Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square‐meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.     

Phylogenetic relationships of Early–Middle Ordovician ostracods of Baltoscandia

Phylogenetic analysis of the Early and early Middle Ordovician (Tremadoc and Arenig) ostracod species of Baltoscandia suggests a polyphyletic origin for the suborder Beyrichiocopa. Binodicopes, leiocopes and eridostracans are excluded from the beyrichiocopide clade. An independent origin from the basal ostracods is suggested for the binodicopes and eridostracans. The palaeocopes form a strongly supported monophyletic clade. Within this suborder, the ctenonotellid and the tetradellid families together form a monophyletic clade. The tetradellids are paraphyletic, being a stem-group for the ctenonotellids. Nanopsis nanella , the earliest known ostracod from the Tremadoc, is a basal palaeocope. The early eridostracans Conchoprimitia and Incisua , with their uncomplicated carapace morphology, might be the most primitive ostracods.     

Biodiversity curves for the Ordovician of Baltoscandia

Biodiversity curves for the Ordovician of Baltoscandia show a substantial increase in taxonomical diversity through the period, as seen also in global data sets. A database of 10,340 records of first and last appearances of species at different localities in the region has been analysed using simple species counts, and partly validated with resampling methods. While the biodiversity curve for all fossil groups combined is probably reasonably accurate except for an unknown scaling constant, taxonomical or geographical subsets may not be sufficiently well sampled to allow precise measurement of their species counts through time. The analysis shows a major diversification event commencing in the middle Arenig (Ibex-Whiterock boundary), and more limited diversification events in the Llanvirn, Caradoc and Ashgill. The Scandinavian (Norwegian and Swedish) biodiversity curves are broadly correlated with major changes in sea level, with low biodiversity at highstands and high biodiversity at lowstands, although this pattern is not clear for all fossil groups. In the Arenig, graptolites and trilobites appear to have higher diversities at high sea levels, while the brachiopods and ostracodes show higher diversities at low sea level. As a consequence, the Arenig diversification is delayed for the latter two groups until the upper end of the interval.     

AnOistodus venustus-like conodont species from the Middle Ordovician of Baltoscandia

Ordovician conodont specimens resemblingOistodus venustus Stauffer, 1935 have been reported from many areas. There is increasing evidence, however, that several lineages with homeomorphic conodont elements have erroneously been referred to one and the same species. I have investigated Baltoscandian conodont elements of this kind in order to find out about their origins and phylogenetic relationships with morphologically comparable elements from other areas. A natural grouping of finds from the Middle and lower Upper Ordovician of Baltoscandia is here described as belonging to a new species of a new genus,Venoistodus balticus n. gen., n. sp. The new species probably evolved paedomorphically fromDrepanoistodus forceps (Lindström, 1955) in the Early Ordovician.     

Early growth-stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia

Five apices of orthoceridan cephalopods from the early Middle Ordovician Holen Limestone of Öland, Sweden that where collected in the late 19th Century by G. Holm provide information on cephalopod evolution in the early Palaeozoic. The apices belong to specimens of the genus Hedstroemoceras Foerste, 1930 and Archigeisonoceras Chen, 1984. The apices are small in comparison with apices of other cephalopods of the Ordovician; the initial chambers of the shells of both genera are hemispherical and approximately 1 mm and 1.5 mm in cross-section diameter, respectively. The apical 2–3 mm of the shell are free from growth-lines and possess no cicatrix, though distinct longitudinal wrinkles are present. There is a slight variability of siphuncle position during early growth in Archigeisonoceras. It can be shown that the structure of the connecting ring of Hedstroemoceras is similar to that of other Orthocerida. Additionally, the hemispherical apex of Lituites perfectus Wahlenberg, 1821 gives evidence for the orthoceridan affinity of lituitidans. The investigation shows that early Middle Ordovician Orthocerida display a characteristic connecting ring structure, a characteristic apex morphology and variable siphuncular positions that differs significantly from other cephalopods of the Ordovician. Based on this evidence it is concluded that a small spherical apex is an autapomorphy of the Orthocerida. Moreover, this evidence supports a splitting of the order Orthocerida in two taxa of different affinities. The Orthocerida sensu stricto comprises orthocones with a tubular siphuncle nearly without endospiphuncular deposits, and a spherical apex. Embryonic shell, orthoceridan ancestry, orthoceridan classification.     

Middle Ordovician Aporthophyla brachiopod fauna from the roof of the World,southern Tibet

A Darriwilian (late Middle Ordovician) brachiopod fauna from the Lower Formation of the Chiatsun Group at Jiacun, northern Nyalam, southern Tibet, consists of ten brachiopod species, forming a distinct Aporthophyla–Paralenorthis Association. Its taxonomic composition is typical of the Aporthophyla Fauna that occupied lower BA2 to upper BA3 benthic environments on sandy lime mud substrates. The occurrence of Paralenorthis in southern Tibet is confirmed for the first time, represented by P. costata sp. nov. Numerical analyses (PCA and CA) of 18 Darriwilian brachiopod faunas from ten palaeoplates or terranes indicate that: (1) the Aporthophyla Fauna was confined to a specific latitudinal belt although it had a wide lateral distribution from the large palaeocontinents of Gondwana to Laurentia; (2) the Saucrorthis Fauna, a typical late Middle Ordovician regional fauna, is limited to a much smaller area, marginal to the Gondwana supercontinent; (3) the strong provincialism persistent in the late Middle Ordovician contributed to increased gamma biodiversity during the Great Ordovician Biodiversification Event.     

The identity of the Ordovician (Darriwilian) graptolite Fucoides dentatus Brongniart, 1828

Abstract: The biostratigraphically important Middle Ordovician (Darriwilian) biserial graptolite Fucoides dentatus Brongniart, 1828 is redescribed and illustrated from its type material and from additional specimens collected at the type locality – Lévis, Quebec, Canada. It is referred to Levisograptus gen. nov., which includes also the austrodentatus group of early axonophoran graptolites. The species has previously been confused with a younger, mid‐Darriwilian species, now referred to as Eoglyptograptus gerhardi sp. nov., and recognized as the type species of the genus Eoglyptograptus Mitchell. Both species can be differentiated easily by their respective proximal development types and show nonoverlapping biostratigraphical ranges. Levisograptus dentatus (Brongniart) is an important biostratigraphical index species in the early Darriwilian. Eoglyptograptus species are found in the higher Darriwilian and are biogeographically restricted to the Atlantic Faunal Realm.     

First report of craniide brachiopods in the Palaeozoic of Iran (Pseudocrania,Ordovician), and Early to Mid‐Ordovician biogeography of the Craniida

Abstract: Lower Ordovician faunas of Bohemia (Perunica), Baltica and North China include the oldest known representatives of the Order Craniida, but otherwise in Gondwana and associated terranes, the record of craniides is sparse. Pseudocrania insperata sp. nov. from the Lashkarak Formation of the Eastern Alborz Mountains is the first and as yet only record of the occurrence of craniides in the Middle Ordovician (Darriwilian) of Iran and temperate to high latitude peri‐Gondwana. Pseudocrania was known hitherto only from the Middle Ordovician of Baltoscandia and the Chu‐Ili terrane of Kazakhstan.     

Volkhov graptolites from the lower-middle Ordovician boundary beds of the St. Petersburg region,Russia

The lower and lowermost Middle Ordovician strata of the Volkhov Stage exposed in the vicinity of St. Petersburg, Russia, yield a graptolite fauna at several stratigraphic levels, being more often confined to the mud-mound clays than to the normally stratified, Condensed carbonates. Overall taxonomic diversity of the Volkhov graptolites is relatively low and associations include no more than seven species in ail localities. TheTetragraptus, Didymograptus andXiphograptus species found are pandemic and have quite extensive stratigraphie ranges within the mid-Arenig Series. Based on the graptolite zonation, the Volkhov Stage can be correlated to thePseudophyllograptus angustifolius elongatus andDidymograptus (Expansograptus) hirundo graptolite biozones of Baltoscandia and to the upperDidymograptus simulans toDidymograptus (Expansograptus) hirundo biozones of the British graptolite succession. TheIsograptus victoriae lunatus lower zonal boundary probably falls within the graptolite-bearing interval of the lower Volkhov Stage. As the result of indirect biostratigraphic and interpretative lithological correlations, the first appearance ofUndulograptus austrodentatus at the base of the Darriwilian Stage is deemed to be close to the boundary of theMegistaspis simon andM. limbata trilobite biozones and the lowerBaltionodus norrlandicus conodont Biozone of the upper Volkhov Stage. Seven dichograptid species belonging to four genera are described, one of them,Azygograptus volkhovensis n. sp., new.      

The earliest known strophomenoids (Brachiopoda) from early Middle Ordovician rocks Of South China

The upper Daguanshan Formation (middle Expansograptus hirundo graptolite biozone, Dapingian, early Middle Ordovician) of the Shuanghe area, Changning County, southern Sichuan Province, contains three new genera and species of strophomenoids: Ochyromena plana, Shuangheella elongata, and Primotimena globula, which are attributed to the Strophomenidae, Rafinesquinidae and Glyptomenidae respectively. These are the earliest known strophomenoids from the South China palaeoplate, and also the oldest rafinesquinid and glyptomenid brachiopods worldwide. Global review of the superfamily Strophomenoidea of Middle Ordovician age suggests that the first diversity peak at the species level occurred in late Darriwilian (Llanvirn) time, mainly as a result of the rapid diversification of the family Strophomenidae. The first appearance datum (FAD) of strophomenoids and their possible westward dispersal were from North China (latest Floian) and/or South China (early Dapingian), through the Chu‐Ili terrane of Kazakhstan, Iran, and Baltica (early Darriwilian), to Avalonia and Laurentia (late Darriwilian). This points to the existence of early diversification hotspots of the strophomenoid superfamily in the North and South China palaeoplates during the early Middle Ordovician in generally shallow water (corresponding to BA2) environments. The high degree of similarity in the external morphology and ventral interior of the three new genera indicates that the early diversification of strophomenoids began with differentiation of the cardinalia, especially in the configuration of the bilobed cardinal process, a key evolutionary novelty for the strophomenoids.     

Functional morphology and evolution of the cystoid Echinosphaerites

Echinosphaerites , known from the Lower and Middle Ordovician, has branched biserial brachioles. Such features are so far unknown in the Rhombifera. Echinosphaerites had a skeletal meshwork like that of other blastozoan echinoderms, with a fine outer mesh layer and an inner coarse mesh layer. During evolution the number and location of brachioles, including the pattern of brachiole branching, changed by increase in the number of brachioles and increased complexity of the branching pattern. The exothecal pore structures increased in the complexity of patterns of tangential canals. The pattern of skeletal growth in Echinosphaerites is discussed. The Echinosphaeritidae and Caryocystitidae families are closely related and show parallel development.     

Actin gene family evolution and the phylogeny of coleoid cephalopods (Mollusca: Cephalopoda)

Phylogenetic analysis conducted on a 784-bp fragment of 82 actin gene sequences of 44 coleoid cephalopod taxa, along with results obtained from genomic Southern blot analysis, confirmed the presence of at least three distinct actin loci in coleoids. Actin isoforms were characteri zed through phylogenetic analysis of representative cephalopod sequences from each of the three isoforms, along with translated actin cDNA sequences from a diverse array of metazoan taxa downloaded from GenBank. One of the three isoforms found in cephalopods was closely related to actin sequences expressed in the muscular tissues of other molluscs. A second isoform was most similar to cytoplasmic-specific actin amino acid sequences. The muscle type actins of molluscs were found to be distinct from those of arthropods, suggesting at least two independent derivations of muscle actins in the protostome lineage, although statistical support for this conclusion was lacking. Parsimony and maximum-likelihood analyses of two of the isoforms from which >30 orthologous coleoid sequences had been obtained (one of the cytoplasmic actins and the muscle actin) supported the monophyly of several higher-level coleoid taxa. These included the superorders Octopodiformes and Decapodiformes, the order Octopoda, the octopod suborder Incirrata, and the teuthoid suborder Myopsida. The monophyly of several taxonomic groups within the Decapodiformes was not supported, including the orders Teuthoidea and Sepioidea and the teuthoid suborder Oegopsida. Parametric bootstrap analysis conducted on the simulated cytoplasmic actin data set provided statistical support to reject the monophyly of the Sepioidea. Although parametric bootstrap analysis of the muscle actin isoform did not reject sepioid monophyly at the 5% level, the results (rejection at P: = 0.068) were certainly suggestive of sepioid nonmonophyly.     

High‐resolution δ13Corg chemostratigraphy links the Decorah impact structure and Winneshiek Konservat‐Lagerstätte to the Darriwilian (Middle Ordovician) global peak influx of meteorites

The precise age of the Winneshiek Shale, a recently discovered Konservat‐Lagerstätte located in a very unusual depositional setting inside the Decorah impact structure, has remained uncertain in the absence of biostratigraphically highly diagnostic fossils. This chemostratigraphical study, based on δ¹³C_org data from 36 drill core samples through the shale, shows that the age ranges from the upper part of a small unnamed δ¹³C excursion in the Dw1 Stage Slice of the Darriwilian Global Stage to the lower part of the MDICE excursion in Stage Slice Dw2 of the same stage. This Dw1–Dw2 interval has an isotopic age of ~464–467 Ma. The gradational contact between the Winneshiek Shale and the underlying, rapidly deposited, impact breccia indicates minimal time difference between the impact event and the Winneshiek Shale. New age data show that the Decorah impact event was coeval with the early Darriwilian abnormally high influx of micrometeorites and meteorites recorded in sections in Baltoscandia, Russia and China and that the Decorah crater can be included among the unusually large number of meteorite craters formed during Middle and early Late Ordovician time. As is commonly the case in black shale deposits, the partly uniquely preserved Winneshiek Shale crater fauna is impoverished taxonomically and adds numerically relatively little to the conspicuous and much discussed Darriwilian global biodiversification increase.     

Observations on the embryonic and early post-embryonic development ofRossia macrosoma (Mollusca,Cephalopoda)

Summary 1. Egg masses ofRossia macrosoma were collected by bottom trawling in the Western Mediterranean. The ultrastructure of the peculiar outer case of the egg is described.2. The embryonic development was followed at a constant temperature (15° C); a graphic presentation of developmental rates throughout the embryogenesis is given. Problems concerning the length and the course of embryonic development are discussed.3. Embryogenesis is briefly described and figured. A few remarks are made on the organogenesis of the arm-funnel complex, the alimentary canal, the shell sac and the vena cava. Emphasis is placed on the dynamics of embryogenesis.4. The mechanism of yolk absorption during later embryonic development includes collecting and transfer (to the hepatopancreas) of nutritive material by means of the yolk vessels (former yolk sinus).5. For hatching, the secretory action of the Hoyle organ is followed by breaking the outer egg case, which is possibly facilitated by the muscular mantle spine.6. Hatched animals were raised in the laboratory for several months, under varying conditions. The observations on growth and behaviour are discussed, earlier observations and field data being considered.

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Observations sur le développement embryonnaire et postembryonnaire deRossia macrosoma (Mollusca, Cephalopoda)

Extrait Des pontes deRossia macrosoma ont été récoltées au large de la côte catalane (Méditerranée occidentale). L'ultrastructure de la «coque» typique de l'oeuf de cette espèce est décrite. Le développement embryonnaire a été suivi à une température constante de 15° C. L'embryogenèse est brièvement décrite, des stades importants sont représentés par des Figures. Quelques processus de l'organogenèse sont décrits et discutés en détail, tout en soulignant la dynamique de l'embryogenèse. L'importance du réseau de lacunes sanguines entourant le sac vitellin interne, pour le transport de matériel nutritif vers l'intérieur de l'hépatopancréas, est mise en évidence. L'éclosion se fait en principe comme chez les autres décapodes (action de «l'organe de Hoyle»), mais la rupture de la «coque» externe résulte, en plus, de la pression appliquée par l'animal; le «rostre» musculaire pourrait y jouer un rôle important. Des animaux éclos ont été élevés en laboratoire, pendant plusieurs mois, sous différents régimes de température et de luminosité. Les observations sur la croissance et le comportement sont comparées avec d'autres résultats obtenus en laboratoire et avec des données indiquées dans la littérature.

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Abbreviations used in figures a arm - bh branchial heart - bd blastodisc - co coecum - e eye - fi fin - fo funnel organ - fp funnel pocket - fr funnel retractor - fu funnel - g gill - go gonad - h systematic heart - ho Holye's organ - hp hepatopancreas - i intestine - k kidney - kp kidney papilla - m mantle - ms mantle septum - mo mouth - o olfactory tuberculum - oe oesophagus - pc pericardium - pl primary lid - s sucker - sc statocyst - se shell epithelium - sg salivary gland - sh shell sac - st stomodaeum - stg stellate ganglion - t tentacle - vc vena cava - y yolk - ye yolk envelope     

On the origin of bactritoids (Cephalopoda)

There is a high probability that bactritoids represent a paraphylum or polyphylum. The initial chambers or protoconchs of the Early-Middle DevonianBactrites Sandberger,Devonobactrites Shimansky, andLobobactrites Schindewolf are elongated spheres with a diameter of 0.3–1.0 mm. The initial chambers are larger in diameter than the slender, smooth shaft located adorally to the initial chamber. Similar apices occur in a number of Late Silurian sphaerorthoceridans with central siphuncles. Sphaerorthoceridans with a bactritoid-like apex and an eccentric siphuncle are known from the Early Devonian. The earliest questionableBactrites occurs in the Pragian (middle Early Devonian). By Emsian time bactritoids are common elements of cephalopod faunas.Bactrites-like orthocones of the Middle Ordovician and Late Silurian are homeomorphs with clearly different early growth stages. Thus, the time interval between the first appearance ofBactrites and the origin of ammonoids can be narrowed down to the Pragian to Early Emsian. The placement of the siphuncle in a ventral marginal position has been used as one of the critical morphologic features in defining the bactritoids. However, the displacement of the siphuncle from subcentral or eccentric positions toward the conch margin occurred at least three times during the Ordovician — Early Devonian evolution of the Orthocerida. Thus, there is a high probability that a marginal shift of the orthocerid siphuncle occurred in post-Emsian times, too.     

The oldest bryozoans of Baltoscandia from the lowermost Floian (Ordovician) of north-western Russia: two new rare,small and simple species of Revalotrypidae

We describe two new species of esthonioporate bryozoans, Revalotrypa inopinata sp. nov. and R. yugaensis sp. nov., from the Lower Ordovician, south-east of Lake Ladoga, north-western Russia. The colonies of the two species are very small and were extracted from limestone nodules found in the lower part of glauconitic sandstone in the Joa Member (lowermost Floian). Revalotrypa inopinata sp. nov. and R. yugaensis sp. nov. are the oldest known bryozoans from Baltoscandia. Study of the two species includes scanning electron microscopy (SEM) imaging and X-ray microtomography (micro-CT). The palaeogeographic distribution of the oldest known bryozoans from the Lower Ordovician and the advantages and disadvantages of micro-CT for the study of Palaeozoic bryozoans are discussed. The authorship of Revalotrypidae, which has been inconsistently allocated in the literature, is discussed.     

Early Ordovician reefs in South China (Chenjiahe section, Hubei Province): deciphering the early evolution of skeletal-dominated reefs

The Lower Ordovician (late Tremadocian–early Floian) Fenhsiang and the overlying Hunghuayuan Formations at the Chenjiahe section in the Three Gorges area of Hubei Province, South China, include four types of reef: microbe-dominated (lithistid sponge–stromatolite and lithistid sponge–calcimicrobial) reefs, and skeletal-dominated (lithistid sponge–bryozoan and bryozoan–pelmatozoan) reefs. The microbe-dominated reefs are characterized by the dominance of microbial sediments that encrusted and bound the surfaces of sponges to reinforce the reef frameworks. In contrast, the skeletal-dominated reefs are distinguished by bryozoans that encrusted frame-building sponges and pelmatozoans, and that grew downward to fill the open spaces available within the frameworks. A series of these reefs shows a temporal succession in reef type, with a decline in the lithistid sponge–stromatolite reefs and an increase in the lithistid sponges and receptaculitids within the lithistid sponge–calcimicrobial reefs in the Hunghuayuan Formation; the lithistid sponge–bryozoan reefs are common in both the Fenhsiang and Hunghuayuan Formations. These features of the Chenjiahe reefs are in marked contrast to other coeval reefs on the Yangtze Platform and elsewhere. Skeletal-dominated reefs first developed in the Three Gorges and adjacent areas, located on the central part of the platform. Likewise, lithistid sponges and receptaculitids first developed in the Three Gorges area and then expanded their range. In contrast, stromatolites declined over time, but remained abundant on a marginal part of the platform. The spatial–temporal distributions of these reefs on the Yangtze Platform reflect the initiation of the Great Ordovician Biodiversification Event and its consequences, although influenced by local environmental conditions. The Three Gorges area was a center for the development of skeletal-dominated reefs, which were established earlier here than elsewhere in the world. These reef types and their spatial–temporal successions provide invaluable clues to the earliest evolution of skeletal-dominated reefs and their ensuing development during the Middle–Late Ordovician.     

Phylogeny and early evolution of the Cynipoidea (Hymenoptera)

Based on several structural and biological characteristics, the Cynipoidea can be divided into two groups, 'macrocynipoids' and 'microcynipoids'. The macrocynipoids (i.e. the family Liopteridae and the genera Austrocynips, Eileenella, Heteribalia and Ibalia ) are generally large insects that parasitize wood- or cone-boring insect larvae. The microcynipoids are smaller insects that are either phytophagous gall inducers and inquilines (Cynipidae) or parasitoids of larvae of Hymenoptera, Neuroptera or Diptera (Figitidae sensu lato , including the families Eucoilidae, Charipidae and Anacharitidae). The phylogenetic relationships among genera of macrocynipoids and between these and a sample of four genera representing the Figitidae and Cynipidae were examined by parsimony analysis of 110 external morphological characters of adults. Within the macrocynipoids, three monophyletic lineages emerged, classified here as different families: the Austrocynipidae, with a single species, Austrocynips mirabilis , the only cynipoid with a true pterostigma; the Ibaliidae, including the genera Eileenella, Ibalia and Heteribalia ; and the Liopteridae, comprising the remaining genera of macrocynipoids. The analysis further supported the monophyly of the microcynipoids and indicated that the macrocynipoids form a paraphyletic group relative to the microcynipoids, with the shortest tree suggesting the relationship (Austrocynipidae, (Ibaliidae, (Liopteridae, microcynipoids))). These results imply that cynipoids were originally parasitoids of wood-boring insect larvae and that the other modes of life evolved secondarily within the group.     

Stages in the ontogeny and a model of the evolution of bivalves (Mollusca)

Comparison of the ontogenies of bivalves of different habitats and systematic position provide two main conclusions. The first is that bivalves stick to a certain basic program of ontogeny which can be divided into six phases. The first three phases are parallel to those of some other invertebrates, the 4th phase is parallel to that of conchiferan molluscs, with, the 5th phase, bivalve characters are acquired, and in the 6th phase, genus-specific adult organization is reached. The second is that within this basic developmental programm specific differences occur. The ontogeny ofTeredora is similar to that of many marine bivalves and is characterized by a strong plasticity. The adult, in contrast, is highly specialized for living within wood. InAnodonta, the opposite is true and embryonic development even includes a parasitic stage, while the adult is of a more general organization. Due to brood-protection and embryonic nursing, the whole organogenesis ofSphaerium becomes directed towards adult organization. Ontogenetic development of a general body plan of the Bivalvia provides a model for the evolution of the first bivalves (protobranchs excluded) from univalves by a one-step alteration connected with shell mineralization and loss of the buccal mass affecting the embryo near end of embryogenesis.