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.     

Brachiopod faunal exchange through an epioceanic-epicontinental transitional area from the Early Jurassic South Iberian platform system

The La Mola region (eastern External Betic Zone) can be regarded as one of the easternmost complete Jurassic successions of the Betic Cordillera in the Iberian Peninsula, but the paleogeographical setting of their outcrops remains widely discussed. Analysis of brachiopod assemblages from the Lower Jurassic improves the accuracy of previous paleogeographical data, enabling identification of a mainly epioceanic transitional area in which influences of epicontinental habitats are also detected. Assemblage 1, mainly with a Mediterranean affinity but also sharing several constituents with the African and Northeastern Iberian basins, typifies the Sinemurian–Pliensbachian transition. Assemblage 2, as a whole, shows a transitional character between epioceanic and epicontinental habitats; it is subdivided into two successive and interrelated sub-assemblages: Ass. 2a (Demonense–Tenuicostatum Zones) reveals a free connection with the epioceanic Subbetic area, whereas Ass. 2b (Uppermost Pliensbachian–Lower Toarcian) shows a closer relationship with epicontinental environments. Assemblage 3 (Uppermost Spinatum–basal Serpentinum Zones) is commonly recorded in the peri-Iberian epicontinental platform system integrated within the NW-European bioprovince, but it can also be regarded as a marginal assemblage that is widespread in the westernmost Tethyan margin prior to the Early Toarcian extinction event. Assemblage 2 constitutes a suitable index for assessing the paleobiogeographical affinity of the La Mola region, as Ass. 2a is progressively replaced by Ass. 2b, thus triggering the arrival of epicontinental taxa to the more intra-epioceanic Subbetic environments, inferring a possible connection through the La Mola transitional slope. Consequently, this region enabled a faunal mixing and exchange between both environments, and La Mola likely remained as an area that would facilitate migration and an effective dispersal seaway or, at least, did not constitute an ecological filter-barrier for brachiopods. Biostratigraphical data from brachiopods and ammonites are correlated for the first time in La Mola, refining and calibrating biochronostratigraphical gaps in the pre-Domerian deposits where biochronological markers are usually scarce, and around the Pliensbachian–Toarcian boundary, a crucial timespan in which the Early Toarcian extinction event took place.     

An eastern Tethyan (Tibetan) record of the Early Jurassic (Toarcian) mass extinction event

A record of the Early Jurassic mass extinction event is reported from eastern Tethyan (Tibetan) locations for the first time. In the Mount Everest region a thick Lower Jurassic carbonate formation, here named the Yungjia Formation, is developed within the predominantly clastic Triassic–Jurassic succession. Within the formation a sharp transition from peloidal packstones/grainstones to thin‐bedded, pyritic micrite‐shales interbeds records a sharp pulse of deepening and development of dysoxic bottom waters. Both the lithiotid bivalves and the lituolid foraminifera are important constituents of the lower Yungjia Formation but they disappear at this flooding surface or a short distance below it. This extinction event is comparable to that seen at the base of the Pliensbachian/Toarcian boundary in western Tethyan platform carbonates but the Tibetan events occurred late in the Toarcian Stage as indicated by nannofossil biostratigraphy and C isotope chemostratigraphy. The Early Jurassic extinction event (and the associated spread of oxygen‐poor waters) was therefore not synchronous throughout the Tethyan region.     

Why was there a delayed radiation after the end‐Palaeozoic extinctions?

Data from widespread dysaerobic facies, carbon/sulphur ratios and cerium anomalies suggest that the early Triassic was a time when anoxic conditions spread widely over epicontinental seas. These conditions, associated with marine transgression following the latest Permian regression, are likely to be a prime cause of the mass extinction of Palaeozoic marine faunas. The occurrence of many Lazarus taxa in the Middle and Upper Triassic indicates, however, that the extinctions at the end of the Permian were less severe than has been widely assumed, and that the turnover from Palaeozoic to Mesozoic faunas was considerably extended in time, being finally accomplished only after the end‐Triassic mass extinction event.     

Multivariate hierarchical analyses of Early Jurassic Ostracoda assemblages

Palaeobiogeographic patterns of Early Jurassic ostracods from the northern and southern hemispheres (96 sections located in Europe, North Africa, Western Australia and North and South America) based on 243 species-level records document global patterns of distribution that can be compared to those previously published on ostracods from the European Epicontinental Sea and Tethyan and South Panthalassa areas. All described records of ostracods from both hemispheres spanning the Hettangian to Early Toarcian have been compiled and verified, and their patterns of origin and distribution have been interpreted. Jaccard coefficient of similarity was used to asses similarities among European, American and Tethyan ostracod shelf faunas. The numerical analysis shows a progressive longitudinal gradient in provincialism through the Early Jurassic, consistent with the northward drift of Tethyan ostracod faunas towards the European Epicontinental Sea and the southward movement of European taxa into Tethys and Panthalassa oceans. The spread of cosmopolitan species and extinction of endemic species, allied to the disappearance of geographical barriers, warmer climate conditions and rising sea levels can explain the reduction in ostracod diversity and the east-west provincialism throughout the Early Jurassic. Interchange between hemispheres, including bipolar distributions, are recognized from the Sinemurian time, pointing out that for most of the studied period, the climate worldwide was warm and tropical.     

Victims of the Early Toarcian anoxic event: the radiation and extinction of Jurassic Koninckinidae (Brachiopoda)

The significant mass extinction attributed to the Early Toarcian anoxic event had a severe impact on the phylum Brachiopoda. Beyond a serious decrease of species diversity, the extinction of the orders Spiriferidina and Athyridida is connected with this episode. The order Athyridida was represented by the family Koninckinidae in the Early Jurassic. The stratigraphical and geographical distribution of the three Early Jurassic koninckinid genera ( Koninckella, Koninckodonta, Amphiclinodonta ) shows a definite radiative pattern. The number of their nominal species increased from 2 to 17 from the Sinemurian to Early Toarcian; in the same time interval, their area increased from the Alpine region to the whole Mediterranean and the NW-European domains. This radiative evolution can be explained as the result of different factors: (1) morphological adaptation to muddy bottoms, (2) fundamental changes in the current pattern in the Tethys/Laurasian Seaway, and, possibly, (3) utilization of methane-based chemosynthesis as alternative food source. The radiation of koninckinids, leading from the cryptic habitats of the Tethyan rocky floors to the extensive muddy bottoms of the open European shelves, was abruptly terminated by the anoxic event in the Early Toarcian Falciferum Zone. The main causes of the extinction might be: (1) the excessive warming of Tethyan deep waters by thermohaline circulation, (2) the anoxic event, which was not survived by the spire-bearers, handicapped by their stiff, calcareous spiralia. Brachiopoda, Early Jurassic, Europe, extinction, Koninckinidae, radiation, Tethys.     

Palaeoecological significance of a new Griesbachian (Early Triassic) gastropod assemblage from Oman

A new Early Triassic (Griesbachian) gastropod fauna from the Al Jil Formation of Oman is described. Early Triassic faunas from elsewhere (e.g. the Italian Dolomites and the western USA) are typically of low diversity and high dominance, usually attributed to environmental stress in the immediate aftermath of the end-Permian mass extinction event. The new Oman fauna has, by contrast, relatively high diversity, low dominance and a more even spread of individuals between taxa. It is the most diverse Griesbachian fauna known to date. This is attributed to the favourable (i.e. well-oxygenated) conditions under which the fauna lived. This uncharacteristic Griesbachian gastropod fauna demonstrates that, in the absence of oceanic anoxia, biotic recovery after the end-Permian extinction event may occur surprisingly quickly (within one conodont zone). The fauna is also partially silicified, which has increased its preservation potential relative to other Griesbachian gastropod assemblages. However, only one reappearing Lazarus genus is present in the Oman fauna. This suggests that there was some other control on the abundance of Lazarus genera at this time, other than the absence of silicified faunas as previously suggested.     

An early Bathonian Tethyan ammonite fauna from Argentina

Evidence for the Lower Bathonian Substage in most of the Pacific area has been lacking or controversial because index ammonoids of the Tethyan Subrealm in southern Europe and North Africa were unknown. During the Bathonian, the East Pacific margin belonged to either the Boreal Realm or the East-Pacific Subrealm of the Tethyan Realm, each with faunas distinctly different from the Eurafrican Tethyan faunas that form the basis for the chronostratigraphical standard. The first representatives of Morphoceras from South America, M. gulisanoi sp. nov., here described from a single locality in southern Mendoza Province, clearly document the Lower Bathonian Substage, probably the Zigzag Zone. Associated ammonoids are the perisphinctoidean Procerites cf. schloenbachi de Grossouvre and the oppeliid Oxycerites ( O. ) cf. aspidoides (Oppel), also typical Tethyan elements but less useful for dating.     

Diversity and density characteristics of Pliensbachian-Toarcian molluscan and brachiopod faunas of the North Atlantic margins

Pliensbachian and Toarcian molluscs and brachiopods of Morocco, Iberia, England, and Greenland are analysed in terms of diversity and density. With regard to local variations, Substrate and food supply are thought to be the major factors controlling the predominance of suspension-feeding bivalves in shallow water deposits while the greater physical stability of deeper water favoured the ammonites; high diversity of the gastropods relates to firm substrates. On a regional scale, both bivalve density and diversity tend to increase from the Tethyan to the Boreal redm, while there is a Variable tendency for a northward diversity reduction in other groups. The bivalve data are incompatible with either temperature or salinity control of faunal provinciality. To provide a better account of this, a model is developed which lays emphasis on the physical instability of a boreal inland sea as compared with the Tethys. Some broader implications for endemism in Jurassic faunas are briefly discussed.     

New data from Oman indicate benthic high biomass productivity coupled with low taxonomic diversity in the aftermath of the Permian–Triassic Boundary mass extinction

A new Early Triassic marine fauna is described from an exotic block (olistolith) from the Ad Daffah conglomerate in eastern Oman (Batain), which provides new insights into the ecology and diversity during the early aftermath of the Permian–Triassic Boundary mass extinction. Based on conodont quantitative biochronology, we assign a middle Griesbachian age to the upper part of this boulder. It was derived from an offshore seamount and yielded both nektonic and benthic faunas, including conodonts, ammonoids, gastropods and crinoid ossicles in mass abundance. This demonstrates that despite the stratigraphically near extinction at the Permian–Triassic Boundary, Crinoidea produced enough biomass to form crinoidal limestone as early as middle Griesbachian time. Baudicrinus, previously placed in Dadocrinidae, is now placed in Holocrinidae; therefore, Dadocrinidae are absent in the Early Triassic, and Holocrinidae remains the most basal crown‐group articulates, originating during the middle Griesbachian in the Tethyan Realm. Abundant gastropods assigned to Naticopsis reached a shell size larger than 20 mm and provide another example against any generalized Lilliput effect during the Griesbachian. Whereas the benthic biomass was as high as to allow the resumption of small carbonate factories, the taxonomic diversity of the benthos remained low compared to post‐Early Triassic times. This slow benthic taxonomic recovery is here attributed to low competition within impoverished post‐extinction faunas.     

The biogeography of Early Triassic ammonoid faunas: Clusters, gradients, and networks

The aim of this paper is to quantitatively investigate the spatial and temporal biogeographical relationships of the recovery of ammonoid faunas after the Permian-Triassic mass extinction using three complementary numerical approaches among which is a new, non-hierarchical clustering strategy. The faunal data set consists of a taxonomically homogenised compilation of the spatial and temporal occurrences of ammonoid genera within 20 Early Triassic Tethyan and Panthalassic sites ranging from 40°S to 70°N in palaeolatitudes. In addition to hierarchical cluster analysis (hCA) and nonmetric multidimensional scaling (NMDS), we introduce a third, new non-hierarchical clustering technique allowing the visualisation of a nonmetric interassemblages similarity structure as a connected network constructed without inferring additional internal nodes. The resulting network, which we call a “Bootstrapped Spanning Network” (BSN), allows the simultaneous identification of partially or totally nested as well as gradational linear or reticulated biogeographical structures.The identified interlocalities relationships indicate that the very beginning of the Early Triassic (Griesbachian) corresponds to a very simple biogeographical context, representing a time of great cosmopolitanism for ammonoids. This context shifts rapidly to a more complex configuration indicative of a more endemic and latitudinally-restricted distribution of the ammonoids during the middle and late Early Triassic (Smithian and Spathian). From an evolutionary dynamic point of view, our results illustrate a very rapid (less than ca. 1.4 myr) Early Triassic recovery of the ammonoid faunas, in contrast to many other marine organisms. This recovery is linked with a marked increase in the overall biogeographical heterogeneity, and parallels the formation of a latitudinal gradient of taxonomic richness, which may be essentially controlled by the progressive intensification of the gradient of sea surface temperature. From a methodological point of view, we show that a BSN is a simple, intuitively legible picture of the nested as well as gradational taxonomic similarity relationships, hence providing a good synthesis (and additional insights) between hierarchical clustering and ordination in reduced space results.     

Devonian conodont biogeography: Quantitative analysis of provinciality

Cluster analysis of conodont faunas from each of 17 Lower‐lower Upper Devonian zones and subzones (data as reported by Klapper and Johnson, 1980) reveal changing patterns of provinciality.

Provinciality, expressed by a differentiation into western Laurussian and proto‐Tethyan biogeographic regions, is moderate in the lower Lochkovian but is low or absent in the upper Lochkovian‐lower Pragian. Provincialism returns in the Pragian and reaches its maximum development during the Emsian. Most Australian faunas are distinct from those of western Laurussia and the proto‐Tethys. Conodont faunas from suspect terranes of western North America display no unusual biogeographic affinities. Provincialism declines during the Eifelian and is only weakly developed in Givetian‐lower Frasnian faunas.

Changing global sea level during the Devonian may explain the development of Devonian conodont provinciality. As proposed by Klapper and Johnson (1980), low provinciality is associated with low stands of sea level. Endemic faunas develop in isolated epeiric seas during intermediate stages of sea level rise. High stands of sea level ultimately drown barriers to faunal exchange and prompt a return to low provinciality conditions.     

The Early Cretaceous of North-East Greenland: A crossroads of belemnite migration

Recent field work in Lower Cretaceous successions of Traill Ø and Wollaston Forland, North-East Greenland, have resulted in c. 2350 belemnite guards collected bed-by-bed from the upper Ryazanian – Hauterivian. The most common belemnite genera observed, Acroteuthis, Pachyteuthis, and Cylindroteuthis are of boreal-arctic affinities and closely related to NW European and Siberian faunas. Other taxa, including Hibolithes (common), Pseudobelus (relatively common) and Duvalia (rare), show faunal links to both NW European and Mediterranean faunas. This paper describes and discusses these findings in their taxonomic, biostratigraphic, palaeobiogeographic, palaeoecologic and palaeoceanographic context. In particular, the occurrence of Pseudobelus which is common in the circum Mediterranean area, is remarkable since it is the first observation of this Tethyan genus in the entire Boreal Realm. The palaeoecological interpretation of these observations result in the recognition of four different palaeobiogeographic belemnite assemblages for the Boreal Realm: 1) North-East Greenland, 2) Spitsbergen, 3) NW Europe and 4) Siberia. In contrast to the other assemblages, the belemnite faunas of North-East Greenland consist of a) Boreal-Arctic elements, b) Boreal-European taxa, c) endemic belemnites of Tethyan ancestry, and d) Tethyan species. These findings make North-East Greenland part of an immigration route from the Tethyan Realm via the north Atlantic to the high Boreal. This allowed Tethyan species, which are otherwise unknown from the Boreal Realm, to reach North-East Greenland. The occurrence of the Tethyan genus Pseudobelus in North-East Greenland also supports the interpretation of this taxon as a hemipelagic dweller, capable of crossing major distances. The belemnite patterns further suggest the existence of a proto Gulf-stream, documenting a south-to-north flow of warm surface waters as far north as Greenland already in the earliest Cretaceous (Valanginian). This has substantial implications for the interpretation of Early Cretaceous climate and oceanic current systems, as well as for the palaeobiology of belemnites.     

Paleobiogeography of Mesozoic brachiopod faunas from Andean–Patagonian areas in a global context

During the Mesozoic, the Andean region has played a hinging role between high- and low-latitude faunas, which are, respectively, characterized by stocks that display long-term fidelity. This paper is aimed at providing an updated review of Late Triassic to Late Cretaceous South American articulated brachiopods in the light of previous knowledge at worldwide scale. Late Triassic brachiopods from the Argentine–Chilean Andes show unmistakable Maorian (or Notal) faunal elements alongside some more cosmopolitan genera, with certain influence of Eastern Pacific taxa. By Early Jurassic times, differentiation of Tethyan and Boreal Realms became progressively evident in Europe. In South America, Hettangian–Sinemurian brachiopod faunules from the Argentinian Andes are somewhat impoverished, with mostly cosmopolitan genera showing certain affinities to Maorian species, and with the addition of some endemics later. Increasingly, diverse Pliensbachian Andean brachiopods denote close relationships to Celto-Swabian taxa, then by Domerian times, a certain degree of endemism was developed, though somewhat delayed Tethyan influences, and persistent links with New Zealand are subordinately recognizable, too; most Toarcian assemblages reveal basically Celto-Swabian and Iberian affinities as well. East-west austral links across the Pacific may have been favored by migratory routes fringing the Gondwana margin, whereas faunal exchange with the western end of the Tethys appears to reflect an intermittent shallow-marine connection through the Hispanic Corridor. During the Middle Jurassic, distinction of Tethyan and Boreal Realms was maintained in the northern Hemisphere, and the differentiation of an Ethiopian or Southern Tethyan fauna became better characterized. Aalenian and Bajocian brachiopods of the Andes display generic affinities mainly with those from western Europe, with some minor endemic developments; brachiopods recorded from the Bathonian–Callovian of Argentina (and Chile) also occur along the northern Tethyan margin, yet with some genera extending into Indo-Ethiopian areas. During the Late Jurassic, Boreal faunas from high-latitudes became even more strongly differentiated from low-latitude, Tethyan ones. Oxfordian and Tithonian brachiopods from the Andes apparently belong to genera of cosmopolitan or northern Tethyan affiliation, yet there are few elements in common with other eastern Pacific areas, such as Mexico. Early Cretaceous brachiopods, in addition to Andean basins of Chile and western Argentina, are known also from Patagonia and Tierra del Fuego. They belong mostly to widely distributed, mainly Tethyan genera, with some quasi-cosmopolitan and circum-Pacific components (some shared with Antarctica become noticeable). Late Cretaceous brachiopods from northern Patagonia show significant affinities to Maastrichtian ones of northwest Europe and central Asia, which calls for further assessing the potential role that may have played the trans-Saharan passageway in such dispersal. Broad aspects of Mesozoic brachiopod paleobiogeography are fairly well understood, yet details of ranking and naming of certain units are still in need of more agreement.     

A longirostrine Temnodontosaurus (Ichthyosauria) with comments on Early Jurassic ichthyosaur niche partitioning and disparity

Abstract: We describe an almost complete ichthyosaur skeleton from the middle Toarcian (Lower Jurassic) of the Beaujolais foothills near Lyon, France, and assign it to Temnodontosaurus azerguensis sp. nov. This new species exhibits cranial peculiarities such as a thin, elongated and possibly edentulous rostrum, as well as a reduced quadrate. These characters indicate dietary preferences that markedly differ from other species referred to Temnodontosaurus, a genus previously considered as the top predator of the Early Jurassic seas. Despite a conservative postcranial skeleton, we propose that Temnodontosaurus is one of the most ecologically disparate genera of ichthyosaurs, including apex predators and now a soft prey longirostrine hunter. Ammonites collected from the same stratigraphic level as the described specimen indicate that the new species is somewhat younger (bifrons ammonite zone) than the most known Toarcian ichthyosaurs and therefore slightly postdates the interval of severe environmental changes and marine invertebrate extinctions known as the Toarcian Oceanic Anoxic Event. The present study therefore raises the question of whether postcrisis recovery of vertebrate faunas, including the radiation of Temnodontosaurus into a new ecological niche, may have been a consequence of marine ecosystem reorganization across this event.     

Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover

The Late Jurassic to Early Cretaceous interval represents a time of environmental upheaval and cataclysmic events, combined with disruptions to terrestrial and marine ecosystems. Historically, the Jurassic/Cretaceous (J/K) boundary was classified as one of eight mass extinctions. However, more recent research has largely overturned this view, revealing a much more complex pattern of biotic and abiotic dynamics than has previously been appreciated. Here, we present a synthesis of our current knowledge of Late Jurassic–Early Cretaceous events, focusing particularly on events closest to the J/K boundary. We find evidence for a combination of short‐term catastrophic events, large‐scale tectonic processes and environmental perturbations, and major clade interactions that led to a seemingly dramatic faunal and ecological turnover in both the marine and terrestrial realms. This is coupled with a great reduction in global biodiversity which might in part be explained by poor sampling. Very few groups appear to have been entirely resilient to this J/K boundary ‘event’, which hints at a ‘cascade model’ of ecosystem changes driving faunal dynamics. Within terrestrial ecosystems, larger, more‐specialised organisms, such as saurischian dinosaurs, appear to have suffered the most. Medium‐sized tetanuran theropods declined, and were replaced by larger‐bodied groups, and basal eusauropods were replaced by neosauropod faunas. The ascent of paravian theropods is emphasised by escalated competition with contemporary pterosaur groups, culminating in the explosive radiation of birds, although the timing of this is obfuscated by biases in sampling. Smaller, more ecologically diverse terrestrial non‐archosaurs, such as lissamphibians and mammaliaforms, were comparatively resilient to extinctions, instead documenting the origination of many extant groups around the J/K boundary. In the marine realm, extinctions were focused on low‐latitude, shallow marine shelf‐dwelling faunas, corresponding to a significant eustatic sea‐level fall in the latest Jurassic. More mobile and ecologically plastic marine groups, such as ichthyosaurs, survived the boundary relatively unscathed. High rates of extinction and turnover in other macropredaceous marine groups, including plesiosaurs, are accompanied by the origin of most major lineages of extant sharks. Groups which occupied both marine and terrestrial ecosystems, including crocodylomorphs, document a selective extinction in shallow marine forms, whereas turtles appear to have diversified. These patterns suggest that different extinction selectivity and ecological processes were operating between marine and terrestrial ecosystems, which were ultimately important in determining the fates of many key groups, as well as the origins of many major extant lineages. We identify a series of potential abiotic candidates for driving these patterns, including multiple bolide impacts, several episodes of flood basalt eruptions, dramatic climate change, and major disruptions to oceanic systems. The J/K transition therefore, although not a mass extinction, represents an important transitional period in the co‐evolutionary history of life on Earth.     

Rapid warming and ostracods mass extinction at the Lower Toarcian (Jurassic) of central Spain

Here we present the results of the study of two Lower Toarcian carbonate sections located in the Iberian Range of central Spain. Analyses of stable isotope on belemnite calcite allowed calculation of seawater palaeotemperature variations, which were compared with the stratigraphical distribution of ostracods. These organisms are particularly sensitive to ratios of temperature and salinity variations and hence are good indicators of climate changes. From a cooling interval, with seawater temperatures of 13.2 °C recorded at the Pliensbachian?Toarcian transition, seawater temperature began to rise in the lowermost Toarcian Tenuicostatum Zone, reaching average temperatures between 14.6 °C and 16.3 °C during the time of deposition of this Zone. Coinciding with this seawater warming, up to 85% of the ostracods species progressively disappeared during a period of approximately 300 kyr, marking the extinction interval. The extinction boundary, located around the Tenuicostatum?Serpentinum zonal boundary, coincides with a marked increase in temperature in the Serpentinum Zone, on which average seawater temperatures of 22 °C have been calculated. Warming continued through part of the Middle Toarcian Bifrons Zone, reaching average temperatures of 24.7 °C. Readjustment of the ostracod population allowed recovery of these faunas in the upper Serpentinum Zone, although the extinction of a major ostracod group, the healdioids, was also recorded. The correlation between mass extinction and warming infers a causal relationship. Comparison of the results with the records of stable isotopes in belemnites and in bulk carbonates, as well as TOC and facies analysis suggests that the anoxia linked to the Early Toarcian oceanic anoxic event was not the main responsible for the ostracod mass extinction.     

A New Genus of Rhynchonellid Brachiopod from the Lower Triassic of South China and Implications for Timing the Recovery of Brachiopoda After the End-Permian Mass Extinction

A new genus, Meishanorhynchia , is proposed based on new material from the Lower Triassic of the Meishan section, South China. It is of a late Griesbachian age based on both associated biozones (ammonoids and bivalves) and radiometric dates of the intercalated volcanic ash beds. Comparison with both Palaeozoic and Mesozoic–Cenozoic-related genera suggests that it may represent the first radiation of progenitor brachiopods in the aftermath of the end-Permian extinction. The lowest brachiopod horizon that contains the genus is estimated to be about 250.1 ± 0.3 Ma. This implies that the initial stage of recovery of Brachiopoda in the Early Triassic was probably about 1.3 ± 0.3 myr after the major pulse of the end-Permian mass extinction (dated as 251.4 ± 0.3 Ma). This is in agreement with Hallam's expectancy that biotic recovery typically begins within one million years or so of major mass extinctions, in contrast to current views on the end-Permian extinction event which propose that the recovery of most if not all biotic groups in the Early Triassic was severely delayed and only began about five million years after the end-Permian extinction.     

An unusually diverse mollusc fauna from the earliest Triassic of South China and its implications for benthic recovery after the end-Permian biotic crisis

A single carbonate coquinoid lens from the Griesbachian (Early Triassic) of Shanggan, South China, yielded 11 bivalve species described in this study in addition to four gastropod and one ammonoid species reported elsewhere. This makes the Shanggan fauna one of the richest mollusc faunas from the early post-extinction interval after the end-Permian mass extinction event. Four of the present genera are long-term survivors, five are holdovers that went extinct at the end of the Griesbachian or later in the Early Triassic, and seven first appear in the Griesbachian. Three new bivalve species are described: Myalinella newelli nov. sp., Scythentolium scutigerulus nov. sp., and Eumorphotis shajingengi nov. sp. The genus Astartella, previously assumed to have vanished at the end of the Permian, is reported for the first time from the Early Triassic, which also removes Astartidae from Early Triassic Lazarus taxa. The small growth size of the Astartella specimens supports an earlier hypothesis that many of the Early Triassic Lazarus taxa did not survive in unknown refuges but were simply overlooked due to the scarcity of easily observable large-sized specimens. Ecologically, a comparatively high proportion of infaunal bivalve species (4/11) is remarkable for the early post-extinction interval, supporting the impression of a relatively advanced recovery state. Moreover, abundance-data of the bivalve-gastropod community reveal a remarkably low dominance index (D = 0.17) that is suggestive for advanced recovery and stable environmental conditions. It is proposed that the Shanggan fauna represents a late Griesbachian benthic recovery event that coincided with the appearance of similarly diverse benthic faunas in Oman and Primorye. A high proportion of genera that have previously not been reported from the Early Triassic indicate that the prevalence of poor preservation conditions is a major obstacle in identifying early phases of recovery from the greatest crisis in the history of metazoan life. The early recovery of benthic faunas reported in this study questions previous claims of a prolonged lag phase as a consequence of the extraordinary extinction magnitude or the persistence of adverse environmental conditions.     

Microbes,mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

The end‐Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic‐matter‐poor sediments during the Triassic to generally organic‐matter‐rich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photic‐zone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better‐known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ¹³C of atmospheric CO₂ and variation in the sources of organic carbon. Regardless of the causes of δ¹³C variability, long‐term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.