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Eoconusphaera hallstattensis sp. nov. and a review of the Rhaetian genus Eoconusphaera
Abstract
The genus Eoconusphaera is among the few calcareous nannofossil genera that occur in the Upper Triassic. The calcareous nannofossil assemblages of three Rhaetian sections–two from the Austrian Northern Calcareous Alps and one from offshore north-western Australia–were studied using scanning electron and/or transmitted-light microscopy. Significant structural differences were observed in the inner structures of conical Rhaetian forms belonging to Eoconusphaera, which prompted a revision of Eoconusphaera zlambachensis and the description of a new species, E. hallstattensis sp. nov.
... Calcareous nannofossils are known since the Late Triassic to the Present and have become important primary producers of marine phytoplankton since the Early Jurassic (Bown et al., 2004;Payne & van de Schootbrugge, 2007). In the Late Triassic, they were represented by few species and were abundant only in some carbonate shelves in the Tethys (Moshkovitz, 1982;Jafar, 1983;Bown, 1987;Posch & Stradner, 1987;Bown & Lord, 1990;Bralower et al., 1991; 150 Clémence et al., 2010;Gardin et al., 2012;Preto et al., 2012Preto et al., , 2013Bottini et al., 2016;Sinnyovsky, 2016;Holcová & Michalík, 2017;Fraguas et al., 2019;Demangel et al., 2020Demangel et al., , 2021Rifl & Holcová, 2020), where they proved to be reliable biostratigraphic markers for the Norian and Rhaetian (Bralower et al., 1991;Bown, 1998;Gardin et al., 2012;Demangel et al., 2021). Outside the Tethyan Realm, Late Triassic calcareous nannofossils are only known in three other regions in the Eastern Pacific basins of North and South America (Pérez Panera et al., 2021a). ...
... Calcareous nannofossils are known since the Late Triassic to the Present and have become important primary producers of marine phytoplankton since the Early Jurassic (Bown et al., 2004;Payne & van de Schootbrugge, 2007). In the Late Triassic, they were represented by few species and were abundant only in some carbonate shelves in the Tethys (Moshkovitz, 1982;Jafar, 1983;Bown, 1987;Posch & Stradner, 1987;Bown & Lord, 1990;Bralower et al., 1991; 150 Clémence et al., 2010;Gardin et al., 2012;Preto et al., 2012Preto et al., , 2013Bottini et al., 2016;Sinnyovsky, 2016;Holcová & Michalík, 2017;Fraguas et al., 2019;Demangel et al., 2020Demangel et al., , 2021Rifl & Holcová, 2020), where they proved to be reliable biostratigraphic markers for the Norian and Rhaetian (Bralower et al., 1991;Bown, 1998;Gardin et al., 2012;Demangel et al., 2021). Outside the Tethyan Realm, Late Triassic calcareous nannofossils are only known in three other regions in the Eastern Pacific basins of North and South America (Pérez Panera et al., 2021a). ...
... In the Western Tethys Calcareous Alps, this species has its first occurrence (FO) in the middle Norian and dominates the assemblages up to the late Norian (Gardin et al., 2012;Demangel et al., 2020). In the South- eastern Pacific, there are no records from middle Norian marine successions, but during the late Norian, Prinsiosphaera triassica constitutes monospecific assemblages (Bralower et al., 1991(Bralower et al., , 1992Bown, 1992;Demangel et al., 2020Demangel et al., , 2021Pérez Panera et al., 2021a, 2021bsee Fig. 5). Only a few species of calcareous dinocysts may co-occur in the assemblages (Thoracosphaera, Orthopithonella, and Pirumella). ...
The Arroyo Malo Formation represents the first marine ingression during the Late Triassic in the Neuquén Basin, west-central Argentina. The presence of calcareous nannofossils in this unit provides independent age constraint of a late Norian to Rhaetian age for its lower and middle parts, in agreement with known fossil invertebrates. Furthermore, this represents one of the only three records of this group known so far outside the Tethyan Realm. The presence of monospecific assemblages of Prinsiosphaera triassica in the late Norian and assemblages characterized by P. triassica and Eoconusphaera zlambachensis in the Rhaetian, together with a review of other known records worldwide, allows the formal proposal of a global biozonation for the Late Triassic. Prinsiosphaera triassica Biozone, from middle Norian to late Norian, and Crucirhabdus primulus Biozone, spanning the Rhaetian (comprising two sub-biozones in the Tethys) are defined. On the basis of the new record from the Upper Triassic of the Neuquén Basin, we interpret that calcareous nannofossils originated during the middle Norian in the Western Tethys, and we suggest that they then rapidly dispersed to the Southeastern Tethys, and from there to the Eastern Pacific across Panthalassa by southern mid-latitude easterly oceanic currents.
... 4). Two conical forms are present with Eoconusphaera hallstattensis (Demangel et al. 2021;Fig. 5) and then Eoconusphaera zlambachensis (Moshkovitz 1982;Fig. ...
... stuerzenbaumi Zone), Eoconusphaera zlambachensis, are present in the western and southern Tethys Ocean. We recently revised their taxonomy (Demangel et al. 2021). In the Zlambach section, E. zlambachensis is more abundant than E. hallstattensis, reaching maximum abundances in the middle Rhaetian (V. ...
Calcareous nannofossils evolved in the global ocean from the Carnian (early Late Triassic) and have contributed to the accumulation of biogenic calcium carbonate in marine sediments since then. Bio-diversification and bio-productivity became more
significant in the Rhaetian (Late Triassic), representing an important period to understand the dynamics of calcareous nannofossil evolution. The calcareous nannofossil content of the Zlambach Formation, Northern Calcareous Alps, Austria, was qualitatively and quantitatively investigated using both scanning electron and light microscopy. The nannolith, Prinsiosphaera triassica triassica, dominates the assemblage in most samples
and increases slightly in abundance in the lower Rhaetian, followed by a small-scale short-term increase during the middle Rhaetian and reaches rock-forming abundance in the upper Rhaetian. A systematic size decrease is recorded from the lower Rhaetian onwards, possibly due to changes in the palaeo-environment. The abundance of P. triassica triassica is slightly affected by the occurrence of Eoconusphaera hallstattensis and
strongly by E. zlambachensis. E. hallstattensis is constrained to a relatively short interval, from the upper Paracochloceras suessi Zone (lower Rhaetian) and disappeared during the lower Vandaites stuerzenbaumi Zone (middle Rhaetian), after the occurrence of a second Eoconusphaeraceae species: E. zlambachensis. The last occurrence of E. hallstattensis comes along with the subspecies Prinsiosphaera triassica crenulata showing characteristic parallel-oriented calcite lamellae. Those three species are suggested as good biostratigraphical markers for the Upper Triassic. The coccolithophorids are present in low abundance, increasing slightly in the middle Rhaetian. After the first record of coccoliths in the middle Norian (Alaunian), the oldest Crucirhabdus minutus and Archaeozygodiscus koessenensis were observed in the upper Norian (Sevatian) and the first occurrence of Crucirhabdus primulus was recorded in the lower Rhaetian. These observations suggest a rather slow temporal diversification of the first coccolithophorids.
... The coccolithophores Crucirhabdus minutus and Archaeozygodiscus koessenensis sporadically appear since the late Norian in the same area and P. triassica dispersed to the Southeastern Tethys and the Eastern Pacific (Demangel et al., 2020;Bown, 1992). By the early Rhaetian, Crucuirhabdus primulus and Eoconusphaera hallstattensis first appear in the Western and Southeastern Tethys, followed by Eocnousphera zlambachensis (Demangel et al., 2021). In the Southeastern Pacific, recently investigated Late Triassic successions in the Neuquén Basin, Argentina, and in the Pucrará Group, northwestern Perú, revealed the presence of P. triassica in the late Norian and Rhaetian and E. zlambachensis and C. minutus in the Rhaetian (Pérez Panera et al., 2021a;2021b). ...
Calcareous nannofossils first appear in the middle Norian in the Western Tethys, Northern Calcareous Alps, with the nannolith Prinsiospjhaera triassica and small unidentified coccolith-like rims (Demangel et al., 2020). The coccolithophores Crucirhabdus minutus and Archaeozygodiscus koessenensis sporadically appear since the late Norian in the same area and P. triassica dispersed to the Southeastern Tethys and the Eastern Pacific (Demangel et al., 2020; Bown, 1992). By the early Rhaetian, Crucuirhabdus primulus and Eoconusphaera hallstattensis first appear in the Western and Southeastern Tethys, followed by Eocnousphera zlambachensis (Demangel et al., 2021). In the Southeastern Pacific, recently investigated Late Triassic successions in the Neuquén Basin, Argentina, and in the Pucrará Group, northwestern Perú, revealed the presence of P. triassica in the late Norian and Rhaetian and E. zlambachensis and C. minutus in the Rhaetian (Pérez Panera et al., 2021a; 2021b). These findings correspond to the second record of this group outside de Tethyan Realm, after the finding of P. triassica and C. minutus in the Norian and Rhaetian in the Queen Charlotte Islands, western Canada (Bown, 1992).
Tethyan assemblages have high species richness and younger records than the Eastern Pacific ones, suggesting a Tethyan origin and diversification, and a continuous dispersal through the Panthalassa to the Eastern Pacific. According to oceanic circulation models for the Late Triassic, southern easterly cool-water currents could connect the Southeastern Tethys with the Southeastern Pacific, dispersing the calcareous nannoplankton. Once the nannoplankton reached the Southeastern Pacific continental margins, they could disperse to the north transported by contouritic currents. This hypothesis supposes a permanent connection through the Late Triassic, and could explain the reduced species richness in the Eastern Pacific due to a progressive loss in diversity across the dispersal route. Western and Southeastern Tethys, which were connected along continental shelves and were settled in tropical to subtropical latitudes, share equal species richness values. A first loss in species richness occurred from the Southeastern Tethys to the Southeastern Pacific (Neuquén Basin and Pucará Group), with the absence of Eoconusphaera hallstattensis, Archaeozygodiscus koessenensis and Crucirhabdus primulus (although this species is latter recorded in the Early Jurassic). This loss represents 50% of the species richness and makes sense, assuming that this part of the dispersal pathway involved passive transport across the open ocean and by cool-water currents. From the Southeastern Pacific, nannoplankton had a relatively easy path to the Northeastern Pacific. Even so, loss of species richness continues, giving the absence of Eoconousphaera zlambachensis in Queen Charlotte Island sections (Bown, 1992). Although Southeastern and Northeastern Pacific basins were probably continuously connected by continental shelves, the strong equatorial westerly oceanic currents could act as an important barrier against dispersion between these areas.
... The calcareous nannofossils content was analysed using scanning electron microscopy. Eight samples were prepared following the method of Demangel et al. (2020Demangel et al. ( , 2021. Blocks with a surface of 1 cm 2 were cut into the fresh sample surface perpendicular to the bedding plane to reduce the effect of sample heterogeneity (Chayes, 1954). ...
The end-Triassic Mass Extinction (ETME) is generally regarded as a consequence of the environmental changes associated with the emplacement of the Central Atlantic Magmatic Province (CAMP) and ranks among the 'big five' mass extinctions in Earth history. A notable feature of the ETME is a halt in marine carbonate deposition followed by the formation of unusual facies such as carbonate cement fans and oolites in the early aftermath of the event. The ETME time interval has been well studied over the last few decades, in contrast to a few minor extinction events that preceded it, among them the extinctions associated with the Norian-Rhaetian boundary (NRB). This study provides new insights into these extinction events with complete mid-Norian to Hettangian δ 18 O carb and δ 13 C carb record from a key section at Wadi Milaha (Ras Al Khaimah Emirate, United Arab Emirates). Ooids are important proxies for palaeoenvironmental reconstruction. The post ETME oolite horizon is documented providing morphological classification as well as a detailed modal analysis of rock components and different types of coated grains. Through a multi-technique approach, we argue for the stability of the carbon cycle across the NRB extinction event and the existence of a hiatus at the TJB (Triassic-Jurassic Boundary) in Wadi Milaha. Our new morphological classification of post-extinction ooids is compatible with a major role for seawater geochemistry with respect to sedimentological processes, by example in the peculiar way ooids diversify and alternate with other kinds of coated grains.
A peculiar record of previously undescribed and/or rarely described nannoliths from the Hettangian to lower Pliensbachian of the Llanbdr (Mochras Farm) core, Wales (UK) is documented here. Some of the observed morphotypes are assigned to didemnid ascidians, commonly observed in the upper Rhaetian, as evidence of the resilience of ascidian tunicates during the end‐Triassic mass extinction. One newly described nannolith present throughout the Hettangian and Sinemurian, Quadrifolium hesselboi , is similar to forms previously observed in the upper Rhaetian, and is therefore considered to be a new nannofossil survivor from the end‐Triassic mass extinction. High‐amplitude changes observed in the absolute and relative abundances of nannoliths, schizosphaerellids and coccoliths across the Hettangian suggest a recovery interval from the end‐Triassic crisis spanning the planorbis – liasicus ammonite zones in four distinct steps with high volatility in environmental conditions, followed by the establishment of more stable environmental conditions in the angulata ammonite zone.
The Triassic is bound by two mass extinctions that coincide with vast outpourings of volcanic flood basalts. The Mesozoic begins with a gradual recovery of plant and animal life after the end-Permian mass extinction. Conodonts and ammonoids are the main correlation tools for marine deposits. The Pangea supercontinent has no known glacial episodes during the Triassic, but the modulation of its monsoonal climate by Milankovitch cycles left sedimentary signatures useful for high-resolution scaling. Dinosaurs begin to dominate the terrestrial ecosystems in latest Triassic. In contrast to the rapid evolution and pronounced environmental changes that characterize the Early Triassic through Carnian, the Norian–Rhaetian of the Late Triassic was an unusually long interval of stability in Earth history.
In this work, calcareous nannofossils are identified for the first time in the uppermost Triassic sequence of the Lombardy Basin (Southern Calcareous Alps, Italy). Two zones are recognized, namely the NT2b (latest Triassic) and the NJT1 (earliest Jurassic). Two species resulted to be good markers to constrain the TJB interval: Prinsiosphaera triassica and Schizosphaerella punctulata. Nannofossil data are calibrated with C isotopic chemostratigraphy obtained for carbonate and organic matter. Size reduction of P. triassica and a decline in the abundance of Triassic nannofossils are detected soon after the “precursor Carbon Isotope Excursion (CIE) and culminated during the “initial negative CIE” characterized by lowest nannofossil abundances and small-sized P. triassica. The extinction of Triassic nannofossils occurred in distinctive steps within the “initial negative CIE”, while the Jurassic S. punctulata is first observed at the base of the “main negative CIE”. The latest Triassic nannofossil decline in abundance, size reduction and extinctions, represent a progressive deterioration associated to the Central Atlantic Magmatic Province (CAMP) volcanism. Our findings are consistent with nannofossil changes at supraregional scale and indicate that the massive CAMP flood basalts were preceded by initial volcanic pulses. We speculate that a combination of climate change, fertilization and ocean acidification started to influence the calcification process prior to the “initial negative CIE”. Nannoplankton extinctions were not simultaneous and might imply limited capacity for adaptation in the early stages of evolutionary history. However, originations of new taxa soon after the disappearance of Triassic forms suggest the ability to rapidly overcame extreme stressing conditions.
Why do you ask, when the Abstract is already there in the Text?
The onset of pelagic biomineralization marked a milestone in the history
of the long term inorganic carbon cycle: as soon as calcareous
nannofossils became major limestone producers, the pH and
supersaturation state of the global ocean were stabilized (the so-called
Mid Mesozoic Revolution). But although it is known that calcareous
nannofossils were abundant already by the end of the Triassic, no
estimates exist on their contribution to hemipelagic carbonate
sedimentation. With this work, we estimate the volume proportion of
Prinsiosphaera, the dominant Late Triassic calcareous nannofossil, in
hemipelagic and pelagic carbonates of western Tethys. The investigated
Upper Triassic lime mudstones are composed essentially of microspar and
tests of calcareous nannofossils, plus minor bioclasts. Prinsiosphaera
became a significant component of lime mudstones since the late Norian,
and was contributing up to ca. 60% of the carbonate by the late Rhaetian
in periplatform environments with hemipelagic sedimentation. The
increasing proportion of Prinsiosphaera in upper Rhaetian hemipelagic
lime mudstones is paralleled by a increase of the δ13C
of bulk carbonate. We interpreted this isotopic trend as related to the
diagenesis of microspar, which incorporated respired organic carbon with
a low δ13C when it formed during shallow burial. As the
proportion of nannofossil tests increased, the contribution of microspar
with low δ13C diminished, determining the isotopic
trend. We suggest that a similar diagenetic effect may be observed in
many Mesozoic limestones with a significant, but not yet dominant,
proportion of calcareous plankton.
Abundant calcispheres occur in Upper Carnian and Norian hemipelagic limestone successions of the Southern Apennines and Sicily. They exhibit a variety of morphologies that were investigated with optical and scanning electron microscopy (SEM). The most common morphology is that of a full solid sphere of radiaxial calcite crystals, 20–22 μm in diameter on average, with or without a minor hollow in the center. Smaller forms may be clusters of sub-micron crystals, only rarely disposed as to form a spherical test with a diameter of 10 μm or less. Larger forms are similar to small forms (clusters or spheres of sub-micron crystals) with an epitaxial calcite overgrowth. The taxonomic attribution of these calcispheres is uncertain, mostly because of their poor preservation, but a comparison is possible with some Mesozoic calcispheres attributed to calcareous dinocysts. The amount of epitaxial overgrowth is variable, but in most cases much larger than the original sphere. This prevents a significant evaluation of the contribution of calcispheres to carbonate pelagic sedimentation by point counting in thin-section. However, it can be shown that calcispheres become abundant only after a major climatic perturbation dated at the end of the Early Carnian, known as the Carnian Pluvial Event (CPE). This event involved a strong and prolonged enhancement of the hydrological cycle, with consequent supply of excess hydrogen carbonate to the oceans and increased seawater alkalinity. Although calcispheres of this type are known at least from the Middle Triassic, it is only shortly after the CPE that they become abundant, and the first common occurrence of calcareous nannoplankton in the western Tethys is thus Late Carnian in age.
New high-precision U/Pb geochronology from volcanic ashes shows that the Triassic-Jurassic boundary and end-Triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial flood volcanism in the Central Atlantic Magmatic Province to <150 ka. This narrows the correlation between volcanism and mass extinction by an order of magnitude for any such catastrophe in Earth history. We also show that a concomitant drop and rise in sea level and negative δ13C spike in the very latest Triassic occurred locally in <290 ka. Such rapid sea-level fluctuations on a global scale require that global cooling and glaciation were closely associated with the end-Triassic extinction and potentially driven by Central Atlantic Magmatic Province volcanism.
2004-08 Scotese, C.R., 2004. A continental drift flipbook, Journal of Geology, v. 112, issue 6, p. 729-741. (95)
Abstract: Forty-six miniature plate tectonic reconstructions are presented that can be assembled into a "flipbook" that illustrates the movement of the continents since the Late Precambrian, 750 m.yr. ago. Six principal lines of evidence have been used to reconstruct the past positions of the continents: (1) linear magnetic anomalies produced by sea floor spreading, (2) paleomagnetism, (3) hotspot tracks and large igneous provinces, (4) the tectonic fabric of the ocean floor mapped by satellite altimetry, (5) lithologic indicators of climate (e.g., coals, salt deposits, tillites), and (6) the geologic record of plate tectonic history. I discuss the probable uncertainties associated with the plate tectonic reconstructions and give an estimate of the uncertainty in the positions of the continents back through time.
A new version of global and regional palaeogeographic maps is presented for two time intervals. These maps depict the plate tectonic configuration, palaeoenvironment and lithofacies during the Late Triassic (Carnian–Norian) and Early Jurassic (Hettangian–Toarcian) time. The individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the Absaroka sequence of Sloss. During Triassic time Pangaea began to stretch, initiating the rifting and future break-up of the supercontinent. The continued northward drift of the Cimmerian continent corresponded with the progressive closure and consumption of Palaeotethys oceanic crust, and the opening of the Neotethys Ocean. The most significant Late Triassic convergent event was the Indosinian orogeny, occurring as a result of the consolidation of South China and North China blocks. Also, Indochina and Indonesia were sutured to South China. At the same time the Qiangtang block approached the Eurasian margin. The consolidation of the North Chinese and Amurian blocks left open a large embayment of Panthalassa, between Amuria and Laurasia, the so-called Mongol-Okhotsk Ocean. Active subduction existed along the margin of this ocean, dipping cratonwards towards East Siberia. The last collisional events of the Uralian orogeny took place during the Triassic and Early Jurassic time. The conclusion of the Uralian orogeny was accompanied by uplift of the adjacent areas of Eastern Europe and Western Siberia. During the Early Jurassic the Palaeotethys Ocean was finally closed and the Cimmerian continent collided with Asia causing the Cimmerian orogeny. The time around the Triassic–Jurassic boundary marked an important biotic extinction event. Plate tectonic activity caused palaeogeographic and palaeoclimatic change, which may have contributed to the mass extinction. From the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction: 1) the closure of Palaeotethys and assembly of the Asian part of Pangaea; 2) the break-up of Pangaea in the future Central Atlantic area and transition from rifting to drifting; and 3) the very extensive basaltic volcanism of the Central Atlantic Magmatic Province.
COCCOLITHS, the calcite plates formed by unicellular phyto-planktonic
algae (coccolithophores, phylum Prymnesiophyta)1, are
produced in enormous quantities and probably constitute the largest
single carbonate sink in oceanic biogeochemical cycles. They are major
sediment formers, are of great value to geologists as biostratigraphic
indicators and have been extensively studied as models for
biomineralization2,3. We have applied the understanding of
coccolith biomineralization to the fossil record4 and present
evidence from electron and optical microscopy that there has been a
conserved mechanism of crystal nucleation throughout the 230 million
year history of coccolithophores. This fundamental feature of coccolith
growth, which we term the V/R model, involves the assembly of a ring of
single crystals with alternating orientations, radial (R) and vertical
(V), and provides a powerful tool for tracing phylogenies, identifying
homologous structures and rationalizing the higher taxonomy of the
group. The living cocco-lithophorid, Emiliania huxleyi, seemed anomalous
because only R crystals had been observed; transmission electron
microscopy of proto-coccolith rings, however, revealed relict V crystals
which are overgrown by preferential development of R units in complete
coccoliths. A speculative model based on a plicated macromolecular
template is presented as a possible explanation for the conserved
nucleation process.
The Carnian pluvial event (CPE) is considered as the drastic environmental change that occurred in the western Tethys during the Carnian (Late Triassic). Although some studies have suggested that the CPE was a global event, no studies have referred to pelagic environment related to the CPE. To reveal the pelagic environment during the Carnian, X-ray absorption fine structure (XAFS) analysis was used to determine the iron (Fe) species in chert samples collected from the Ladinian–Carnian strata in the Tamba–Mino–Ashio Belt, Japan. The X-ray absorption near-edge structure (XANES) analysis shows that all the red cherts contain hematite with a combination of clay minerals, such as illite, chlorite, and smectite. The existence of these minerals is identified using extended XAFS (EXAFS). The XANES analysis of the green and the white cherts does not show any hematite signatures, while the EXAFS analysis indicated increasing amounts of chlorite.
Zusammenfassung: Eoconusphaera zlambachensis und Prinsiosphaera triassica, die wohl häufigsten Nannofossilien der obersten Trias, werden in verschiedenen Erhaltungsstadien aus weiteren, neuen Fundpunkten rhätischer Zlambachmergel der Nördli- chen Kalkalpen vorgestellt. Erstmals wird Eoconusphaera jansae, die bisher erst ab dem Unter-Sinemur bekannt ge- wesen war, unerwarteterweise aus rhätischen Zlambachmergeln der Ostalpen nachgewiesen. Auch in Rhätmergeln des Wombat-Plateaus NW vor Australien ist ihr Auftreten evident, womit ihre tethysweite Verbreitung schon in der obersten Trias erwiesen ist. Abstract: Eoconusphaera zlambachensis and Prinsiosphaera triassica, surely the most common nannofossils of the Uppermost Triassic, are presented in variable stages of preservation from new sites of Rhaetian Zlambach marls of the Northern Calcareous Alps. Eoconusphaera jansae, hitherto known only since the Lower Sinemurian, has now been found in the Rhaetian Zlambach marls of the Eastern Alps. They also occur in the Rhaetian marls of the Wombat plateau NW of Australia. Thus their occurrence throughout the whole of the Tethys, beginning in the Upper Triassic, is demonstrated. 1. Einleitung Zweck der vorliegenden kleinen Studie ist es, weitere Beobachtungen und Bemerkungen zum Vorkommen, zur Erhaltung und zur Artabgren- zung von Vertretern der häufigsten im Raum der Tethys vorkommenden obersttriadischen kalki- gen Nannofossilien darzulegen. Wie anhand neuerer Untersuchungen von rhätischen Zlam- bachmergeln im Bereich der Nördlichen Kalkal- pen festgestellt werden konnte, sind es in erster Linie die kugeligen und konischen Vertreter, die zufolge ihrer Robustheit und vielleicht auch ver- hältnismäßigen Größe diagenetische Verände- rungen eher überstanden haben als die wesent- lich zarteren Coccolithen. So ist es doch immer wieder möglich, Angehörige der Gattungen Eo- conusphaera und Prinsiosphaera identifizieren zu können, wenn sie auch schon in einem dia- genetisch weit umgewandelten Stadium vorhan- den sind oder auch schon knapp vor dem Zerfall stehen. Da inzwischen auch für die obersttriadi-
Gardin, S., Krystyn, L., Richoz, S., Bartolini, A. & Galbrun, B. 2012: Where and when the earliest coccolithophores? Lethaia, Vol. 45, pp. 507–523.
New calcareous nannofossil analyses from the Northern Calcareous Alps of Austria are herein used to update and improve the state of knowledge about the oldest occurrence of coccolithophores reported in the literature. Previously reported Norian occurrences of coccoliths were based on an obsolete Triassic chronostratigraphy, in which the Rhaetian stage was subsumed into the Norian (‘Sevatian 2’). The oldest stratigraphical record of coccoliths spp. lies just below the Norian-Rhaetian boundary and the first coccolith species, Crucirhabdus minutus, is recorded from the base of Rhaetian stage. The latter bio-event is located just above the First Occurrence of the conodont Misikella posthersteni and the first occurrence of the ammonoid Paracochloceras suessi in the Steinbergkogel section (Austria), Global Stratotype Section and Point (GSSP) candidate for the Norian–Rhaetian boundary. The appearance of the coccolith Crucirhabdus minutus is seen as a robust biochronological datum that will provide useful constraints for Triassic biostratigraphy, palaeoclimatic modelling and phylogenetic reconstructions. The new calcareous nannofossil biochronology of Steinbergkogel that we present herein completes the existing biostratigraphic characterization of the Norian-Rhaetian transition based on conodonts and ammonoids and strengthens the position of Steinbergkogel as the best GSSP proposed section for the base of the Rhaetian. The record of coccolithophores across the Norian–Rhaetian boundary at Steinbergkogel takes place along with a discernible increase in abundance of Prinsiosphaera triassica, as well as the appearance of Euconusphaera zlambachensis, which are the two most important Rhaetian pelagic carbonate producers. This succession of bio-events is interpreted as the initiation of the pelagic carbonate production driven by the successful spreading of calcareous nannofossils in the Western Tethys during the Rhaetian. □Austria, Calcareous Alps, coccolithophores, Triassic.
A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofossils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagenetic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated byPrinsiosphaera triassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, withCrucirhabdus primulus as the ancestor. This species gave rise toC. minutus andArchaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences ofC. primulus andEoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphometric changes in this time-interval which possess biostratigraphic utility: (1)P. triassica increases in diameter from an average of 6 μm to over 9 μm; (2)E. zlambachensis evolves from a stubby to an elongated shape; and (3)C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).
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