Abstract The classification of the Pteriomorphia, a major division of the Bivalvia with Ordovician origins, is controversial both among palaeontologists and neontologists. To elucidate phylogenetic relationships new near-complete 18S rDNA sequences of 26 Pteriomorphia, three Protobranchia, three Heterodonta, one Anomalodesmata and three Scaphopoda were obtained, aligned with 71 other published molluscan sequences, and analysed with parsimony, maximum likelihood and spectral analysis. Although Bivalvia appear diphyletic due to heterogeneity of substitution rates among lineages, monophyly of Protobranchia, Heteroconchia and Pteriomorphia is supported. The heteroconch Lucinida, Myoida and Venerida are not monophyletic, and Anomalodesmata arise from within Heteroconchia. The basal nodes of Pteriomorphia have little support but two major clades, [Pinnoidea (Ostreoidea + Pterioidea)] and [(Anomioidea + Plicatuloidea) + (Limoidea + Pectinoidea)], are resolved with more confidence. The strongly supported clade of Anomioidea + Plicatuloidea, the separation of Pinnoidea from Pterioidea and most of the intrafamiliar relationships are not in accordance with morphological classifications. Combining these results with selected morphological characters, a phylogenetic hypothesis is proposed showing Mytiloidea and Arcoidea as the basal pteriomorph groups, the latter giving rise to the clade uniting the pinnoid-ostreoid-pterioid and the anomioid-limoid-pectinoid lines.

Abstract Prodissoconchs of Middle Jurassic oysters possess key characters for a new understanding of the hinge evolution within the Ostreoidea. The ancestral larval hinge most likely evolved from a mytilid-like provinculum through accelerated growth in an antero-ventral direction along a flat, helico-spiral trajectory. The posterior denticles moved into the centre of the hinge axis while the central and anterior denticles were reduced in size and eventually disappeared. The remaining, now central, posterior denticles evolved further into a secondary symmetrical hinge. In most oysters, except for example Tiostrea, the larval ligament maintained its relative position between posterior and anterior denticles, even though this is not obvious in the Ostreidae because of the loss of the anterior denticles. Larval and adult ligaments are continuous. The examined specimens provide further evidence for the plesiomorphic state of planktotrophy within the superfamily. Although additional direct evidence is lacking, consideration of all new available data favour the hypothesis that the postero-dorsal notch is uniquely derived by oysters. The ideas on character evolution put forward in this study are consistent with phylogeny hypotheses based on palaeontological–biological data and recent genetical studies.

Abstract Pteriomorphian spermatozoa, like those of most other bivalves, are of the classic aquasperm type (conical acrosomal vesicle, short to rod-shaped nucleus, short midpiece composed of two centrioles and a ring of spherical mitochondria, a simple flagellum). Whereas most other bivalve subclasses show at least some defining acrosomal feature(s), this does not appear to be the case within the Pteriomorphia. While this could indicate non-monophyletic status, it also correlates with the fact that the Pteriomorphia are a very old and very successful group of bivalves. Acrosomal similarities suggest a close link between the Ostreoidea and Limoidea (acrosomal vesicle with wedge-shaped apical zone; radiating plates present but not well developed); and between the Pterioidea, Pinnoidea and Pectinoidea (dense anterior layer; very well developed radiating plates). For supposedly closely related taxa, the Arcoidea and Limopsoidea (both Arcoida) differ markedly from each other in acrosomal shape and substructure. The affinities of the Anomioidea and even more so the Mytiloida remain uncertain, the latter possibly connected with the Pterioida or, more likely, removed from the rest of the Pteriomorphia (mytiloid acrosomes show concentric lamellae). A very close relationship between the Pectinidae and Spondylidae of the Pectinoidea is demonstrated (dense anterior layer of acrosome recurved). Within the Mytilidae (Mytiloidea) there is substantial variation in sperm morphology between supraspecific taxa especially at the subfamial level.

Abstract Parsimony analysis suggests derivation of the Bivalvia from monoplacophorans rather than from rostroconchs, and additionally indicates that a phylogenetic classification of the Bivalvia can be achieved by erecting the superorder Nuculaniformii nov. and the order Nuculanoida nov. for the superfamily Nuculanoidea; relegating all other palaeotaxodonts to the superorder Nuculiformii; restricting the order Nuculoida to the families Nuculidae and Pristiglomidae; expanding the order Solemyoida to include ctenodontid genera as basal plesions; restricting the superorder Heteroconchia to palaeoheterodonts and heterodonts, exclusive of the Modiomorphidae; relegating the new family Evyanidae, the Colpomyidae, Matheriidae and Modiolodontidae to near-basal plesion status within the superorder Pteriomorphia; restricting the Mytiloida to the superfamily Mytiloidea, inclusive of modiolopsid genera as basal plesions; placing Ortonella as a basal plesion within the Cyrtodontoida; expanding the order Pectinoida to include the Myodakryotidae and the suborders Limina and Pectinina; and expanding the superfamily Arcoidea to include the frejid genera and Catamarcaia as basal plesions, and the family Glyptarcidae. Modiomorphid anomalodesmatans appear to be more closely related to the Pteriomorphia than to the Heteroconchia, and Evyana lies close to the common ancestry of modiomorphids and colpomyid pteriomorphians. Arcoids may have evolved from left–right symmetrical but otherwise rhombopteriid-like ancestors, rather than from actinodontoids or directly from cyrtodontids. The new family Eodonidae is proposed to distinguish the nacreous genus Eodon from the non-nacreous Astartidae within the superfamily Crassatelloidea.

Abstract Despite widespread agreement on the monophyly of several major taxa of bivalves, others remain uncertain and the relationships among them are debated. The present study compares new and published morphological phylogenies with new analyses based on 18S gene sequences. All but one family and all superfamilies in the Bivalvia were monophyletic in all the analyses. Several higher taxa, including most subclasses and orders, were also resolved as monophyletic. Only Myoida shows strong evidence for polyphyly, with at least two origins from Veneroida. Autobranchia was supported as monophyletic in the parsimony analyses. Within Pteriomorphia, Ostreoida is the sister taxon of Pterioida, if not derived from within it, rather than closest to Pectinoida. The numerous points of agreement with morphology based analyses suggests that both types of evidence are converging on a common phylogeny; however, differences remain to be resolved by further study.

Abstract In the 30 years since publication of the bivalve Treatise , (Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology. Part N. Mollusca 6, Bivalvia , Geological Society of America and University of Kansas) important new faunas have been described from the early and mid Cambrian and from the early and mid Ordovician. These contain significant new forms, including some long-ranging intermediate groups, that indicate the relationships between the principal bivalve clades, but lack of fossils from the late Cambrian and earliest Ordovician is a major hindrance. The principal phase of bivalve diversification followed on from the evolution of the filibranch gill in the latest Cambrian or earliest Ordovician. The fundamental division of the class is into two subclasses, Protobranchia and Autolamelli branchiata; links between the two can be demonstrated in the early Ordovician. Major divisions of each subclass are recognized as superorders. Within the Protobranchia, the Nuculoida developed specialist food-gathering palps and an enlarged foot. Diverging early from the protobranch stock were other bivalves that lived symbiotically with sulphur-oxidizing chemoautotrophic bacteria; this allowed colonization of anaerobic substrates and produced two distinct stocks: the deeply infaunal anteriorly elongate Solemyoida and the shallower infaunal Nucinelloida. The Autolamellibranchiata, initially identified by strongly asymmetrical hinges, diversified in three directions, each characterized by distinctive hinges. The Trigonioida were characterized by ligamental nymphs and frequently denticulate teeth, and rapidly regained greater symmetry; the Anomalodesmata also developed a strong ligamental insertion within nymphs and largely lost their dentition, whilst the Heteroconchia, principally with a shell including a complex crossed-lamellar structure, had various combinations of cardinal and lateral teeth. Heteroconch diversifications were mainly in the Mesozoic and Cenozoic, but one Ordovician group, the Glyptarcoidea, is a good ancestor for the Pteriomorphia. The following new taxa are proposed: Cardiolarioidea superfam. nov., Eritropidae fam. nov. and Catamarcaidae fam. nov.