Early Toarcian Mass Extinction Research Articles

Long duration of benthic ecological recovery from the early Toarcian (Early Jurassic) mass extinction event in the Cleveland Basin, UK

The Cleveland Basin of Yorkshire, UK hosts one of the most iconic Lower Jurassic rock successions for studying the Toarcian oceanic anoxic event and the associated mass extinction, yet our understanding of the subsequent ecological recovery is limited. This study documents for the first time the full extent and nature of benthic macrofaunal recovery from the early Toarcian mass extinction event within the Cleveland Basin. Benthic oxygen levels remained low following the extinction event, allowing specialist communities that were tolerant to low oxygen levels to dominate. Recovery began properly once the seafloor ventilation had begun to improve and was first expressed by an expanded ecological tiering structure. Recovery progressed slowly thereafter with the possible return to oxygen-restricted environments. As sea-levels fell and sand-dominated deposition again occurred within the basin, the recovery accelerated, with ecological and species richness being reattained and even exceeding pre-extinction levels. Full recovery occurred, at the latest, c. 7 myr after the extinction event; this duration is on par with estimates of recovery rates from the largest mass extinction event of the Phanerozoic (the end-Permian mass extinction event). Recovery within the Cleveland Basin was likely to have been strongly influenced by local sea-levels and the continuation of challenging environmental conditions after the extinction event. Supplementary material: Benthic fossil occurrence data are available at https://doi.org/10.6084/m9.figshare.c.6332986

The last representatives of the Superfamily Wellerelloidea (Brachiopoda, Rhynchonellida) in the westernmost Tethys (Iberian paleomargins) prior to their demise in the early Toarcian Mass Extinction Event

AbstractThe last clade-level extinction episode affecting the Phylum Brachiopoda has been long-established in the Early Toarcian Mass Extinction Event (ETMEE) around the Pliensbachian-Toarcian transition, when several rhynchonellide groups became extinct and others underwent a notable renewal in the western Tethys. Among them, Wellerelloidea is a long-ranging superfamily severely affected by this environmental crisis, embodying the subfamily Cirpinae as the last wellerelloids worldwide, prior to their global extinction in the Pb-To transition. The profuse record of Lower Jurassic cirpines in the peri-Iberian paleomargins provides an opportunity to clarify the taxonomy of wellerelloid species in the pre-extinction interval. A new species (Cirpa lucentina) is erected and the revision of the Cirpinae taxa around the ETMEE is carried out. Morphometric analysis and the study of internal structures of the shells support separation of the genera Cirpa and Salgirella, adding new supplementary diagnostic criteria. The biogeographic distribution of this clade in the western Tethys and its evolutionary history in the Early Jurassic reveal a pervasive colonization pattern of both epicontinental and epioceanic habitats. The Mediterranean origin of the last representatives of this group is ascertained, but while diversification of Salgirella took place in epioceanic habitats, speciation of Cirpa is unrelated to biochorema boundaries, even colonizing epicontinental seas until their extinction prior to the hyperwarming event that occurred in the basal Serpentinum Zone. A rhynchonellide morphogroup epitomized by cirpines was resilient to this event in the epicontinental seas. This morphogroup is also recorded after the extinction interval by means of the genus Pseudogibbirhynchia, thus postulating potential pre- and post-extinction phyletic relationships.UUID: http://zoobank.org/100b06c0-a803-4f6f-b0fa-2d9e2d6678d2

Two pulses of extinction of larger benthic foraminifera during the Pliensbachian-Toarcian and early Toarcian environmental crises

The early Toarcian (Early Jurassic, ~183 Ma) was characterized by a pronounced oceanic anoxic event (OAE), global warming, major changes in hydrological cycling and a second order mass extinction. Most studies of early Toarcian geobiology have focused on the Northern Hemisphere, and only a few studies have been carried out on strata from the southwestern Tethys Ocean and Southern Hemisphere. This limits our understanding of the geographic extent of the early Toarcian mass extinction (ETME). To address this issue, in this study we have analyzed foraminiferal assemblages from the East Selong and Nianduo sections of South Tibet, China. Our results show that 14 species disappeared during the ETME, accounting for 21.9% (14/64) of overall taxonomic richness. Of these, nine of the victims were larger benthic foraminifera (LBF), accounting for 75% (9/12) of LBF species. We also find that close to the Pliensbachian-Toarcian boundary, six species of foraminifera disappear, and five of these victims were LBF (5/17). Our work thus demonstrates two extinction pulses of LBF in South Tibet, i.e. at the Pliensbachian-Toarcian boundary, and coeval the early Toarcian OAE. The results suggest LBF were more vulnerable to environmental stress than smaller foraminifera, which is consistent with other mass extinctions such as the Permian-Triassic extinction. Our findings confirm that the Southern Hemisphere, like the Northern Hemisphere, was severely affected by environmental changes across the late Pliensbachian to early Toarcian interval, and that extinctions were synchronous with the Northern Hemisphere.

Upper Toarcian (Lower Jurassic) marine gastropods from the Cleveland Basin, England: systematics, palaeobiogeography and contribution to biotic recovery from the early Toarcian extinction event

AbstractHere we describe a new upper Toarcian (Lower Jurassic) marine gastropod fauna from rocks of the Cleveland Basin exposed on the North Yorkshire coast of England. The fossil assemblage consists of 16 species, of which three are new:Katosira?bicarinatasp. nov.,Turritelloidea stephenisp. nov. andStriactaenonina eleganssp. nov. Four species are described in open nomenclature asTricarilda? sp.,Jurildasp.,Cylindrobullinasp. andCossmanninasp. The other species have previously been described:Coelodiscus minutus(SchüblerinZieten),Procerithium quadrilineatum(Römer),Pseudokatosira undulata(Benzinvon Zieten),Palaeorissoinaaff.acuminata(Gründel),Pietteia unicarinata(Hudleston),Globulariacf.canina(Hudleston),Striactaeoninacf.richterorumSchulbert & Nützel,Striactaenoninaaff.tenuistriata(Hudleston) andSulcoactaeon sedgvici(Phillips). Most of these species are the earliest records of their respective genera and show palaeobiogeographical connections with contemporary gastropod associations from other regions of Europe and South America. The taxonomic composition of the upper Toarcian Cleveland Basin gastropod assemblage differs substantially from the faunas of the upper Pliensbachian and lower ToarcianTenuicostatumZone, showing the strong effect of the early Toarcian mass extinction event on the marine gastropod communities in the basin. Only a few gastropod species are shared between the late Toarcian faunas and the much more diverse Aalenian gastropod faunas in the Cleveland Basin, suggesting that there was a facies control on gastropod occurrences at that time. This is also a potential explanation for the taxonomic differences between the late Toarcian gastropod faunas in the Cleveland Basin and those in France, and northern and southern Germany.

Changes in brachiopod body size prior to the Early Toarcian (Jurassic) Mass Extinction

Herein we analyze the shell length as estimation of the body size of several brachiopod assemblages recorded into the extinction interval prior to the Early Toarcian Mass Extinction. They were collected from representative localities around the Iberian Massif (Spain and Portugal) presenting different paleoenvironmental conditions. The analysis performed has revealed two different trends of spatial and temporal changes in body size. (1) Mean size decreases in the assemblages following an anticlockwise pattern from Iberian to Lusitanian basins, in relation to the progressive environmental stress caused by shifts on terrigenous input and in depth. This diminution results from changes in the relative abundance of bigger or smaller species in each area rather than from miniaturization of the taxa. (2) Some new evolved or more resilient species show increasing sizes over time along the extinction interval. This increase affects both big spiriferinides in the Iberian Basin and small rhynchonellides and koninckinides in the Lusitanian Basin, and runs in parallel to the drop in diversity and the progressive warming of the sea water in this interval in all the localities. This increase in body size has been related to the changes in diversity that could have favored the access to resources for resilient or new evolved species. The observed patterns are comparable with those detected in the end-Permian Crisis, and might indicate a generalizable response to similar biotic crises.

Episodic events in long-term geological processes: A new classification and its applications

Long-term geological processes are usually described with curves reflecting continuous changes in the characteristic parameters through the geological history, and such curves can be employed directly for recognition of episodic (relatively long-term) events linked to these changes. The episodic events can be classified into several categories according to their scale (ordinary and anomalous events), “shape” (positive, negative, and neutral events), and relation to long-term trend change (successive, interruptive, facilitative, stabilizing, transformative, increasing, and decreasing). Many types of these events can be defined depending on the combination of the above-mentioned patterns. Of course, spatial rank, duration, and origin can be also considered in description of these events. The proposed classification can be applied to events in some real long-term geological processes, which include global sea-level changes, biodiversity dynamics, lithospheric plate number changes, and palaeoclimate changes. Several case examples prove the usefulness of the classification. It is established that the Early Valanginian (Early Cretaceous) eustatic lowstand (the lowest position of the sea level in the entire Cretaceous) was negative, but ordinary and only interruptive event. In the other case, it becomes clear that the only end-Ordovician and the Permian/Triassic mass extinctions transformed the trends of the biodiversity dynamics (from increase to decrease and from decrease to increase respectively), and the only Cretaceous/Paleogene mass extinction was really anomalous event on the Phanerozoic biodiversity curve. The new palaeontological data are employed to reconstruct the diversity dynamics of brachiopods in Germany (without the Alps) and the Swiss Jura Mountains. The further interpretation of the both diversity curves implies that the Early Toarcian mass extinction affected the regional brachiopod faunas strongly, but this event was only decreasing (biotic radiation continued after it, although “restarted” from the lower point) similarly to the end-Triassic and Cretaceous/Paleogene mass extinctions. The number of lithospheric plates decreased in the Early Cretaceous; however, the previous trend to increase in this number re-established after the noted event. The Oi-1 glaciation in the very beginning of the Oligocene was anomalous event, but it only stabilized the earlier trend of temperature decline and did not transform it. Further development of the comprehensive classification of geological events is necessary. For instance, it has become clear that the Silurian environmental perturbations and the Cretaceous oceanic anoxic events should be classified as discrete events that differ essentially from episodic events, the latter of which are relevant to continuous changes in geological processes.

Arab-Madagascan brachiopod dispersal along the North-Gondwana paleomargin towards the Western Tethys Ocean during the Early Toarcian (Jurassic)

Multiple approaches, mainly focused on assessing the depositional environments, paleotemperature, chronostratigraphical and paleobiogeographical data, morphometrical analyses and the study of the internal structure of spiriferinide brachiopods assigned to the genus Calyptoria, have revealed that this brachiopod fauna migrated from their Arab-Madagascan homeland along the North-Gondwana paleomargin into the peri-Iberian epicontinental platform system, in the same well-known spreading episode suffered by the Arab-Madagascan Bouleiceras ammonite fauna over the worldwide-distributed early Toarcian platforms. This brachiopod fauna has been so far overlooked as potential Arab-Madagascan immigrant fauna, as it was integrated together with diverse assemblages well-established in the peri-Iberian basins. Dispersal of Calyptoria stock was conditioned by the interplay of several factors, such as the tectonic framework, the development of epicontinental seas on both Tethyan margins, the early Toarcian transgression, the prevailing ocean current pattern, and their limited ability for dispersal. The revised chronostratigraphical framework of this dispersal episode reveals the coincidence with the thermal maximum recorded prior to the Early Toarcian Mass Extinction Event (ETMEE) in the westernmost Tethys Ocean. The concurrence of these factors played a significant role in the Calyptoria spreading, showing an inter-tropical distribution between 0° and 30° in both hemispheres. The taxonomical updating performed in the light of the current systematic trends support this adaptive strategy carried out by Calyptoria species, consisting of a sudden and practically synchronous dispersal without outstanding morphological changes instead of the diverse evolution of different brachiopod taxa in response to the environmental changes related to the ETMEE. Updating and rearranging of the species attributed to the genus Calyptoria suggest a new systematic scheme for several former attributions, supporting that lower Toarcian occurrences of L. undulata from westernmost Tethyan areas belong to Calyptoria, thus pointing towards the dispersal from the Northern peri-Gondwana seas to the westernmost Tethyan epicontinental platforms. Similar adaptive strategies were linked to other extinction events, supporting a possible pattern in the response of certain brachiopod populations to such biotic crises.

New deep-water brachiopod resilient assemblage from the South-Iberian Palaeomargin (Western Tethys) and its significance for the brachiopod adaptive strategies around the Early Toarcian Mass Extinction Event

The Pliensbachian-Toarcian transition was a period of changes in long-term environmental conditions leading up to the Early Toarcian Mass Extinction Event (ETMEE), which resulted in a noticeable extinction and turnover in the marine biota. The westernmost Tethyan basins, especially the peri-Iberian platforms, provide an exceptional brachiopod record to better understand the adaptive strategies and the severe ecological effects of these faunas within the marine ecosystems. This event marks a critical interval in the evolutionary history of the Phylum Brachiopoda as two orders, the Athyridida and Spiriferinida, became extinct. Evolutionary patterns displayed by several taxa from these groups and some rhynchonellids typifying deep-water habitats are analyzed across this biotic crisis spanning several Mediterranean and NW-European basins. New work performed in La Cerradura section, a deep pelagic trough from the South-Iberian palaeomargin, reveals two new taxa (Koninckodonta sumuntanensis and Atychorhynchia falsiorigo) herein described. This newly documented fauna supports pre-extinction dwarfing and resilience in deep refugia linked to the ETMEE, and an episode of speciation which is interpreted in terms of a pre-extinction radiation. In the ETMEE repopulation phase an opportunistic strategy occurs typified by Soaresirhynchia bouchardi, and a case of homoplasy involving post-extinction pioneers (Elvis taxon) is detected. Similar adaptive strategies occurred associated with other mass extinctions such as the Permian/Triassic and the Cretaceous/Paleocene events, supporting a possible standard pattern in the response of the brachiopod fauna to such biotic crises and shedding light on the ecological effects of the mass extinction events.

Cisnerospira(Brachiopoda, Spiriferinida), an atypical Early Jurassic spire bearer from the Subbetic Zone (SE Spain) and its significance

AbstractThe order Spiriferinida represented a significant group whose extinction is linked to the early Toarcian mass extinction event. The genusCisnerospiraManceñido, 2004, conspicuous representative of this group in the Early Jurassic of the western Tethys, is analyzed from a systematic standpoint, grounded mainly on evidence from the Subbetic domain, and its initial diagnosis is revised accordingly. A definitive suprageneric position within the subfamily Paralaballinae is formally proposed in the light of new data herein provided. Both external and internal diagnostic features and the generic and intraspecific variability are described through the analysis of theCisnerospiraspecies recorded in the easternmost Subbetic area, i.e.,Cisnerospira adscendens(Deslongchamps, 1858), C. aff.adscendens, C. angulata(Oppel, 1861), andC.?sylvia(Gemmellaro,1882). In addition, their interrelation with other records from several Tethyan basins is addressed, and the generic spectrum has been extended to include several species with high morphological affinity. This characterization thus contributes to clarify certain ambiguities in the systematics of the spiriferinids, which entails a complex taxonomy mainly based on the external features, where the ribbing pattern was given foremost classificatory value due to the lack of more reliable generic diagnostic criteria. Furthemore, a morphofunctional analysis performed inCisnerospirareveals a presumable epibenthonic libero-sessile way of life, and two alternative adaptive strategies are discussed: resting on and/or sticking in substrates with different degree of consolidation, providing a significant hydrodynamic stability to the shell.

Palaeoclimatic oscillations in the Pliensbachian (Early Jurassic) of the Asturian Basin (Northern Spain)

Abstract. One of the main controversial themes in palaeoclimatology involves elucidating whether climate during the Jurassic was warmer than the present day and if it was the same over Pangaea, with no major latitudinal gradients. There has been an abundance of evidence of oscillations in seawater temperature throughout the Jurassic. The Pliensbachian (Early Jurassic) constitutes a distinctive time interval for which several seawater temperature oscillations, including an exceptional cooling event, have been documented. To constrain the timing and magnitude of these climate changes, the Rodiles section of the Asturian Basin (Northern Spain), a well exposed succession of the uppermost Sinemurian, Pliensbachian and Lower Toarcian deposits, has been studied. A total of 562 beds were measured and sampled for ammonites, for biochronostratigraphical purposes, and for belemnites, to determine the palaeoclimatic evolution through stable isotope studies. Comparison of the recorded latest Sinemurian, Pliensbachian and Early Toarcian changes in seawater palaeotemperature with other European sections allows characterization of several climatic changes that are likely of a global extent. A warming interval partly coinciding with a δ13Cbel negative excursion was recorded at the Late Sinemurian. After a “normal” temperature interval, with temperatures close to average values of the Late Sinemurian–Early Toarcian period, a new warming interval containing a short-lived positive δ13Cbel peak, developed during the Early–Late Pliensbachian transition. The Late Pliensbachian represents an outstanding cooling interval containing a δ13Cbel positive excursion interrupted by a small negative δ13Cbel peak. Finally, the Early Toarcian represented an exceptional warming period, which has been pointed out as being responsible for the prominent Early Toarcian mass extinction.

Diversity dynamics of Toarcian–Aalenian (Jurassic) ammonites and transgressions/regressions in the Greater Caucasus Basin (Caucasian Sea, northern Neo-Tethys Ocean): An evidence of partial dependence

Possible dependence of ammonite diversity dynamics on shoreline shifts, i.e., transgressions and regressions, is yet to be understood. Data from the Greater Caucasus Basin, which was located on the northern active margin of the Neo-Tethys Ocean in the Mesozoic, permit to analyze the noted dependence at the Toarcian–Aalenian time interval. Distribution of 161 species and 40 genera in each regional ammonite-based biozone of this interval allows to evaluate the diversity dynamics of ammonites. The total number of both species and genera changed cyclically. The diversity maxima occurred in the H. bifrons, G. thouarsense, and L. murchisonae zones. The diversity minima occurred in the D. semicelatum, H. variabilis, P. aalensis, and G. concavum zones. Semi-quantitative analysis of facies distribution in 28 areas of the basin indicates a stepwise early Toarcian–middle Aalenian transgression, followed by a late Aalenian regression. The general trend towards species diversification through the study interval coincided with the noted transgression. This is an evidence of partial dependence, but no dependence between the generic diversity dynamics and shoreline shifts is visible. It is hypothesized that transgressions facilitated the growth of the ammonite diversity, but, when they stopped, the diversity was controlled by other factors. Changes in the spatial distribution of deep-marine palaeoenvironments might also affect the ammonite diversity dynamics, but no regional evidence of importance of oxygen depletion and seawater temperature fluctuations is found. The minimal ammonite diversity in the D. semicelatum Zone could be a consequence of the Early Toarcian mass extinction. In the Greater Caucasus Basin, the latter occurred at a peak of regression.

Biostratigraphy and paleobiogeographic affinities of the Jurassic brachiopod assemblages from Sierra Espuña (Maláguide Complex, Internal Betic Zones, Spain)

The assemblages of Early Jurassic brachiopods (Pliensbachian - Toarcian) from Sierra Espuña (Murcia Province, SE Spain) are described. This is the only area in the Internal Zones of the Betic Cordillera, corresponding to the margins of the Alborán Terrane, where Jurassic brachiopods are known to occur. In the tectonic Unit of Morrón de Totana (more southward located) assemblage MT1 of Late Pliensbachian age has been characterized. This assemblage has been subdivided into three successive sub-assemblages: MT1a (Algovianum Zone), MT1b (Emaciatum Zone, Solare Subzone) and MT1c (Emaciatum Zone, Elisa Subzone). Northward, in the Perona tectonic Unit two distinct assemblages, P1 (Latest Sinemurian - Early Pliensbachian) and P2 (Early Toarcian, Serpentinum Zone) have been recognized. Differences between the assemblages from the two tectonic units are evident after the paleobiogeographical analysis. In the Morrón de Totana Unit, taxa with Mediterranean affinities occur. MT1 assemblage is very similar to assemblages previously known in the Eastern Subbetic as well as in other areas of the Mediterranean Province. In the Perona Unit the Mediterranean affinity of the assemblages is not so evident. P1 Assemblage consists of widely distributed taxa, lacking in the most characteristic elements of the Mediterranean Province which, however, are present in neighbouring Betic areas. P2 Assemblage belongs to the Spanish Province that develops in Western Tethys after the Early Toarcian Mass Extinction Event. The occurrence in this assemblage of Prionorhynchia aff. msougari Rousselle, until now only found in North Africa, indicates a closer connection of the Perona Unit with the African paleomargin of the Tethys than with the South Iberian paleomargin. The paleobiogeographical data suggest a more southern and marginal (close to epicontinental areas) position of the Perona Unit than the Morrón de Totana Unit.

Mass extinction and recovery of the Early Toarcian (Early Jurassic) brachiopods linked to climate change in Northern and Central Spain

The Early Toarcian mass extinction event represented the most important Mesozoic and Cenozoic turnover of the population of brachiopods and severely affected other benthic fauna. Two main hypotheses have been proposed to explain the synchronous and global mass extinction: an oceanic anoxic event or a warming episode. To test both hypotheses, the dynamics of the brachiopod assemblages below and above the extinction boundary are analyzed and compared with the seawater paleotemperature variations, calculated from the δ 18O data recorded in belemnite rostra. Five sections from Northern and Central Spain, well dated with ammonites, have been selected for this study. The sections show no indication of sedimentary breaks and contain abundant brachiopods, which have been grouped into four assemblages. The changes observed in the brachiopod assemblages show a close correlation with the changes in the seawater paleotemperatures. The oldest assemblage (assemblage 1) coincides with a cooling interval recorded to have taken place in the latest Pliensbachian. Paleobiogeographical reconstruction shows that this assemblage was distributed at paleolatitudes between 30 and 45°N, with a preference for relatively cool waters. With the rise of temperatures that took place during the earliest Toarcian Tenuicostatum Zone, assemblage 1 was substituted by assemblage 2, which composed of different species of the same genera but considerably restricted to the warmer waters of lower paleolatitudes, between 28 and 35°N. Coinciding with the rapid and pronounced increase in seawater temperature, recorded at the Tenuicostatum–Serpentinum zonal boundary, all of these brachiopod species disappeared in the studied localities, marking clearly the extinction boundary. Predominant southward currents through the Laurasian Seaway precluded the possible migration of the brachiopods to cooler northern waters. The brachiopods' disappearance is independent from the oxygenation degree of the sea bottom, and therefore the rapid warming seems to be the most plausible cause of the mass extinction. After the extinction event, the recovery of the brachiopods was uneven. Subsequent to a brief pause, recovery was rapid in Central Spain and in other southern areas of Western Tethys, whereas in northern Spain and in the whole of Europe north of the French Central Massif, brachiopods did not recover until the Mid to Late Toarcian times.

New range data for marine invertebrate species across the early Toarcian (Early Jurassic) mass extinction

Abstract: We have reinvestigated the marine mass extinction interval that occurred during the early Toarcian, which was a time of widespread marine anoxia. The ranges of marine benthic invertebrates are significantly altered using new observations from the Cleveland Basin, UK. Goniomya rhombifera is reported for the first time from the Whitby Mudstone Formation and together with an increased epifaunal bivalve diversity indicates a brief, relatively oxygenated period towards the end of the event. The new data, together with published results, suggest three apparent extinction horizons on a global scale; the first is just above the Pliensbachian–Toarcian boundary, and the following two are in the semicelatum ammonite Subzone. As a result of the Signor–Lipps effect there may be only one, or possibly two, true extinctions. The youngest extinction horizon coincides with the first of the abrupt carbon isotope shifts that characterize this interval, and with increases in sea surface temperature, continental weathering rates, and seawater anoxia. Pseudomytiloides dubius is the only abundant benthic macroinvertebrate during the most hostile environmental conditions but it and all other benthic species are almost entirely absent for many thousands of years immediately after each abrupt negative carbon isotope shift. Supplementary material: A compilation of the Late Pliensbachian and Early Toarcian marine biotic changes and a discussion of the systematics of the key faunal elements are available at http://www.geolsoc.org.uk/SUP18353 .

Seawater temperature and carbon isotope variations in belemnites linked to mass extinction during the Toarcian (Early Jurassic) in Central and Northern Spain. Comparison with other European sections

The Early Toarcian mass extinction marks one of the critical events in the history of the Earth. Many of these events have been linked to important climate changes. Two sections of the Toarcian showing high-resolution ammonite-based biostratigraphy are studied in Central and Northern Spain. Stable isotope datasets, based on the analysis of 192 diagenetically screened belemnite calcite and 41 bulk carbonates, allowed the construction of δ 13C curves and a δ 18O-based palaeotemperature. Comparison of the extinction pattern with other sections in Europe and northern Africa shows that the Early Toarcian mass extinction boundary occurred at the Tenuicostatum–Serpentinum transition, and that the organic-rich facies linked to the Oceanic Anoxic Event and the associated negative δ 13C excursion are diachronous. From a latest Pliensbachian cooling interval, a first increment of seawater temperature averaging about 4.5 °C, started around the Pliensbachian–Toarcian boundary and developed during the earliest Toarcian Tenuicostatum Biochron, marking the beginning of the main extinction interval. From the Tenuicostatum–Serpentinum transition up to the Bifrons Biochron, a rise in seawater temperature averaging 5.7 °C to 7.8 °C was recorded. This warming interval, which started rapidly and which seems to be synchronous at least in Western Europe, is considered one of the main factors responsible for mass extinction. For some authors this rapid warming was probably due to a massive injection of greenhouse gases into the atmosphere, but it does not seem to be recorded in belemnite calcite, and the origin of these possible gases is largely debated in the literature. Additional isotope excursions were found in the studied sections in Spain during the Middle and Late Toarcian. A negative δ 13C bel excursion has been recorded at the latest Bifrons Biochron. Above this shift, the Illustris–Vitiosa subzones thermal peak, which represents a 2–3 °C Δ T, could be linked to one of the tectonomagmatic activity peaks recorded in the Karoo Basin. A renewal in the ammonite and brachipod faunas coincident with this climatic change has been recognized in NW Europe and Western Tethys. An interesting thermal peak has also been detected in belemnites of the Insigne Subzone. Δ T is in the order of 3 °C, and in both sections the thermal peak is included into a δ 13C bel negative excursion of about − 1.5‰. Relative synchrony with the new age for the Karoo main magmatic activity (178–180 Ma) indicates that the δ 13C negative anomaly and the warming interval could be caused by the release of volcanogenic greenhouse gases. At this short interval, noteworthy changes in the abundance and diversity of the recorded assemblages in several faunal groups of NW Europe and Tethys are observed. The uppermost Levesquei Subzone thermal peak has only been recognized in the deposits of the section located in Central Spain and coincides with a positive δ 13C excursion.

Biodiversity and biogeography of middle–late liassic ammonoids: implications for the early Toarcian mass extinction

In Western Tethyan areas, the Toarcian stage begins with two important evolutionary events in ammonite faunas: (1) the disruption of Tethyan–Boreal provinciality; (2) a biological crisis linked with the oceanic anoxic event OAE. The analysis of these events has been addressed by constructing curves of ammonoid diversity (species richness, origination and extinction rates) in the Late Pliensbachian (= Domerian)–Early Toarcian interval in selected localities. Two diversity drops are recognized. The first one is recorded at the end of the Dactylioceras mirabile subzone and reflects the disruption of Tethyan–Boreal provinciality, through the progressive extinction of the Boreal endemic family Amaltheidae that occupied the north-western European seas during the whole Pliensbachian on the one hand, and the extinction of Late Domerian Ammonitina endemic to the Mediterranean areas on the other hand. The Early Toarcian homogeneization of Mediterranean and north-western European ammonoid faunas was reached via elimination of both Boreal and Mediterranean endemics with differential rates of extinction in the two palaeogeographic domains and the subsequent geographical expansion of Tethyan-derived ammonoids. The second, dramatic drop in ammonite diversity in the upper part of the Dactylioceras semicelatum subzone coincided with the onset of OAE. It also affected epioceanic ammonoid clades like Phyllocerataceae and Lytocerataceae. These two drops are interpreted as two distinct extinctions and not as episodes of a single, stepwise event. Complex relations between ammonoid diversity and sea-level changes are suggested by trends in endemism, which may be reversed during either a single transgression or a single regression.