Early Jurassic anoxia triggered the evolution of the oldest holoplanktonic gastropod Coelodiscus minutus by means of heterochrony

The tiny gastropod Coelodiscus minutus is superabundant in concretions of the Early Jurassic Posidonia Shale of South Germany which were formed under anoxic or extremely dysoxic conditions. Previous suggestions that C. minutus was a holoplanktonic organism are corroborated based on new evidence from exceptionally well-preserved specimens. The measurements of shell thickness show that the shell of Coelodiscus is very thin (mean 11 mu m). In contrast to previous suggestions, the shell of Coelodiscus was not formed in three ontogenetic phases (embryonic, larval and adult shell) but in two phases comprising an embryonic and a secondary shell, the latter forming during an extended larval phase. Hostile conditions on the sea floor, absence or extreme scarcity of epibenthic animals as well as the small size also argue against a benthic life style of this gastropod. Coelodiscus minutus is the oldest known holoplanktonic gastropod. We speculate that Coelodiscus evolved during the Early Jurassic from a benthic precursor, which had a planktotrophic larval development. Probably under the influence of increasing frequency of dysoxic episodes along with hostile benthic conditions, the larval phase was extended neotenously and eventually, a holoplanktonic species evolved. During the Early Toarcian anoxic event, C. minutus was highly abundant in the plankton and dead shells rained down to the anoxic or dysoxic sea bottom. These thin and fragile shells formed an ooze similar to the pteropod ooze in the modern deep sea. The shells were preserved due to the absence or low level of deposit feeding and bioturbation as well as the formation of early diagenetic concretions

Latest Triassic (Sevatian–Rhaetian) reef carbonates from the Northern Calcareous Alps (Austria), their mollusc dwellers, and their fate at the end-Triassic extinction event

A reef-associated mollusc fauna (gastropods and bivalves) and its facies context are described from latest Triassic (Sevatian–Rhaetian) reef carbonates of Austria (Rötelwand reef at Gaissau and Gosaukamm near Hallstatt). The studied carbonates from the Rötelwand reef consist of mollusc-rich rudstones, partly boundstones, which contain branched corals (Cycliphyllia and Retiophylia, Pinacophyllum), whereas coralline sponges are absent. The rich foraminiferid fauna that is associated with the reef builders consists of 11 genera; eight of these genera became extinct until the end of the Rhaetian. Associated with small patch reefs was a rich mollusc fauna with 19 gastropod species and 8 epifaunal bivalve species. The gastropod fauna is dominated by Microschiza rhaetica, Trochotoma praecursor, and the large growing Purpuroidea moosleitneri. Six gastropod species are new to science: Angulomphalus senowbarii sp. nov., Stuorella zapfei sp. nov., Hologyra callosa sp. nov., Microschiza rhaetica sp. nov., Angularia corallina sp. nov., and Purpuroidea moosleitneri sp. nov. Four Triassic gastropod species are placed in other genera (new combinations): Tylotrochus diversicostatus Wolff, 1967 and Eucycloscala epitoniformis Nützel and Senowbari-Daryan, 1999 are placed in Sadkia, Praelittorina sepkoskii Nützel and Erwin, 2004 in Microschiza, and Purpuroidea? minioi Leonardi, 1935 in Angularia Koken, 1892. Reversal of precedence is proposed for Angularia Koken, 1892 (Gastropoda) and Angularia Busk, 1881 (Bryozoa) under ICZN Art. 23.9. Although reefs suffered a catastrophic decline at the end of the Triassic, most of the studied reef-associated bivalve and gastropod genera survived into the Jurassic, indicating a considerable ecological plasticity of these groups. Only 12 out of 47 reef-associated mollusc genera became extinct (25.5%). This observation is at variance with earlier suggestions that taxa that were associated with reefs and carbonate substrata had a significantly higher extinction risk than level-bottom dwellers. However, extinction at the species level appears more severe; only three bivalve species but no gastropod species recorded in this fauna have records from the Jurassic

Gastropoden aus dem oberen Pliensbachium (Amaltheenton-Formation) NE Bayerns (Umgebung von Stauf/Dörlbach/Altdorf) (Franken, Süddeutschland)

Aus dem nordöstlichen Bayern (Oberpfalz, Umgebung von Stauf, Dörlbach, Altdorf) wird eine Gastropodenfauna des oberen Pliensbachiums beschrieben. Sie umfasst 42 Arten, von denen fünf neu sind (Anodomaria staufensis, Nodosotrochus tricostatus, Crossostoma spiralocostata, Costataphrus weissmuelleri und Camponaxis? jaegeri). 15 Arten werden unter offener Nomenklatur beschrieben. Neu ist die Gattung Nodosotrochus. Die Fauna enthält eine Reihe von Arten, die bereits von anderen Fundorten gleichalter Schichten in Franken bekannt sind (z.B. von Kalchreuth und Buttenheim). Andere Arten fehlen dagegen in diesen Gemeinschaften. Auch die Häufigkeiten der Arten sind in beiden Bereichen sehr unterschiedlich. Diese Differenzen werden durch unterschiedliche fazielle und ökologische Bedingungen erklärt. Die hier beschriebene Fauna lebte wahrscheinlich in flacherem Wasser in der Nähe der damaligen Küste.Gründel J & Nützel A: Gastropods from the Upper Pliensbachian (Amaltheenton-Formation) from NE Bavaria (vicinity of Stauf/Dörlbach/Altdorf) (Franconia, Southern Germany). A Late Pliensbachian gastropod fauna is decribed from NE Bavaria (Oberpfalz, vicinity of Neumarkt, Stauf, Dörlbach). It consists of 42 species including five new species. The new species are Anodomaria staufensis, Nodosotrochus tricostatus, Crossostoma spiralocostata, Costataphrus weissmuelleri and Camponaxis? jaegeri. 15 species are described unter open nomenclature. The new genus Nodosotrochus is described. The present fauna shares several species with previously reported gastropod faunas of the same age from Franconia (for example Kalchreuth and Buttenheim). However, several other species are not present in these assemblages and the frequency distributions of the species differ strongly from each other. This is interpreted as result of different facies and ecology. The fauna described here is interpreted to have lived in shallower waters and closer to the palaeo-coastline

Microbial-, fusulinid limestones with large gastropods and calcareous algae: an unusual facies from the Early Permian Khao Khad Formation of Central Thailand

<jats:title>Abstract</jats:title><jats:p>The Early Permian (Kungurian) Khao Khad Formation of Central Thailand consists mostly of carbonates deposited on the western margin of the Indochina Terrane. This formation has yielded unusual microbial-fusulinid limestones with large gastropods which contribute most to the rock volume. With a height of more than 6 cm, the gastropods are amongst the largest Early Permian gastropods ever reported. Gastropods as major rock formers are rare in the Palaeozoic. This, and other recently reported invertebrate faunas from Thailand show that gastropods may dominate Permian fossil assemblages not only in diversity, but also regarding abundance and in some cases also regarding biomass. Besides gastropods, fusulinids, various calcareous algae, intraclasts and thick microbial-cyanobacterial (<jats:italic>Girvanella</jats:italic> and <jats:italic>Archaeolithoporella</jats:italic>) coatings and reticular microbial patches as well as thick inter- and intragranular radial fibrous cement crusts are present. The gastropods represent at least four species and belong probably to undescribed taxa. The fusulinid genus <jats:italic>Pseudofusulina</jats:italic> and <jats:italic>Misellina</jats:italic> (<jats:italic>M</jats:italic>.) <jats:italic>termieri</jats:italic> are reported from the Khao Khad Formation for the first time and indicate a Bolorian age. Calcareous algae are dominated by dasycladaceans followed by gymnocodiaceans and solenoporaceans. The studied limestone almost completely lacks metazoan reef builders such as corals and sponges. Likewise, brachiopods and bivalves are absent in the studied samples and echinoderms are very scarce. The carbonate is interpreted as product of shallow water, back-reef lagoonal platform community with a high productivity providing the large gastropods with sufficient food. However, conditions were too eutrophic for sessile filter feeders including metazoan reef builders.</jats:p&gt

The earliest example of sexual dimorphism in bivalves—evidence from the astartid Nicaniella (Lower Jurassic, southern Germany)

Protandrous sex change (sex change from male to female) is one of the diverse sexual expressions exhibited by bivalves, but its expression in the shell is quite rare. Previous studies on living and fossil astartids suggest a relationship between protandrous sex change and the formation of crenulations on the ventral shell margin at later ontogenetic stages. Here we report the formation of such crenulations in the Early Jurassic astartid Nicaniella rakoveci (Kuhn, 1935) from the Amaltheenton Formation at Buttenheim, Franconia. This is the earliest known record of protandrous hermaphroditism in fossil bivalves, predating previous reports by at least 13 Myr. A principal component analysis of linear size measurements and Fourier shape analysis of the shell outlines revealed that the outline of Nicaniella rakoveci specimens varies from subquadrate to subcircular, but this variation is independent of the presence or absence of crenulations and therefore not associated with sex. Crenulated specimens exhibit a lower height/inflation ratio than non-crenulated ones, suggesting that the protandrous females have more inflated valves than the males. The formation of crenulations was probably related to allocation of resources for reproduction. The most likely function of the crenulations was to increase the internal shell volume in the female stage to accommodate more eggs rather than being an adaptation against predation as often assumed for other bivalves. The formation of crenulations is part of the protandrous life history and probably controlled by a genetic mechanism that is also responsible for sex change