Letteratura scientifica selezionata sul tema "Orthocerida"

Abstract. The annulated orthoceridans of the Middle and Late Ordovician of Baltoscandia are described and their systematic frame is revised. The revision of these nautiloids, which are part of the Orthocerida and Pseudorthocerida, is based on the investigation of characters of the septal neck, the siphuncular tube, and the apex. An unequivocal terminology of these characters is suggested and applied. The shape of the septal neck and the siphuncular tube are described for the first time in Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum Eichwald, 1860, and Gorbyoceras textumaraneum (Roemer, 1861). Ctenoceras sweeti n. sp. is erected. The apex of Dawsonoceras barrandei Horný, 1956 is figured and described for the first time. The distribution of the character states of the apex and the septal neck support the emendation of the families Orthoceratidae, Dawsonoceratidae, and Proteoceratidae. The analysis shows also that the families Kionoceratidae, and Leuroceratidae must be refused because they represent not natural groups. However, it is also shown that the present knowledge is not sufficient to establish an unequivocal classification of the Middle, and Late Ordovician annulate cephalopods. Die orthoceriden Cephalopoden des Mittleren bis Späten Ordoviziums im Baltoskandium werden beschrieben und revidiert. Die Revision dieser Cephalopoden, welche zu den Orthocerida und Pseudorthocerida gehören, stützt sich auf die Untersuchung der Apikalenden, der Septalduten und der Form der Siphonalröhre. Eine eindeutige Terminologie für diese Merkmale wird vorgeschlagen und angewandt. Die Form der Septalduten und der Siphonalröhre von Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum, Eichwald, 1860 und Gorbyoceras textumaraneum (Roemer, 1861) wird erstmals beschrieben. Die Art Ctenoceras sweeti n. sp. wird aufgestellt. Der Apex von Dawsonoceras barrandei Horný, 1956 wird erstmals beschrieben und dargestellt. Die neu gefundenen Merkmale stützen eine Emendation der Familien Orthoceratidae, Dawsonoceratidae und Proteoceratidae. Es wird daher dafür plädiert, die Familien Kionoceratidae (Hyatt, 1900) und Leuroceratidae (Sweet, 1964) nicht mehr zu verwenden, da diese keine natürlichen Gruppen repräsentieren. Die Untersuchung zeigt aber auch, dass es derzeit noch nicht möglich ist die annulaten Cephalopoden des Mittleren und Oberen Ordoviziums zweifelsfrei zu klassifizieren. doi:10.1002/mmng.200600005

The Early and Middle Ordovician Orthocerida and Lituitida of Precordilleran Argentina are described, and their systematics and paleogeographic significance are revised. These cephalopods show a strong affinity to coeval faunas of North China, suggesting a location of the Precordillera at middle latitudes in the Southern Hemisphere east of the North China block and relatively close to the Gondwanan margin during the early Middle Ordovician. The descriptive terminology of characters of the septal necks, the position and shape of the siphuncule, and the shape of the connecting ring is improved. The distribution of these characters support an emendation of the Baltoceratidae, Sactorthoceratidae, and Proteoceratidae. Braulioceras n. gen. (Sactorthoceratidae) and Palorthoceras n. gen. (Orthoceratidae) are erected. The new species Braulioceras sanjuanense, Eosomichelinoceras baldisii, Gangshanoceras villicumense, and Rhynchorthoceras minor are proposed. Palorthoceras n. gen. from the Lower Ordovician Oepikodus evae Zone represents the earliest known orthocerid.

The Early Ordovician successions of the southern Montagne Noire consist of a thick sequence of predominantly siliciclastic sediments of which the late Tremadocian St. Chinian Formation and the earliest Floian La Maurerie Formation contain a comparatively rich and abundant cephalopod association. The cephalopods of the St. Chinian and La Maurerie Formation are interpreted as generally authochthonous, representing a fauna which originally lived in the open water above the sediments or related to the sea bottom. The cephalopod associations of the St. Chinian and La Maurerie formations are similar to other contemporaneous assemblages known from higher palaeolatitudes and associated with deeper depositional settings. They are composed almost exclusively of longiconic orthocones, in this case predominantly of eothinoceratids and baltocerids. The occurrences of <i>Annbactrocera</i>, and <i>Bactroceras</i> in the St. Chinian Formation are at present the earliest unambiguous reports of the Orthocerida. The available data suggest an initial expansion of orthoceroid cephalopod faunas from open water habitats of high paleo-latitudes, and a subsequent expansion on the carbonate platforms during the Floian. The presence of the eothinoceratid <i>Saloceras</i> in abundance demonstrates the Gondwanan affinity of the assemblage whilst adding further support for the presence of a "<i>Saloceras realm</i>" that may have extended along the margins of East and West Gondwana at least into intermediate latitudes. The following new taxa are proposed: <i>Annbactroceras</i> n. gen., <i>Annbactroceras felinense</i> n. sp., <i>Cyclostomiceras thorali</i> n. sp., <i>Felinoceras</i> n. gen., <i>Felinoceras constrictum</i> n. sp., <i>Lobendoceras undulatum</i> n. sp., Rioceratidae n. fam., <i>Saloceras murvielense</i> n. sp., <i>Thoraloceras</i> n. gen., <i>Thoraloceras bactroceroides</i> n. sp. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201000013" target="_blank">10.1002/mmng.201000013</a>

The historical and prevailing view regarding the distribution of nautiloid cephalopods is one of cosmopolitanism. There are several objections to such a sweeping view of this major group of marine invertebrates, but only the most significant are addressed here. First, unlike endemism, there is no clear agreement on the meaning of the term cosmopolitanism as used in biogeography. It is thus extremely difficult to gain a historical perspective without access to original data. I have found the term used for as few as four occurrences on four modern landmasses without reference to the paleogeographic relationships of these landmasses. Second, while a few nautiloid groups did compile impressive dispersal statistics, the fossil record clearly reveals that such periods of dispersal were generally brief in geological terms and that the group or groups involved did not colonize all available landmasses. Third, nautiloids were incapable of developing cosmopolitan distributions unless climatic constraints were removed by changes in the global system or by positioning all landmasses within the sub-tropical to tropical latitudes. Since there is no convincing evidence that either event occurred during the 520 million years of nautiloid evolution, it is perhaps more appropriate to view the distribution of nautiloids in terms of the number of landmasses colonized relative to the number of landmasses available for colonization. For nautiloids, the number of landmasses available for colonization was always fewer than the number of landmasses comprising the global paleogeography during any one slice of geologic time. Nautiloid genera restricted to one landmass are considered endemic, a condition exhibited by 65% of the Ordovician and Silurian genera and 81% of the Devonian genera. The maximum number of landmasses colonized by any one nautiloid genus for any one particular period of time was four, two fewer than the six available landmasses.The basic biogeographic unit for nautiloid cephalopods is the genus. This is so because the dispersive potential of nautiloids was low when compared with true pelagic groups such as conodonts. Thus for nautiloid groups capable of dispersal among landmasses, the time needed to effect dispersal and insure permanence in the stratigraphic record was something greater than the longevity of the typical nautiloid species but less than the longevity of most genera. It seems reasonable that the best chance for the occurrence of cosmopolitan nautiloid genera would be at or near the zenith of those groups with attributes most suitable for dispersal. However, the fossil record for nautiloids shows that such periods rarely coincide with the peak intervals of total nautiloid diversity for the Lower and Middle Paleozoic (Arenig, Wenlock and Eifelian) occurring instead during succeeding intervals of time. Such events are generally confined to periods of modal diversity within each group. The lowest percentages of endemic genera and the intervals in which they occurred for the major nautiloid groups are: Ellesmerocerida (57%) and Endocerida (60%) for the Llanvirn, Actinocerida (36%) and Tarphycerida (45%) in the Llandeilo, Orthocerida (52%, 47%, 55%) and Oncocerida (66%, 66%, 75%) for the Caradoc, Ludlow, and Givetian, Discosorida (67%) in the Wenlock and Nautilida (62%) for the Givetian. While the low percentage of endemics for the Actinocerida and Tarphycerida translate into the highest percentages of genera found on more than three separate landmasses (20%), similar percentages of endemics for the Orthocerida do not. Nonendemic members of the Orthocerida are more common to two or three of the available landmasses with approximately 20% occurring in either of these configurations. The fossil record also shows that Devonian nautiloids were the most restricted with the majority occurring on no more than two landmasses.These and other criteria indicate that the distributions of nautiloid cephalopods do not conform to the general perception of cosmopolitanism. At the generic level the group is largely endemic with reasonably large numbers of genera occurring on two or three landmasses with no genus occurring on all available landmasses during a given interval of time. Because of the type and manner of biogeographic barriers imposed on nautiloids, their distributions or patterns tend to have well defined limits with considerable predictive powers.

A diverse, well-preserved assemblage of nautiloid cephalopods was collected from the Treptoceras duseri shale, a 1.5-m-thick claystone within the Waynesville Formation (Late Ordovician, early Richmondian) exposed in southwest Ohio. The strata, the enclosed fauna, and its taphonomy indicate deposition in a low-energy, mud-bottom marine environment, in water depths of 20–25 m, below wave base but within the zone of storm-current reworking.Nautiloid specimens consist of complete conchs that have been replaced by calcite. Twelve species of nautiloids, belonging to eight genera, representative of four orders, have been collected from the shale in southwest Ohio. Longiconic orthocones are clearly the dominant nautiloid morphotype present, with the assemblage dominated by three species of the longiconic orthocerid Treptoceras and with fewer numbers of the endocerid Cameroceras and the slender orthocerid Isorthoceras?, the cyrtoconic oncocerids Oncoceras and Manitoulinoceras, and rare specimens of the orthocerid Gorbyoceras, the oncocerid Zittelloceras, and the ascocerid Schuchertoceras.Nautiloid taphonomy, the diversity of nautiloid taxa present, the lack of postmortem buoyancy in the shells of the more common taxa, the recurrent nature of this assemblage, and the restricted distribution of this Treptoceras–Cameroceras fauna to portions of eastern North America in the Late Ordovician suggest that this nautiloid assemblage represents an in-situ accumulation of nautiloids representative of a living assemblage. These nautiloids were important elements associated with benthic communities in these epeiric sea mud-bottom environments and not simply assemblages of drifted, necroplanktonic shells.

Abstract. The study of the apertural complex in the type species of the reussellid genera leads to a reassessment of the classification of these taxa. The genus Reussella has so many characteristics in common with Bulimina that it is reclassified in the Buliminidae. The family Trimosinidae is retained and redescribed to contain the genera Trimosina, Mimosina and Fijiella only. Pyramidina is retained in the Turrilinidae on account of the praebulimine toothplate.The absence of an internal toothplate and the very un-bulimine apertural face in Bifarinella, Chrysalidinella, Cifellia, Finlayina, Orthocerina, Pavonina and Valvohifarina species is the main argument to remove these genera from the Buliminacea and reclassify them in the superfamily Pavoninacea herein proposed.The genus Orthocerina d’Orbigny, 1839 is reinstated and shown to be closely related to Chrysalidinella and Cifellia.The genus Compressigerina is reclassified in the Uvigerinidae, close to Trifarina because of the presence of an apertural neck with toothplate.

Volume V features the first 304 of the total 632 colour plates of the series, each consisting of 8 photographs and generally illustrating 4 species in dorsal and lateral aspects, respectively. It covers all the primitive families (Orthoceri) from Anthribidae to Apionidae, as well as the Heteromorphi and the subfamilies Amycterinae and Entiminae (Adelognatha) of the Gonatoceri. Two taxonomic indices, one by genus to species and the other by species to genus, conclude the volume.

This second volume of colour plates features the remaining 328 plates of the series, or 2624 individual photographs to bring the total number of coloured illustrations of the series to 5036. It includes further illustrations of the families Anthribidae, Caridae, Rhynchitidae, Attelabidae, Brentidae and Apionidae of Orthoceri and of the Erirhinidae of Heteromorphi, but the bulk of the plates deals with the remaining subfamilies of the Gonatoceri. Two similar indices as in Volume V, but combined for both colour plate volumes, conclude Volume VI.

This volume covers the remainder of the primitive weevils (Division Orthoceri), namely the families Brentidae, Eurhynchidae and Apionidae. It catalogues 43 genera and 173 species and features almost 2000 individual drawings and black-and-white photographs. These illustrations are augmented by 270 full-colour habitus photographs in Volumes V and VI. The volume also includes an important chapter on the Immature Stages of Australian Curculionoidea by Brenda May, New Zealand, which describes the larval and pupal stages of 158 species of Australian weevils and features overviews of larval characters and their nomenclature as well as of rearing and preservation techniques applicable to weevil larvae. More than half the drawings in the volume accompany this chapter.