Comparative neuroanatomy of Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda (Mollusca) and its phylogenetic implications

Polarization sensitivity and decentralized visual processing in an animal with a distributed visual system

The marine mollusc Acanthopleura granulata (Mollusca; Polyplacophora) has a distributed visual array composed of hundreds of small image-forming eyes embedded within its eight dorsal shell plates. As in other animals with distributed visual systems, we still have a poor understanding of the visual capabilities of A. granulata and we have yet to learn where and how it processes visual information. Using behavioral trials involving isoluminant looming visual stimuli, we found that A. granulata demonstrates spatial vision with an angular resolution of 6 deg. We also found that A. granulata responds to looming stimuli defined by contrasting angles of linear polarization. To learn where and how A. granulata processes visual information, we traced optic nerves using fluorescent lipophilic dyes. We found that the optic nerves innervate the underlying lateral neuropil, a neural tissue layer that circumnavigates the body. Adjacent optic nerves innervate the lateral neuropil with highly overlapping arborizations, suggesting it is the site of an integrated visuotopic map. Using immunohistochemistry, we found that the lateral neuropil of A. granulata is subdivided into two separate layers. In comparison, we found that a chiton with eyespots (Chiton tuberculatus) and two eyeless chitons (Ischnochiton papillosus and Chaetopleura apiculata) have lateral neuropil that is a singular circular layer without subdivision, findings consistent with previous work on chiton neuroanatomy. Overall, our results suggest that A. granulata effectuates its visually mediated behaviors using a unique processing scheme: it extracts spatial and polarization information using a distributed visual system, and then integrates and processes that information using decentralized neural circuits.

First Immunodetection of Sensory and Nervous Systems of Parasitic Larvae (Glochidia) of Freshwater Bivalve Nodularia douglasiae

Most freshwater mussels have an unusual life cycle that requires host fish species for larval (glochidia) development and dispersal. Glochidia have a unique morphological structure that adapts to parasitic lifestyles and survival. The morphology of the glochidial shells of most Unionoidea, a group of freshwater bivalve mollusks, has been studied in detail using light and scanning electron microscopy. This study summarizes our data on the glochidia shell morphology of the Asian mussel Nodularia douglasiae from two localities in the Primorsky Territory, the Russian Far East. In contrast to the shell morphology of glochidia, little is known about the neurodevelopment of the Unionoidea. Herein, we first demonstrate that the structures of the sensory, muscle, and nervous systems of the glochidia larvae of N. douglasiae differ dramatically from those of the comparable larval systems of marine bivalve species, as revealed through alpha-acetylated tubulin, serotonin (5-HT), and FMRFamide antibodies as well as phalloidin for detection of F-actin and whole-mount confocal microscopy. We found that the glochidia sensory system included four pairs of tubulin-lir multicilia hair cells. Non-ciliar tubulin-lir cells synthesize the neuropeptide FMRFamide and are identified as afferent neurons collecting information from peripheral tubulin-lir hair sensory cells to nervous regulators. The glochidia’s muscular system was represented by a smooth adductor, retractors, and minor muscle bundles associated with the shell and visceral organs. The 5-HT-lir larval system is arranged most simply and consists of two immunopositive neurons innervating the adductor. The FMRFamide-lir system is more complicated and consists of several neuronal centers comprising neuronal bodies and their neurites in different areas of the larva. The FMRFamide-lir neurons are closely associated with sensory hair cells, and others, together with 5-HT-lir neurons, may be involved in the anlagen of adult ganglia. Thus, the nervous system of N. douglasiae glochidia is drastically different from other mollusks and lophotrochozoans because of the absence of an apical organ and the location and composition of FMRFamide and 5-HT cells. Morphological, molecular, and behavioral investigations of Unionoidea taxa need to be further conducted to investigate the parasite-host relationship, nerve-dependent regulation of parasite behavior, and evolution of mollusks.

Extensive conservation of the proneuropeptide and peptide prohormone complement in mollusks

As one of the most diverse groups of invertebrate animals, mollusks represent powerful models for neurobiological and developmental studies. Neuropeptides and peptide hormones are a heterogeneous class of signalling molecules involved in chemical communication between neurons and in neuroendocrine regulation. Here we present a fine-grained view of the molluscan neuropeptide and peptide hormone toolkit. Our results expand the distribution of several peptide families (e.g., prokineticin, insulin-related peptides, prohormone-4, LFRFamide) within Lophotrochozoa and provide evidence for an early origin of others (e.g., GNXQN/prohormone-2, neuroparsin). We identified a new peptide family broadly distributed among conchiferan mollusks, the PXRX family. We found the Wnt antagonist dickkopf1/2/4 ortholog in lophotrochozoans and nematodes and reveal that the egg-laying hormone family is a DH44 homolog restricted to gastropods. Our data demonstrate that numerous peptides evolved much earlier than previously assumed and that key signalling elements are extensively conserved among extant mollusks.

The neuroanatomy of the siboglinid Riftia pachyptila highlights sedentarian annelid nervous system evolution

Tracing the evolution of the siboglinid group, peculiar group of marine gutless annelids, requires the detailed study of the fragmentarily explored central nervous system of vestimentiferans and other siboglinids. 3D reconstructions of the neuroanatomy of Riftia revealed that the "brain" of adult vestimentiferans is a fusion product of the supraesophageal and subesophageal ganglia. The supraesophageal ganglion-like area contains the following neural structures that are homologous to the annelid elements: the peripheral perikarya of the brain lobes, two main transverse commissures, mushroom-like structures, commissural cell cluster, and the circumesophageal connectives with two roots which give rise to the palp neurites. Three pairs of giant perikarya are located in the supraesophageal ganglion, giving rise to the paired giant axons. The circumesophageal connectives run to the VNC. The subesophageal ganglion-like area contains a tripartite ventral aggregation of perikarya (= the postoral ganglion of the VNC) interconnected by the subenteral commissure. The paired VNC is intraepidermal, not ganglionated over most of its length, associated with the ciliary field, and comprises the giant axons. The pairs of VNC and the giant axons fuse posteriorly. Within siboglinids, the vestimentiferans are distinguished by a large and considerably differentiated brain. This reflects the derived development of the tentacle crown. The tentacles of vestimentiferans are homologous to the annelid palps based on their innervation from the dorsal and ventral roots of the circumesophageal connectives. Neuroanatomy of the vestimentiferan brains is close to the brains of Cirratuliiformia and Spionida/Sabellida, which have several transverse commissures, specific position of the giant somata (if any), and palp nerve roots (if any). The palps and palp neurite roots originally developed in all main annelid clades (basally branching, errantian and sedentarian annelids), show the greatest diversity in their number in sedentarian species. Over the course of evolution of Sedentaria, the number of palps and their nerve roots either dramatically increased (as in vestimentiferan siboglinids) or were lost.

Synopsis of the knowledge on the Brazilian aplacophorans (Mollusca: Caudofoveata & Solenogastres)

Abstract: Aplacophorans are well known as exclusively marine benthic molluscs with a vermiform body covered by aragonitic sclerites (also called spicules), whose species are widely distributed from the sublittoral down to the abyss. Currently, only nine species are known from Brazil (one Solenogastres and eight Caudofoveata), but these very few records are no longer a reflection of an existing low diversity. Sampling in deep waters has been conducted recently in oil-rich areas of the Brazilian coast, and the museum collections have now many aplacophoran lots. There is a need to learn about and/or install some microscopical facilities in Brazilian institutions, to form expertise for the investigations on these generally small animals. With studies on taxonomy, phylogeny, biogeography and ecology, important questions will be surely answered about the diversity, distribution, and the relationship among the deep-sea fauna from Brazil and from other places.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

Designing Brains for Pain: Human to Mollusc

There is compelling evidence that the "what it feels like" subjective experience of sensory stimuli arises in the cerebral cortex in both humans as well as mammalian experimental animal models. Humans are alone in their ability to verbally communicate their experience of the external environment. In other species, sensory awareness is extrapolated on the basis of behavioral indicators. For instance, cephalopods have been claimed to be sentient on the basis of their complex behavior and anecdotal reports of human-like intelligence. We have interrogated the findings of avoidance learning behavioral paradigms and classical brain lesion studies and conclude that there is no evidence for cephalopods feeling pain. This analysis highlighted the questionable nature of anthropometric assumptions about sensory experience with increased phylogenetic distance from humans. We contend that understanding whether invertebrates such as molluscs are sentient should first begin with defining the computational processes and neural circuitries underpinning subjective awareness. Using fundamental design principles, we advance the notion that subjective awareness is dependent on observer neural networks (networks that in some sense introspect the neural processing generating neural representations of sensory stimuli). This introspective process allows the observer network to create an internal model that predicts the neural processing taking place in the network being surveyed. Predictions arising from the internal model form the basis of a rudimentary form of awareness. We develop an algorithm built on parallel observer networks that generates multiple levels of sensory awareness. A network of cortical regions in the human brain has the appropriate functional properties and neural interconnectivity that is consistent with the predicted circuitry of the algorithm generating pain awareness. By contrast, the cephalopod brain lacks the necessary neural circuitry to implement such an algorithm. In conclusion, we find no compelling behavioral, functional, or neuroanatomical evidence to indicate that cephalopods feel pain.

Do chitons have a brain? New evidence for diversity and complexity in the polyplacophoran central nervous system

Molluscs demonstrate astonishing morphological diversity, and the relationships among clades have been debated for more than a century. Molluscan nervous systems range from simple 'ladder-like' cords to the complex brains of cephalopods. Chitons (Polyplacophora) are assumed to retain many molluscan plesiomorphies, lacking neural condensation and ganglionic structure, and therefore a brain. We reconstructed three-dimensional anatomical models of the nervous system in eight species of chitons in an attempt to clarify chiton neuroarchitecture and its variability. We combined new data with digitised historic slide material originally used by malacologist Johannes Thiele (1860-1935). Reconstructions of whole nervous systems in Acanthochitona fascicularis, Callochiton septemvalvis, Chiton olivaceus, Hemiarthrum setulosum, Lepidochitona cinerea, Lepidopleurus cajetanus and Leptochiton asellus, and the anterior nervous system of Schizoplax brandtii, demonstrated consistent and substantial anterior neural concentration in the circumoesophageal nerve ring. This is further organised into three concentric tracts, corresponding to the lateral, ventral and cerebral nerve cords. These represent homologues to the three main pairs of ganglia in other molluscs. Their relative size, shape and organisation are highly variable among the examined taxa, but consistent with previous studies of select species, and we formulated a set of neuroanatomical characters for chitons. These support anatomical transitions at the ordinal and subordinal levels; the identification of robust homologies in neural architecture will be central to future comparisons across Mollusca and, more broadly, Lophotrochozoa. Contrary to almost all previous descriptions, the size and structure of the chiton anterior nerve ring unambiguously qualify it as a true brain with cordal substructure.

Detailed reconstruction of the nervous and muscular system of Lobatocerebridae with an evaluation of its annelid affinity

BACKGROUND

The microscopic worm group Lobatocerebridae has been regarded a 'problematicum', with the systematic relationship being highly debated until a recent phylogenomic study placed them within annelids (Curr Biol 25: 2000-2006, 2015). To date, a morphological comparison with other spiralian taxa lacks detailed information on the nervous and muscular system, which is here presented for Lobatocerebrum riegeri n. sp. based on immunohistochemistry and confocal laser scanning microscopy, supported by TEM and live observations.

RESULTS

The musculature is organized as a grid of longitudinal muscles and transverse muscular ring complexes in the trunk. The rostrum is supplied by longitudinal muscles and only a few transverse muscles. The intraepidermal central nervous system consists of a big, multi-lobed brain, nine major nerve bundles extending anteriorly into the rostrum and two lateral and one median cord extending posteriorly to the anus, connected by five commissures. The glandular epidermis has at least three types of mucus secreting glands and one type of adhesive unicellular glands.

CONCLUSIONS

No exclusive "annelid characters" could be found in the neuromuscular system of Lobatocerebridae, except for perhaps the mid-ventral nerve. However, none of the observed structures disputes its position within this group. The neuromuscular and glandular system of L. riegeri n. sp. shows similarities to those of meiofaunal annelids such as Dinophilidae and Protodrilidae, yet likewise to Gnathostomulida and catenulid Platyhelminthes, all living in the restrictive interstitial environment among sand grains. It therefore suggests an extreme evolutionary plasticity of annelid nervous and muscular architecture, previously regarded as highly conservative organ systems throughout metazoan evolution.

Evidence for a cordal, not ganglionic, pattern of cephalopod brain neurogenesis

INTRODUCTION

From the large-brained cephalopods to the acephalic bivalves, molluscs show a vast range of nervous system centralization patterns. Despite this diversity, molluscan nervous systems, broadly considered, are organized either as medullary cords, as seen in chitons, or as ganglia, which are typical of gastropods and bivalves. The cephalopod brain is exceptional not just in terms of its size; its relationship to a molluscan cordal or ganglionic plan has not been resolved from the study of its compacted adult structure. One approach to clarifying this puzzle is to investigate the patterns of early cephalopod brain neurogenesis, where molecular markers for cephalopod neural development may be informative.

RESULTS

We report here on early brain pattern formation in the California two-spot octopus, Octopus bimaculoides. Employing gene expression analysis with the pan-bilaterian neuronal marker ELAV and the atonal-related neuronal differentiation genes NEUROGENIN and NEUROD, as well as immunostaining using a Distalless-like homeoprotein antibody, we found that the octopus central brain forms from concentric cords rather than bilaterally distributed pairs of ganglia.

CONCLUSION

We conclude that the cephalopod brain, despite its great size and elaborate specializations, retains in its development the hypothesized ancestral molluscan nervous system plan of medullary cords, as described for chitons and other aculiferan molluscs.

Unexpected co-linearity of Hox gene expression in an aculiferan mollusk

BACKGROUND

Mollusca is an extremely diverse animal phylum that includes the aculiferans (worm-like aplacophorans and eight-shelled polyplacophorans) and their sister group, the conchiferans, comprising monoplacophorans, bivalves (clams, mussels), gastropods (snails, slugs), scaphopods (tusk shells) and cephalopods (squids, octopuses). Studies on mollusks have revealed an overall number of 11 Hox genes in seven out of eight molluscan "class"-level taxa, but expression data of key developmental regulators such as homeotic genes are only available for three gastropod and two cephalopod species. These show that Hox genes are involved in the formation of specific features including shell, foot, funnel or tentacles and not in antero-posterior body plan patterning as in most other bilaterian animals. The role of Hox genes in non-conchiferan (i.e., aculiferan) mollusks remains entirely unknown.

RESULTS

Here we present the first data on the expression of seven Hox genes in apolyplacophoran mollusk, Acanthochitona crinita. In A. crinita the Hox genes Acr-Hox1-5, Hox7 and Post2 are expressed in a co-linear pattern along the antero-posterior axis, but not in molluscan-specific features such as the shell or the foot. The expression pattern is restricted to the post-trochal region and the transcripts are present in ecto-, endo- and mesodermal cell layers. Contrary to the situation in gastropods and cephalopods, we did neither find Hox gene expression in distinct neural subsets of A. crinita, nor in its developing shell plates.

CONCLUSIONS

Our analysis and comparison with other lophotrochozoans indicate that the basal role of Hox genes is in antero-posterior axis patterning in mollusks, similar to the vast majority of bilaterian animals, and that this role has been conserved in polyplacophorans, while co-option into patterning of evolutionary novelties emerged either at the base of Conchifera or independently in gastropods and cephalopods. These morphological innovations most likely contributed to the evolutionary success of its representatives, as exemplified by, e.g., the wide ecological range and species richness of gastropods.

Development of the nervous system in Solenogastres (Mollusca) reveals putative ancestral spiralian features

Background

The Solenogastres (or Neomeniomorpha) are a taxon of aplacophoran molluscs with contentious phylogenetic placement. Since available developmental data on non-conchiferan (that is, aculiferan) molluscs mainly stem from polyplacophorans, data on aplacophorans are needed to clarify evolutionary questions concerning the morphological features of the last common ancestor (LCA) of the Aculifera and the entire Mollusca. We therefore investigated the development of the nervous system in two solenogasters, Wirenia argentea and Gymnomenia pellucida, using immunocytochemistry and electron microscopy.

Results

Nervous system formation starts simultaneously from the apical and abapical pole of the larva with the development of a few cells of the apical organ and a posterior neurogenic domain. A pair of neurite bundles grows out from both the neuropil of the apical organ and the posterior neurogenic domain. After their fusion in the region of the prototroch, which is innervated by an underlying serotonin-like immunoreactive (−LIR) plexus, the larva exhibits two longitudinal neurite bundles - the future lateral nerve cords. The apical organ in its fully developed state exhibits approximately 8 to 10 flask-shaped cells but no peripheral cells. The entire ventral nervous system, which includes a pair of longitudinal neurite bundles (the future ventral nerve cords) and a serotonin-LIR ventromedian nerve plexus, appears simultaneously and is established after the lateral nervous system. During metamorphosis the apical organ and the prototrochal nerve plexus are lost.

Conclusions

The development of the nervous system in early solenogaster larvae shows striking similarities to other spiralians, especially polychaetes, in exhibiting an apical organ with flask-shaped cells, a single pair of longitudinal neurite bundles, a serotonin-LIR innervation of the prototroch, and formation of these structures from an anterior and a posterior neurogenic domain. This provides evidence for an ancestral spiralian pattern of early nervous system development and a LCA of the Spiralia with a single pair of nerve cords. In later nervous system development, however, the annelids deviate from all other spiralians including solenogasters in forming a posterior growth zone, which initiates teloblastic growth. Since this mode of organogenesis is confined to annelids, we conclude that the LCA of both molluscs and spiralians was unsegmented.

Sensing deep extreme environments: the receptor cell types, brain centers, and multi-layer neural packaging of hydrothermal vent endemic worms

INTRODUCTION

Deep-sea alvinellid worm species endemic to hydrothermal vents, such as Alvinella and Paralvinella, are considered to be among the most thermotolerant animals known with their adaptability to toxic heavy metals, and tolerance of highly reductive and oxidative stressful environments. Despite the number of recent studies focused on their overall transcriptomic, proteomic, and metabolic stabilities, little is known regarding their sensory receptor cells and electrically active neuro-processing centers, and how these can tolerate and function in such harsh conditions.

RESULTS

We examined the extra- and intracellular organizations of the epidermal ciliated sensory cells and their higher centers in the central nervous system through immunocytochemical, ultrastructural, and neurotracing analyses. We observed that these cells were rich in mitochondria and possessed many electron-dense granules, and identified specialized glial cells and serial myelin-like repeats in the head sensory systems of Paralvinella hessleri. Additionally, we identified the major epidermal sensory pathways, in which a pair of distinct mushroom bodies-like or small interneuron clusters was observed. These sensory learning and memory systems are commonly found in insects and annelids, but the alvinellid inputs are unlikely derived from the sensory ciliary cells of the dorsal head regions.

CONCLUSIONS

Our evidence provides insight into the cellular and system-wide adaptive structure used to sense, process, and combat the deep-sea hydrothermal vent environment. The alvinellid sensory cells exhibit characteristics of annelid ciliary types, and among the most unique features were the head sensory inputs and structure of the neural cell bodies of the brain, which were surrounded by multiple membranes. We speculated that such enhanced protection is required for the production of normal electrical signals, and to avoid the breakdown of the membrane surrounding metabolically fragile neurons from oxidative stress. Such pivotal acquisition is not broadly found in the all body parts, suggesting the head sensory inputs are specific, and these heterogenetic protection mechanisms may be present in alvinellid worms.

The complete mitochondrial genome of Scutopus ventrolineatus (Mollusca: Chaetodermomorpha) supports the Aculifera hypothesis

Background

With more than 100000 living species, mollusks are the second most diverse metazoan phylum. The current taxonomic classification of mollusks recognizes eight classes (Neomeniomorpha, Chaetodermomorpha, Polyplacophora, Monoplacophora, Cephalopoda, Gastropoda, Bivalvia, and Scaphopoda) that exhibit very distinct body plans. In the past, phylogenetic relationships among mollusk classes have been contentious due to the lack of indisputable morphological synapomorphies. Fortunately, recent phylogenetic analyses based on multi-gene data sets are rendering promising results. In this regard, mitochondrial genomes have been widely used to reconstruct deep phylogenies. For mollusks, complete mitochondrial genomes are mostly available for gastropods, bivalves, and cephalopods, whereas other less-diverse lineages have few or none reported.

Results

The complete DNA sequence (14662 bp) of the mitochondrial genome of the chaetodermomorph Scutopus ventrolineatus Salvini-Plawen, 1968 was determined. Compared with other mollusks, the relative position of protein-coding genes in the mitochondrial genome of S. ventrolineatus is very similar to those reported for Polyplacophora, Cephalopoda and early-diverging lineages of Bivalvia and Gastropoda (Vetigastropoda and Neritimorpha; but not Patellogastropoda). The reconstructed phylogenetic tree based on combined mitochondrial and nuclear sequence data recovered monophyletic Aplacophora, Aculifera, and Conchifera. Within the latter, Cephalopoda was the sister group of Gastropoda and Bivalvia + Scaphopoda.

Conclusions

Phylogenetic analyses of mitochondrial sequences showed strong among-lineage rate heterogeneity that produced long-branch attraction biases. Removal of long branches (namely those of bivalves and patellogastropods) ameliorated but not fully resolved the problem. Best results in terms of statistical support were achieved when mitochondrial and nuclear sequence data were concatenated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0197-9) contains supplementary material, which is available to authorized users.

A new sensory organ in "primitive" molluscs (Polyplacophora: Lepidopleurida), and its context in the nervous system of chitons

Introduction

Chitons (Polyplacophora) are molluscs considered to have a simple nervous system without cephalisation. The position of the class within Mollusca is the topic of extensive debate and neuroanatomical characters can provide new sources of phylogenetic data as well as insights into the fundamental biology of the organisms. We report a new discrete anterior sensory structure in chitons, occurring throughout Lepidopleurida, the order of living chitons that retains plesiomorphic characteristics.

Results

The novel “Schwabe organ” is clearly visible on living animals as a pair of streaks of brown or purplish pigment on the roof of the pallial cavity, lateral to or partly covered by the mouth lappets. We describe the histology and ultrastructure of the anterior nervous system, including the Schwabe organ, in two lepidopleuran chitons using light and electron microscopy. The oesophageal nerve ring is greatly enlarged and displays ganglionic structure, with the neuropil surrounded by neural somata. The Schwabe organ is innervated by the lateral nerve cord, and dense bundles of nerve fibres running through the Schwabe organ epithelium are frequently surrounded by the pigment granules which characterise the organ. Basal cells projecting to the epithelial surface and cells bearing a large number of ciliary structures may be indicative of sensory function. The Schwabe organ is present in all genera within Lepidopleurida (and absent throughout Chitonida) and represents a novel anatomical synapomorphy of the clade.

Conclusions

The Schwabe organ is a pigmented sensory organ, found on the ventral surface of deep-sea and shallow water chitons; although its anatomy is well understood, its function remains unknown. The anterior commissure of the chiton oesophagial nerve ring can be considered a brain. Our thorough review of the chiton central nervous system, and particularly the sensory organs of the pallial cavity, provides a context to interpret neuroanatomical homology and assess this new sense organ.

The continuing debate on deep molluscan phylogeny: evidence for Serialia (Mollusca, Monoplacophora + Polyplacophora)

Molluscs are a diverse animal phylum with a formidable fossil record. Although there is little doubt about the monophyly of the eight extant classes, relationships between these groups are controversial. We analysed a comprehensive multilocus molecular data set for molluscs, the first to include multiple species from all classes, including five monoplacophorans in both extant families. Our analyses of five markers resolve two major clades: the first includes gastropods and bivalves sister to Serialia (monoplacophorans and chitons), and the second comprises scaphopods sister to aplacophorans and cephalopods. Traditional groupings such as Testaria, Aculifera, and Conchifera are rejected by our data with significant Approximately Unbiased (AU) test values. A new molecular clock indicates that molluscs had a terminal Precambrian origin with rapid divergence of all eight extant classes in the Cambrian. The recovery of Serialia as a derived, Late Cambrian clade is potentially in line with the stratigraphic chronology of morphologically heterogeneous early mollusc fossils. Serialia is in conflict with traditional molluscan classifications and recent phylogenomic data. Yet our hypothesis, as others from molecular data, implies frequent molluscan shell and body transformations by heterochronic shifts in development and multiple convergent adaptations, leading to the variable shells and body plans in extant lineages.