Aalenian Bajocian Boundary Research Articles

Catutosaurus gaspariniae, gen. et sp. nov. (Ichthyosauria, Thunnosauria) of the Upper Jurassic of Patagonia and the evolution of the ophthalmosaurids

ABSTRACT Thunnosaurian ichthyosaurs were one of the most successful lineages of Mesozoic tetrapods in terms of adaptation to a pelagic lifestyle. Among them, the ophthalmosaurids are of particular interest. Their fossils document an early widespread distribution, including high northern and southern latitudes soon after they appear at the Aalenian–Bajocian boundary, becoming almost the only surviving lineage of ichthyosaurs for approximately 76 million years until their extinction at the end of the Cenomanian (Late Cretaceous). Evolutionary innovations that could have been involved in their remarkable evolutionary success are still poorly understood. Two regions of the skeleton, involved in two vital functions – breathing and swimming – depict notable variation among ophthalmosaurids: the narial region and the proximal forefin configuration (i.e., stylopodium-zeugopodium-mesopodium pattern). Here, we describe Catutosaurus gaspariniae gen. et sp. nov., a new Tithonian (Late Jurassic) ophthalmosaurid from the northwest of Patagonia, Argentina. The new taxon depicts a peculiar pattern of the narial opening and forefin. Catutosaurus has an ‘ophthalmosaurine-like’ skull with reduced cheek, triangular squamosal, slender and delicate snout and rounded tooth roots; and ‘platypterygiine-like’ forefin composed by the typical polygonal and tightly packed elements and a broad articulation between the stylopodium and zeugopodial row. Phylogenetic analysis retrieves Catutosaurus nested within basal ophthalmosaurids, at the base of a large clade named here as Panplatypterygiine. New information provided by Catutosaurus suggests that changes in the narial region and complexity of the stylopodium–zeugopodium articulation occurred independently among ophthalmosaurids and not in a stepwise manner along with the evolution of the clade. http://zoobank.org/urn:lsid:zoobank.org:pub:FDB6C301-BAE5-4212-9310-BE09E24605F9

Interacción entre vulcanismo, tectónica y sedimentación en una plataforma carbonatada somera: Ejemplo en el Tethys occidental (Jurásico Medio, sureste de la Cordillera Ibérica)

En el sureste de la Cordillera Ibérica (España) existen una serie de afloramientos de rocas volcánicas origi­nadas por erupciones submarinas, principalmente de tipo volcanoclástico, interestratificadas en las sucesiones carbonatadas del Jurásico Inferior y Medio. La cartografía geológica de los afloramientos de rocas volcánicas revela que se encuentran alineados a lo largo de tres elementos de debilidad cortical bien definidos (zonas de falla de Teruel, de Caudiel y de Alcublas), originados probablemente por fracturas relacionadas con la apertura y expansión del Tethys Ligur y del Tethys Alpino. En este trabajo se presentan los resultados del estudio de la evolución tectonosedimentaria de un ambiente submarino somero afectado por la presencia de una acumulación volcánica que tuvo lugar en el Jurásico Medio (límite Aaleniense‒Bajociense). Se trata del afloramiento volcánico de Caudiel, que es el más suroriental de todos los afloramientos localizados en uno de los tres elementos de debilidad estructural mencionados: la Zona de Falla de Caudiel. La presencia de un relieve positivo en el fondo de la plataforma, ya de por sí bastante somera, condiciona la sedimentación carbonatada, muy sensible a la pro­fundidad, hasta el punto de llegar a inducir cambios significativos de facies con respecto a otras áreas cercanas o de impedir la sedimentación. Los movimientos episódicos del substrato generaron una topografía irregular y cambiante a lo largo del tiempo, con tasas variables de hundimiento y levantamiento en diferentes áreas. El aflo­ramiento de Caudiel constituye un buen ejemplo fósil de interacción entre varios factores (vulcanismo, tectónica y sedimentación) en plataformas carbonatadas someras, en las que los cambios de acomodación y las tasas de sedimentación parecen estar subordinadas a perturbaciones tectónicas de carácter local.

Benthic foraminiferal assemblages across the Aalenian–Bajocian boundary in the Iberian and Basque-Cantabrian basins (North Spain)

The aim of this work is to describe the benthic foraminiferal assemblages recorded across the Aalenian–Bajocian boundary (Middle Jurassic) in two sections of the Iberian and Basque-Cantabrian basins (North Spain), well calibrated with the standard ammonite zonation, in order to establish a biostratigraphic scale based on the foraminiferal record, and to infer the palaeoecological conditions that affected their development. A total of 29 samples were collected in the studied sections, and the composition of the foraminiferal assemblages recorded throughout the studied stratigraphic interval was analysed using the stratigraphic distribution of the identified species, the quantitative data obtained through the estimation of relative abundances and the calculation of several diversity indexes. The studied assemblages are moderately abundant and diverse, and they are constituted by a total of 5435 specimens, corresponding to 5 suborders, 13 families, 25 genera and 51 species. Their composition displays close similarities to those already described for the Jurassic carbonate platforms of the Boreal Realm (Type-A), Boreal Atlantic subdivision. For the studied stratigraphic interval, it was recognised the Lenticulina quenstedti Foraminiferal Zone, in which the recorded assemblages developed in intermediate part of a marine platform environmental conditions, well oxygenated and with normal salinity. In the Aalenian–Bajocian transition, it was possible to identify a sharp decrease in the abundance and diversity of the foraminiferal assemblages that has also been recognised in other coeval basins of the Iberian Plate palaeomargin, thus representing a bioevent of the Northern Hemisphere.

Carbon and oxygen isotope stratigraphy of Jurassic platform carbonates from Saudi Arabia: Implications for diagenesis, correlations and global paleoenvironmental changes

Major carbon cycle disturbances reflected by carbon isotope excursions (CIE) have been reported in the Jurassic successions of Euro-Boreal and Mediterranean basins. However, sedimentary and geochemical data from other domains are still required to unravel the global or regional nature of paleoenvironmental disturbances and identify the potential triggers. Here, we present biostratigraphically well-constrained (middle Toarcian to lower Kimmeridgian) bulk-rock δ13C and δ18O data from central Saudi Arabia. Five major negative δ18O excursions are identified in back-reef facies below sequence boundaries and disconformities (Aalenian-Bajocian boundary, upper Bajocian, middle Bathonian, upper Callovian and lower Kimmeridgian), suggesting that these carbonates underwent intense meteoric diagenesis during partial or major subaerial exposures of the Arabian Platform. The δ13C record is less altered and shows correlations with the regional relative sea-level changes influencing the productivity levels and inflows of fluvial (12C-enriched) or oceanic (12C-depleted) waters onto the platform. Overall, most of the δ13C variations recorded at the scale of the Arabian Platform within the lower and middle Jurassic are in agreement with NW Tethyan data. Among these variations, the increase in δ13C values recorded in the middle Toarcian, lower Bajocian, and lower Bathonian zigzag Zone indicate a considerable increase of productivity and 12Corg burial concomitant with global sea-level rises. Conversely, the occurrence of major but diachronous negative CIEs at the Aalenian-Bajocian boundary and around the lower-upper Bajocian transition are interpreted as accelerations of terrestrial 12Corg transfer to the ocean and atmosphere linked to regional but widespread emergences of landmasses. Whether global (eustatic) or regional (tectonic), all these sea-level changes and carbon transfers could be distal consequences of successive episodes of the Pangean breakup.

Carbon cycle history through the Middle Jurassic (Aalenian – Bathonian) of the Mecsek Mountains, Southern Hungary

AbstractA carbonate carbon isotope curve from the Aalenian–Bathonian interval is presented from the Óbánya valley, of the Mecsek Mountains, Hungary. This interval is certainly less well constrained and studied than other Jurassic time slices. The Óbánya valley lies in the eastern part of the Mecsek Mountains, between Óbánya and Kisújbánya and provides exposures of an Aalenian to Lower Cretaceous sequence. It is not strongly affected by tectonics, as compared to other sections of eastern Mecsek of the same age. In parts, a rich fossil assemblage has been collected, with Bathonian ammonites being especially valuable at this locality. The pelagic Middle Jurassic is represented by the Komló Calcareous Marl Formation and thin-bedded limestones of the Óbánya Limestone Formation. These are overlain by Upper Jurassic siliceous limestones and radiolarites of the Fonyászó Limestone Formation. Our new data indicate a series of carbon isotope anomalies within the late Aalenian and early-middle Bajocian. In particular, analysis of the Komló Calcareous Marl Formation reveals a negative carbon isotope excursion followed by positive values that occurs near the base of the section (across the Aalenian–Bajocian boundary). The origin of this carbon-isotope anomaly is interpreted to lie in significant changes to carbon fluxes potentially stemming from reduced run off, lowering the fertility of surface waters which in turn leads to lessened primary production and a negative δ13C shift. These data are comparable with carbonate carbon isotope records from other Tethyan margin sediments. Our integrated biostratigraphy and carbon isotope stratigraphy enable us to improve stratigraphic correlation and age determination of the examined strata. Therefore, this study of the Komló Calcareous Marl Formation confirms that the existing carbon isotope curves serve as a global standard for Aalenian–Bathonian δ13C variation.

Paleoecological quantitative analysis based on benthic foraminifera of the Aalenian–Bajocian boundary (upper Bradfordensis–lower Discites) in the Barranco de Agua Larga section (Betic Cordillera, Southern Spain)

To report, for the first time, the benthic foraminiferal assemblages recorded across the upper Bradfordensis–lower Discites ammonite zones (Aalenian–Bajocian boundary, Middle Jurassic) in the Barranco de Agua Larga section (Betic Cordillera, Southern Spain),in order to infer the paleecological conditions that affected their development. A total of 17 samples were collected in this reference section, and the composition of the foraminiferal assemblages recorded throughout the studied stratigraphical was analyzed using quantitative data obtained through the estimation of relative abundances and the calculation of several diversity indexes. The assemblages are abundant (a total of 3, 139 well-preserved specimens) and diverse(82 species). Their composition displays close similarities to those already described for the Jurassic carbonate platforms of the Boreal Realm (Type-A), Boreal Atlantic subdivision. The studied assemblages were developed in distal marine environmental conditions, at a depth situated above the level of the calcite compensation, well oxygenated and with normal salinity, although for certain times (Aalenian–Bajocian boundary) unfavorable environmental conditions could have developed. The sharp decrease in the abundance and diversity that was recorded during the Aalenian–Bajocian transition has also been recognized in other coeval basins of the Iberian Plate paleomargin, thus representing a bioevent of regional rank.

High resolution ammonite–benthic foraminiferal biostratigraphy across the Aalenian–Bajocian boundary in the Lusitanian Basin (Portugal)

This work describes the ammonite and benthic foraminiferal assemblages recorded across the Aalenian–Bajocian boundary of the Serra da Boa Viagem II section, located about 6 km to the east of the Bajocian GSSP (Murtinheira, Portugal), and calibrated to the standard ammonite zonation previously established for the Lusitanian Basin. A total of 220 ammonite specimens referred to 30 fossiliferous levels were collected and identified throughout the section, enabling the recognition of the Concavum Zone (Concavum and Limitatum subzones) of the upper Aalenian, and the Discites Zone of the lower Bajocian. A total of 2356 foraminifers were obtained from the 16 samples collected along the section, corresponding to 4 suborders, 8 families, 16 genera and 44 species.The occurrence of Lenticulina quenstedti (Gümbel) has enabled the recognition of the Lenticulina quenstedti Zone, ranging from the Bradfordensis Zone (middle Aalenian) to the lower Discites Zone (lower Bajocian). The first record of Ramulina spandeli Paalzow, whose occurrence, up to now, was limited in the Lusitanian Basin to the Murtinheira section (the Bajocian GSSP), highlights the usefulness of the Ramulina spandeli Zone, with its lower boundary referred to the lower Discites Zone (lower Bajocian). Other bioevents displaying local, basinal or regional biostratigraphic interest have also been identified.The benthic foraminiferal record here presented, accurately calibrated with the ammonite record, aims at contributing to support the recognition of the Lenticulina quenstedti Zone and the Ramulina spandeli Zone as formal biostratigraphic units integrating the biostratigraphic scale based on benthic foraminifers for the Aalenian–Bajocian boundary in the Lusitanian Basin (Portugal). Copyright © 2014 John Wiley & Sons, Ltd.

Ophthalmosaurian (Ichthyosauria) records from the Aalenian–Bajocian of Patagonia (Argentina): an overview

AbstractThe oldest ophthalmosaurian records worldwide have been recovered from the Aalenian–Bajocian boundary of the Neuquén Basin in Central-West Argentina (Mendoza and Neuquén provinces). Although scarce, they document a poorly known period in the evolutionary history of parvipelvian ichthyosaurs. In this contribution we present updated information on these fossils, including a phylogenetic analysis, and a redescription of ‘Stenopterygius grandis’ Cabrera, 1939. Patagonian ichthyosaur occurrences indicate that during the Bajocian the Neuquén Basin palaeogulf, on the southern margins of the Palaeopacific Ocean, was inhabited by at least three morphologically discrete taxa: the slenderStenopterygius cayi, robust ophthalmosaurianMollesaurus periallusand another indeterminate ichthyosaurian. Rib bone tissue structure indicates that rib cages of Bajocian ichthyosaurs included forms with dense rib microstructure (Mollesaurus) and forms with an ‘osteoporotic-like’ pattern (Stenopterygius cayi).

First Diagnostic Marine Reptile Remains from the Aalenian (Middle Jurassic): A New Ichthyosaur from Southwestern Germany

BackgroundThe Middle Jurassic was a critical time in the evolutionary history of ichthyosaurs. During this time interval, the diverse, well-studied faunas of the Lower Jurassic were entirely replaced by ophthalmosaurids, a new group that arose sometime prior to the Aalenian-Bajocian boundary and by the latest middle Jurassic comprised the only surviving group of ichthyosaurs. Thus, the Middle Jurassic Aalenian-Bathonian interval (176–165 million years ago) comprises the time frame during which ophthalmosaurids not only originated but also achieved taxonomic dominance. However, diagnostic ichthyosaur remains have been described previously from only a single locality from this interval, from the Bajocian of Argentina.Methodology/Principal FindingsIn this paper, we describe a new species of ichthyosaur based on a partial articulated specimen from the Middle Jurassic of southwestern Germany. This specimen was recovered from the Opalinuston Formation (early Aalenian) and is referable to Stenopterygius aaleniensis sp. nov. reflecting features of the skull and forefin. The genus Stenopterygius is diverse and abundant in the Lower Jurassic of Europe, but its presence has not previously been confirmed in younger (Middle Jurassic) rocks from the northern hemisphere.Conclusions/SignificanceThis specimen represents the only diagnostic ichthyosaur remains reported from the Aalenian. It bears numerous similarities in size and in morphology to the Lower Jurassic species of the genus Stenopterygius and provides additional evidence that the major ecological changes hypothesized to have occurred at the end of the Toarcian took place sometime after this point and most likely did not occur suddenly. There is currently no evidence for the presence of ophthalmosaurids in the northern hemisphere during the Aalenian-Bathonian interval.

Investigating boron isotopes in a middle Jurassic micritic sequence: Primary vs. diagenetic signal

Boron isotope abundance in carbonates provides a powerful proxy for seawater pH reconstructions. Nevertheless, application of this proxy on bulk rocks has never been clearly investigated. Herein, boron isotopes were measured in early Bajocian samples from the Terminilletto section (Italy) to explore the meaning of bulk rock δ11B values in paleo-environmental investigations. In the southwestern Tethys, the early Bajocian is marked by a change in sedimentation from calcareous to radiolaritic lithologies, concomitant with a δ13C positive excursion and a sedimentary hiatus in several calcareous sections. The isotopic compositions of boron extracted from Terminilletto samples range from 10‰ to 12‰ and boron content from 0.75 to 1.45 ppm, both lower than modern ranges. This implies either diagenesis or a lower boron concentration in Jurassic seawater. Boron isotopic compositions are compared to major and trace element analyses to investigate potential alteration of the carbonate primary signal. Fe and Al contents provide useful information on both clay contamination and sedimentation style. The δ11B values measured in carbonates constrain the boron isotopic composition of the precipitating fluid, either seawater or derived from seawater, regarding that both δ11B and δ18O are consistent with marine signatures. Jurassic seawater δ11B might have been lower than the modern one. The δ11B variations, consistent with micropaleontological observations, suggest a seawater alkalinization reflecting the atmospheric CO2 decrease after a probable phase of volcanism intensification prior to the Aalenian–Bajocian boundary. The carbonate crisis itself might also have promoted pH rise through progressive [CO32−] accumulation in seawater.

Carbon-isotope stratigraphy and temperature change during the Early–Middle Jurassic (Toarcian–Aalenian), Raasay, Scotland, UK

The data presented here provide the first high-resolution investigation of carbon and oxygen isotope derived from Boreal Aalenian–Bajocian belemnites from the Isle of Raasay, Scotland, UK. Further isotope data has been gathered from the Toarcian. The carbon-isotope data reveal a positive carbon-isotope excursion in the early Aalenian Scissum Zone (equivalent to the Tethyan Comptum Subzone). The correspondence between this record and Tethyan records lends support that this excursion is global in origin. Although the excursion is coeval with a transgressive interval less obvious is an association with the deposition of organic-rich deposits. An equivocal excursion toward light isotopic close to the Aalenian–Bajocian boundary may be linked to a major pulse of subduction related magmatism. Light oxygen isotopes (and high temperatures) are derived from belemnites from the Toarcian Falciferum Zone. Akin to coeval data from elsewhere surface waters during the Falciferum Zone may have been episodically reduced. Oxygen isotope data from the Aalenian are interpreted to show a cooling trend to a minimum at the base in the Opalinum–Scissum zonal boundary. This major step in cooling is however not entirely coincident with sea-level data and significantly the inferred cooling occurs prior to the positive carbon-isotope excursion. Hence a carbon drawdown and burial scenario that has been interpreted from positive carbon excursions elsewhere cannot drive a precursory cooling episode.

Palaeoclimatic and biotic changes during the Aalenian (Middle Jurassic) at the southern Laurasian Seaway (Basque–Cantabrian Basin, northern Spain)

The uppermost Toarcian–lowermost Bajocian deposits have been studied in 10 sections located in the western part of the Basque–Cantabrian Basin (northern Spain). The studied 276 successive recorded ammonite assemblages allowed detailed biostratigraphical subdivision and the correlation of the sections at the ammonite zone and subzone scale. The foraminifers were studied in 96 samples and 55 diagenetically screened belemnite rostra were analyzed for C and O isotope. Facies and thickness distribution of the Aalenian sediments suggest that deposition took place in a nearly symmetrical sub-basin included into an intraplate shallow platform, developed over continental crust, on which flexure was the main control responsible for subsidence and basin evolution. Noteworthy negative δ 13C bel excursions, coinciding with seawater temperature changes, were recorded during the Bradfordensis Biochron, around the Concavum–Limitatum biochron boundary, and around the Aalenian–Bajocian boundary. A close relationship between the changes in seawater temperature and the biotic changes observed in the foraminiferal and in the ammonoids assemblages has been evidenced. During the Comptum Biochron, the δ 18O bel-based palaeotemperature shows a remarkable cooling interval with an average seawater temperature of 15.7 °C. This cooling favoured the immigration of species of foraminifers that thrived in platforms with colder seawater temperatures located north of the Basque–Cantabrian Basin. As a consequence, a strong increase in the diversity of the foraminiferal assemblages (28.5% of first appearances) occurred. This interval also coincides with the highest ammonoid abundance recorded during the Aalenian. A notable increase in temperature with peak values up to 24.3 °C, was measured during the Bradfordensis Biochron. This warming marks the beginning of progressive loss of foraminifer diversity and the decrease in the abundance of ammonoids. However, the increase in the relative abundances of some foraminiferal taxa such as the genus Spirillina during the warming phase is remarkable. Some of the foraminiferal species that had their last occurrence during this interval seem to be immigrants from NW Europe that arrived during the Comptum cooling interval, and that did not survive to rise of the seawater temperature. Another drop in temperature was recorded during the late Gigantea and the early Concavum biochrons. Coinciding with this cooling interval nearly 20% of the foraminiferal species disappeared while the ammonoid abundance increased. A new and significant warming episode occurred during the latest Aalenian (Limitatum Biochron), extending to the earliest Bajocian (Discites Biochron). Biotic response to this new Δ T of 2.5 °C is remarkable, marking one of the most important crises of the Aalenian. More than 30% of the foraminiferal species disappeared and no new appearances were recorded. Ammonoids show a decrease in the specimen abundance during the Limitatum–Discites warming, preceding a drastic decrease in the species abundance. The remarkable biotic changes linked to the variations of seawater temperature recorded in the Basque–Cantabrian Basin during the Aalenian, open the possibility that some of these faunal turnovers, specially marked in the benthic foraminifers, were of global extent and mainly forced by climate changes.

Early Permian to Middle Jurassic rifting and sedimentation in East Africa and Madagascar

Pre-drift sediments of Madagascar (Early Permian-Middle Jurassic) have been studied palynologically. These studies resulted in a more precise dating of individual stratigraphic units and the recognition of minor and major breaks in the succession. Palynostratigraphic and physical evidence of unconformities have been used to subdivide the pre-drift sediments into depositional cycles and to infer rifting events. A comparison with equivalent strata of Mozambique, Tanzania, Kenya, Ethiopia and Somalia shows a general correspondence and provides additional information for the construction of a generalized framework for the East African/Madagascan region which demonstrates the relationships between rifting and sedimentation and elucidates the prehistory of the break-up of Gondwana into a western and eastern fragment during the Middle Jurassic. The predrift succession of East Africa/Madagascar can be subdivided into two major sequences, a Karoo sequence (cycles 1–5, Asselian-early Smithian) and a transitional sequence (cycles 6–9, Ladinian-early Bajocian). The two sequences are separeted by a late Scythian to Anisian hiatus which indicates extensive uplift and erosion before a major rifting event initiated the resumption of sedimentation in the Ladinian. This Middle Triassic event marks the transition from the intracratonic rift stage to the pericratonic basin stage and correlates with a significant event in the Pangaean history, the transition from final coalescence to initial dispersal. The onset of the southward drift of Madagascar is believed to have occurred about 60 Ma later near the Aalenian-Bajocian boundary, contemporaneous with or immediately after the deposition of syntectonic sediments of cycle 9.

Corrélations biostratigraphiques a la limite supérieure de l'Aalénien

In Western Europe, the boundary between Aalenian and Bajocian is as a general rule based on the evolution ofGraphoceratidae: disappearing of Graphoceras and appearance of Toxolioceras.. In Tethys provinces, Graphoceratidae reduce in importance and in boreal and pacific provinces, they are seldom seen or even non-existent. A group with pratically a world wide distribution, sub-family Hammatoceratinae, may be used to establish correlations between paleogeographic provinces. In this sub-family, genus Euaptetoceras ( Buckman)) appears at the top of Aalenian. The species E. amplectens and E. amaltheiforme,, relatively involute forms ending the phylum, have a rather short life: they are situated at the top of Formosum-Limitatum Subzone. This Amplectens Horizon has a wide geographical distribution, since it has been located in Western Europe (England, Suabia, Swiss and French Jura, Portugal); also it has been found in Tethys province (Alaska, Canada) and in Pacific province (Argentine Republic). In conclusion, the Amplectens Horizon reveals as a very good reference to define Aalenian-Bajocian boundary in a world scale. Sometimes placed at the top of Concavum Zone, sometimes at the bottom of Discites Zone, I propose for it a limiting horizon at the top of Aalenian.

Recherches palynologiques dans le Jurassique d'Alsace (France)

In the Alsace, the Jurassic is to be found in a complete section ranging from the Hettangian to the Lower Kimmeridgian. The palynological study of abundant material sampled from outcrops and drillings has yielded numerous spores, pollen grains, dinocysts and acritarchs. Thorough analysis of the successive assemblages shows a drastic change in their composition across the Aalenian-Bajocian boundary, where dinocysts dominant associations replace the Liassic spores-pollen dominant associations. Moreover, it helps outlining the stratigraphical evolution of the microflora through Jurassic times.