Renato Posenato

THE PRATI DI STUORES/STUORES WIESEN SECTION (DOLOMITES, ITALY): A CANDIDATE GLOBAL STRATOTYPE SECTION AND POINT FOR THE BASE OF THE CARNIAN STAGE

The Prati di Stuores/Stuores Wiesen section (Dolomites, Italy) is proposed as a candidate Global Stratotype Section and Point for the base of the Carnian Stage. In addition to being a famous, richly fossiliferous locality, it includes the type-section of the Cordevolian substage. The section is located near Pralongia, along the southern slope of the crest separating the Badia/Abtei and Cordevole valleys. Below the levels with Trachyceras aon , the section contains a rich ammonoid fauna that chacterizes the lower part of the Regoledanus Subzone and subsequently records the first appearances of the mid-high latitude genus Daxatina ( Daxatina sp., D. cf. canadensis ) and of traditional Trachyceras with species different from T. aon . Moreover, the Daxatina cf. canadensis Subzone is recognised above the Regoledanus Subzone. Very rare conodonts of the Budurovignatus group and species of Gladigondolella from the diebeli Assemblage Zone occur. Gondolella polygnatyformis, already known from the Aon Subzone, is absent. Palynomorphs, foraminifers, gastropods, bivalves, brachiopods, microcrinoids and holothurian sclerites were studied. Variations in frequency and taxonomic diversity of these faunas suggest anaerobic-disaerobic bottom conditions for the lower-middle part of the section (0-105 m), followed by a more stable oxygen content in the upper portion. Magnetostratigraphy showed four intervals with normal polarity and three intervals with reversed polarity. The Daxatina cf. canadensis Subzone falls close to the normal polarity interval S2n. The present study proposes the FAD of the cosmopolitan genus Daxatina as a marker of the base of the Carnian Stage, placing it at a lower stratigraphic level than previously indicated in the Stuores area. The Prati di Stuores section is proposed as GSSP of the Ladinian-Carnian boundary.

The Anisian macroflora from the Northern Dolomites (Kühwiesenkopf / Monte Prà della Vacca, Braies): a first report

The knowledge of Triassic macroflora from the Dolomites mostly concerns the Ladinian, while literature data on Anisian plants are scarce. This gap is filled by the discovery, reported here, of a rich plant deposit from Kuhwiesenkopf / Monte Pra della Vacca (Prags/Braies Dolomites). The fossils occur in a horizon, about 1 m thick, from the lower part of the Dont Formation, a basinal unit mostly constituted by hemipelagic, terrigenous-carbonatic sediments of Pelsonian - Illyrian age. The stratigrafic interval with the plant horizon is Pelsonian in age. A preliminary systematic analysis of the numerous and well preserved specimens has allowed the identification of at least 17 genera. The taxa belong primarily to the Pteridophyta ( Anomopteris, Neuropteridium, Cladophlebis, Crematopteris, ? Marattiopsis ), subordinately to the Cycadophyta ( Bjuvia, Taeniopteris, Dioonitocarpidium, Pterophyllum / Nilssonia ). Coniferophyta are represented by Voltzia , ? Voltzia and Albertia ; the latter genus is recorded herein for the first time in the Middle Triassic of the Dolomites. Besides, two Lycophyta genera (? Isoetites , Lycophyta new taxon), three Pteridospermae genera (? Sagenopteris , Scytophyllum and Peltaspermum ), and one Sphenophyta genus ( Equisetites ) have also been recognized.

PALEOECOLOGICAL DYNAMICS OF SHALLOW-WATER BIVALVE CARPETS FROM A LOWER JURASSIC LAGOONAL SETTING, NORTHEAST ITALY

Abstract Shell beds are products of complex biological, taphonomic, and sedimentological factors. Paleoecological analysis of Pliensbachian shell accumulations from northeast Italy, Southern Alps, records successive phases of colonization and demise, each of which are characterized by specific taphonomic features and sedimentary fabrics. Field studies were augmented by serial sectioning and thin sections revealing microfacies and microtaphofacies, as well as scanning electro-microscopy (SEM) and electron backscatter diffraction (EBSD). The term bivalve carpet is introduced to describe the original geometry of the bivalve accumulation colonizing the substrate. Within a lagoonal depositional setting, small, thin-shelled Isognomonidae-like bivalves lived as endo-byssate, non-siphonate suspension feeders. These bivalves, interpreted as juveniles, constructed bivalve carpets with a primary lateral extension while lacking a distinct vertical growth. Bivalves colonized firm, dysoxic carbonate substrates consisting of a mixture of peloids, bioclasts, and chemolithotrophic iron-oxidizing bacteria. Oxygen-rich bottom currents and microbial activity stabilized the lagoon floor thus allowing for the episodic settlement of fixosessile bivalves and their autochthonous preservation. Short-term anoxic phases interpreted to reflect seasonal decreases in water energy resulted in the mass mortalities of the juvenile bivalves. The abundance of articulated, vertical orientated valve pairs in the shell accumulations indicate that the bivalve carpets were embedded soon after the death of the organisms without prior physical disturbance.

Tomographic Analysis for Bioerosion Signatures in Shallow-Water Rhodoliths from the Abrolhos Bank, Brazil

Abstract LEAL, R.N.; BASSI, D.; POSENATO, R., and AMADO-FILHO, G.M., 2012. Tomographic analysis for bioerosion signatures in shallow-water rhodoliths from the Abrolhos Bank, Brazil. Bioerosion at all scales is now recognized as playing a major role in facies interpretation. Macroscopic and microscopic borings can provide an indication of water depth, if they can be attributed to the action of specific borers. Tomographic analysis is a new method for assessing bioerosion, for identifying boring taxa, as well as for calculating the volume and porosity in present-day rhodoliths. The tomographic system provides also a quantification of the calcium carbonate produced by bioerosion. Recent rhodoliths collected at 20-m water depth on the Abrolhos Bank, Brazil, have been multiscann analyzed. The study shows that rhodoliths from this site are characterized by a highly diversified assemblage of boring bivalves and sponges associated producing the ichnogenera Gastrochaenolites and Entobia. The fauna from this boring assemblage can remove up to 10% of the rhodolith volume. The method can be expected to yield similar results as applied to both modern and fossil rhodoliths from other localities and time frames.

DISCOVERY OF PARATIROLITES FROM THE BELLEROPHON FORMATION (UPPER PERMIAN, DOLOMITES, ITALY)

The first discovery of Paratirolites in the western Tethys is here described. The ammonoid was collected in the debris of the Bellerophon Fm. near Santa Cristina Valgardena (Bolzano, Dolomites). The specimen is contained in a small slab made of marly dark grey limestone with a biomicrofacies dominated by ostracods. It probably comes from the Ostracod Assemblage beds (Badiota Facies), which are situated few metres below the Comelicania beds. The occurrence of Paratirolites allow us to date this segment of the upper Bellerophon Fm. to the late Dorashamian, and to define the Comelicania and Nankinella beds of the Dolomites as younger than the Phisonites - Comelicania (= Gruntallina ) beds of the Transcaucasia (basal Dorashamian). This assemblage is equivalent to or younger than the Paratirolites Zone (late Dorashamian).

Carbon-isotope anomalies and demise of carbonate platforms in the Sinemurian (Early Jurassic) of the Tethyan region: evidence from the Southern Alps (Northern Italy)

Despite its global impact on ecosystems, the Triassic/Jurassic boundary event had only a modest effect on the carbonate depositional systems of the Southern Alps, whereas a fundamental reorganization of the same palaeogeographic area took place during the Sinemurian Age. This paper investigates whether or not the well-documented demise of Sinemurian carbonate platforms in the Tethyan region was a response to a global event by examination of carbon-isotope anomalies in successions of different facies that record this interval of time. A chemostratigraphic transect from Lake Garda up to the eastern Italian border is illustrated by four stratigraphic sections; high-resolution (20 cm over key intervals) chemostratigraphic sampling allowed detection of a major negative δ 13 C anomaly of ~ 1.5‰, preceded by a positive excursion, both in shallow- and deep-water successions, over the stratigraphical range of the ammonite genus Arnioceras . A comparison with sections from the UK suggests that the positive excursion belongs to the turneri Zone and the succeeding negative excursion falls within the obtusum Zone. In the deep-water Belluno Basin, the negative anomaly occurs in a biogenic chert-rich unit recording the onset of mesotrophic conditions in the basin. In the platform-carbonate successions, this major negative carbon-isotope excursion is developed within a calcarenitic unit corresponding to the lowest occurrence of the foraminifer Paleomayncina termieri . This evidence for deepening and transgression across the carbonate platform suggests pre-conditioning for drowning. Hence, rather than tectonic subsidence alone, environmental factors may have aided the demise of Tethyan carbonate platforms during the Early Jurassic Sinemurian Age.

Latest Changhsingian Orthotetid Brachiopods in the Dolomites (Southern Alps, Italy): Ecological Opportunists at the Peak of the End-Permian Mass Extinction

Abstract The majority of Changhsingian orthotetid brachiopod species from the Southern Alps were described by the end of the nineteenth century, but were then neglected by subsequent authors who proposed further new species, giving rise to great taxonomical confusion. An examination of type-specimens and newly collected material permits the consideration of Orthothetina ladina (Stache, 1878) and Ombonia tirolensis (Stache, 1878), type-species of Ombonia, as the only valid species and describe Teserina nerii as a new genus and species. The former two species are mostly located within a short stratigraphic interval, which is a few centimetres thick, limited by the trigger and peak of the end-Permian mass extinction. Teserina nerii n. gen. n. sp. occurs 2–3 m above the extinction peak and represents one of the last Permian rhynchonelliform brachiopod holdovers in the Southern Alps.

Bivalve borings in Lower Jurassic Lithiotis fauna from northeastern Italy and its palaeoecological interpretation

Abstract Random shell sections of the Pliensbachian (Early Jurassic) larger bivalve Opisoma from columns within the Main Post Office building of Ferrara, northern Italy, have been discovered to bear neat clavate-shape boreholes. These boreholes belong to the ichnogenus Gastrochaenolites Leymerie and represent bivalve borings. Opisoma is a subordinate component of the Lithiotis fauna characterised by aberrant shells thriving in tropical lagoonal settings which were widespread throughout the Tethyan and Panthalassa coasts. Although the Lithiotis fauna is well known in the palaeontological literature, no bivalve boring have been so far been formally described. The uniqueness of the morphology, size and substrate of these borings merits the designation of the new species Gastrochaenolites messisbugi ichnosp. nov. which thereby represents the first ichnospecies described from this fauna. The morphology of the boreholes and the included bivalves allows the boring activity to be ascribed to a mytilid bivalve. Palaeoecological and taphonomic analyses allowed the presence of the boreholes to be correlated to the Opisoma mode of life (epifaunal, free-living form) as well as to generally low sedimentation rates and seasonal mesotrophic conditions during an overall oligotrophic regime. http://zoobank.org/urn:lsid:zoobank.org:act:CCF77B32-D459-4305-BC86-93F228852E50

SYSTEMATICS OF LINGULIDE BRACHIOPODS FROM THE END-PERMIAN MASS EXTINCTION INTERVAL

The systematics of lingulide brachiopods, from the end-Permian mass extinction interval, is here studied and discussed. The material has been collected from upper Permian (Changhsingian) beds of Southern Alps and Lower Triassic beds of several Tethyan localities, where the surviving phase following the peak the end-Permian mass extinction is recorded. The study contributes to fill the gap of knowledge regarding the lingulide systematics during a time lapse crucial for the fate of the Mesozoic and Cenozoic marine organisms. The systematics is based both on inner shell morphology and shell microstructure, which are considered to be the most useful taxonomical characters to study the lingulide phylogeny. The specimens have been referred to species of the new genus Trentingula, which is characterized by a shell with a secondary layer virgose fabric and a primitive disposition of the ventral muscle umbonal scar in the Lingulidae phylogeny. Trentingula n. gen. comprises four species: T. lorigae n. gen. n. sp . , type-species, T. borealis (Bittner), T. mazzinensis n. gen. n. sp . , and T. prinothi n. gen. n. sp. The type-species is late Griesbachian – Dienerian in age and has a wide geographic distribution in the western Tethys (Southern Alps and Hungary). Trentingula prinothi n. gen. n. sp. occurs in the Upper Permian Bellerophon Formation of the Dolomites; it has a large shell with a short mantle cavity. Trentingula mazzinensis n. gen. n . sp. occurs in the Griesbachian Mazzin Member of Werfen Formation and is characterized by a small sized shell, about half of the type species, which records the “Lilliput effect” related to the aftermath of the end-Permian mass extinction.