Susana E. Damborenea

First record of the Early Toarcian Oceanic Anoxic Event from the Southern Hemisphere, Neuquén Basin, Argentina

Abstract: The first record of the Early Toarcian Oceanic Anoxic Event (c. 183 Ma) from the Southern Hemisphere is described from the Neuquén Basin, Argentina, identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon-isotope excursion (δ13Corg) in bulk rock and fossil wood. The negative excursion of −6‰ in bulk organic carbon (falling to −31.3‰) crosses the boundary of the tenuicostatum–hoelderi Andean ammonite Zones, equivalent to the tenuicostatum–falciferum/serpentinum zones of Europe. These data indicate that the Early Toarcian Oceanic Anoxic Event was a global phenomenon. Supplementary material: A detailed stratigraphic log, chemostratigraphic data and nannofossil data are available at http://www.geolsoc.org.uk/SUP18411.

Jurassic evolution of Southern Hemisphere marine palaeobiogeographic units based on benthonic bivalves

Abstract The distribution of benthonic Jurassic bivalve genera in the Southern Hemisphere is analysed here. For this region, palaeobiogeographic units (biochoremas) are quantitatively characterized according to their biologic contents (mainly levels of endemism). Their evolution through time is followed from the latest Triassic to the earliest Cretaceous. The Tethyan Realm is undoubtedly the most mature and persistent through time, with three subordinate units: an Australian unit restricted to the Late Triassic, a North Andean unit, which appears sporadically as an endemic centre, and an East African unit which is recognisable from Bajocian times onwards. From Late Triassic times, a South Pacific Realm has been recognised, with a Maorian Province mostly based on the distribution of monotoid genera. A South Andean unit is also recognisable through most of the Jurassic, and its reference either to the South Pacific unit or to the Tethyan Realm is a matter of debate. Being a transitional biogeographic setting between Tethyan and South Pacific first-order units, it is included in the South Pacific unit due to the common presence of antitropical (didemic) genera. The East African unit is included within the Tethyan Realm during the Jurassic, but during Early Cretaceous times, it splits into two units, one of which was regarded as part of the “South Temperate Realm” by Kauffman. The rank of all these units changed with time. Throughout the Jurassic, the ecotone between South Pacific and Tethyan palaeobiogeographic units fluctuated in position with time. The approximate latitudinal location of the ecotonal boundary area and its shift through time are recognised on the basis of faunal composition along the Andean region.

Hettangian and Sinemurian (lower Jurassic) biostratigraphy of Argentina

Abstract On the northern bank of the Rio Atuel, Mendoza province, Argentina, the marine fossiliferous Mesozoic is now known to begin with Hettangian-lower Sinemurian strata. Traditionally, the marine Jurassic sequence in the area was believed to start with upper Sinemurian levels. Representatives of the genera Psiloceras Hyatt, Caloceras Hyatt, Alsatites Haug, Waehneroceras Hyatt, Schlotheimia Bayle, Sulciferites Spath, Badouxia Geux and Taylor, Vermiceras Hyatt, Coroniceras Hyatt, Agassiceras Hyatt, Euagassiceras Spath, and Arnioceras Hyatt thus indicate the presence of beds equivalent to the Planorbis, Liasicus, Angulata, Bucklandi, Semicostatum, and Turneri zones of the standard international chronostratigraphic scale. Hence, in Argentina, the marine Hettangian, Sinemurian, and lowermost Pliensbachian have been proved only in the neighborhood of the Rio Atuel region, where marine Triassic could also be represented. Elsewhere in Argentina, the oldest Jurassic ammonite-bearing levels usually belong to the upper lower or the upper Pliensbachian. Regionally, these findings imply a substantial modification to previous paleogeographic reconstructions for the Early Jurassic of South America.

The Toarcian Oceanic Anoxic Event (Early Jurassic) in the Neuquén Basin, Argentina: A Reassessment of Age and Carbon Isotope Stratigraphy

The Toarcian oceanic anoxic event (T-OAE) is recorded by the presence of globally distributed marine organic carbon–rich black shales and a negative carbon isotope shift, with δ13Corg values as low as −33‰, interrupting an overarching positive excursion. Here we present new biostratigraphic data and high-resolution δ13Corg data from two Southern Hemisphere localities: Arroyo Serrucho in the north and Arroyo Lapa in the south of the Neuquén Basin, Argentina. Previous studies at these localities aimed to provide an accurate numerical age for the T-OAE and characterization of its carbon isotope stratigraphy. The new carbon isotope data and ammonite biostratigraphy presented here from Arroyo Serrucho show the T-OAE to be recorded lower in the section than supposed by previous authors, thus calling into question the published age of the T-OAE in this section. A newly investigated exposure at Arroyo Lapa North shows a complex carbon isotope record with at least three high-amplitude fluctuations in the hoelderi zone (equivalent to the serpentinum zone in northwestern Europe), with δ13Corg values of <−28‰, and two intervening positive isotope excursions, with δ13Corg values around −24‰. At Arroyo Lapa South, the characteristic major stepped negative carbon isotope excursion is recorded, with δ13Corg values of <−30‰ and total organic-carbon contents increasing to 11%; above this level an erosional surface of a submarine channel truncates the section. These new data are globally correlative and unambiguously illustrate the global reach of the T-OAE.

Southern Hemisphere Palaeobiogeography of Triassic-Jurassic Marine Bivalves

Bivalves have proven to have a great potential for paleobiogeographic analyses due to their relatively complete fossil record, especially for Mesozoic and Cenozoic times. Being mostly benthonic, they have a large variety of life habits which should be taken into account, particularly in detailed paleobiogeographic studies. We will analyze marine bivalve distribution in the Southern Hemisphere during several successive time slices within the Triassic and Jurassic, an epoch marked by critical geologic and biotic events. This period covers both the biotic recovery after the harshest diversity crisis ever (the Permian/Triassic extinction event), and later also the biotic reaction to another severe crisis at the Triassic/ Jurassic boundary. This allows the opportunity to evaluate the response of paleobiogeographic patterns to such events. The Earth’s configuration drastically changed from a concentration of land masses in a unique supercontinent (Pangea) and two oceans (Tethys and Panthalassa), to a fragmented series of continental land masses. These began to disperse, opening sea corridors which largely affected not only the global distribution of biotas but also paleoclimate and sea paleocurrents as well. This dynamic paleogeography adds an interesting ingredient to the study of past distributions of benthic organisms making it possible to frame them into a physically and biologically changing scenario. ‘Every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants, must have been interested in the singular facts which it presents... Of late years, ... a great light has been thrown upon the subject by geological investigations, which have shown that the present state of the earth, and the organisms now inhabiting it, are but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions.’ Alfred Russel Wallace 1855 S. E. Damborenea et al., Southern Hemisphere Palaeobiogeography of Triassic-Jurassic Marine Bivalves, SpringerBriefs Seaways and Landbridges: Southern Hemisphere Biogeographic Connections Through Time, DOI: 10.1007/978-94-007-5098-2_1, The Author(s) 2013 1 Alfred Russel Wallace, considered the nineteenth century’s leading expert on the geographic distribution of animal species and sometimes called the ‘‘father of biogeography’’, already recognized the importance of studying the history of biotas long before moving continents and plate tectonics were heard or even thought of. On the other hand Darwin, who initially recognized the importance of geographic isolation to speciation in his unpublished notebooks (see Lieberman 2003, 2008), did not mention this in his later publications. The study of global biodiversity changes is a hot issue these days, as we humans become aware of the fragility of the Earth system and the urgent need to understand it better to keep it going. One of the key aspects of biodiversity is the distribution of organisms, and biogeography is the discipline which tries to recognize and characterize the causes and patterns of distribution. Biogeography is closely linked to ecology, since the distribution of organisms is governed by ecologic factors, but it cannot ignore other matters, such as the origin of species, their dispersal, and extinction, and thus it can be considered a historic science. Biologists are beginning to investigate the causes of the great global biodiversity changes that are now taking place on the Earth. But paleontologists, who possess a much more extensive time perspective, are constantly observing and surveying the changes in biodiversity produced at various times in the past, and they have the most precise access possible to this very important dimension: time. Thus, paleobiogeography, which studies the distribution of organisms in the past, is a very complex subject that combines information from both biology and the Earth sciences (Cecca 2002), and ‘‘paleobiogeographers can actually monitor how the Earth and its biota have evolved and coevolved’’ (Lieberman 2000). The data provided by the fossil record are increasingly being used in combination with other sorts of data in modern biogeographic analysis. The relationship between geology and biogeography is then unavoidable, and should be based on a reciprocal illumination approach (Morrone 2009). Similarly to biogeography, different approaches can be recognized for paleobiogeography (Rosen 1992, 1995): 1. Descriptive paleobiogeography: recognition and description of the distribution of organisms. The outcome is the definition of biogeographic units or biochoremas. Both quantitative (more frequent in neobiogeography) and qualitative (more subjective) methods can be used. 2. Causal paleobiogeography: examines the causes of the observed distributions. There are many arguments related to theoretic biogeography and the philosophic approaches, which will not be discussed further here (for a good synthesis see Cecca 2009). According to the temporal scale of the processes involved, two main viewpoints allow distinction between ecologic biogeography, with a temporal scale related to biologic cycles, and historic biogeography when long-term processes are analyzed. 3. Applied paleobiogeography: the distribution of organisms can be used to infer the role of ecologic factors, the relation between phylogeny and provinciality, or paleogeographic patterns. 2

THE TRIASSIC/JURASSIC BOUNDARY IN THE ANDES OF ARGENTINA

The Arroyo Malo Formation at Alumbre Creek, on the northern bank of the Atuel River, west central Argentina, comprises a c. 300 m thick continuous marine succession across the Triassic-Jurassic System boundary, consisting of massive and laminated pelites indicative of a slope depositional environment. Late Triassic invertebrates, including ammonoids, nautiloids, bivalves, gastropods, brachiopods and corals are restricted to the lower 150 m. Beds between 125-135 m from the bottom yield Choristoceras cf . marshi Hauer, a species found in the Marshi/Crickmayi Zone of Europe and North America, together with loose fragments of Psiloceras cf. pressum Hillebrandt, coeval with the lower to middle part of the Hettangian Planorbis Zone. About 80 m higher are beds yielding Psiloceras cf. rectocostatum Hillebrandt, a species that gives name to an Andean biozone partially coeval with the Johnstoni and Plicatulum Subzones, upper Planorbis Zone. Other fossils recorded in the Rhaetian strata of this section are foraminifers, ostracods and plant remains identified as Zuberia cf. zuberi (Szaj.) Freng. and Clathropteris sp. The section was also sampled for conodonts and radiolarians, thus far with negative results. A palaeomagnetic study is underway.

The genera Calliotropis Seguenza and Ambercyclus n. gen. (Vetigastropoda, Eucyclidae) from the Early Jurassic of Argentina

Abstract The systematic position of the fossil species referred to Calliotropis is currently under debate due to the striking resemblance between these forms with their extant counterparts in general shell morphology and ornament pattern. We propose two temporal subgenera of Calliotropis: Calliotropis (Riselloidea) for Mesozoic species and Calliotropis (Calliotropis) for Cenozoic and Recent taxa. We also synonymize the families Eucyclidae and Calliotropidae, and redescribe the type genus of Eucylidae based on topotypic material of E. obeliscus from Normandy. We argue that the generic name Amberleya should be restricted to its type species Amberleya bathonica. For the species that were traditionally included in Amberleya, we propose the new genus Ambercyclus, with its type species Amberleya orbignyana. The present paper also provides descriptions of three Calliotropis species from the Early Jurassic marine deposits of Argentina. The occurrence of Calliotropis (Riselloidea) keideli n. sp., Calliotropis (Riselloidea) cf. C. (R.) keideli and Calliotropis (Riselloidea) sp. in the Jurassic of Chubut and Neuquén provinces represents a new record of the genus in Early Jurassic sediments of Argentina and South America. Moreover, two species of Ambercyclus n. gen., such as Ambercyclus espinosus and Ambercyclus? isabelensis n. sp., are described from the same deposits. Eucyclus, Amberleya, Ambercyclus, and Calliotropis are included into Eucyclidae, which we consider to be a family of Seguenzioidea.

Middle Jurassic Biostratigraphyof Argentina

Abstract The Middle Jurassic of west-central Argentina comprises several stratigraphic sequences characterized on thebasis of ammonoids, bivalves, brachiopods, foraminifers and ostracods. Aalenian and Bajocian ammonoids belong in the Bredyia manflasensis, “Zurcheria” groeberi Puchenquia malarguensis, Pseudotoites singularis, Emileia giebeli, Stephanoceras humphriesianum and Megasphaeroceras rotundum Zones. Three independent series of four biozonal units each, respectively based on bivalves, brachiopods and microfauna, are recognized in this interval. Bathonian and Callovian ammonoids are allocated to the Cadomites--Tulitidae, Lilloettia steinmanni, Eurycephalites vergarensis, Neuqueniceras (Frickites) bodenbenderi, Hecticoceras proximum Zones and Rehmannia (Loczyceras) patagoniensis Horizon. At the same time, further biostratigraphic subdivisions are distinguished by other taxa, totalling two for bivalves, three for brachiopods and one for calcareous microfossils. Diversity trends of the different faunal groups and sequence distribution are explained by changes in relative extension of platform and basinal areas.

Barremian Bivalves from the Huitrín Formation, West-Central Argentina: Taxonomy and Paleoecology of a Restricted Marine Association

Abstract The Cretaceous Huitrín Formation in west-central Argentina records the final connection of the Neuquén Basin to the Pacific Ocean. This formation is comprised of a variety of continental to marginal-marine sediments deposited behind an Andean volcanic arc under warm, arid paleoclimatic conditions. Here we focus on a bivalve fauna from carbonate ramp deposits within the Barremian La Tosca Member of the Huitrín Formation. This fauna is very abundant and widely distributed within the basin but, surprisingly, it has not yet been studied in detail. In addition, paleoenvironmental affinities remain unresolved, with the fauna variously interpreted as having freshwater, brackish, and marine affinities. We studied the faunas taxonomy and paleoecology based on more than 500 specimens collected at ten fossil localities in combination with new field observations. The bivalve assemblage was recorded from middle to outer carbonate ramp deposits and is composed of five taxa of marine affinity: Phelopteria huitriniana n. sp., Isognomon cf. I. nanus (Behrendsen), Placunopsis? pichi n. sp., Anthonya jarai n. sp., and Argenticyprina mulensis n. gen. n. sp.; the first three may be regarded as eurytopic and/or opportunistic. Reduced diversity, low evenness, overall small size (length <4 cm), thin shells, eurytopic or opportunistic life strategies, and high endemism point to a restricted marine setting for the La Tosca Member. The most important limiting factors likely were low primary productivity and fluctuating salinity and temperature, as conditions inferred for the unit include high evaporation rates combined with low continental runoff and reduced rainfall. Thick evaporite deposits below and above La Tosca Member and thin intercalated gypsum beds support a restricted, hypersaline setting.

Bivalves from the Triassic−Jurassic transition in northern Spain (Asturias and western Basque−Cantabrian Basin)

Abstract. Bivalve mollusks from the Triassic-Jurassic transition collected in eight localities in Asturias and the western Basque-Cantabrian Basin (Palencia province) are systematically revised. Preservation is poor at all localities. The dominant Rhaetian bivalves are Isocyprina concentrica (Moore) and Bakevellia (Bakevelloides) praecursor (Quenstedt). These species, together with Isocyprina cf. ewaldi (Bornemann), Pteromya cf. crowcombeia (Moore), Pseudoplacunopsis alpina (Winkler), and Modiolus? sp. (cf. minimus J. Sowerby), with a specimen of Arcestidae (?), belong to an assemblage similar to that found in the Westbury and Lilstock formations (Penarth Group) in the late Rhaetian of southern England. The most abundant Hettangian species is Isocyprina (Eotrapezium) germari (Dunker). Others are referred to Cuneigervillia rhombica (Cossmann), Sphaeriola? sp., Eomiodon? sp. and Pteromya cf. tatei (Richardson and Tutcher). All Hettangian shell beds examined are monotypic or have very low diversity, a biological indication that they may belong to a restricted marine environment, with high environmental stress levels. Even the more diverse assemblage (Pteromya-Cuneigervillia-Eomiodon) was probably also salinity controlled. The fauna analyzed here clearly belongs to the same facies and environment as those described from Aquitaine (France) and the Pyrenees and is different from coeval bivalve assemblages from other European Hettangian localities. The Triassic-Jurassic boundary cannot be precisely located at the studied sections on the basis of the bivalve faunas alone, but these indicate that the transition beds in Asturias were deposited in a marginal marine environment and the benthic fauna was dominated by shallow burrowing, suspensivorous bivalves.