A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors

Aplacophorans have long been argued to be basal molluscs. We present a molecular phylogeny, including the aplacophorans Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata), which recovered instead the clade Aculifera (Aplacophora + Polyplacophora). Our relaxed Bayesian molecular clock estimates an Early Ordovician appearance of the aculiferan crown group consistent with the presence of chiton-like molluscs with seven or eight dorsal shell plates by the Late Cambrian (approx. 501-490 Ma). Molecular, embryological and palaeontological data indicate that aplacophorans, as well as chitons, evolved from a paraphyletic assemblage of chiton-like ancestors. The recovery of cephalopods as a sister group to aculiferans suggests that the plesiomorphic condition in molluscs might be a morphology similar to that found in monoplacophorans.     

Deep molluscan phylogeny: synthesis of palaeontological and neontological data

The position of the earliest-derived living molluscs, the Polyplacophora (chitons) and shell-less vermiform Aplacophora, remains highly contentious despite many morphological, developmental and molecular studies of extant organisms. These two groups are thought to represent either a basal molluscan grade or a clade (Aculifera) sister to the 'higher' molluscs (Conchifera). These incompatible hypotheses result in very different predictions about the earliest molluscs. A new cladistic analysis incorporating both Palaeozoic and extant molluscs is presented here. Our results support the monophyly of Aculifera and suggest that extant aplacophorans and polyplacophorans both derive from a disparate group of multivalved molluscs in two major clades. Reanalysis of the critical Ordovician taxon 'Helminthochiton' thraivensis shows that this animal lacks a true foot despite bearing polyplacophoran-like valves. Its position within our phylogenetic reconstruction indicates that many fossil 'polyplacophorans' in the order Palaeoloricata are likely to represent footless stem-group aplacophorans. 'H.' thraivensis and similar forms such as Acaenoplax may be morphological stepping stones between chitons and the shell-less aplacophorans. Our results imply that crown-group molluscan synapomorphies include serial repetition, the presence of a foot, a mineralized scleritome and a creeping rather than worm-like mode of life.     

Silurian craniide brachiopods from Gotland

Seven craniide brachiopod genera are described from the Silurian (Wenlock–Ludlow) of Gotland, including one new genus and five new species. The new genus and species Thulecrania septicostata is unique among Silurian craniides as it possesses solid spines. The new species Lepidocrania multilamellosa is the first Silurian record of this poorly known Permian genus. The problematic North American Propatella Grubbs, 1939 , was originally described as a gastropod, but the new species Propatella palmaria from Gotland shows that it is a craniide with sutured hollow spines of a type not previously recorded from Silurian craniides. The dorsal valves of the new species Valdiviathyris? bicornis are remarkably similar to those of the type species and represent the first possible Palaeozoic record of this poorly known extant craniide. This first systematic study of craniide brachiopods from the Silurian of Gotland shows that the diversity is relatively high as compared to other known Silurian craniide faunas, but a more thorough comparison is not possible due to the lack of data from most parts of the world. The new data from Gotland support the view that the craniides were not affected by the end‐Ordovician extinction.     

Facies and fauna of the Pennsylvanian Buckhorn Asphalt Quarry deposit: a review and new data on an important Palaeozoic fossil <Emphasis Type="Italic">Lagerstätte</Emphasis> with aragonite preservation

The Pennsylvanian Buckhorn Asphalt Quarry contains the best-preserved Palaeozoic mollusc fauna in the world. Early impregnation of mixed siliciclastic–carbonate rocks (mudstones, pack to grainstones, shell beds, and conglomerates) with hydrocarbons prevented aragonite destruction (“Impregnation Fossil Lagerstätte”). The exceptional preservation comprises shell microstructures, microornaments and early ontogenetic shells. Most gastropods had planktotrophic larval development indicating a high primary production although the remains of phytoplankton are very rare in this and other Late Palaeozoic deposits. Deposition occurred close to a shallow-water coastal area. Mass flow processes (density currents) triggered by storms were involved in the transport mechanisms of some units. Shells of benthic molluscs yield the most diverse known Palaeozoic microboring assemblage, indicating at least partly euphotic conditions. The invertebrate fauna comprises about 160 species and is dominated by molluscs, which is unusual for a Palaeozoic deposit, suggesting that aragonite dissolution produces a major bias in the fossil record. However, most mollusc genera in the Buckhorn deposit are also known from other Pennsylvanian occurrences as recrystallised shells. This shows that preservation bias via preferential aragonite dissolution may be overestimated.     

How the mollusc got its scales: convergent evolution of the molluscan scleritome

Radiation of dramatically disparate forms among the phylum Mollusca remains a key question in metazoan evolution, and requires careful evaluation of homology of hard parts throughout the deep fossil record. Enigmatic early Cambrian taxa such as Halkieria and Wiwaxia (in the clade Halwaxiida) have been proposed to represent stem‐group aculiferan molluscs (Caudofoveata + Solenogastres + Polyplacophora), as complex scleritomes were considered to be unique to aculiferans among extant molluscs. The ‘scaly‐foot gastropod’ (Neomphalina: Peltospiridae) from hydrothermal vents of the Indian Ocean, however, also carries dermal sclerites and thus challenges this inferred homology. Despite superficial similarities to various mollusc sclerites, the scaly‐foot gastropod sclerites are secreted in layers covering outpockets of epithelium and are largely proteinaceous, while chiton (Polyplacophora: Chitonida) sclerites are secreted to fill an invaginated cuticular chamber and are largely calcareous. Marked differences in the underlying epithelium of the scaly‐foot gastropod sclerites and operculum suggest that the sclerites do not originate from multiplication of the operculum. This convergence in different classes highlights the ability of molluscs to adapt mineralized dermal structures, as supported by the extensive early fossil record of molluscs with scleritomes. Sclerites of halwaxiids are morphologically variable, undermining the assumed affinity of specific taxa with chitons, or the larger putative clade Aculifera. Comparisons with independently derived similar structures in living molluscs are essential for determining homology among fossils and their position with respect to the enigmatic evolution of molluscan shell forms in deep time. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 949–954.     

A nearshore–offshore trend in acritarch distribution from the Early–Middle Ordovician of the Yangtze Platform, South China

The stratigraphical interval of the late Early Ordovician Didymograptus deflexus and the early Middle Ordovician Azygograptus suecicus graptolite Biozones was investigated from seven sections from the upper Yangtze Platform, southern China. These are located on different parts of the platform, between the nearshore environments of the Kunming area, Yunnan Province, and the offshore carbonate shelf of the Yichang area, Hubei Province. The assemblages recovered from the different parts of the platform vary both in terms of diversity and composition. The nearshore environments show low diversity assemblages with about 10 acritarch species, whereas the offshore shelf environments reflect higher diversities with about 40 species. The composition of the assemblages also changes from simple morphologies (micrhystrids, leiosphaerids, fusiform acritarchs) in nearshore environments to specimens with longer and more complexly branched processes on the shelf. The polygonomorph acritarchs are common over all the upper Yantze Platform, while the acanthomorph genera Baltisphaeridium and Peteinosphaeridium are the most abundant taxa on the offshore carbonate shelf area. Phylogenetic or sea-level changes are probably not responsible for the compositional and diversity changes that occurred during the investigated interval. This study confirms previous interpretations that poorly diversified Palaeozoic acritarch assemblages occur in neritic environments and more complex, highly diversified assemblages are found on the shelf.     

A novel respiratory architecture in the Silurian mollusc Acaenoplax

Extant aplacophorans, a group of shell‐less vermiform molluscs, respire through appendages within or projecting from a posterior cavity. Respiratory structures differ between the subclasses Caudofoveata (ctenidia within the cavity) and Solenogastres (folds of the mantle itself). Acaenoplax hayae, a Silurian vermiform mollusc from the Herefordshire Lagerstätte, England, exhibits characteristics of both these groups. While recent work places it within the crown group Aplacophora, near the caudofoveates, initial observations suggested that its respiratory structures were closer to those of the solenogastres. Here, we present new reconstructions of the posterior of Acaenoplax prepared with the aim of resolving features obscured when prior studies were undertaken. These reconstructions detail a novel posterior architecture, not closely comparable to that of either extant aplacophoran group, in which respiratory projections arise from a membrane that partly encloses a central posterior cavity. The posterior membrane is flanked by small spherical projections; both membrane and spherical projections are apparently unique within the Aplacophora. The existence of this previously undocumented respiratory system underlines the diversity of the aplacophoran clade during the Palaeozoic.     

Earliest ontogeny of Early Palaeozoic Craniiformea: compelling evidence for lecithotrophy

Popov, L.E., Bassett, M.G. & Holmer, L.E. 2012: Earliest ontogeny of Early Palaeozoic Craniiformea: compelling evidence for lecithotrophy. Lethaia, Vol. 45, pp. 566–573. The early ontogeny of Palaeozoic Craniiformea (Brachiopoda) remains controversial, with conflicting reports of evidence indicating lecithotrophic versus planktotrophic larval stages. Further compelling evidence for lecithotrophy in Palaeozoic craniiforms is described here. Newly obtained, well‐preserved Silurian specimens of craniiforms, including Craniops (Craniopsida), and Lepidocrania? and Orthisocrania (Craniida) from Gotland and the St. Petersburg region, form the basis for this study. The new material demonstrates that the characters of shell structure and shell formation provide evidence of early differentiation of an adult dorsal mantle, and the presence of a distinctive primary layer with a characteristic lath‐like pattern indicates that these craniiforms underwent a lecithotrophic larval stage, more or less identical to that of living. □Novocrania. Brachiopoda, Craniiformea, ontogeny, phylogeny, Early Palaeozoic.     

Shell Structure And Inferred Growth, Functions And Affinities Of The Sclerites Of The Problematic Micrina

The stratiform laminae of Micrina sclerites originally consisted of rheomorphic successions of monolayers of micrometric–sized, apatitic tablets, presumably interleaved with chitin and glycosaminoglycans (GAGs). Paired laminae enclose slot–like chambers swelling into lobes distally that originally contained GAGs and deposits of spherulitic and prismatic apatite. The laminae are pervaded by apatitic tubes, apparently secreted by microvillous setoblasts and containing, at the surface, chitinous setae. Internal markings suggest that the triangular (sellate) sclerite supported a pair of muscles and the planospiral (mitral) sclerite, a medial muscle and gonadal sacs flanked by a pair of crescentic muscle bases. Both sclerites were secreted by a mantle with a circumferential fold. The sellate and mitral sclerites are homologized with the anterior and posterior shells of Halkieria and could have become the dorsal and ventral valves of the ancestral brachiopod by a sequence of transformations. These include: the folding of the halkieriid body axis; accelerated mixoperipheral growth of the anterior (dorsal) shell to enclose, with the posterior (ventral) shell, a mantle cavity lined with modified ciliated epithelium of the foot; reduction of sclerite–secreting epithelium to the locus of the brachiopod pedicle epithelium; and the anterior (dorsal) spread of gonadal lamellae.     

Ordovician and Silurian coralline red algae

Solenopora gotlandica Rothpletz, hitherto placed in the Solenoporaceae Pia, is locally common in the mid-Silurian of Gotland and Wales. It possesses small cells, radial monomerous thallus organization, trichocytes, and sporangia arranged in irregular sori. Solenopora gotlandica is not congeneric with S. spongioides , the type species of Solenopora. It closely resembles the extant coralline red alga Sporolithon Heydrich. S. gotlandica is here transferred to Craticula gen. nov., and Craticula is placed in the Craticulaceae fam. nov., which morphologically closely resembles the Mesozoic-Cenozoic family Sporolithaceae. The Craticulaceae is placed in the extant rhodophyte order Corallinales. Middle-Late Ordovician Petrophyton kiaeri Høeg resembles Late Ordovician Solenopora richmondense Blackwell et al. , which has sporangia in calcified compartments arranged in sori. These fossils may be craticulaceans. The apparent gap in the fossil record between Silurian Craticula and Cretaceous Sporolithon favours separation of these genera at family level. However, future work may demonstrate that the Craticulaceae is a junior synonym of the Sporolithaceae. Recognition of the Craticulaceae extends the earliest record of the Corallinales from Cretaceous to Silurian, and possibly Ordovician.     

Llandovery sporomorphs and graptolites from the Manbo Formation,the Mojiang County,Yunnan, China

Llandovery sporomorphs and graptolites have been recovered from the Manbo Formation, Mojiang area, western Yunnan, China, which belonged to the Indo-China Palaeoplate in the Palaeozoic. The graptolite fauna, including 8 genera and 12 species, is considered Mid Telychian, Llandovery in geological age, confirming the existence of Llandovery rocks in this region. The Silurian stratigraphical sequence of this area is reconsidered as in the ascending order: Manbo Formation (Llandovery–early Wenlock), Shuiqing Formation (late Wenlock–Ludlow) and unnamed formation (Pridoli?). The sporomorph assemblage from the Manbo Formation includes seven species in five genera. The dominant members of this assemblage are Tetrahedraletes medinensis and Laevolancis chibrikovae. Based on the low diversity and low abundance, the geological age of this assemblage is considered to be approximately the Telychian, Llandovery. The parent plants of sporomorphs probably inhabited the landmass near Mojiang area, these sporomorph-producing plants probably included bryophyte-like land plants and primitive vascular land plants. The sporomorph evidence shows that the South China and Indo-China palaeoplates may have been in close proximity (maybe with some continental bridges linking them) to each other at least in the Llandovery. They were also closely related with Gondwanaland in the Llandovery.     

SCAPHOPODIZATION IN PALAEOZOIC MOLLUSCS

Abstract:  The relationship of a variety of problematic Early Palaeozoic fossils to the true scaphopods (Late Palaeozoic–Recent) is clarified by the recognition that a trend towards the development of a tubular shell is a recurrent theme within Palaeozoic benthic molluscs. This trend is here termed scaphopodization and can be recognized already in the Cambrian in helcionelloids such as Yochelcionella and Eotebenna . The problematic Early Palaeozoic tubular fossils Janospira , Jinonicella and Rhytiodentalium are interpreted as pseudo-scaphopod derivatives of the apparently exogastric ribeirioid rostroconchs. The direction of coiling of the protoconch indicates that they are not closely related to the true scaphopods, which were derived from endogastric concocardioidean rostroconchs in the Devonian or Carboniferous. Scaphopods show a comparable underlying morphological blueprint to these conocardioideans, but they represent a distinct stream-lining to the infaunal habitat.     

Taxonomy and phylogeny of cementing Triassic bivalves (families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae)

Based on new material from the Upper Triassic Nayband Formation of east-central Iran and on type material from the Alpine Triassic, the taxonomy of the cementing bivalve families Prospondylidae, Plicatulidae, Dimyidae and Ostreidae is examined and their phylogenetic relations are discussed. The Prospondylidae are characterized by the presence of an early pectiniform stage in their Palaeozoic genera which disappeared in most later forms due to ontogenetic pre-displacement of cementation. The Plicatulidae probably evolved from an ancestor within the Prospondylidae by the formation of strong crura, which allowed the reduction of the lateral part of the ligament. Their hinge was later modified by shifting resilifer and crura in a ventral direction and by forming a secondary ligament dorsally. The emended genera Eoplicatula Pseudoplacunopsis represent different early stages of this development. The species Eoplicatula parvadehensis sp. nov. and Pseudoplacunopsis asymmetrica sp. nov. from the Nayband Formation are described. The shell of some early Ostreidae is characterized by the lack of structural chambers and by the presence of an originally aragonitic inner shell layer. For such forms, the new genus Umbrostrea is proposed, and the new species Umbrostrea emamii Umbrostrea iranica are described. The available data on shell microstructure as well as most conchological characters do not support a close relationship between Ostreidae, Plicatulidae and Dimyidae.     

Mississippian crinoid biodiversity,biogeography and macroevolution

Abstract: The biodiversity and biogeography of 217 genera of Mississippian crinoids from North America and the British Isles shed light on the macroevolutionary turnover between the Middle Palaeozoic and Late Palaeozoic Crinoid Evolutionary Faunas. This turnover resulted from steady differential extinction among clades during the middle Mississippian after crinoids reached their Phanerozoic peak of generic richness during the early Mississippian. This peak richness was primarily a function of Mississippian originations rather than Devonian–holdover taxa. North America had 100 per cent higher generic richness than the British Isles, but rarefaction analysis adjusts the difference to only 37 per cent higher. Rarefaction demonstrated that North America had increased biodiversity, compared to the British Isles, almost entirely among monobathrid camerates, disparids and primitive cladids. In contrast, diplobathrid camerates, advanced cladids and flexibles had the same generic biodiversity between regions, when compared using rarefaction. The early Mississippian radiation resulted from two primary causes: (1) the expansion of Tournaisian carbonate ramps following the Frasnian mass extinction of reef faunas and (2) the predatory release in the Tournaisian following the end‐Famennian Hangenberg extinction of durophagous fishes. A majority of crinoid genera from the British Isles are cosmopolitan. When combined with rarefaction analysis and evidence for more first occurrences in North America, this suggests higher origination rates in North America, especially when carbonate ramps were widespread. With the gradual reduction in the area of carbonate ramps from the early to late Mississippian, in conjunction with the radiation of new durophagous fishes, camerate crinoids in particular experienced continuous background extinction, without replacement, beginning during the earliest Viséan (late Osagean). By middle Viséan time (late Meramecian) advanced cladids were dominant in all settings. This resulted in the transition from the Middle Palaeozoic to the Late Palaeozoic Crinoid Macroevolutionary Fauna.     

A cryptic record of Burgess Shale‐type diversity from the early Cambrian of Baltica

Exceptionally preserved ‘Burgess Shale‐type’ fossil assemblages from the Cambrian of Laurentia, South China and Australia record a diverse array of non‐biomineralizing organisms. During this time, the palaeocontinent Baltica was geographically isolated from these regions, and is conspicuously lacking in terms of comparable accessible early Cambrian Lagerstätten. Here we report a diverse assemblage of small carbonaceous fossils (SCFs) from the early Cambrian (Stage 4) File Haidar Formation of southeast Sweden and surrounding areas of the Baltoscandian Basin, including exceptionally preserved remains of Burgess Shale‐type metazoans and other organisms. Recovered SCFs include taxonomically resolvable ecdysozoan elements (priapulid and palaeoscolecid worms), lophotrochozoan elements (annelid chaetae and wiwaxiid sclerites), as well as ‘protoconodonts’, denticulate feeding structures, and a background of filamentous and spheroidal microbes. The annelids, wiwaxiids and priapulids are the first recorded from the Cambrian of Baltica. The File Haidar SCF assemblage is broadly comparable to those recovered from Cambrian basins in Laurentia and South China, though differences at lower taxonomic levels point to possible environmental or palaeogeographical controls on taxon ranges. These data reveal a fundamentally expanded picture of early Cambrian diversity on Baltica, and provide key insights into high‐latitude Cambrian faunas and patterns of SCF preservation. We establish three new taxa based on large populations of distinctive SCFs: Baltiscalida njorda gen. et sp. nov. (a priapulid), Baltichaeta jormunganda gen. et sp. nov. (an annelid) and Baltinema rana gen. et sp. nov. (a filamentous problematicum).     

Latitudinal diversity gradients for brachiopod genera during late Palaeozoic time: links between climate, biogeography and evolutionary rates

Aim The latitudinal diversity gradient, in which taxonomic richness is greatest at low latitudes and declines towards the poles, is a pervasive feature of the biota through geological time. This study utilizes fossil data to examine how the latitudinal diversity gradient and associated spatial patterns covaried through the major climate shifts at the onset and end of the late Palaeozoic ice age. Location Data were acquired from fossil localities from around the world. Methods Latitudinal patterns of diversity, mean geographical range size and macroevolutionary rates were constructed from a literature‐derived data base of occurrences of fossil brachiopod genera in space and time. The literature search resulted in a total of 18,596 occurrences for 991 genera from 2320 localities. Results Climate changes associated with the onset of the late Palaeozoic ice age (c. 327 Ma) altered the biogeographical structure of the brachiopod fauna by the preferential elimination of narrowly distributed, largely tropical genera when glaciation began. Because the oceans were left populated primarily with widespread genera, the slope of the diversity gradient became gentle at this time, and the gradient of average latitudinal range size weakened. In addition, because narrowly distributed genera had intrinsically high rates of origination and extinction, the gradients of both of these macroevolutionary rates were also reduced. These patterns were reversed when the ice age climate abated in early Permian time (c. 290 Ma): narrowly distributed genera rediversified at low latitudes, restoring steep gradients of diversity, average latitudinal range size and macroevolutionary rates. Main conclusions During late Palaeozoic time, these latitudinal gradients for brachiopods may have been linked by the increased magnitude of seasonality during the late Palaeozoic ice age. Pronounced seasonality would have prevented the existence of genera with narrow latitudinal ranges. These results for the late Palaeozoic ice age suggest a climatic basis for the present‐day latitudinal diversity gradient.     

The tommotiid Kelanella and associated fauna from the early Cambrian of southern Montagne Noire (France): implications for camenellan phylogeny

A carbonate bed of the Pardailhan Formation, early Cambrian, southern Montagne Noire (southern France), provided microfossils such as Hyolithellus sp., Torellella cf. mutila and Torellella sp. along with numerous disarticulated pieces of composite skeletons such as valves of the brachiopod Eoobolus priscus and of the bradoriid Monceretia erisylvia, and chancelloriid sclerites (Chancelloria sp.). The assemblage also furnished a rich set of sclerites from the tommotiid Kelanella altaica. Five morphological variations of the latter have been identified. The presence of concentric ribs formed by distal inflation of selected shell laminae in Kelanella supports its assignment to the camenellans. More particularly, the presence of transverse structures within the internal cavity (septa) of Kelanella suggests a close relationship with the Lapworthellidae. However, the latter differ from Kelanella by the continuous morphological variation along their scleritome which is also composed of simple conical elements with uniform ornamentation. Several forms of Kelanella are similar to mitral and sellate sclerites of Camenella, whereas some other forms are comparable to Kennardia. The new material suggests that Kelanella occupies a transitional position between Lapworthellidae and the grouping of Tommotiidae and Kennardiidae. Such a phylogenetic position also implies that the number of sclerite morphotypes tends to decrease within the camenellan scleritome during evolution.     

Taphonomic windows and molluscan preservation

Recent studies on silicified fossil biotas have suggested that substantial skewing of the molluscan record resulted from early aragonite dissolution in mid-outer carbonate ramp settings. If those rare skeletal lagerstätten are representative, then the quality and completeness of the molluscan record are thrown into doubt. Yet database studies suggest that the bivalve fossil record is actually relatively complete. If so, then biodiversity must be captured by other processes that preserved shells vulnerable to early dissolution, and which operated on a relatively high frequency, i.e., less than the species duration for bivalves.Storm beds, shell plasters and submarine hardgrounds are identified as fossil deposits that can preserve the labile aragonitic component of the fauna and thus represent potential taphonomic windows. Many storm event beds include rich accumulations of shelly benthos. Differences between storm bed faunas and those of the background facies could reflect transportation effects. However, some storm bed assemblages are rich in originally aragonitic infaunal bivalves that are not represented in background facies or more proximal shelf equivalents, and here rapid burial and removal of organic matter by winnowing may be the keys to aragonite shell preservation. Despite Palaeozoic to Cenozoic changes in the thickness and frequency of shell beds that reflect the predominant bioclast producers, shallow infaunas are commonly concentrated together with epifauna in such deposits.Some low energy, organic-rich mud-dominated settings are associated with preservation of aragonitic molluscs. Infaunal bivalves are a prominent component of shell plasters or pavements in such settings, linked to episodic bottom water anoxia. Decaying algal blooms drew the redox boundary up above the sediment–water interface, and brought populations of infaunal bivalves to the surface where they died. Isolated from the oxic taphonomically active zone, the shells were not dissolved and were buried as thin shell layers. In similar settings, aragonitic shells were preserved as moulds through early pyritisation, or even through preservation of original shell aragonite.In oxic environments, bioturbational reworking of surface sediment destroyed moulds of aragonitic shells after early dissolution. In some hardgrounds, these delicate moulds were preserved due to synsedimentary cementation, probably using carbonate released by aragonite dissolution. The examples included here come from both intervals of “calcite” and “aragonite” seas, and it is not possible to assess whether the saturation state (with respect to aragonite) of the ambient sea water played a role in the selective removal of aragonitic shells.While taphonomic windows may have captured the diversity of individual groups, it is clear from quantitative data involving skeletal lagerstätten that the scale of loss from early aragonite dissolution has drastically altered the trophic composition of some fossil assemblages commonly used as the basis for reconstructions of past communities.