomatic arches. During loads that simulate feeding (bilateral molar biting), the three species exhibit similar stress levels. It is not clear whether L. silvicolum has a skull shape that is stronger under the loads imposed by excavation, but it does exhibit relatively higher bilateral canine bite forces that are generated via relatively larger temporalis muscles. Based on the muscle data, our study suggests that the feeding apparatus of mammals can exhibit performance and morphological adaptations to functions other than feeding. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 1–10.     

Relationships of diversity,disparity, and their evolutionary rates in squirrels (Sciuridae)

Several theories predict that rapidly diversifying clades will also rapidly diverge phenotypically; yet, there are also reasons for suspecting that diversification and divergence might not be correlated. In the widely distributed squirrel clade (Sciuridae), we test for correlations between per lineage speciation rates, species richness, disparity, and a time‐invariant measure of disparity that allows for comparing rates when evolutionary modes differ, as they do in squirrels. We find that species richness and speciation rates are not correlated with clade age or with each other. Disparity appears to be positively correlated with clade age because young, rapidly diversifying Nearctic grassland clades are strongly pulled to a single stable optimum but older, slowly diversifying Paleotropical forest clades contain lineages that diverge along multiple ecological and morphological lines. That contrast is likely due to both the environments they inhabit and their phylogenetic community structure. Our results argue against a shared explanation for diversity and disparity in favor of geographically mediated modes of speciation and ecologically mediated modes of phenotypic evolution.     

Multiple origins of secondary temporal fenestrae and orbitozygomatic junctions in birds

Orbitozygomatic junctions (bony connections between the postorbital and zygomatic processes) and secondary temporal fenestrae have long been known to occur in a few avian species, but no comprehensive study of this phenomenon has ever been published. Having surveyed all non‐passerine and most passerine families, we established that the orbitozygomatic junction evolved 18–20 times independently in Cracidae, Phasianoidea (Odontophoridae and Phasianidae), Gastornis, Columbidae (three times), Pteroclidae (Syrrhaptes), Aptornis, Thinocoridae (Thinocorus), Scolopacidae (possibly twice), Ciconiidae (twice), Brachypteraciidae (Uratelornis), Picidae (Picus spp.), Psittacidae (Melopsittacus), Cacatuidae, and Alaudidae (twice). The junction arises in evolution as a result of either elongation of the two processes that meet at angles or the appearance of a bony cross‐bridge in place of a ligament or aponeurosis. In the first case, the junction is initially non‐adaptive, as indicated by its extreme variation (e.g., in Cracinae and Ciconiidae), and may or may not prove functional as an exaptation. Whenever adaptive, the junction supports an expansion of the adductor mandibulae externus (primarily its pars media). In addition, a rostral extension of the tympanic wing has come to cross the temporal fossa in Strigidae (at least twice) and probably Podargus. Altogether, secondary bony connections across the temporal fossa evolved independently at least 21–23 times in neornithine birds.     

Niche evolution and diversification in a Neotropical radiation of birds (Aves: Furnariidae)

Rapid diversification may be caused by ecological adaptive radiation via niche divergence. In this model, speciation is coupled with niche divergence and lineage diversification is predicted to be correlated with rates of niche evolution. Studies of the role of niche evolution in diversification have generally focused on ecomorphological diversification but climatic‐niche evolution may also be important. We tested these alternatives using a phylogeny of 298 species of ovenbirds (Aves: Furnariidae). We found that within Furnariidae, variation in species richness and diversification rates of subclades were best predicted by rate of climatic‐niche evolution than ecomorphological evolution. Although both are clearly important, univariate regression and multivariate model averaging more consistently supported the climatic‐niche as the best predictor of lineage diversification. Our study adds to the growing body of evidence, suggesting that climatic‐niche divergence may be an important driver of rapid diversification in addition to ecomorphological evolution. However, this pattern may depend on the phylogenetic scale at which rate heterogeneity is examined.     

The evolution and evolutionary consequences of marginal thermostability in proteins

When we seek to explain the characteristics of living systems in their evolutionary context, we are often interested in understanding how and why certain properties arose through evolution, and how these properties then affected the continuing evolutionary process. This endeavor has been assisted by the use of simple computational models that have properties characteristic of natural living systems but allow simulations over evolutionary timescales with full transparency. We examine a model of the evolution of a gene under selective pressure to code for a protein that exists in a prespecified folded state at a given growth temperature. We observe the emergence of proteins with modest stabilities far below those possible with the model, with a denaturation temperature tracking the simulation temperature, despite the absence of selective pressure for such marginal stability. This demonstrates that neither observations of marginally stable proteins, nor even instances where increased stability interferes with function, provide evidence that marginal stability is an adaptation. Instead the marginal stability is the result of a balance between predominantly destabilizing mutations and selection that shifts depending on effective population size. Even if marginal stability is not an adaptation, the natural tendency of proteins toward marginal stability, and the range of stabilities that occur during evolution, may have significant effect on the evolutionary process.     

Evidence of a neo-sex chromosome in birds

Neo-sex chromosomes often originate from sex chromosome–autosome fusions and constitute an important basis for the study of gene degeneration and expression in a sex chromosomal context. Neo-sex chromosomes are known from many animal and plant lineages, but have not been reported in birds, a group in which genome organization seems particularly stable. Following indications of sex linkage and unexpected sex-biased gene expression in warblers (Sylvioidea; Passeriformes), we have conducted an extensive marker analysis targeting 31 orthologues of loci on zebra finch chromosome 4a in five species, representative of independent branches of Passerida. We identified a region of sex linkage covering approximately the first half (10 Mb) of chromosome 4a, and associated to both Z and W chromosomes, in three Sylvioidea passerine species. Linkage analysis in an extended pedigree of one species additionally confirmed the association between this part of chromosome 4a and the Z chromosome. Markers located between 10 and 21 Mb of chromosome 4a showed no signs of sex linkage, suggesting that only half of the chromosome was involved in this transition. No sex linkage was observed in non-Sylvioidea passerines, indicating that the neo-sex chromosome arose at the base of the Sylvioidea branch of the avian phylogeny, at 47.4–37.6 millions years ago (MYA), substantially later than the ancestral sex chromosomes (150 MYA). We hypothesize that the gene content of chromosome 4a might be relevant in its transition to a sex chromosome, based on the presence of genes (for example, the androgen receptor) that could offer a selective advantage when associated to Z-linked sex determination loci.     

Feather function and the evolution of birds

The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life-history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.     

Epidemiological and evolutionary consequences of targeted vaccination

Recent theory has examined the way in which vaccination strategies are expected to influence the evolution of parasite virulence. Most of this work has assumed that vaccination is imposed on a homogeneous host population. However, host populations are typically composed of different types of individuals, with each type responding differently to infection. Moreover, actual interventions often focus treatment on those hosts that are likely to suffer the most ill effects of a particular disease. Here we consider the epidemiological and evolutionary consequences of interventions that focus vaccination on individuals expressing the greatest susceptibility to infection and/or the greatest vulnerability to mortality once infected. Our results indicate that predictions are very sensitive to the nature and degree of heterogeneity in susceptibility and vulnerability. They further suggest that accounting for realistic kinds of heterogeneity when contemplating targeted treatment plans and policies might provide a new tool in the design of more effective virulence management strategies.     

On the evolutionary interplay between dispersal and local adaptation in heterogeneous environments

Dispersal, whether in the form of a dandelion seed drifting on the breeze, or a salmon migrating upstream to breed in a nonnatal stream, transports genes between locations. At these locations, local adaptation modifies the gene frequencies so their carriers are better suited to particular conditions, be those of newly disturbed soil or a quiet river pool. Both dispersal and local adaptation are major drivers of population structure; however, in general, their respective roles are not independent and the two may often be at odds with one another evolutionarily, each one exhibiting negative feedback on the evolution of the other. Here, we investigate their joint evolution within a simple, discrete‐time, metapopulation model. Depending on environmental conditions, their evolutionary interplay leads to either a monomorphic population of highly dispersing generalists or a collection of rarely dispersing, locally adapted, polymorphic sub‐populations, each adapted to a particular habitat type. A critical value of environmental heterogeneity divides these two selection regimes and the nature of the transition between them is determined by the level of kin competition. When kin competition is low, at the transition we observe discontinuities, bistability, and hysteresis in the evolved strategies; however, when high, kin competition moderates the evolutionary feedback and the transition is smooth.     

Chondrocranium and cranial muscle morphology in Lysapsus and Pseudis tadpoles (Anura: Hylidae: Hylinae)

The chondrocranium and cranial muscles of Lysapsus limellum, Pseudis cardosoi, P. minuta and four subspecies of P. paradoxa (caribensis, occidentalis, paradoxa and platensis) are described. Lysapsus and Pseudis are related to the Hylidae by: (1) vertical processus anterolateralis of the larval crista parotica, and (2) presence of m. mandibulolabialis inferior and superior (the latter reversed in P. cardosoi). We confirm some larval synapomorphies previously proposed for the pseudids, except the m. subarcualis obliquus II having one head and the absence of copula anterior. We add three new features for the pseudids: (1) the distal end of ceratobranchiales III and IV projects towards either the capsula auditiva (Lysapsus) or the processus basicapsularis – new term – (Pseudis); (2) the arcus subocularis is twice as wide at the level of the processus ascendens as it is just posterior to the processus muscularis quadrati, and (3) the lateral corner of the processus articularis quadrati largely projects laterally. We diagnose Lysapsus larvae (the states of Pseudis in parentheses) as follows: (1) short processus pseudopterygoideus (long), (2) absence of lateral development of larval crista parotica (presence), (3) absence of commissura proximalis II (presence), (4) presence of processus lateralis hyalis – new term – (absence), (5) absence of processus basicapsularis (presence), (6) processus ascendens slightly curved along its entire extent (curved at the union with the basi cranii only), and (7) absence of m. tympanopharyngeus (presence).     

念珠藻属植物hetR基因的适应性进化分析

以念珠藻属(Nostoc)及其近缘类群hetR基因的51条序列为研究对象,对hetR基因的编码蛋白进行生物信息学分析和系统发育分析,并使用分支模型、位点模型和分支-位点模型进行该基因位点的适应性进化研究。系统发育分析结果显示,51条hetR基因蛋白序列可分为4个大分支。适应性进化分析结果表明,在3种进化模型中,大多数分支及藻株都没有检测到统计学上具有显著性的正选择位点,说明检测的位点大多处于负选择压力下。但在普通念珠藻(Nostoc commune,CHAB2802)中检测到正选择位点(126T),提示念珠藻属植物hetR基因发生了适应性改变。     

Comparative finite element analysis of the cranial performance of four herbivorous marsupials

Marsupial herbivores exhibit a wide variety of skull shapes and sizes to exploit different ecological niches. Several studies on teeth, dentaries, and jaw adductor muscles indicate that marsupial herbivores exhibit different specializations for grazing and browsing. No studies, however, have examined the skulls of marsupial herbivores to determine the relationship between stress and strain, and the evolution of skull shape. The relationship between skull morphology, biomechanical performance, and diet was tested by applying the finite element method to the skulls of four marsupial herbivores: the common wombat (Vombatus ursinus), koala (Phascolarctos cinereus), swamp wallaby (Wallabia bicolor), and red kangaroo (Macropus rufus). It was hypothesized that grazers, requiring stronger skulls to process tougher food, would have higher biomechanical performance than browsers. This was true when comparing the koala and wallaby (browsers) to the wombat (a grazer). The cranial model of the wombat resulted in low stress and high mechanical efficiency in relation to a robust skull capable of generating high bite forces. However, the kangaroo, also a grazer, has evolved a very different strategy to process tough food. The cranium is much more gracile and has higher stress and lower mechanical efficiency, but they adopt a different method of processing food by having a curved tooth row to concentrate force in a smaller area and molar progression to remove worn teeth from the tooth row. Therefore, the position of the bite is crucial for the structural performance of the kangaroo skull, while it is not for the wombat which process food along the entire tooth row. In accordance with previous studies, the results from this study show the mammalian skull is optimized to resist forces generated during feeding. However, other factors, including the lifestyle of the animal and its environment, also affect selection for skull morphology to meet multiple functional demands. J. Morphol. 276:1230–1243, 2015. © 2015 Wiley Periodicals, Inc.     

Variations in the hypotarsus morphology of birds and their evolutionary significance

Crown group (neornithine birds) exhibit a great variation in the morphology of the hypotarsus, a structure on the proximal end of the tarsometatarsus, which guides the tendons of the flexor muscles of the toes. Hypotarsus morphology is of significance for the identification of fossil taxa, and several extant groups show characteristic patterns that are of phylogenetic interest. So far, however, the diversity of hypotarsus morphologies has been little studied, and there are no comprehensive studies across all neornithine birds. In particular, the identities of the involved canals and sulci remain elusive, and some confusion exists about their correct homologies. In this study, hypotarsus morphologies are for the first time surveyed among all extant birds, and basic patterns are characterized. Instances are identified, in which particular hypotarsus morphologies are correlated with certain locomotion types, inferences are made about possible ancestral morphologies, and some patterns of phylogenetic interest are discussed.     

Precursors of mesotocin and vasotocin in birds: Identification of VLDV- and MSEL-neurophysins in chicken,goose, and ostrich

Precursors of neurohypophysial hormones are small proteins processed into nonapeptide hormones and neurophysins during axonal transport to the neurohypophysis. In mammals, oxytocin is associated with VLDV-neurophysin and vasopressin with MSEL-neurophysin. In birds, mesotocin and vasotocin are found instead of mammalian oxytocin and vasopressin. From goose, chicken and ostrich posterior pituitary glands, two types of neurophysins related to mammalian VLDV-and MSEL-neurophysins, respectively, have been identified by their N-terminal sequences. It is assumed that, as in mammals, hormonal peptide and the first 9 residues of the corresponding neurophysin are encoded by a common exon and that mesotocin and vasotocin, evolutionary predecessors of oxytocin and vasopressin, are associated in the precursors with VLDV-neurophysin and MSEL-neurophysin, respectively.     

Cretaceous origins of the vibrotactile bill-tip organ in birds

Some probe-foraging birds locate their buried prey by detecting mechanical vibrations in the substrate using a specialized tactile bill-tip organ comprising mechanoreceptors embedded in densely clustered pits in the bone at the tip of their beak. This remarkable sensory modality is known as ‘remote touch’, and the associated bill-tip organ is found in probe-foraging taxa belonging to both the palaeognathous (in kiwi) and neognathous (in ibises and shorebirds) clades of modern birds. Intriguingly, a structurally similar bill-tip organ is also present in the beaks of extant, non-probing palaeognathous birds (e.g. emu and ostriches) that do not use remote touch. By comparison with our comprehensive sample representing all orders of extant modern birds (Neornithes), we provide evidence that the lithornithids (the most basal known palaeognathous birds which evolved in the Cretaceous period) had the ability to use remote touch. This finding suggests that the occurrence of the vestigial bony bill-tip organ in all modern non-probing palaeognathous birds represents a plesiomorphic condition. Furthermore, our results show that remote-touch probe foraging evolved very early among the Neornithes and it may even have predated the palaeognathous–neognathous divergence. We postulate that the tactile bony bill-tip organ in Neornithes may have originated from other snout tactile specializations of their non-avian theropod ancestors.     

Quantification of cranial convergences in arvicolids (Rodentia)

Cranial convergence in nine species of arvicolids is quantified phenetically using geometric morphometry. In a preliminary step, a hypothesis about phyletic relationships is proposed as a framework against which to examine morphological comparisons. The cranial morphology is then depicted in three series of 37, 20 and 13 landmarks characterizing the lower, upper and lateral sides of the skull respectively. Superimposition (Procrustes) methods are used to quantify shape differences and establish phenograms for the three sides of the skull. The phenogram obtained for lateral sides reveals a strong connection between skull profile and mode of life: surface dwelling forms have elongated skulls whereas burrowers have angular skulls. The phenogram of the lower sides show no clear correlation between shape and mode of life. Analysis of the upper cranial sides revealed a substantial difference between hard-substrate burrowers and other ecological groups. The results of the morphological analyses are compared with the phyletic hypothesis and with ecological data to explore how convergences take place in the evolution of arvicolids. Analyses pointed out three instances of morphological convergence: one between Synaptomys cooperi and voles, one between Lagurus lagurus and lemmings, and one between Myopus schisticolor and some voles. Four main ecological events are discussed.     

Linking form and function of the fibrous joints in the skull: a new quantification scheme for cranial sutures using the extant fish Polypterus endlicherii

The aim of this study is to connect specific sutural morphologies with the specific types of deformation they experience. To meet this goal, we quantified the morphologies of the interfrontal (IF), interparietal (IP), and frontoparietal (FP) sutures in the extant fish Polypterus endlicherii, and used our published measurements of in vivo deformation of these sutures during feeding to infer how suture morphology and function are connected. Specifically, we found that three relatively simple measures of cross-sectional suture complexity (i.e., the ratio of total sutural length to its shortest end-to-end length; amount of sutural overlap; and size of the largest interdigitation) can be used to distinguish between the IF, FP, and IP sutures, which exhibit very different cross-sectional shapes and responses to loading. Interestingly, these differences in cross-sectional morphology are not reflected by the linear traces of these sutures on the surface of the skull, implying that cross-sectional shape of a suture must be known to infer the loading conditions it experiences. Plotting the three cross-sectional metrics against one another to yield a sutural morphospace shows that the IF, IP, and FP sutures define regions that are largely distinct from one another. Our previous measurements of strain across these sutures suggested that the FP region would lie between the IF and IP regions; instead, the FP region is largely set apart from the other two fields. Based on this discovery, and on the locations of cranial muscles, we propose a new model of deformation in the skull of P. endlicherii during feeding, in which rotation parallel to the skull roof is combined with bending, subjecting the FP suture to complex shearing. Finally, although the sutures of P. endlicherii appear to be significantly less complex than those of mammals, these fish sutures show a similar range of morphologies and perform similar functions as do mammalian sutures.