Middle Tremadocian Research Articles

A New Genus of Middle Tremadocian Orthoceratoids and the Early Ordovician Origin of Orthoceratoid Cephalopods

The cephalopods of the subclass Orthoceratoidea, which are termed “orthoceratoids” herein, are a group that remains “the last unexplored wilderness in the Cephalopoda” (Flower 1962: 23). After 45 years this statement still holds true because phylogeny reconstructions are hindered by their morphologi− cal simplicity, numerous homeomorphies and iterative evolu− tion. The Orthocerida, straight cephalopods that are charac− terised by a wide chamber spacing, a thin tubular siphuncle and a small spherical initial chamber, lacking a cicatrix (Kroger 2006) were the ancestors of bactritoids, ammonoids, and coleoids (Engeser 1996). The origin of the Orthocerida is poorly understood. The earliest unequivocal Orthocerida are known from the Floian (Early Ordovician). A number of poorly known possible Orthocerida and/or stem group Ortho− cerida are known from the Tremadocian. Here, I reassign the long known middle Tremadocian “Orthoceras attavus ”t o the new genus Slemmestadoceras belonging to a group of world− wide distributed orthoceratoids. The presence of Slemmesta− doceras with a thin, probable tubular siphuncle and small ini− tial chambers in the middle Tremadocian suggests that the Orthocerida may have originated already at that time. The comparison of Slemmestadoceras with following late Trema− docian and Floian orthoceratoids demonstrates that a higher level taxon comprising these forms, such as the subclass Orthoceratoidea may constitute a paraphylum. The Orthoceratoidae comprise a wide range of orthocones with short septal necks and tubular or expanded siphuncles of the orders Ascocerida, Dissidocerida, Lituitida, Orthocerida, and Pseudorthocerida (Sweet 1964; Dzik 1984; Zhuravleva 1994; Kroger et al. 2007). Two main groups are recognisable within the Orthoceratoidae, orthocones with a conical apex, having a cicatrix (Ascocerida, Pseudorthocerida) and orthocones with small spherical apex lacking a cicatrix (Lituitida, Orthocerida). The Ascocerida and Lituitida are very characteristic orders that appear in the Middle Ordovician (Furnish and Glenister 1964; Dzik 1984). The Dissidocerida include orthocones with slightly expanded tubular siphuncles and rod−like endosiphuncular de− posits known from the Silurian Dissidoceratidae and several Early Ordovician families (Zhuravleva 1994; Evans 2005). The Orthocerida was emended by Kroger and Isakar (2006: 143) and comprises now only orthoceratoids with a spherical apex and a simple siphuncle. By contrast, the Orthocerida sensu Sweet (1964: K223) comprise a wide range of orthoceratoids classified today within the Orthocerida, Pseudorthocerida, and Dissido− cerida. Herein I always refer to the emended order Orthocerida sensu Kroger and Isakar, 2006 as orthocerids. The origin of the Orthocerida and Pseudorthocerida is poorly understood. In the Early Ordovician a number of orthocones occurred that are ei− ther poorly known proper members of these latter orders or stem group members, respectively. Classically the origin of Orthocerida was sought within the middle Floian (Flower 1962; Hook and Flower 1977) when the first straight orthocerids with wide chamber spacing and a cen− tral, narrow, empty siphuncle appear. Recently, Evans (2005) described the new genus Semiannuloceras from the early Floian (Moridunian) of Wales and classified it within the orthocerid family Baltoceratidae. Furthermore, Evans (2005) formulated a scenario of the early evolution of orthoceratoid cephalopods and the origin of Orthocerida, in which he suggests an ancestry of the Orthocerida from the Troedssonellidae and/or Polymeridae,

Graptolite biostratigraphy and its correlation of the Mungok and Yeonghung formations at Yeongwol area, Korea

Based on graptolites from the Mungok and Yeonghung formations at Yeongwol, Korea and recent taxonomy on graptolites, the previous three graptolite zones are refined as theAnisograptus matanensis, Adelograptus cf.tenellus, Callograptus spp. zones, and two zones are firstly proposed for the Tremadocian rocks in the area: theParadelograptus antiquus andAorograptus victoriae zones, in ascending order: TheAnisograptus matanensis Zone yieldsAnisograptus matanensis indicating an early Tremadoc age. TheAdelograptus cf.tenellus Zone is erected by the occurrence ofAdelograptus cf.tenellus andPsigraptus jacksoni which represent lowermost Late Tremadocian in age. TheCallograptus spp. Zone yielding plenty of dendroids includingCallograptus andDendrograptus, is tentative but useful to correlate with other rocks in the Sino-Korean Block. TheParadelograptus antiquus Zone is recognized by the occurrence ofParadelograptus antiquus andClonograptus aureus and is correlated with the middle Late Tremadoc of Yukon, Canada. TheAorograptus victoriae Zone consists mainly ofAorograptus victoriae and subordinately ofKiaerograptus andClonograptus and corresponds to the eponymous zones elsewhere. The boundary between the Mungok and the Yeonghung formations consists of ribbon rock and grainstone to packstone facies of the Mungok Formation and laminated dolomite, argillaceous limestone, and wackestone to grainstone facies of the Yeonghung Formation and is drawn within theAorograptus victoriae Zone which indicates middle Late Tremadocian.

EARLIEST ORDOVICIAN (EARLY TO MIDDLE TREMADOCIAN) RADIOLARIAN FAUNAS OF THE COW HEAD GROUP, WESTERN NEWFOUNDLAND

Well-preserved earliest Ordovician (early to middle Tremadocian) radiolarian faunas were recovered from carbonate rocks of the Cow Head Group of the Great Northern Peninsula of the island of Newfoundland, Canada. The earliest Ordovician faunal assemblages are from Green Point, Martin Point, Broom Point North and South, and St. Paul's Inlet in Gros Morne National Park. Latest Cambrian faunas were also recovered from Green Point and St. Paul's Inlet, but are extremely low in both abundance and diversity. The radiolarian faunas include five families, 10 genera, and 24 species. Of these, one family Aspiculumidae, one genus, and 19 species are new. The new family and new genus are Aspiculumidae and Aspiculum, respectively. The new species are Pararcheoentactinia? cowheadensis, Aspiculum eccentricum, Aspiculum? angulatum, Parechidnina delicata, P. variospina, Curvechidnina multiramosa, Echidnina conexa, E. laxa, E. semiconexa, E. severedeformis, Echidnina? immanis, Palaeospiculum curvum, P. multifurcatum, P. neofurcatum, P. tetractium, Protoentactinia deformis, P. kozuriana, P. primigena, and P. transformis. The Aspiculumidae is established on the basis of the new genus Aspiculum and on Parechidnina, whose family-level assignments were previously indeterminate. The new family Aspiculumidae is distinguished from the other four families by the absence of the spicule system.All genera of the earliest Ordovician radiolarian faunas can be placed in the families Aspiculumidae, Archeoentactiniidae, Echidninidae, Palaeospiculumidae, and Protoentactiniidae, as can the genera of the Cambrian radiolarian faunas. However, echidninids from Cambrian faunas are generally characterized by interlocked or fused spicules whose original structure is recognizable, while those from the earliest Ordovician are commonly characterized by fused and/or modified spicules. Also, the very rare protoentactinids of the Late Cambrian are extremely abundant and diverse in the earliest Ordovician faunas described herein. Specimens of the families Palaeospiculumidae and Archaeoentactinidae are less diverse and/or less plentiful in the earliest Ordovician compared to those in Cambrian. The genus Parechidnina, which now belongs to Aspiculumidae, is more plentiful and very diverse in the earliest Ordovician, and, at the same time, lineages of the new genus Aspiculum and a related not-yet-named genus began to evolve.The detailed biostratigraphic ages of the earliest Ordovician radiolarian faunas were determined mainly by the co-occurring conodonts. The age range of the earliest Ordovician faunas represented extends from the Cordylodus lindstromi Zone through the C. angulatus Zone to the Rossodus manitouensis Zone.

Tempo of earliest Ordovician graptolite faunal succession: conodont-based correlations from the Tremadocian of Quebec

Successive Tremadocian planktic dendroid graptolite assemblages from continental slope sequences in Quebec can be correlated with North American platform biozonations on the basis of conodonts. Anisograptid-bearing (Assemblage 2), middle Tremadocian "Matane faunas" are associated with Early Ordovician Rossodus manitouensis Zone (new designation) conodonts. Younger middle Tremadocian faunas with adelograptids (Assemblage 3) are no younger than the Rossodus manitouensis Zone. Key dendroid evolutionary–immigration events take place within the lower conodont Fauna B interval. Rooted dendroids near Cap des Rosiers, Quebec, and in eastern New York State occur with lower Fauna B conodonts and the trilobites Pareuloma and Borthaspidella. However, the earliest Tremadocian (and earliest Ordovician) dendroid immigration event, represented by the local lowest occurrence of faunas with Dictyonema flabelliforme s.l. at localities in western Newfoundland, eastern New York State, Norway, and eastern China, also lies within the lower Fauna B interval. Finally, the lowest occurrence of key Assemblage 2 dendroid taxa falls within the lower Fauna B interval at the latter localities.The Rossodus manitouensis Zone is proposed as a new designation for a biostratigraphic unit that is appropriate for North American marginal and open shelf sequences. This zone is approximately equivalent to the "Loxodus bransoni Interval" of other authors and is characterized by Fauna C conodonts. Newly described taxa include Rossodus? highgatensis n. sp., Scolopodus? praecornuformis n. sp., and Variabiloconus n. gen.

Observations sur deux niveaux mineralises dans le Paleozoique inferieur des monts du Minervois (Montagne Noire, Aude); mineralisations, lithologie et stratigraphie; contribution au debat syngenese-epigenese

Abstract A sparsely mineralized zone consisting essentially of galena, pyrite, and barite occurs at the top of an upper Georgian (Cambrian) calcareous dolomite sequence in the Minervois mountains, southwestern Montagne Noire region, France, and can be followed for more than 12 kilometers. Its constant association with a particular lithofacies suggests that the mineralization is syngenetic. Another zone of galena and sphalerite mineralization is localized in recrystallized limestone and nodular calcareous schists, attributed to the middle Tremadocian, and can be followed for more than three kilometers. this zone also appears to begenetically correlative with sedimentologic and paleogeographic conditions.