Tempo and mode in the evolution of morphological disparity in the Neotropical fern genus Pleopeltis

Species diversity and morphological disparity are two measures to examine the diversity of life. Evidence based on the fossil record suggests a complex relation between these two parameters of biodiversity including frequent decoupling of their assembly through time. However, rather few studies explored the correlation of these two measurements by studying extant plant species. This study was designed to explore the accumulation of morphological disparity of the derived Neotropical fern genus Pleopeltis. To explore the relationship of species diversity and morphological disparity, we employed several approaches including divergence time estimates based on DNA sequence variation, reconstruction of character state changes based on a morphological matrix comprising 41 discrete characters, and exploration of the phylomorphospace. Accumulation of species diversity and morphological disparity was found to be concordant although the assumption of independence was not rejected for the accumulation of genetic and morphological variation. The phylomorphospace reconstruction provided further evidence for clade‐specific morphospace expansion that imply developmental pathways and competition among clades as major factors shaping the assembly of morphological disparity over time.     

Morphological variability in time and space: an example of patterns within buchiid bivalves (Bivalvia,Buchiidae)

Abstract: We studied morphological variation of the bivalve Buchia over its geographical and temporal range. Buchia was widely distributed during the Late Jurassic and Early Cretaceous, and, while previous quantitative studies have shown that species are characterized by large amounts of variation, there have been no prior attempts to measure how morphology varies geographically. We employed traditional morphometric techniques using nine linear/angular measurements on 1855 buchiid shells from eight localities taken from widely separated, but mostly coeval, sections across its range. Principal component and canonical variate analyses indicate that geographical morphospace of buchiids varied significantly, but we did not find evidence of a latitudinal gradient in shell shape. The amount of variation between localities was similar to the amount of variation between species, indicating the importance of geographical effects on morphology. Disparity (morphological diversity within a taxon, calculated by the sum of variances) and diversity (number of species) were calculated for each location and time period (age). Disparity and diversity reached ultimate lows just before the genus’ extinction in the Hauterivian, and is suggestive that extinction was morphologically selective. We did not find significant trends for either metric, but there were discordances throughout its temporal range. Latitudinal trends of disparity and diversity within Buchia are not apparent. This research adds to the growing body of work on geographical variation and is a preliminary step to understanding the nature and variation of buchiid species and of biodiversity in general.     

Morphospace occupation in thalattosuchian crocodylomorphs: skull shape variation, species delineation and temporal patterns

Abstract: Skull shape variation in thalattosuchians is examined using geometric morphometric techniques in order to delineate species, especially with respect to the classification of Callovian species, and to explore patterns of disparity during their evolutionary history. The pattern of morphological diversity in thalattosuchian skulls was found to be very similar to modern crocodilians: the main sources of variation are the length and the width of the snout, but these broad changes are correlated with size of supratemporal fenestra and frontal bone, length of the nasal bone, size of the orbit and premaxilla and position of the narial opening. Patterns of shape variation, in combination with discrete‐state morphology and stratigraphic and geographic range data were used to distinguish nine species of teleosaurid and 14 species of metriorhynchid, with the four currently recognized Callovian species being split into eight. Metriorhynchids were found to be more disparate from the average shape of morphospace than teleosaurids. However, short‐snouted metriorhynchids and long‐snouted teleosaurids showed the greatest amount of disparity with respect to snout morphotypes, indicating that each group tended to explore opposite areas of morphospace. Phylogeny was found to have a moderate influence on the pattern of morphospace occupation in metriorhynchids, but little effect in teleosaurids suggesting that other factors or constraints control the pattern of skull shape variation in thalattosuchians. A comparison of thalattosuchians with dyrosaur/pholidosaurids shows that thalattosuchians have a unique skull morphology, implying that there are multiple ways to construct a ‘long snout’. Moreover, the skull geometry of the problematic species Pelagosaurus typus was found to converge on the teleosaurid area of morphospace. Finally, the temporal distribution of thalattosuchian species and morphotypes demonstrate a clear and highly correlated relationship with sea level curves and mass extinction events through the Jurassic and the Early Cretaceous.     

Inferring the accumulation of morphological disparity in epiphyllous liverworts

Phylogenetic studies of lineages growing in extreme environments have frequently recovered evidence not only of high level of homoplasy but also of discordance of morphological disparity and species diversity. It has been suggested that this divergence may be caused by developmental constraints and/or natural selection. Here we explored these hypotheses by inferring the phenotypic evolution of the derived liverwort genus Cololejeunea. These liverworts occur preferentially on the surface of leaves or other aerial parts of vascular plants growing in wet forests. The evolution of 12 morphological characters was studied using a phylogenetic framework comprising 70 species of Cololejeunea. The phylogeny was reconstructed using DNA sequences of one nuclear and two plastid regions and enabled the inference of the evolution of the studied morphological characters by determining the frequency of homoplasy. Mantel tests were used to test for correlations of morphological disparity?×?species diversity and morphological disparity?×?epiphytism. The phylogenetic informativeness of each binary character was estimated by the D metric of the Fritz and Purvis test, and the relationship between each character and epiphytism was inferred by Pearson’s coefficient. We evaluated the morphospace occupation using principal coordinate analyses. Our results not only recovered high levels of homoplasy but also weak correlations of morphological disparity and species diversity. Morphological disparity was not linked to epiphytism, although positive or negative relationships between some characters and epiphytism were found. The Brownian model of character evolution was not rejected for the studied morphological disparity in Cololejeunea with the exception of asexual propagules. The observations support the prediction that iterative evolution in a well-defined morphospace may result in rampant homoplasy and the observed divergence of disparity and diversity.     

Bathymetric patterns of morphological disparity in deep-sea gastropods from the western North Atlantic basin

Understanding patterns of species richness requires knowledge of the individual roles species play in community structure. Here, I use gastropod shells as a source of information about both their ecological and their evolutionary functions in generating bathymetric gradients of diversity. Specifically, morphological disparity of shell architecture in deep-sea gastropods is evaluated over a depth gradient in the western North Atlantic by constructing an empirical morphospace based on an eigenshape analysis. Morphological disparity is quantified by calculating the centroid, total range, and dispersion of the morphospace at each station along the depth gradient. The results indicate that local faunas are drawn from a regional pool with the same variance but that average dissimilarity in forms reflects the number of species in the sample. The range of the morphospace at local scales is also less than at regional scales, resulting from the variability of the morphospace centroid over depth. Although the position of the morphospace changes with depth, morphological disparity remains unaffected. Despite the lack of bathymetric patterns in variance, patterns in nearest neighbor distance persist. The findings suggest the importance of interacting ecological and evolutionary processes at varying spatiotemporal scales for both morphological disparity and species richness.     

Patterns of cranial ontogeny in lacertid lizards: morphological and allometric disparity

We explored the ontogenetic dynamics of the morphological and allometric disparity in the cranium shapes of twelve lacertid lizard species. The analysed species (Darevskia praticola, Dinarolacerta mosorensis, Iberolacerta horvathi, Lacerta agilis, L. trilineata, L. viridis, Podarcis erhardii, P. melisellensis, P. muralis, P. sicula, P. taurica and Zootoca vivipara) can be classified into different ecomorphs: terrestrial lizards that inhabit vegetated habitats (habitats with lush or sparse vegetation), saxicolous and shrub‐climbing lizards. We observed that there was an overall increase in the morphological disparity (MD) during the ontogeny of the lacertid lizards. The ventral cranium, which is involved in the mechanics of jaw movement and feeding, showed higher levels of MD, an ontogenetic shift in the morphospace planes and more variable allometric patterns than more conserved dorsal crania. With respect to ecology, the allometric trajectories of the shrub‐climbing species tended to cluster together, whereas the allometric trajectories of the saxicolous species were highly dispersed. Our results indicate that the ontogenetic patterns of morphological and allometric disparity in the lacertid lizards are modified by ecology and functional constraints and that the identical mechanisms that lead to intraspecific morphological variation also produce morphological divergence at higher taxonomic levels.     

Adaptive landscape and functional diversity of Neotropical cichlids: implications for the ecology and evolution of Cichlinae (Cichlidae; Cichliformes)

Morphological, lineage and ecological diversity can vary substantially even among closely related lineages. Factors that influence morphological diversification, especially in functionally relevant traits, can help to explain the modern distribution of disparity across phylogenies and communities. Multivariate axes of feeding functional morphology from 75 species of Neotropical cichlid and a stepwise‐AIC algorithm were used to estimate the adaptive landscape of functional morphospace in Cichlinae. Adaptive landscape complexity and convergence, as well as the functional diversity of Cichlinae, were compared with expectations under null evolutionary models. Neotropical cichlid feeding function varied primarily between traits associated with ram feeding vs. suction feeding/biting and secondarily with oral jaw muscle size and pharyngeal crushing capacity. The number of changes in selective regimes and the amount of convergence between lineages was higher than expected under a null model of evolution, but convergence was not higher than expected under a similarly complex adaptive landscape. Functional disparity was compatible with an adaptive landscape model, whereas the distribution of evolutionary change through morphospace corresponded with a process of evolution towards a single adaptive peak. The continentally distributed Neotropical cichlids have evolved relatively rapidly towards a number of adaptive peaks in functional trait space. Selection in Cichlinae functional morphospace is more complex than expected under null evolutionary models. The complexity of selective constraints in feeding morphology has likely been a significant contributor to the diversity of feeding ecology in this clade.     

Comparing assemblages of Asteraceae and their insect herbivores under different land‐use regimens

Accelerated tropical landscape changes occurring over recent decades have produced environmental mosaics comprising remaining isolated green areas and mixed land‐use types. Our objective was to study the effects of alterations in the natural landscape on the species composition and structure of assemblages of Asteraceae and their endophagous insects through comparisons between cerrado (savanna), pastures and Eucalyptus stands. We first investigated whether similarities between assemblages of Asteraceae and their insects varied among land uses or localities. Secondly, we asked whether assemblages of Eucalyptus stands and pastures are subsets of those within the cerrado. We sampled within randomly deployed transects in 15 areas. Land use was found to be an important factor in determining plant composition similarity; however, locality did not exert any significant influence. Pastures were less similar to one another, suggesting high beta diversity. Similarities among insect assemblages were correlated with plant assemblage composition, but not with land use or locality. Species of Tephritidae were distributed along localities independently of land use. High beta diversity in Asteraceae assemblages among cerrados and pastures was supported by nestedness analysis. Plant assemblages in Eucalyptus stands were subsets of cerrado, but pasture assemblages were only partial subsets. A higher degree of nestedness in insect assemblages than in plant assemblages indicated lower beta diversity within these herbivores. Our data indicate that many herbivores are specialized on widely distributed plant genera. Conservation of Asteraceae species and their flower head insects depends not only on maintenance of landscape fragments but also on the correct matching of management form and land use. Such management may contribute to reducing isolation of plant and insect species by increasing the connectivity of remaining cerrado tracts, allowing population maintenance even at low abundances.     

The complex effects of mass extinctions on morphological disparity

Studies of biodiversity through deep time have been a staple for biologists and paleontologists for over 60 years. Investigations of species richness (diversity) revealed that at least five mass extinctions punctuated the last half billion years, each seeing the rapid demise of a large proportion of contemporary taxa. In contrast to diversity, the response of morphological diversity (disparity) to mass extinctions is unclear. Generally, diversity and disparity are decoupled, such that diversity may decline as morphological disparity increases, and vice versa. Here, we develop simulations to model disparity changes across mass extinctions using continuous traits and birth-death trees. We find no simple null for disparity change following a mass extinction but do observe general patterns. The range of trait values decreases following either random or trait-selective mass extinctions, whereas variance and the density of morphospace occupation only decline following trait-selective events. General trends may differentiate random and trait-selective mass extinctions, but methods struggle to identify trait selectivity. Long-term effects of mass extinction trait selectivity change support for phylogenetic comparative methods away from the simulated Brownian motion toward Ornstein-Uhlenbeck and Early Burst models. We find that morphological change over mass extinction is best studied by quantifying multiple aspects of morphospace occupation.     

Sex‐specific responses of phenotypic diversity to environmental variation

Identifying the factors generating ecomorphological diversity within species can provide a window into the nascent stages of ecological radiation. Sexual dimorphism is an obvious axis of intraspecific morphological diversity that could affect how environmental variation leads to ecological divergence among populations. In this paper we test for sex‐specific responses in how environmental variation generates phenotypic diversity within species, using the generalist lizard Gallotia galloti on Tenerife (Canary Islands). We evaluate two hypotheses: the first proposes that different environments have different phenotypic optima, leading to shifts in the positions of populations in morphospace between environments; the second posits that the strength of trait‐filtering differs between environments, predicting changes in the volume of morphospace occupied by populations in different environments. We found that intraspecific morphological diversity, provided it is adaptive, arises from both shifts in populations’ position in morphospace and differences in the strength of environmental filtering among environments, especially at high elevations. However, effects were found only in males; morphological diversity of females responded little to environmental variation. These results within G. galloti suggest natural selection is not the sole source of phenotypic diversity across environments, but rather that variation in the strength of, or response to, sexual selection may play an important role in generating morphological diversity in environmentally diverse settings. More generally, disparities in trait–environment relationships among males and females also suggest that ignoring sex differences in studies of trait dispersion and clustering may produce misleading inferences.     

Morphological vs. molecular delineation of taxa across montane regions in Europe: the case study of Gammarus balcanicus Schäferna, (Crustacea: Amphipoda)

Mountainous areas are characterized by substantial biodiversity and endemicity due to their complex geological history and habitat fragmentation. Hence, it can be assumed that particularly high species richness can be found in organisms with limited dispersal capabilities that inhabit mountain streams. A number of scientific papers focus on molecular phylogeography or traditional taxonomy of species or species groups inhabiting such habitats. However, there is a lack of studies that integrate morphological and molecular data to identify and delineate cryptic species. For practical reasons, uncovering cryptic diversity is crucial in taxa used in biomonitoring. Distinct species, hard to separate based on morphology only, may have different tolerance ranges towards a variety of factors. Thus, our goal is to combine the two approaches to reveal potential patterns of diversification within a species widely distributed across European mountains: the amphipod crustacean Gammarus balcanicus. The data were obtained from 13 populations spread across the range of the species. Individuals were initially ascribed to G. balcanicus based on conventional fauna key morpho‐anatomical diagnostic features and were further analysed for 23 additional features to explore any putative diversification. Morphometric data were analysed with use of the multiple correspondence analysis and anova . Molecular distances were calculated for 551‐bp‐long COI sequences. Test for isolation by distance was performed for both morphological and molecular data. The morphometric studies showed that some of the analysed features differed significantly between populations, although there was only a weak correlation between the morphological divergence and the between‐population geographical distances. Moreover, high morphological diversity was present within sites. A set of 42 COI haplotypes was identified among the 135 individuals sequenced. No haplotype was shared among populations. The molecular p‐distances within the nine localities presenting more than one haplotype were either almost null (ca. <0.003 for 7 localities) or relatively low (ca. 0.01–0.02 for 2 localities). In opposite, the molecular p‐distances between localities were mostly at a high level (94% of pairwise comparisons being >0.14), similar as between other well‐defined species of the genus Gammarus. Surprisingly, G. balcanicus appears to be polyphyletic based on topology of the neighbour‐joining tree. The level of genetic distance between localities was not correlated with their geographical proximity. Globally, combining spatial patterns of morphological versus molecular divergence indicates a high level of cryptic diversity within a species conventionally defined based upon fauna key morphological features. In this context, the name G. balcanicus should be applied only to the population from locus typicus, while the other populations represent a number of putative distinct species. We may expect that such phenomenon would apply also to other animal taxa with conserved morphology, which are widespread over different mountain ranges in Europe.     

Recognition of a species-poor, geographically restricted but morphologically diverse Cape lineage of diving beetles (Coleoptera: Dytiscidae: Hyphydrini)

Aim To establish the phylogeny and geographical origin of the genera of the diving beetle tribe Hyphydrini in order to investigate the origin of differences in geographical range size, intrageneric species‐richness and morphological disparity. In particular, we tested the hypothesis that the geographically restricted, species‐poor and morphologically deviating genera found in the Cape Region of South Africa are a paraphyletic pool of ‘primitive’ Hyphydrini, from which the morphologically more uniform, species‐rich and geographically widespread genera have originated. Location Worldwide, with special reference to the Cape Region of South Africa. Methods We constructed a genus‐level molecular phylogeny of 10 of the 14 known genera of Hyphydrini, including the five endemic to the Cape Region, using sequences from four gene fragments (two mitochondrial, rrnL and cox1; and two nuclear, 18S rRNA and histone 3, c. 2200 bp). Phylogenies were built with Bayesian methods, and linearized using penalized likelihood. Morphological disparity was characterized by correspondence analysis of a data matrix of 21 binary characters. We compare morphological disparity among groups using distance to the global and local centroids and the total range of morphospace occupied. Geographical range was estimated using the number of 6° longitude × 8° latitude Universal Transverse Mercator squares known to contain any species of each genus. Results Hyphydrini is made up of four well supported clades of similar relative genetic divergence: (1) Hyphydrus (Old World plus Australasia, 133 species), (2) the five endemic genera of the Cape Region, sister to Hovahydrus (Madagascar) (10 species), (3) Desmopachria (America, 92 species), and (4) two Oriental genera (Microdytes and Allopachria, 68 species). The morphological disparity within the Cape Region lineage has apparently increased with time, with the two genera closest to the global centroid paraphyletic and basal with respect to the three more recent, morphologically deviating genera. Differences in the number of species between each of the four lineages were not significant. The correlation between the number of species in each lineage and geographical range extent was highly significant, with the low species number of the Cape Region (six) well within the 95% confidence interval of the regression. Main conclusions Contrary to expectations, the species‐poor, morphologically deviating endemic genera of the Cape Region are not a ‘primitive’ relictual pool from which the widespread, species‐rich and morphologically uniform genera have originated. The morphological disparity within the Cape lineage has increased with time, and the apparent lack of species‐level diversification disappears when species–area relationships are considered. A major unanswered question is why one of the four main lineages of Hyphydrini has remained restricted to a very reduced area (the Cape Region), but despite this evolved the highest degree of morphological diversity seen in the tribe.     

A two-dimensional morphospace for cyanobacteria and microalgae: Morphological diversity,evolutionary relatedness,and size constraints

1. Body metrics are considered as master traits that regulate physiological, behavioural and life history features of planktic cyanobacteria and microalgae. Although the distribution of their morphological traits reflects the various trade-offs and strategies needed for survival in pelagic habitats, previous methods for quantifying phytoplankton body shape do not adequately represent the intricate details of surface variation that are so important for their nutrient- and light-harvesting capabilities. Therefore, here we provide a new framework to quantify and illustrate the morphological diversity of cyanobacteria and microalgae.
2. Essential components of formulae used for surface area (A = Cs × d²) and volume (V = Cv × d³) calculations are provided by the shape-specific surface area and volume constants (Cs, Cv). Cs, the surface shape factor, characterises the coarseness of the object surface, and Cv, the volumetric shape factor, characterises the shape deviation from a sphere. Using these morphologically and biologically relevant variables, we defined a two-dimensional morphological space, in which all three-dimensional objects have well-defined positions.
3. By analysing morphologies of taxa representing various forms in major cyanobacterial and microalgal groups, we demonstrated that these groups show considerable differences in the area occupied within the morphospace and these differences are not affected by evolutionary relatedness. We showed that the ratio of surface and volume constants correlated with organism size, suggesting that the development of basic morphologies is constrained by their linear dimensions.
4. Using surface and volumetric shape factors, we created a two-dimensional Euclidean morphospace in which all three-dimensional objects have a specific position. Positioning uni- and multicellular cyanobacteria and microalgae of various shapes into this morphospace allows their geometrical and ecological limitations to be understood. Because of the close linkage between phytoplankton morphology and ecology, the proposed morphospace may serve as a proxy for an ecospace. Thus, in future the proposed morphospace can be used to visualise current ecological processes such as eutrophication or seasonal succession of phytoplankton.

     

Quantitative Genetics and Evolution of Head Shape in <Emphasis Type="Italic">Plethodon</Emphasis> Salamanders

The evolution of morphological diversity via selection requires that morphological traits display significant heritable genetic variation. In Plethodon salamanders, considerable evidence suggests that head shape evolves in response to selection from interspecific competition, yet the genetic underpinnings of head shape have not been quantitatively examined. Here I used geometric morphometrics and quantitative genetics to assess heritable patterns of head shape variation from hatchling salamanders in two Plethodon species (P. cinereus and P. nettingi). Head shape differed significantly between species and among clutches within species, suggesting that a sizeable proportion of head shape variation was the result of clutch effects. Further, using a full-sib animal model and restricted maximum likelihood (REML), I identified large values of maximal additive heritability for all study localities ($ h_{\max }^{2} > 0.65 $ h_{\max }^{2} > 0.65 ), revealing that Plethodon exhibit considerable heritable genetic variation for head shape. Comparisons of the components of heritable shape variation showed that the magnitude of shape heritability (h_max² h_{\max }^{2} ) did not differ among localities or species. Therefore, the potential microevolutionary shape change displayed by the two species would be similar if they were exposed to comparable selective forces. However, the direction of maximal shape heritability in morphospace differed between P. cinereus and P. nettingi, indicating that potential evolutionary shape change along these heritability trajectories would diverge between the two species. This finding implies that distinct head shapes could evolve in the two species, even if subjected to the same selection pressure. When combined with previous knowledge of patterns of head shape variation among species and ecological selection on head shape, these findings suggest that microevolutionary and macroevolutionary trends of morphological diversification in Plethodon may be explained as a result of the interaction between ecological selection and underlying patterns of genetic covariance for this multi-dimensional trait.     

Genus Colocasiomyia (Drosophilidae: Diptera) in Sabah,Bornean Malaysia: High species diversity and use of host aroid inflorescences

A total of 21 Colocasiomyia species, including 17 undescribed species, are reported from Sabah (Mt. Kinabalu and neighboring areas), Malaysia, based on samples collected from inflorescences of 14 or 15 Araceae species. This number of species is the largest as a local fauna of this genus in the world. The high species diversity is attributed to 12 undescribed species belonging to the Colocasiomyia baechlii species group. A particular breeding habit of Colocasiomyia is sharing of the same inflorescence by a pair of species, with partial niche separation between them: one species uses exclusively the pistillate (lower female‐flower) section of the spadix for oviposition and larval development, whereas the other mostly uses the staminate (upper male‐flower) section. However, the baechlii group species show quite different patterns of host plant use: many (up to eight) species cohabit on the same inflorescence. It is unlikely that they separate their breeding niches micro‐allopatrically within an inflorescence. Instead, species composition and their proportions of individual numbers vary among different localities, seasons and host plants, with partial overlap among them. Such partial separations in local distribution, phenology and host selection would in combination contribute to their coexistence and promote the species diversity of this group.     

Modelling sponge species diversity using a morphological predictor: a tropical test of a temperate model

The contribution of sponges to marine surveys is often underestimated due to problems of identification, synonymous species and limited numbers of specialists in the field. Bell & Barnes (2001) illustrated how sponge morphological diversity (diversity of body forms) might be used as a predictor of sponge species diversity and richness. This study investigated these relationships at six tropical West Indian Ocean localities in a number of habitat types. These habitats included tropical coral reefs, soft substratum (seagrass, mangrove and sand), caves and boulders. Sampling was undertaken at three depth zones in coral reef habitats only (intertidal, 10–15 m and 20–25 m), with the other habitats sampled in less than 10m of water. Species diversity and richness were significantly correlated (P < 0.05) with morphological diversity at all localities and depths in coral reef and soft substratum habitats. However, no significant correlation was found between these variables in cave or boulder habitats. The slope of the linear regression found between morphological diversity and species diversity did not significantly differ between coral reef, soft substratum and temperate reef (data taken from Bell & Barnes 2001) habitats. Similarly coral reefs showed the same relationship between morphological diversity and species richness as temperate reefs, however the relationship between morphological diversity and species richness was significantly different at both habitats compared with soft substratum environments. Sponge morphological diversity therefore may be more useful as a predictor of sponge species diversity, rather than species richness, as the former relationship is common between more habitats than the latter.     

Crustacean disparity through the Phanerozoic: comparing morphological and stratigraphic data

Crustaceans have been an important component of marine diversity and biomass since the earliest Phanerozoic. With a relatively well-documented fossil record, they provide an excellent subject for a continuous study of disparity (? bodyplan variety) from the Cambrian to the Recent. A data base of 135 morphological characters forms the basis for cladistic and morphospace studies at the ordinal and sub-ordinal level. Gross cladistic topology is: (Eumalacostraca + Hoplocarida vs Maxillopoda) vs Phyllopoda (paraphyletic). Each of these groups is of approximately equal disparity, and occupies a distinct region of the morphospace plot. A few problematical fossils (e.g. Waptia and Odaraia) fall close to the base of the tree. Comparison of the cladogram with stratigraphic range data indicates the location of probable ghost lineages, and randomization procedures provide a statistical test of the goodness of fit of a given set of stratigraphic ranges to a given tree topology. Disparity indices are calculated at series and stage intervals. Observed range data indicate that Cambrian disparity was approximately one third its present level. The Earliest Ordovician saw a marked decrease, with an increase and subsequent plateau through rest of the period. Increases through the Silurian and Devonian corresponded to the radiation of branchiopods, cephalocarids, and latterly the Eumalacostraca and Hoplocarida. By the end of the Carboniferous, observed disparity had reached over four fifths of Recent levels, and the remaining history of the group saw a gradual but slightly irregular increase up until the end of the Tertiary. Indices of disparity incorporating ghost lineages exhibit less marked peaks and troughs, with fewer perturbations overall. Cladistically-implied disparity in the Lower Cambrian is estimated at three quarters of that in the Recent. Rarefaction is used to compare actual levels of disparity at each time interval with the mean for a similar number of taxa selected randomly from the list of all realized bodyplans. Most intervals preserved a range of forms more disparate than the mean of random samples drawn from the pool of all the taxa considered. From the Triassic to the Recent this difference was intermittently significant. Once occupied, extremes of morphospace tend not to fall vacant again.     

Evolution of Cranial Shape in Caecilians (Amphibia: Gymnophiona)

Insights into morphological diversification can be obtained from the ways the species of a clade occupy morphospace. Projecting a phylogeny into morphospace provides estimates of evolutionary trajectories as lineages diversified information that can be used to infer the dynamics of evolutionary processes that produced patterns of morphospace occupation. We present here a large-scale investigation into evolution of morphological variation in the skull of caecilian amphibians, a major clade of vertebrates. Because caecilians are limbless, predominantly fossorial animals, diversification of their skull has occurred within a framework imposed by the functional demands of head-first burrowing. We examined cranial shape in 141 species, over half of known species, using X-ray computed tomography and geometric morphometrics. Mapping an existing phylogeny into the cranial morphospace to estimate the history of morphological change (phylomorphospace), we find a striking pattern: most species occupy distinct clusters in cranial morphospace that closely correspond to the main caecilian clades, and each cluster is separated by unoccupied morphospace. The empty spaces in shape space are unlikely to be caused entirely by extinction or incomplete sampling. The main caecilian clades have different amounts of morphological disparity, but neither clade age nor number of species account for this variation. Cranial shape variation is clearly linked to phyletic divergence, but there is also homoplasy, which is attributed to extrinsic factors associated with head-first digging: features of caecilian crania that have been previously argued to correlate with differential microhabitat use and burrowing ability, such as subterminal and terminal mouths, degree of temporal fenestration (stegokrotaphy/zygokrotaphy), and eyes covered by bone, have evolved and many combinations occur in modern species. We find evidence of morphological convergence in cranial shape, among species that have eyes covered by bone, resulting in a narrow bullet-shaped head. These results reveal a complex history, including early expansion of morphospace and both divergent and convergent evolution resulting in the diversity we observe today.     

Lobule shape evolution in Radula (Jungermanniopsida): one rate fits all?

The quantification of lobule shape for Radula spp. shows that there is overlap in lobule shape space occupied by subgenera, such that lobule shape does not always reflect relationships. Morphological convergence caused by lineages repeatedly traversing shared regions of morphospace appears commonplace in Radula, and means that many pairs of relatively unrelated species have similar lobule shapes. When observed over time, as in comparisons between fossil and extant species, this may give the impression of stasis if fossil species resemble modern species by chance, independent of their relatedness. This poses a challenge to relating fossils of known age to extant lineages, particularly when fossils are sterile. Significant rate variation between lineages was identified by Adams' Q‐mode analysis, with the fastest subgenus evolving 23 times more quickly than the slowest. Species of subgenus Volutoradula and subgenus Metaradula are apparently over‐dispersed throughout lobule morphospace according to Sidlauskas' method; morphometric branch lengths and hypervolumes in other subgenera can be explained by a stochastic process. In contrast, Bayesian analysis of macroevolutionary mixtures (BAMM) identified a single evolutionary rate as having the highest posterior probability. Consideration of the three independent accessions into auriculate lobule morphospace by Cladoradula and Radula, wherein convergent lobule shapes result from convergent lobule ontogenies and are correlated with bipinnately branched shoot systems and robust primary stems, leads to an ontogenetic hypothesis driven by structural requirements for light interception, under which auriculate lobules are a spandrel. It is speculated that lobules themselves, however, may be a key innovation facilitating radiation into microsites devoid of or depauperate in fungal endophytes. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 222–242.     

Macroevolutionary patterns in Rhynchocephalia: is the tuatara (Sphenodon punctatus) a living fossil?

The tuatara, Sphenodon punctatus, known from 32 small islands around New Zealand, has often been noted as a classic ‘living fossil’ because of its apparently close resemblance to its Mesozoic forebears and because of a long, low‐diversity history. This designation has been disputed because of the wide diversity of Mesozoic forms and because of derived adaptations in living Sphenodon. We provide a testable definition for ‘living fossils’ based on a slow rate of lineage evolution and a morphology close to the centroid of clade morphospace. We show that through their history since the Triassic, rhynchocephalians had heterogeneous rates of morphological evolution and occupied wide morphospaces during the Triassic and Jurassic, and these then declined in the Cretaceous. In particular, we demonstrate that the extant tuatara underwent unusually slow lineage evolution, and is morphologically conservative, being located near the centre of the morphospace for all Rhynchocephalia.