Auswahl der wissenschaftlichen Literatur zum Thema „Cornulites“

A new cornulitid species Cornulites baranovi sp. nov. is described from the Ohesaare Formation (late Přidoli) of Saaremaa, Estonia. The new species exhibits straight to slightly curved tubes that expand moderately in diameter, a vesicular tube structure and strong annular crests and costae. In addition to C. baranovi sp. nov., Conchicolites sp. also occurs in the Ohesaare Formation. The diversity of cornulitids is low in the Přidoli of Estonia, but not lower than in the Silurian of Estonia in general. C. baranovi often forms a symbiotic association with the bryozoan Fistulipora. It is not sure whether sealing off one cornulitid specimen by host bryozoan was a result of space competition or a post- mortem overgrowth. Specimens of C. baranovi also occur as solitary encrusters and aggregative encrusters in the Ohesaare Formation. C. baranovi presumably represents ecological generalists among the Silurian tentaculitoid tubeworms.

Conoidal shells of Cornulites celatus n. sp. occur commonly within host coralla of Propora conferta Milne-Edwards and Haime, 1851, sensu lato, from the Laframboise Member of the Ellis Bay Formation (Ashgill: Upper Ordovician) at Pointe Laframboise on western Anticosti Island. Examples have also been found at the same locality in the tabulate corals Paleofavosites sp., Acidolites arctatus Dixon, 1986, and A. compactus Dixon, 1986, and the stromatoporoid Ecclimadictyon sp., but not in other associated tabulate coral species. Growth interference between the shells and their hosts indicates a commensal relationship. C.celatus apparently had a more limited paleoenvironmental range than its principal coral host species, which occurs abundantly elsewhere on the island without its endobiotic partner. The diagnosis of Cornulites is emended to include forms having a two-layered shell wall with a distinctive outer layer consistently preserved as prismatic calcite. This new species extends the known stratigraphic range of cornulitids in commensal relationships with corals and stromatoporoids from the Silurian back to the Upper Ordovician.

The Wenlockian lichid trilobite Arctinurus boltoni has been regarded as a classic example of a “snow-shoe” type adaptation. That is, the broad flat doublure has been interpreted as a resting surface, with well-developed terrace ridges serving as either sensory or frictional devices. New data from the Rochester Shale of western New York State indicate that this interpretation is incorrect The discovery of the small, calcareous worm tube Cornulites on complete, articulated specimens of Arctinurus indicates that these trilobites seldom rested their doublure on the sediment surface. Apertures of all specimens of Cornulites are oriented posteriorly, suggesting a rheophilic response to persistent water circulation patterns beneath the trilobite. The presence of suspension feeding organisms on the doublure also indicates that the water currents beneath the trilobite were free of suspended mud, so that Arctinurus may not have processed sediment in search of food.Large specimens of Arctinurus also have numerous epibionts on the dorsal shield, including cyclostome bryozoans, brachiopods (Stegerhynchus) and Cornulites. The presence of these suspension feeders indicates that Arctinurus did not burrow or even partially cover itself with mud. The large number of epibionts on adult specimens of Arctinurus, including several age classes for Stegerhynchus, indicates that molting stopped or at least slowed greatly once a certain size was attained, and that these trilobites may have had fairly long life spans.

Приведены данные о первых находках микроконхид и корнулитид на раковинах брахиопод-атрипид из девонских отложений Закавказья в интервале от эмсского до франского яруса включительно. Эпибионты, представленные гладкими спирально свернутыми трубчатыми раковинами рода Palaeconchus и прямыми трубчатыми раковинами рода Cornulites, примерно в равных пропорциях обнаружены как на брюшных, так и на спинных створках брахиопод. Прикрепление изученных Palaeconchus, скорее всего, происходило посмертно. В пользу этого вывода свидетельствует отсутствие правильности в их размещении. Гирляндный характер нарастания Cornulites, когда отдельные особи имеют примерно одинаковый размер, а их цепочки направлены устьем к передней комиссуре и расположены параллельно отступившему вперед лобному краю, свидетельствует о прижизненном прикреплении. На протяжении девонского периода эпибионты оставались в Закавказье обычными, хотя и достаточно редкими элементами бентосной фауны. Они отличались невысоким уровнем таксономической дивергенции, морфологического разнообразия адаптивных приспособлений и экологической дифференциации, что в целом свидетельствует о стабильности абиотических и биотических условий среды в то время.

Superbly preserved specimens of Eucalyptocrinites tuberculatus from the Waldron Shale (Silurian, Wenlockian) of Indiana have been found with epibionts attached in living position to their stems. The rhynchonellid brachiopod Stegerhynchus indianense, the tabulate coral Favosites forbesi, the annelids? Spirorbis and Cornulites, bryozoans, and rarely the ichnogenus Tremichnus utilized living crinoids as secondary tiering surfaces. Crinozoans also attached to upright crinoid columns, and their holdfasts are the most abundant epibionts on pluricolumnals. Eucalyptocrinites crassus individuals contributed to the success of their epibionts by providing stable attachment surfaces elevated up to one meter above the sediment-water interface. Although crinoid-epibiont relationships are frequently blurred by taphonomic processes, articulated crinoid specimens and encrusters on pluricolumnals suggest that secondary tiering was a significant paleoecological element in the Waldron Shale.

In situ colonies of Bulmanicrusta? sp. encrusting the surface of a hardground from the Upper Ordovician Bull Fork Formation provide the first glimpse of the full colony form, habitat, and faunal associates of a crustoid graptolite. Bulmanicrusta? sp. exhibits a runner-type colony form suited to rapid expansion over hard substrates, indicating it was an opportunistic member of the hardground community. This community also included two bryozoans (“Proboscina” and Amplexopora), Cornulites, and crinoids. The Bulmanicrusta? sp. specimens contain numerous graptoblasts (small, thick-walled vesicles) in organic connection with the colony. These objects are located at branch termini and confirm that graptoblasts were not pathological features, but probably were resting cysts produced by the crustoid colony as a normal part of its life cycle. Thus, the presence of graptoblasts supports the interpretation that the Caesar Creek crustoid was adapted to ephemeral or disturbance-prone environments.

Cette thèse explore la sédimentologie de l'Ordovicien de la plateforme saharienne dans la Daoura, au SW de l'Algérie. Les derniers travaux dans ce secteur, ayant conduit à la cartographie grande échelle, au découpage des formations, et à leur attribution stratigraphique au niveau des systèmes, datent des années 60. Pourtant, cette région, au sud de l'Ougarta mieux connu, est un relais essentiel entre l'intérieur du Craton Ouest Africain et la marge préservée au niveau de l'Anti-Atlas. L'objectif de ce travail est de documenter les environnements de dépôt et la stratigraphie séquentielle, et leurs relations avec l'apparition et le développement des biota fossilifères. En particulier, l'accent est mis sur les passages des environnements littoraux dominés par les tempêtes à des secteurs dominés par les marées, sans que des vallées incisées aient pu être mises en évidence jusque là. Deux coupes à haute résolution ont été réalisées dans les chaînons de Tabelbala et du Djebel Ben Tadjine, recoupant la limite Cambrien-Ordovicien et la limite entre l'Ordovicien supérieur marin et glaciaire. Ces coupes montrent une bonne corrélation entre elles et avec la Saoura et confirment l'absence de vallées incisées au moins jusque dans l'Ordovicien moyen. Les faciès indiquent un profil de dépôt de type rampe dominée par les vagues, essentiellement préservés sous forme de paraséquences. Sur une coupe détaillée du Darriwillien, des niveaux à lumachelles riches en Brachiopodes et Cornulites sont plus particulièrement étudiés. Ils sont préservés dans des tempestites localisées au-dessus des surfaces de ravinement transgressif qui coiffent les paraséquences. L'architecture de ces tempestites suggère une influence indirecte de la marée, qui se traduit, d'une part, par des interstratifications de lits plans supérieurs et d'HCS anisotropiques et, d'autre part, par des rythmes haute fréquence dans le litage des lumachelles. L'augmentation du marnage que cette évolution implique sur un profil de rampe serait favorisée par l'augmentation d'accommodation générée dans le shoreface au début des transgressions. Cette modification de la dynamique côtière et l'extension associée du shoreface serait favorable aux biota, qui étaient absents dans les intervalles régressifs sous-jacents et qui pourraient alors proliférer jusque dans l'offshore supérieur. Les mécanismes de cette adaptation restent à élucider mais l'étude tomographique des lumachelles montre que les Cornulites, lophophoriens sessiles, résistent étonnament à la dynamique des vagues et peuvent rapidement recoloniser le shoreface après les tempêtes
This thesis explores the Ordovician sedimentology of the Saharan platform in the Daoura, SW Algeria. The latest work in this sector, leading to large-scale mapping, the division of formations, and their stratigraphic attribution at the system level, dates from the 1960s. However, this region, south to the better known Ougarta Range, is a bridge between the interior of the West African Craton and the Anti-Atlas basin. The objective of this work is to document depositional environments and sequence stratigraphy, and their relationship with the development of fossiliferous biota. In particular, attention is paid on the transitions from coastal environments dominated by storms to those dominated by tides, in the absence of evidence of incised valleys. Two high-resolution sections were made in the Tabelbala and Djebel Ben Tadjine ranges, crossing the Cambrian-Ordovician limit and the limit between the marine and glacial Upper Ordovician. These sections show a good correlation between them and with the Saoura and confirm the absence of incised valleys at least until the Middle Ordovician. The facies indicate a wave-dominated ramp-type depositional profile, mainly preserved as parasequences. On a detailed section of the Darriwillian, shell beds rich in Brachiopods and Cornulites are more particularly studied. They are preserved in tempestites located above the transgressive ravinement surfaces which cap the parasequences. The architecture of these tempestites suggests an indirect influence of the tide, which results, on the one hand, in interstratifications of upper plane beds and anisotropic HCS and, on the other hand, in rhythmic bedding of the shell beds. The increase in tidal range that this evolution implies on a ramp profile would be favored by the increase in accommodation generated in the shoreface at the start of the transgressions. This modification of coastal dynamics and the associated shoreface extension would be favorable to biota, which were absent in the underlying regressive intervals and which could then proliferate into the upper offshore. The mechanisms of this adaptation remain to be elucidated but the tomographic study of shell beds shows that Cornulites, sessile lophophorians, are surprisingly resistant to wave dynamics and can quickly recolonize the shoreface after storms