Morphospace Analysis Research Articles

Use and misuse of discrete character data for morphospace and disparity analyses

AbstractThe measurement of morphological variation in macroevolutionary studies is increasingly based on morphospaces constructed from discrete character data. This trend mostly results from the appropriation of phylogenetic data matrices as character spaces for carrying out disparity analyses. Phylogenetic matrices provide morphological descriptions of taxa as combinations of character states and thus appear, if not conceptually, at least mathematically, comparable to discrete character datasets found in numerical taxonomy or built for disparity purposes. Hence, phylogenetic matrices seem to constitute an abundant source of data readily available for morphospace analyses. Discrete character spaces have generally been described as more flexible than morphospaces capturing continuous shape variation. The discrete coding of morphology allows morphospaces to accommodate more disparate morphologies and the ability of discrete character schemes to handle missing data is also often emphasized. This flexibility comes at a cost, however. Multivariate ordinations of such spaces often provide deceptive visualizations and may invite the use of inappropriate methodologies for their exploration. The large amount of missing data that typifies many phylogenetic datasets is also problematic for the measurement of dissimilarity among taxa and can therefore be detrimental to the assessment of morphological disparity. Here, the properties of discrete character spaces are described and common pitfalls discussed. Graphical and methodological approaches are suggested to circumvent or limit their impact, and greater caution is recommended when using discrete character data for morphospace and disparity inferences.

Beyond buzz-pollination - departures from an adaptive plateau lead to new pollination syndromes.

Summary Pollination syndromes describe recurring adaptation to selection imposed by distinct pollinators. We tested for pollination syndromes in Merianieae (Melastomataceae), which contain bee‐ (buzz‐), hummingbird‐, flowerpiercer‐, passerine‐, bat‐ and rodent‐pollinated species. Further, we explored trait changes correlated with the repeated shifts away from buzz‐pollination, which represents an ‘adaptive plateau’ in Melastomataceae.We used random forest analyses to identify key traits associated with the different pollinators of 19 Merianieae species and estimated the pollination syndromes of 42 more species. We employed morphospace analyses to compare the morphological diversity (disparity) among syndromes.We identified three pollination syndromes (‘buzz‐bee’, ‘mixed‐vertebrate’ and ‘passerine’), characterized by different pollen expulsion mechanisms and reward types, but not by traditional syndrome characters. Further, we found that ‘efficiency’ rather than ‘attraction’ traits were important for syndrome circumscription. Contrary to syndrome theory, our study supports the pooling of different pollinators (hummingbirds, bats, rodents and flowerpiercers) into the ‘mixed‐vertebrate’ syndrome, and we found that disparity was highest in the ‘buzz‐bee’ syndrome.We conclude that the highly adaptive buzz‐pollination system may have prevented shifts towards classical pollination syndromes, but provided the starting point for the evolution of a novel set of distinct syndromes, all having retained multifunctional stamens that provide pollen expulsion, reward and attraction.

Study of morphological variation of northern Neotropical Ariidae reveals conservatism despite macrohabitat transitions

BackgroundMorphological convergence triggered by trophic adaptations is a common pattern in adaptive radiations. The study of shape variation in an evolutionary context is usually restricted to well-studied fish models. We take advantage of the recently revised systematics of New World Ariidae and investigate skull shape evolution in six genera of northern Neotropical Ariidae. They constitute a lineage that diversified in the marine habitat but repeatedly adapted to freshwater habitats. 3D geometric morphometrics was applied for the first time in catfish skulls and phylogenetically informed statistical analyses were performed to test for the impact of habitat on skull diversification after habitat transition in this lineage.ResultsWe found that skull shape is conserved throughout phylogeny. A morphospace analysis revealed that freshwater and marine species occupy extreme ends of the first principal component axis and that they exhibit similar Procrustes variances. Yet freshwater species occupy the smallest shape space compared to marine and brackish species (based on partial disparity), and marine and freshwater species have the largest Procrustes distance to each other. We observed a single case of shape convergence as derived from ‘C-metrics’, which cannot be explained by the occupation of the same habitat.ConclusionsAlthough Ariidae occupy such a broad spectrum of different habitats from sea to freshwater, the morphospace analysis and analyses of shape and co-variation with habitat in a phylogenetic context shows that conservatism dominates skull shape evolution among ariid genera.

How (much) do flowers vary? Unbalanced disparity among flower functional modules and a mosaic pattern of morphospace occupation in the order Ericales.

The staggering diversity of angiosperms and their flowers has fascinated scientists for centuries. However, the quantitative distribution of floral morphological diversity (disparity) among lineages and the relative contribution of functional modules (perianth, androecium and gynoecium) to total floral disparity have rarely been addressed. Focusing on a major angiosperm order (Ericales), we compiled a dataset of 37 floral traits scored for 381 extant species and nine fossils. We conducted morphospace analyses to explore phylogenetic, temporal and functional patterns of disparity. We found that the floral morphospace is organized as a continuous cloud in which most clades occupy distinct regions in a mosaic pattern, that disparity increases with clade size rather than age, and that fossils fall in a narrow portion of the space. Surprisingly, our study also revealed that among functional modules, it is the androecium that contributes most to total floral disparity in Ericales. We discuss our findings in the light of clade history, selective regimes as well as developmental and functional constraints acting on the evolution of the flower and thereby demonstrate that quantitative analyses such as the ones used here are a powerful tool to gain novel insights into the evolution and diversity of flowers.

Earliest microbial trace fossils in Archaean pillow lavas under scrutiny: new micro-X-ray absorption near-edge spectroscopy, metamorphic and morphological constraints

Abstract Filament-shaped titanite microtextures in early Archaean ( c. 3.47 Ga) metabasalts from the Barberton Greenstone Belt of South Africa have been argued to represent Earth's oldest microbial trace fossils, but petrological data on the environmental conditions that led to the formation of these microtextures are sparse. We present here new metamorphic constraints on the equilibrium mineral assemblage containing the titanite microtextures, together with Fe speciation redox variations in the chlorite matrix surrounding the microtextures and morphospace analysis of the microtextures to test their biogenicity. Thermodynamic phase diagram modelling using a calculated rock microdomain composition indicated that the titanite mineral assemblage is stable at temperature conditions of T =240–360°C. The high-resolution quantitative mapping results combined with micro-X-ray absorption near-edge spectroscopy at the Fe K -edge in chlorite revealed that the titanite microtextures are located in low-temperature, high XFe 3+ chlorite bands, veins and microdomains, supporting an origin as abiotic, retrograde mineral cooling textures. These new data, combined with a late c. 2.9 Ga U–Pb date for the titanite, are incompatible with the existing biological model, which invokes microbial micro-tunnel formation followed by titanite ‘infilling’ and preservation in the early Archaean sub-seafloor. The continuum of titanite morphologies reported herein indicates that titanite morphology cannot be used as reliable evidence in support of a biogenic origin in these metavolcanic rocks. It is proposed that the titanite microtextures of purported biogenic origin from other greenstone belts, such as the Pilbara Craton of Western Australia, also deserve to be scrutinized by high-resolution petrological investigations. Supplementary material: Representative electron microprobe analyses of retrograde minerals and µXANES spectra and corresponding fitted pre-edge peaks for all spot analyses are available at https://doi.org/10.6084/m9.figshare.c.3498465

Modern theoretical and technical approaches in plant morphology

Morphology is deeply rooted in organismal biology, which in recent years has gone through a steady decline in interest both at research institutions and funding agencies. In parallel with this development, morphology as a discipline has been marginalized and nowadays many think of it as just a classical and largely obsolete field of research. However, this is far from the truth. Thanks to modern theoretical concepts and novel technical applications, plant morphology has much to contribute to modern botanical and evolutionary research.In our presentation, we will first outline the application of High Resolution X-Ray Computed Tomography (HRXCT) to the study of plant structure. The ideal way to describe the morphological phenotype of a given organism is to build a three dimensional (3D) model, which may then be interpreted with respect to other types of data, e.g., metabolite content or functional groups of pollinators. We have developed simple but efficient lab protocols using contrasting agents such as phosphotungstate and bismuth tartrate that allow for the streamlined acquisition of high resolution phenotypic data and 3D-representations even of soft plant tissues such as floral organs, ovules, and meristematic tissues. To illustrate this, we will outline selected ongoing studies in comparative plant science that make use of high resolution tomography.In the second part of our talk, we will present a project on the floral morphospace. A striking feature of morphological variation is that due to developmental, functional, and phylogenetic constraints, not all theoretically possible architectures have been explored during evolution. A modern approach to studying the evolution of realized forms among possible ones is to construct morphospaces, i.e. theoretical, mathematical spaces describing and relating organismal phenotypes. Although widely applied in zoology, morphospace analyses and related approaches have so far been largely disregarded in botany, with notable exceptions in the field of pollination biology. Here, we use a morphospace approach to describe and quantify the morphological diversity (disparity) of flowers in the asterid order Ericales. To do so, we have built a dataset containing 37 floral characters for more than 380 species (275 genera) representative of the entire order. We have used non-parametric representations and statistics methods based on distance matrices to build and analyze a morphospace, in which we compare the relative positions of the different ericalean families. We quantify and interpret the disparity among these groups in the light of their taxonomic diversity, their evolutionary history, and their ecology. In addition, we analyze patterns of disparity between sterile, male, and female floral organs.

A new species of Gladiopycnodus (Coccodontoidea, Pycnodontomorpha) from the Cretaceous of Lebanon provides new insights about the morphological diversification of pycnodont fishes through time

The Coccondontoidea is a distinctive superfamily of pycnodont fishes characterized by a series of derived characters, including thick and well-ossified cranial bones supporting horns and spines and a hypertrophied pectoral girdle sutured to the skull forming a sort of cephalo-torax. A new distinctive coccodontoid species, Gladiopycnodus byrnei n. sp., is described from the Cenomanian locality of Hjoûla, Lebanon. This new species exhibits a unique combination of features that clearly support its inclusion within the coccodontoid family Gladiopycnodontidae, including an extremely elongate snout forming a sword-shaped rostrum, thick dermal bones ornamented with rounded tubercles, large supracleithrum and cleithrum extensively sutured to the skull, and irregularly imbricated scales covering the entire body. The new taxon differs from type species G. karami by having different meristic counts and body proportions, dermal bones strongly ornamentated, skull roof with partially fused bones, scales ornamented with small tubercles, and rounded scales covering the caudal peduncle absent. The presence of pectoral fins in our exquisitely well-preserved specimen allowed us to redefine the morphology of the genus Gladiopycnodus, also modifying the diagnosis of the family. In order to interpret the patterns of morphospace occupation and quantify the morphological diversification of pycnodonts through time, the geometric morphometric approach was used. The morphospace analysis revealed that a significant increase in morphological disparity of pycnodonts during the Late Cretaceous was related, at least in part, to the appearance of the representatives of the superfamily Coccodontoidea. The rapid evolutionary radiation of these well-armored pycnodonts was interpreted as an adaptive solution in response to the large predatory-prey escalation in the context of the Mesozoic Marine Revolution.

Limits in the evolution of biological form: a theoretical morphologic perspective.

Limits in the evolution of biological form can be empirically demonstrated by using theoretical morphospace analyses, and actual analytic examples are given for univalved ammonoid shell form, bivalved brachiopod shell form and helical bryozoan colony form. Limits in the evolution of form in these animal groups can be shown to be due to functional and developmental constraints on possible evolutionary trajectories in morphospace. Future evolutionary-limit research is needed to analyse the possible existence of temporal constraint in the evolution of biological form on Earth, and in the search for the possible existence of functional alien life forms on Titan and Triton that are developmentally impossible for Earth life.

Morphological differentiation between species of Myrmelachista Roger (Formicidae: Formicinae) in Atlantic Forest areas of the Alto Tietê (São Paulo)

Myrmelachista is an exclusively Neotropical ant genus. The ants from this genus nest and forage in live or dead trees, but data on its life cycle are relatively scarce. The taxonomy of this genus is considerably complex. The morphological characters of taxonomic importance are not well defined, and combining characteristics from the largest possible number of castes is thus essential. The goal of the present study was to present the results of multidimensional morphological analyses conducted based on morphometric measurements of workers and alates to help with the identification of Myrmelachista species. For that purpose, we gathered data from 277 specimens (workers, males and gynes) from eight species cohabiting Atlantic Forest areas. Eighteen morphometric variables were measured on workers and 20 on reproductive ants. Measurements related to eye size and mandible width were the variables more strongly associated with the two morphological clusters obtained based on the morphology of workers. However, the morphospace described by workers cannot be used to delimit the species of Myrmelachista . Mandible width, petiole height and eye size (eye length and ocelli width) were the most informative variables associated with the four clusters of morphology that clearly delimit males of M. arthuri , M. ruszkii /sp.7, and in part, M. catharinae . Finally, petiole length and head length were the best descriptors of the six morphological clusters recognized for gynes. M. catharinae , M. sp.7, M. nodigera and M. gallicola were clearly delimited in the gyne morphological space. M. ruszkii showed high phenotypic variability, and this species was classified in two morphological clusters based on gyne specimens. Species identification employing multiple gyne morphological traits exhibited the best results. Our results show that morphospace analysis can be useful for delimitation of Myrmelachista species.

Theoretical morphology of colonial meshworks in the Fenestrata (Bryozoa): modelling hydraulic-resistance constraints and minimum fenestrule widths

Ancient fenestrate bryozoans (Fenestellidae, Polyporidae) exhibit a range of fenestrule widths that can be investigated to gain insights about constraints on meshwork morphology. Theoretical morphospace analyses of branch width and spacing reveal that there is a minimum fenestrule width common to both groups. We propose that the minimum fenestrule width observed in fenestrate colonies was constrained by hydraulic resistance (the hydraulic-resistance-constraint hypothesis). Modelling that considers both the viscous and inertial components of fluid-flow resistance through hypothetical fenestrate meshworks corroborates this hypothesis for the smaller fenestrate meshworks of the biserial fenestellids and for polyporids with branch widths less than about 0.6 mm. However, the geometry of the polyporid meshworks in species with branches wider than 0.6 mm does not appear to have been determined by a hydraulic-resistance constraint. Instead, maximum branch densities in the larger-branched polyserial polyporid colonies appear to have been constrained by a limit on the number of zooid rows that these colonies could develop on their branches (the zooid-row-limit hypothesis). The observed decrease in the minimum fenestrule width in these colonies is the indirect geometric result of the inability of the larger polyserial polyporid colonies to develop meshworks with high branch densities.

Fossils and astrobiology: new protocols for cell evolution in deep time

AbstractThe study of life remote in space has strong parallels with the study of life remote in time. Both are dependent on decoding those historic phenomena called ‘fossils’, here taken to include biogenic traces of activity and waste products. There is the shared problem of data restoration from incomplete data sets; the importance of contextual analysis of potentially viable habitats; the centrality of cell theory; the need to reject the null hypothesis of an abiogenic origin for candidate cells via morphospace analysis; the need to demonstrate biology-like behaviour (e.g., association with biofilm-like structures; tendency to form clusters and ‘mats’; and a preference for certain substrates), and of metabolism-like behaviour (e.g., within the candidate cell wall; within surrounding ‘waste products’; evidence for syntrophy and metabolic cycles; and evidence for metabolic tiers). We combine these ideas into a robust protocol for demonstrating ancient or extra-terrestrial life, drawing examples from Earth's early geological record, in particular from the earliest known freshwater communities of the 1.0 Ga Torridonian of Scotland, from the 1.9 Ga Gunflint Chert of Canada, from the 3.4 Ga Strelley Pool sandstone of Australia, and from the 3.46 Ga Apex Chert also of Australia.

A new peculiar dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov. from the Upper Cretaceous coastal deposits of the Aren Formation, south‐central Pyrenees, Lleida, Catalonia, Spain

Abstract: A new type of small, ovoid dinosaur egg, Sankofa pyrenaica oogen. nov. oosp. nov., with a prismatic type eggshell is described from upper Cretaceous (upper Campanian–Maastrichtian) deposits of the Montsec area, South Pyrenean Central Unit, Lleida, Catalonia, Spain. This egg type was sub‐vertically laid in only two rich monospecific sites of a single stratigraphic layer from coastal deposits of the Aren Formation, interpreted as an emerged beach ridge of a barrier island – lagoon depositional system. The size and shape of these eggs with their asymmetric poles are roughly similar to modern hen eggs, which is unusual in the Cretaceous fossil egg record. Its phylogenetic position clusters with bird and Troodontid eggs. A morphospace analysis of egg shapes shows the similarity of the new egg to a Campanian fossil bird egg from Argentina, both being intermediate between modern‐bird eggs and extinct nonavian theropod eggs. However, the eggshell microstructure of Sankofa pyrenaica differs from that of bird eggs in its incipient squamatic texture. It has a peculiar pattern of interlocking small crystals in the middle of the palisade layer, instead of the thick squamatic structure commonly present in modern avian eggshells. This new egg type is attributed to a small theropod, probably with a single oviduct like birds and whose mosaic distribution of features is a combination between that of birds and nonavian theropods. This enhances the arguments supporting the close phylogenetic relationships between both groups.

Functional constraints on coiling geometry and aperture inclination in gastropods

We studied the morphological diversity of gastropod shell forms from the viewpoint of theoretical morphology, emphasizing the relationships of shell form to postural stability and the available space for soft body, which we assessed in terms of the moment of force and soft-tissue ratio calculations, respectively. The results of computer simulations suggest a functional trade-off between postural stability and available space for soft body: a compact shell possessing a low spire and small umbilicus exhibits high postural stability, whereas a less overlapped shell form with a high spire and large umbilicus makes available space for soft body. A functional morphospace analysis using theoretical models reveals that outward and downward inclination of the aperture moderates the functional trade-off between these parameter values and permits compatibility between stable posture and efficient shell construction. The hypothetical optimum that realizes this compatibility is consistent with the observed range of forms estimated from 359 extant gastropod species. The biometric results also suggest that land snails are more highly constrained than marine species in achieving a balance between postural stability and available space for soft body.

A flourishing of fish forms

According to an innovative exercise in 'morphospace analysis', modern fish owe their stunning diversity in part to an ecological cleaning of the slate by the mass extinction at the end of the Cretaceous.

Medusan morphospace: phylogenetic constraints, biomechanical solutions, and ecological consequences

Abstract. Medusae were the earliest animals to evolve muscle‐powered swimming in the seas. Although medusae have achieved diverse and prominent ecological roles throughout the world's oceans, we argue that the primitive organization of cnidarian muscle tissue limits force production and, hence, the mechanical alternatives for swimming bell function. We use a recently developed model comparing the potential force production with the hydrodynamic requirements of jet propulsion, and conclude that jet production is possible only at relatively small bell diameters. In contrast, production of a more complex wake via what we term rowing propulsion permits much larger sizes but requires a different suite of morphological features. Analysis of morphometric data from all medusan taxa independently confirms size‐dependent patterns of bell forms that correspond with model predictions. Further, morphospace analysis indicates that various lineages within the Medusozoa have proceeded along either of two evolutionary trajectories. The first alternative involved restriction of jet‐propelled medusan bell diameters to small dimensions. These medusae may be either solitary individuals (characteristic of Anthomedusae and Trachymedusae) or aggregates of small individual medusan units into larger colonial forms (characteristic of the nectophores of many members of the Siphonophorae). The second trajectory involved use of rowing propulsion (characteristic of Scyphozoa and some hydromedusan lineages such as the Leptomedusae and Narcomedusae) that allows much larger bell sizes. Convergence on either of the differing propulsive alternatives within the Medusozoa has emerged via parallel evolution among different medusan lineages. The distinctions between propulsive modes have important ecological ramifications because swimming and foraging are interdependent activities for medusae. Rowing swimmers are characteristically cruising predators that select different prey types from those selected by jet‐propelled medusae, which are predominantly ambush predators. These relationships indicate that the different biomechanical solutions to constraints on bell function have entailed ecological consequences that are evident in the prey selection patterns and trophic impacts of contemporary medusan lineages.

The flourishing diversity of models in theoretical morphology: from current practices to future macroevolutionary and bioenvironmental challenges

For decades, theoretical morphological studies of different groups of organisms have been successfully pursued in biological, paleontological, and computational contexts, often with distinct modeling approaches and research questions. A regular influx of new perspectives and varied expertise has contributed to the emergence of a veritable multidisciplinary outlook for theoretical morphology. The broadening of this discipline is reflected in a substantial increase in the number of models, leading to a bewildering diversity that has yet to be scrutinized. In this work, we tackle this issue in a synthetic fashion, with a quantitative meta-analysis that allows an objective comparison of theoretical morphological models treated as entities. By analogy with empirical morphospace analyses of actual organisms, we performed a multivariate ordination of a representative sample of models, producing a metaspace of models in which patterns of similarity and difference are visualized. A phenetic tree was used to characterize the relationships between models. Four major groups have been identified, and their disparity analyzed. We suggest this typology as a useful starting point to identify a core set of fundamental principles and protocols for better interpretation of the plethora of current models and for more efficient construction of models in the future. This in turn can help in diversifying the scope of macroevolutionary, developmental, and bioenvironmental questions in theoretical morphology.

Towards a morphospace for the Ediacara biota

Abstract The Ediacara biota of the late Neoproterozoic is justly famous as a biological puzzle. Studies of Ediacaran biology have commonly used analogy with living organisms as a cipher for the decoding of biological affinity, and consequently the life mode and habit. Here, we discuss the problems of using such analogous reasoning and put forward our alternative approach, that of using Morphospace Analysis for the study of growth, form and phylogeny. This tool, we suggest, has the potential to be used for testing the unity of an evolutionary clade, such as ‘rangeomorphs’ and ‘dickinsoniomorphs’. Preliminary data from the members of the Ediacara biota do indeed show such a unity within our preliminary morphospace model (all k values are low). This method reveals no clear relationships, between these forms and more recent biological groups such as the sea pens or the Foraminifera.

Functional morphology and light-gathering ability of podocopid ostracod eyes and the palaeontological implications

The optical features of lateral ocelli of the eye were examined in 29 Recent species of the major ostracod group Podocopida using a theoretical morphological model. A cuticular lens–tapetum model was used for this purpose. Ray tracing was simulated on each model in order to assess the light-gathering abilities of the various forms of eyes. The results of computer simulations and morphospace analyses indicated that the light-gathering ability of the eye is dominantly affected by the thickness and curvature of the outer surface of the lens. On the basis of a combination of form and light-gathering ability, four eye types (LG1, LG2, MG, and HG) were recognized. The results of the phototactic experiment and the light intensity from each microhabitat were concordant with estimated scores on the light-gathering abilities using the theoretical models. Phylogenetic analyses and the fossil record indicate that the MG type is the most plesiomorphic, and that the other types derive from this. The present study also suggests that the optical structure that determines the light-gathering ability is closely related to the development of surface ornamentation on the valve. © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society, 2006, 147, 97–108.

Effects of morphometric descriptor changes on statistical classification and morphospaces

Ten morphometric descriptors (five pairs of form and shape parameters) are used to describe the complex morphology of the first lower molar of two morphologically similar species, Microtus arvalis and M. agrestis. These descriptors are derived either from linear measurements or from outline analysis. The effects of these different descriptors on classical analysis as used in biology or palaeobiology are explored. First, the reliability of results in statistical classification is assessed. All of the descriptors discriminate well between the two species. The initial morphometric scheme (linear or outline) does not induce marked differences in statistical classification and the major discrepancies are between standardized and non-standardized versions of descriptors, and between amplitude- and coefficient-based or linear-based descriptors. Subsequently, the similarity of morphospaces based on partial least squares analysis and of intraspecific variance (estimated from the morphospace analysis) are observed. This is done within a morphospace-disparity framework and procedures used here for testing are directed at this research area. Similarities between morphospaces are relatively high. In this case, the initial morphometric scheme is a major factor inducing dissimilarity. However, the patterns of intraspecific dispersion inferred from morphospaces are roughly similar. Major differences in results correspond to the two classes of form or shape descriptors. Similarity of intraspecific variance is obtained when standardized descriptors are used (except for amplitude-based descriptors); conversely, dissimilarity is obtained when non-standardized descriptors are used. In many cases, the results of the various analyses are robust despite changes in descriptor. Moreover, the developmental pathway of vole teeth can frequently explain major dissimilarity or even similarity.

Theoretical morphology of hinge and shell form in Bivalvia: geometric constraints derived from space conflict between umbones

Geometric properties of the shells of 123 species of extant Bivalvia were analyzed from the viewpoint of theoretical morphology. The effects of shell form and the structure of ligament on the interumbonal space and the maximum shell opening received particular attention. The results of computer simulation and morphospace analysis indicate that possessing both prosogyrous shell form and an extended hinge without the parivincular ligament tends to cause space conflict between umbones or dorsal shell margins of right and left valves. To a large degree, a prosogyrous shell form with a long parivincular ligament helps shell opening without umbonal conflict, if the shell is flat enough to avoid the mutual interference between dorsal shell margins of closed valves. Extension of the ligament and plunging of the anterior part of the coiling axis into the ventral side provide enough space along the dorsal shell margins in which a parivincular ligament and its substrata are developed.