An odontopleurid trilobite remain is described for the fi rst time from Hirnantian (uppermost Ord... more An odontopleurid trilobite remain is described for the fi rst time from Hirnantian (uppermost Ordovician) rocks of Western Gondwana. Very rare material, represented by a single left librigena, comes from a new fossil locality of the Don Braulio Formation in the Eastern Argentine Precordillera. Based on an updated systematic discussion, the fossil is tentatively referred to Eoleonaspis Sheng. Environmental and paleogeographical settings of this fi nding suggest that the genus would be eurytopic, having a cosmopolitan geographical distribution during the Ordovician-Silurian interval. In addition, the new evidence reinforces that scarce odontopleurids, often represented by Eoleonaspis, characterize a trilobite association together with Mucronaspis Destombes, strengthening it as a worldwide Hirnantian biostratigraphic indicator.
ABSTRACT Examination of newly collected brachiopods from the Eusebio Ayala Formation of Paraguay ... more ABSTRACT Examination of newly collected brachiopods from the Eusebio Ayala Formation of Paraguay reveals the occur-rence of Arenorthis paranaensis sp. nov., Plectothyrella? itacu-rubiensis sp. nov., Hindella sp. and Eostropheodonta conradii (Harrington). Associated graptolites of the N. persculptus Zone indicate that the age of the fossiliferous beds is Hirnan-tian. The overall generic composition of the fauna is similar to that of the atypical Hirnantia Fauna of the Bani Province. The record in Paraguay of Arenorthis, hitherto only known from North Africa, together with species of Plectothyrella? and Eostropheodonta different from those recorded in the Kosov Province, emphasizing the affinities between the Paraguayan fauna and the low-diversity African assemblages. Stratigraphic and faunal evidence indicates that biogeographical links between South America and Africa already existed by the end of the Ordovician when most of the intra-cratonic basins of Gondwana were flooded during the postglacial sea level rise.
Silurian and Devonian corals occur abundantly in siliciclastic rocks of the Argentine Precordille... more Silurian and Devonian corals occur abundantly in siliciclastic rocks of the Argentine Precordillera. They are distributed along a Silurian stormdominated shallow-water platform, a transitional restricted nearshore environment, and a Devonian muddy platform setting. Four main associations of rugose and tabulate corals have been recorded: a Silurian association, a lowermost Devonian and two associations in Lower Devonian rocks. The Silurian shallow-water platform coral association shows abundant colonies of a tabulate pleurodictyform coral. The type and the domical form of corals indicate high-energy environments. The lowermost Devonian coral association shows the abundance of the tabulate coral Argentinella argentina, which could have formed patches in protected low-energy environments. The Lower Devonian coral associations correspond to a muddy platform environment. The first one includes rugose and hemispherical tabulate corals and the second is composed of tabulate corals assigned to Parastriatopora sanjuanina, and Parastriatopora sp. They are long stick-like, fasciculate forms, grouped in apparently life position. Rugose and tabulate corals in the Upper Silurian and Lower Devonian of Argentina constitute lowdiversity associations of western Gondwana, developed in mid-to high-latitudes. The morphologies acquired by corals are related to water energy. Laminar or encrusting forms were more competitive in the Silurian shallow-water, high-energy environments. Slender branching forms with a high profile typically inhabited Devonian relatively deeper water or protected settings. The stratigraphic distribution observed partially followed the well-known pattern in which shallow-water faunas disappeared during regressive events and are replaced by new forms in the subsequent transgressive sequence. The turnover pattern detected across the Silurian-Devonian transitional interval is in agreement with that previously recognized on the basis of the accompanying fauna.
Two bryozoan taxa occurring in the Hirnatian (Upper Ordovician) deposits in western Argentina doc... more Two bryozoan taxa occurring in the Hirnatian (Upper Ordovician) deposits in western Argentina document a first postglacial community associated with a mid-to high-latitude brachiopod assemblage, known as the typical Hirnantia fauna, in the Argentine Precordillera. Helopora fragilis Hall and an indeterminate phyloporinid occur within a thin but conspicuous shell bed that overlies diamictitic deposits from the lower member of the Don Braulio Formation. The abundance of well preserved specimens of bryozoans together with the dominance of suspension feeders suggests a mid-shelf setting (offshore transition) with an intermediate to low sedimentation rate, low turbidity, and nutrient-rich conditions. Hirnantian bryozoan assemblages identified from tropical and subtropical regions are rather rare, and this assemblage represents the first high-latitude Hirnantian record. The low diversity of bryozoans may be related to high-latitude location of Argentina during the Late Ordovician. Helopora occurs mainly in Laurentia with several species occurring in the Katian and with a few Hirnatian occurrences in Gondwana. This broad distribution may imply that this genus had a broad environmental tolerance. It is one of the genera that successfully crossed the Ordovician/Silurian boundary. However, although this genus shows a post-extinction diversification and a more widespread distribution in Laurentia (with numerous species recorded in the Silurian and Devonian of Canada, Russia and China), it became extinct in Gondwana. We hypothesize that environmental conditions after the Ordovician/Silurian boundary may have prevented the settlement of bryozoans and caused the extinction of Helopora in the Argentine Precordillera. Key words. Bryozoans. Upper Ordovician. Hirnantian. Argentine Precordillera. Paleoecology. Paleobiogeography.
An odontopleurid trilobite remain is described for the fi rst time from Hirnantian (uppermost Ord... more An odontopleurid trilobite remain is described for the fi rst time from Hirnantian (uppermost Ordovician) rocks of Western Gondwana. Very rare material, represented by a single left librigena, comes from a new fossil locality of the Don Braulio Formation in the Eastern Argentine Precordillera. Based on an updated systematic discussion, the fossil is tentatively referred to Eoleonaspis Sheng. Environmental and paleogeographical settings of this fi nding suggest that the genus would be eurytopic, having a cosmopolitan geographical distribution during the Ordovician-Silurian interval. In addition, the new evidence reinforces that scarce odontopleurids, often represented by Eoleonaspis, characterize a trilobite association together with Mucronaspis Destombes, strengthening it as a worldwide Hirnantian biostratigraphic indicator.
ABSTRACT Examination of newly collected brachiopods from the Eusebio Ayala Formation of Paraguay ... more ABSTRACT Examination of newly collected brachiopods from the Eusebio Ayala Formation of Paraguay reveals the occur-rence of Arenorthis paranaensis sp. nov., Plectothyrella? itacu-rubiensis sp. nov., Hindella sp. and Eostropheodonta conradii (Harrington). Associated graptolites of the N. persculptus Zone indicate that the age of the fossiliferous beds is Hirnan-tian. The overall generic composition of the fauna is similar to that of the atypical Hirnantia Fauna of the Bani Province. The record in Paraguay of Arenorthis, hitherto only known from North Africa, together with species of Plectothyrella? and Eostropheodonta different from those recorded in the Kosov Province, emphasizing the affinities between the Paraguayan fauna and the low-diversity African assemblages. Stratigraphic and faunal evidence indicates that biogeographical links between South America and Africa already existed by the end of the Ordovician when most of the intra-cratonic basins of Gondwana were flooded during the postglacial sea level rise.
Silurian and Devonian corals occur abundantly in siliciclastic rocks of the Argentine Precordille... more Silurian and Devonian corals occur abundantly in siliciclastic rocks of the Argentine Precordillera. They are distributed along a Silurian stormdominated shallow-water platform, a transitional restricted nearshore environment, and a Devonian muddy platform setting. Four main associations of rugose and tabulate corals have been recorded: a Silurian association, a lowermost Devonian and two associations in Lower Devonian rocks. The Silurian shallow-water platform coral association shows abundant colonies of a tabulate pleurodictyform coral. The type and the domical form of corals indicate high-energy environments. The lowermost Devonian coral association shows the abundance of the tabulate coral Argentinella argentina, which could have formed patches in protected low-energy environments. The Lower Devonian coral associations correspond to a muddy platform environment. The first one includes rugose and hemispherical tabulate corals and the second is composed of tabulate corals assigned to Parastriatopora sanjuanina, and Parastriatopora sp. They are long stick-like, fasciculate forms, grouped in apparently life position. Rugose and tabulate corals in the Upper Silurian and Lower Devonian of Argentina constitute lowdiversity associations of western Gondwana, developed in mid-to high-latitudes. The morphologies acquired by corals are related to water energy. Laminar or encrusting forms were more competitive in the Silurian shallow-water, high-energy environments. Slender branching forms with a high profile typically inhabited Devonian relatively deeper water or protected settings. The stratigraphic distribution observed partially followed the well-known pattern in which shallow-water faunas disappeared during regressive events and are replaced by new forms in the subsequent transgressive sequence. The turnover pattern detected across the Silurian-Devonian transitional interval is in agreement with that previously recognized on the basis of the accompanying fauna.
Two bryozoan taxa occurring in the Hirnatian (Upper Ordovician) deposits in western Argentina doc... more Two bryozoan taxa occurring in the Hirnatian (Upper Ordovician) deposits in western Argentina document a first postglacial community associated with a mid-to high-latitude brachiopod assemblage, known as the typical Hirnantia fauna, in the Argentine Precordillera. Helopora fragilis Hall and an indeterminate phyloporinid occur within a thin but conspicuous shell bed that overlies diamictitic deposits from the lower member of the Don Braulio Formation. The abundance of well preserved specimens of bryozoans together with the dominance of suspension feeders suggests a mid-shelf setting (offshore transition) with an intermediate to low sedimentation rate, low turbidity, and nutrient-rich conditions. Hirnantian bryozoan assemblages identified from tropical and subtropical regions are rather rare, and this assemblage represents the first high-latitude Hirnantian record. The low diversity of bryozoans may be related to high-latitude location of Argentina during the Late Ordovician. Helopora occurs mainly in Laurentia with several species occurring in the Katian and with a few Hirnatian occurrences in Gondwana. This broad distribution may imply that this genus had a broad environmental tolerance. It is one of the genera that successfully crossed the Ordovician/Silurian boundary. However, although this genus shows a post-extinction diversification and a more widespread distribution in Laurentia (with numerous species recorded in the Silurian and Devonian of Canada, Russia and China), it became extinct in Gondwana. We hypothesize that environmental conditions after the Ordovician/Silurian boundary may have prevented the settlement of bryozoans and caused the extinction of Helopora in the Argentine Precordillera. Key words. Bryozoans. Upper Ordovician. Hirnantian. Argentine Precordillera. Paleoecology. Paleobiogeography.
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