Molluscan Classes Research Articles

Larval ecology and morphology in fossil gastropods

AbstractThe shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.

The contribution of molecular data to our understanding of cephalopod evolution and systematics: a review

The first DNA sequence of a cephalopod was published in 1983 and the first molecular paper focusing on cephalopods was published in 1994. In this review we trace progress in the field. We examine the placement of Cephalopoda with respect to other molluscan classes and we examine relationships within Cephalopoda. We provide a summary tree of the relationships between cephalopod orders and we examine relationships of taxa within each of these orders. Although much knowledge has been gained over the past 20 years, deeper-level relationships are still not well understood and there is still much scope for further research in this field. Genomic studies are likely to contribute significantly to our knowledge in the future.

An ancient process in a modern mollusc: early development of the shell in Lymnaea stagnalis

BackgroundThe morphological variety displayed by the molluscan shell underlies much of the evolutionary success of this phylum. However, the broad diversity of shell forms, sizes, ornamentations and functions contrasts with a deep conservation of early cell movements associated with the initiation of shell construction. This process begins during early embryogenesis with a thickening of an ectodermal, ‘dorsal’ (opposite the blastopore) population of cells, which then invaginates into the blastocoel to form the shell gland. The shell gland evaginates to form the shell field, which then expands and further differentiates to eventually become the adult shell-secreting organ commonly known as the mantle. Despite the deep conservation of the early shell forming developmental program across molluscan classes, little is known about the fine-scale cellular or molecular processes that underlie molluscan shell development.ResultsUsing modern imaging techniques we provide here a description of the morphogenesis of a gastropod shell gland and shell field using the pulmonate gastropod Lymnaea stagnalis as a model. We find supporting evidence for a hypothesis of molluscan shell gland specification proposed over 60 years ago, and present histochemical assays that can be used to identify a variety of larval shell stages and distinct cell populations in whole mounts.ConclusionsBy providing a detailed spatial and temporal map of cell movements and differentiation events during early shell development in L. stagnalis we have established a platform for future work aimed at elucidation of the molecular mechanisms and regulatory networks that underlie the evo-devo of the molluscan shell.

Mitogenomics does not resolve deep molluscan relationships (yet?)

The origin of molluscs among lophotrochozoan metazoans is unresolved and interclass relationships are contradictory between morphology-based, multi-locus, and recent phylogenomic analyses. Within the “Deep Metazoan Phylogeny” framework, all available molluscan mitochondrial genomes were compiled, covering 6 of 8 classes. Genomes were reannotated, and 13 protein coding genes (PCGs) were analyzed in various taxon settings, under multiple masking and coding regimes. Maximum Likelihood based methods were used for phylogenetic reconstructions. In all cases, molluscs result mixed up with lophotrochozoan outgroups, and most molluscan classes with more than single representatives available are non-monophyletic. We discuss systematic errors such as long branch attraction to cause aberrant, basal positions of fast evolving ingroups such as scaphopods, patellogastropods and, in particular, the gastropod subgroup Heterobranchia. Mitochondrial sequences analyzed either as amino acids or nucleotides may perform well in some (Cephalopoda) but not in other palaeozoic molluscan groups; they are not suitable to reconstruct deep (Cambrian) molluscan evolution.Supposedly “rare” mitochondrial genome level features have long been promoted as phylogenetically informative. In our newly annotated data set, features such as genome size, transcription on one or both strands, and certain coupled pairs of PCGs show a homoplastic, but obviously non-random distribution. Apparently congruent (but not unambiguous) signal for non-trivial subclades, e.g. for a clade composed of pteriomorph and heterodont bivalves, needs confirmation from a more comprehensive bivalve sampling. We found that larger clusters not only of PCGs but also of rRNAs and even tRNAs can bear local phylogenetic signal; adding trnG–trnE to the end of the ancestral cluster trnM–trnC–trnY–trnW–trnQ might be synapomorphic for Mollusca. Mitochondrial gene arrangement and other genome level features explored and reviewed herein thus failed as golden bullets, but are promising as additional characters or evidence supporting deep molluscan clades revealed by other data sets. A representative and dense sampling of molluscan subgroups may contribute to resolve contentious interclass relationships in the future, and is vital for exploring the evolution of especially diverse mitochondrial genomes in molluscs.

On the evolutionary ecology of symbioses between chemosynthetic bacteria and bivalves

Mutualistic associations between bacteria and eukaryotes occur ubiquitously in nature, forming the basis for key ecological and evolutionary innovations. Some of the most prominent examples of these symbioses are chemosynthetic bacteria and marine invertebrates living in the absence of sunlight at deep-sea hydrothermal vents and in sediments rich in reduced sulfur compounds. Here, chemosynthetic bacteria living in close association with their hosts convert CO2 or CH4 into organic compounds and provide the host with necessary nutrients. The dominant macrofauna of hydrothermal vent and cold seep ecosystems all depend on the metabolic activity of chemosynthetic bacteria, which accounts for almost all primary production in these complex ecosystems. Many of these enigmatic mutualistic associations are found within the molluscan class Bivalvia. Currently, chemosynthetic symbioses have been reported from five distinct bivalve families (Lucinidae, Mytilidae, Solemyidae, Thyasiridae, and Vesicomyidae). This brief review aims to provide an overview of the diverse physiological and genetic adaptations of symbiotic chemosynthetic bacteria and their bivalve hosts.

Treatise Online no. 39: Part N, Revised, Volume 1, Chapter 25: Early Triassic–Early Jurassic bivalve diversity dynamics

Bivalves are a highly diversified molluscan class, with a long history dating from early Cambrian times (Cope, 2000). Although the group already showed a steady diversification trend during the Paleozoic, it only became highly successful and expanded rapidly from the Mesozoic onward. The Triassic was, for bivalves, first a recovery period and later a biotic diversification event. It was also the time bivalves first fully exploited their evolutionary novelties.

Core structure of aligned chitin fibers within the interlamellar framework extracted from Haliotis rufescens nacre. Part I: Implications for growth and mechanical response

By means of consecutive alkaline and proteolytic treatments of the organic framework?s interlamellar layers extracted from the nacre of H. rufescens, we have exposed a core of aligned parallel chitin fibers. Our findings both verify basic elements of the interlamellar layer structural model of Levi-Kalisman et al. (2001) and extend the more detailed model of Bezares et al. (2008, 2010). We observe via SEM imaging of square millimeter sized samples, which include numerous interlamellar layers and micron sized, yet nanocrystalline, CaCO3 tiles whose native orientation within the shell was first documented, that the chitin fibers in all layers are aligned normal to the growth direction of the shell. Similar alignment has been suggested in the literature for two other classes of mollusks, viz. N. rupertus and P. martensii (Weiner and Traub, 1983; Wada, 1958), suggesting that this may be a more general motif. We find that in order to expose the chitin core it is necessary to first remove protein by an alkaline treatment followed by enzymatic digestion with proteinase-K. We also observe what appear to be the points of traversal of the exposed chitin core by mineral bridges. The implications of these findings touch directly and most specifically upon the expected mechanical properties of organic framework layers such as stiffness and relaxation time constants, viz. they should be planeorthotropic. Single interlamellar layers extracted from nacre should, by implication, also exhibit an orthotropic stiffness. These novel findings provide the structural picture required for a complete anisotropic, time dependent, constitutive description of nacre long thought to be a paradigm of structural optimization. Such a model is briefly described herein and is developed, in full, in Part II of this series.

A multi-gene phylogeny of Cephalopoda supports convergent morphological evolution in association with multiple habitat shifts in the marine environment.

BackgroundThe marine environment is comprised of numerous divergent organisms living under similar selective pressures, often resulting in the evolution of convergent structures such as the fusiform body shape of pelagic squids, fishes, and some marine mammals. However, little is known about the frequency of, and circumstances leading to, convergent evolution in the open ocean. Here, we present a comparative study of the molluscan class Cephalopoda, a marine group known to occupy habitats from the intertidal to the deep sea. Several lineages bear features that may coincide with a benthic or pelagic existence, making this a valuable group for testing hypotheses of correlated evolution. To test for convergence and correlation, we generate the most taxonomically comprehensive multi-gene phylogeny of cephalopods to date. We then create a character matrix of habitat type and morphological characters, which we use to infer ancestral character states and test for correlation between habitat and morphology.ResultsOur study utilizes a taxonomically well-sampled phylogeny to show convergent evolution in all six morphological characters we analyzed. Three of these characters also correlate with habitat. The presence of an autogenic photophore (those relying upon autonomous enzymatic light reactions) is correlated with a pelagic habitat, while the cornea and accessory nidamental gland correlate with a benthic lifestyle. Here, we present the first statistical tests for correlation between convergent traits and habitat in cephalopods to better understand the evolutionary history of characters that are adaptive in benthic or pelagic environments, respectively.DiscussionOur study supports the hypothesis that habitat has influenced convergent evolution in the marine environment: benthic organisms tend to exhibit similar characteristics that confer protection from invasion by other benthic taxa, while pelagic organisms possess features that facilitate crypsis and communication in an environment lacking physical refuges. Features that have originated multiple times in distantly related lineages are likely adaptive for the organisms inhabiting a particular environment: studying the frequency and evolutionary history of such convergent characters can increase understanding of the underlying forces driving ecological and evolutionary transitions in the marine environment.

BIOMIMETIC DESIGN OF A MULTI-LAYERED DUST PROTECTION SYSTEM FOR OPTICAL INSTRUMENTS OPERATING IN THE LUNAR ENVIRONMENT

A method of identifying biologically meaningful keywords not obviously related to engineering keywords was developed to enhance discovery of relevant biological analogies for design problems. This paper reports the use of biologically meaningful keywords to identify biological analogies to generate solutions for protection required during lunar exploration. In lunar exploration, dust poses a significant problem due to its pervasiveness, adherence, and abrasiveness, causing premature failure of space suits and mechanisms. In this paper, biomimetic concepts are developed to protect a laser/telescope system. The resulting design is comprised of two subsystems. An antagonistic bending Shape Memory Alloy (SMA) actuator system, inspired by bivalves (a class of molluscs that include scallops, clams, oysters and mussels), is used to control the opening and closing of a two-piece lid system, while a high-voltage DC field generator prevents charged dust particles from approaching the optical surfaces. Preliminary results indicate that the SMA actuation system is capable of greater than one-hundred repeatable lid-opening and closing cycles. In addition, the high-voltage DC field was capable of controlling and deflecting 98% of incoming charged polystyrene particles (Dmean = 1 mm) away from a representative surface. The method of using biologically meaningful keywords to identify analogies was successfully applied in this case and could be applied in a variety of settings to generate useful solutions.

Mollusks in Phanerozoic marine communities: Implications from the analysis of global paleontological databases

Evolutionary history of three mollusk classes (Bivalvia, Gastropoda, and Cephalopoda), regarded as components of the Phanerozoic marine biota, is discussed based on the comparison of dynamics of quantitative parameters obtained from the analysis of the global paleontological databases. The main trends in the evolution of the role of mollusks in Phanerozoic marine ecosystems and relationships between the diversification of this group and biodiversity of paleocommunities are considered. Certain parameters show similarity between the diversity dynamics of mollusks and the whole marine biota, including the paleolatitudinal distribution of diversity. At the same time, mollusk classes differ considerably in certain aspects. The evolutionary history of Bivalvia, Gastropoda, and Cephalopoda was different and determined presumably by deep ecological divergence which occurred as early as the Early Paleozoic adaptive radiation. Bivalves and gastropods followed the trend of a gradual and constant increase in their role in marine communities; they are characterized by high and constantly growing duration of genera, high (and also growing) frequency in paleontological collections. Cephalopods show more chaotic macroevolutionary dynamics, relatively low mean duration of genera and low relative frequency.

Cambrian origin of all skeletalized metazoan phyla—Discovery of Earth's oldest bryozoans (Upper Cambrian, southern Mexico)

Exquisite Pywackia baileyi Landing n. gen. and sp. specimens from the lower Tinu Formation, southern Mexico, extend the bryozoan record into the Upper Cambrian. They are ∼8 m.y. older than the purported oldest bryozoans from South China, and show that all skeletalized metazoan phyla appeared in the Cambrian. The new form differs from similar, twig-like cryptostomes by its shallow autozooecia and an elongate axial zooid, which may be homologous to the stolon in nonmineralized ctenostomes. It may morphologically resemble mineralized stem group bryozoans that retained a stolon-like individual, although an ability to bud was acquired by the feeding individuals (autozooids). The latest Cambrian origin of bryozoans, several mollusk classes (polyplacophorans, cephalopods), and euconodonts was a major evolutionary development and can be considered the onset of the Ordovician radiation of more complex marine communities.

Fast evolving 18S rRNA sequences from Solenogastres (Mollusca) resist standard PCR amplification and give new insights into mollusk substitution rate heterogeneity.

BackgroundThe 18S rRNA gene is one of the most important molecular markers, used in diverse applications such as molecular phylogenetic analyses and biodiversity screening. The Mollusca is the second largest phylum within the animal kingdom and mollusks show an outstanding high diversity in body plans and ecological adaptations. Although an enormous amount of 18S data is available for higher mollusks, data on some early branching lineages are still limited. Despite of some partial success in obtaining these data from Solenogastres, by some regarded to be the most "basal" mollusks, this taxon still remained problematic due to contamination with food organisms and general amplification difficulties.ResultsWe report here the first authentic 18S genes of three Solenogastres species (Mollusca), each possessing a unique sequence composition with regions conspicuously rich in guanine and cytosine. For these GC-rich regions we calculated strong secondary structures. The observed high intra-molecular forces hamper standard amplification and appear to increase formation of chimerical sequences caused by contaminating foreign DNAs from potential prey organisms. In our analyses, contamination was avoided by using RNA as a template. Indication for contamination of previously published Solenogastres sequences is presented. Detailed phylogenetic analyses were conducted using RNA specific models that account for compensatory substitutions in stem regions.ConclusionsThe extreme morphological diversity of mollusks is mirrored in the molecular 18S data and shows elevated substitution rates mainly in three higher taxa: true limpets (Patellogastropoda), Cephalopoda and Solenogastres. Our phylogenetic tree based on 123 species, including representatives of all mollusk classes, shows limited resolution at the class level but illustrates the pitfalls of artificial groupings formed due to shared biased sequence composition.

Alteration in the status of glutathione transferase of the water snail, Bulinus globosus, during aestivation and recovery

Abstract The varying status of glutathione transferases (GSTs) in water snail, Bulinus globosus, an intermediate host of disease-causing Schistosoma haematobium (Bilharz 1852) has been investigated. The expression of GST isoenzymes in the water snail appears seasonal with about three isoenzymes appearing during raining season, when the organism is active, which may reduce to a single peak of one isoenzyme during aestivation, when the organism is inactive. GST isoenzyme is present in high concentration in all the tissues investigated namely: haemolymph, foot muscle and hepatopancreas with specific activities of 0.006 ± 0.002, 0.45 ± 0.021 and 1.33 ± 0.103 units/mg protein respectively for 1-chloro-2,4-dinitrobenzene (CDNB) as substrate. With this substrate, the specific activity of GST from the hepatopancreas appears higher than the specific activities that have been previously reported for GSTs from molluscs. Partial purification of the isoenzymes using Tris acrylic acid-based resins enabled us to observe that GST appears to be the major protein in the hepatopancreas of this organism. We also found indications for the presence of an endogenous GST inhibitor in the cytosol, whose function is yet unknown. All the traditional GST inhibitors such as cibacron blue, hematin, bromosulfophthalein and S-hexylglutathione were able to inhibit the isoenzymes effectively, with cibacron blue being the most potent. The isoenzymes however have narrow substrate specificity. We conclude that different isoenzymes of GST are expressed in the same class of molluscs, even when they belong to the same genus or species, and that the expression may depend on whether the snails are on aestivation or not.

The Mitochondrial Genomes of Two Scallops, Argopecten irradians and Chlamys farreri (Mollusca: Bivalvia): The Most Highly Rearranged Gene Order in the Family Pectinidae

Molluscs in general and bivalves in particular, exhibit an extraordinary degree of mitochondrial gene order variation when compared with other metazoans. Here, we determined the mitochondrial genomes of two scallops Argopecten irradians and Chlamys farreri. The complete mitochondrial genome of A. irradians is 16,211 nts in length and the nearly complete mitochondrial genome of C. farreri is 20,789 nts in length. Both of the genomes contain 35 genes including 12 protein-coding genes, 2 ribosomal RNAs, and 21 transfer RNAs. In contrast to the typical animal mitochondrial genome, both of them lack one protein-coding gene atp8 and two trnSs, but show an additional copy of trnF in A. irradians and of trnM in C. farreri, respectively. Gene order and genome content were compared among the four sequenced scallops. Gene arrangement of C. farreri closely resembles that of Mizuhopecten yessoensis. However, two genomes of C. farreri and A. irradians show only three small identical gene blocks and two genomes of A. irradians and Placopecten magellanicus share only one gene block. Comparison of the gene arrangement demonstrated that the four scallops share few identical gene blocks although they belong to the same family. This feature is seldom observed in Metazoa, even in other molluscan classes. The dramatic gene rearrangement often occurs in bivalves, especially in marine bivalves. In addition, comparisons of genomic character among bivalves are also presented.

Involvement of Hox genes in shell morphogenesis in the encapsulated development of a top shell gastropod (Gibbula varia L.)

Regulatory gene expression during the patterning of molluscan shells has only recently drawn the attention of scientists. We show that several Hox genes are expressed in association with the shell gland and the mantle in the marine vetigastropod Gibbula varia (L.). The expression of Gva-Hox1, Gva-Post2, and Gva-Post1 is initially detected in the trochophore larval stage in the area of the shell field during formation of embryonic shell. Later, during development, these genes are expressed in the mantle demonstrating their continuous role in larval shell formation and differentiation of mantle edge that secretes the adult shell. Gva-Hox4 is expressed only late during the development of the veliger-like larva and may also be involved in the adult shell morphogenesis. Additionally, this gene also seems to be associated with secretion of another extracellular structure, the operculum. Our data provide further support for association of Hox genes with shell formation which suggest that the molecular mechanisms underlying shell synthesis may consist of numerous conserved pattern-formation genes. In cephalopods, the only other molluscan class in which Hox gene expression has been studied, no involvement of Hox genes in shell formation has been reported. Thus, our results suggest that Hox genes are coopted to various functions in molluscs.

Assessing the molluscan hypothesis Serialia (Monoplacophora+Polyplacophora) using novel molecular data

A consensus on molluscan relationships has yet to be achieved, largely because of conflicting morphological and molecular hypotheses. Monoplacophora show marked seriality of ctenidia, atria, muscles and nephridia and this has been interpreted as plesiomorphic for Mollusca, reflecting a segmented ancestry. More recently this seriality, also partly seen in Polyplacophora, has been seen as a derived condition. Analysis of the first published monoplacophoran DNA sequence from Laevilipilina antarctica Warén & Hain, 1992 [Giribet, G., Okusu, A., Lindgren, A.R., Huff, S., Schrödl, M., Nishiguchi, M.K., 2006. Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons. Proc. Natl. Acad. Sci. USA 103, 7723-7728. 10.1073/pnas.0602578103], showed Monoplacophora inside Polyplacophora. These taxa were then grouped under the name Serialia, reflecting the hypothesis that their seriality is a synapomorphy. Subsequent examination revealed that part of the L. antarctica published sequence was the result of contamination with Polyplacophora (Giribet, Supplementary Material S1). We collected and sequenced another monoplacophoran, Laevipilina hyalina McLean, 1979, resulting in the first multi-gene dataset representing all molluscan classes. Our analyses did not show unambiguous support for Serialia. Model-based approaches strongly supported Serialia as a clade, however, parsimony analyses under dynamic and static homology did not resolve the position of Monoplacophora. Although our study provides support for Serialia and none for Conchifera, it appears that further resolution of molluscan relationships will require large increases of data.

The oldest cephalopods from east Laurentia

Cambrian Cephalopods are presently reported only from tropical, carbonate platform successions that occur on a number of paleocontinents. Outside of West Gondwanan occurrences on the eastern Sino-Korean Platform in China, the record of Cambrian cephalopods is limited, and information on the early evolution and habitats of this molluscan class has grown slowly over the last century.

The oldest cephalopods from east Laurentia

Cambrian Cephalopods are presently reported only from tropical, carbonate platform successions that occur on a number of paleocontinents. Outside of West Gondwanan occurrences on the eastern Sino-Korean Platform in China, the record of Cambrian cephalopods is limited, and information on the early evolution and habitats of this molluscan class has grown slowly over the last century.

Ultrastructure of epidermal cilia and ciliary rootlets in Scaphopoda

Ciliary structure in Scaphopoda is hitherto unknown and may provide information useful for phylogenetic analyses. Here we describe the ultrastructure of the ciliary apparatus of multiciliated epidermal cells of four species of Scaphopoda: Antalis entalis, Antalis occidentalis, Entalina tetragona and Cadulus propinquus, revealed by transmission electron microscopy. In all studied species the cilia have long whip-like distal ends. The rootlet apparatus consists of a basal foot, a short anterior ciliary rootlet and a long vertical rootlet. In other molluscan classes, the presence of an anterior rootlet has previously only been shown in species of the Neomeniomorpha, Chaetodermomorpha and Polyplacophora, while such a rootlet is absent in Gastropoda, Bivalvia and Cephalopoda. Twin rootlets, such as present in species of lamellibranch Bivalvia and postembryonic Cephalopoda probably represent a split vertical rootlet. The discovery of an anterior rootlet in Scaphopoda shows that the presence of paired ciliary rootlets is not a synapomorphy of a clade comprising the aplacophoran Neomeniomorpha and Chaetodermomorpha and the Polyplacophora, but that it represents a plesiomorphy of the Mollusca.

Nacre and false nacre (foliated aragonite) in extant monoplacophorans (=Tryblidiida: Mollusca)

Extant monoplacophorans (Tryblidiida, Mollusca) have traditionally been reported as having an internal nacreous layer, thus representing the ancestral molluscan condition. The examination of this layer in three species of Neopilinidae (Rokopella euglypta, Veleropilina zografi, and Micropilina arntzi) reveals that only V. zografi secretes an internal layer of true nacre, which occupies only part of the internal shell surface. The rest of the internal surface of V. zografi and the whole internal surfaces of the other two species examined are covered by a material consisting of lath-like, instead of brick-like, crystals, which are arranged into lamellae. In all cases examined, the crystallographic c-axis in this lamellar material is perpendicular to the surface of laths and the a-axis is parallel to their long dimension. The differences between taxa relate to the frequency of twins, which is much higher in Micropilina. In general, the material is well ordered, particularly towards the margin, where lamellae pile up at a small step size, which is most likely due to processes of crystal competition. Given its morphological resemblance to the foliated calcite of bivalves, we propose the name foliated aragonite for this previously undescribed biomaterial secreted by monoplacophorans. We conclude that the foliated aragonite probably lacks preformed interlamellar membranes and is therefore not a variant of nacre. A review of the existing literature reveals that previous reports of nacre in the group were instead of the aragonitic foliated layer and that our report of nacre in V. zografi is the first undisputed evidence of nacre in monoplacophorans. From the evolutionary viewpoint, the foliated aragonite could easily have been derived from nacre. Assuming that nacre represents the ancestral condition, as in other molluscan classes, it has been replaced by foliated aragonite along the tryblidiidan lineage, although the fossil record does not presently provide evidence as to when this replacement took place.