Uppermost Pliensbachian-Lowermost Toarcian ostracod assemblages from the Western Tethys: Comparison between Traras Mountains (Algeria), Subbetic (Spain), and Algarve (Portugal)

Major changes in ostracod assemblages have been recorded in the Lower Jurassic of Western Tethys related to biotic crises of the Pliensbachian-Toarcian boundary and the Polymorphum-Levisoni boundary (Toarcian Oceanic Anoxic Event). The study is focused on the ostracods assemblages from Benzerka section (Traras Mountains, Algeria) and their comparison with sections from External Subbetic and Algarve Basin (South Iberian Palaeomargin). Three stratigraphic intervals coincident with the ammonite zones have been differentiated with important fluctuations in the composition of ostracod assemblages. In the ostracod assemblages from the Traras Mountains the turnover related to this benthic biotic crisis is shown by assemblages previous to the biotic crisis (uppermost Pliensbachian) mainly composed by families Healdiidae and Cytherellidae to assemblages just after the T-OAE (upper part of the Levisoni Zone) with assemblages dominated by families Protocytheridae and Polycopidae and the extinction of family Healdiidae (suborder Metacopina). The Toarcian Oceanic Anoxic Event (T-OAE) probably resulted in restricted conditions but a barren interval due to oxygen-depleted conditions in this setting is not recorded. In this context, the change of components of ostracod assemblages looks gradual. In the Southiberian Palaeomargin, represented by the External Subbetic the turnover of ostracods assemblages is also recorded but the families involved are different than in Traras Mountains. Therefore, different phases have been differentiated in the ostracod turnover occurring from Upper Pliensbachian (assemblages dominated by families Pontocyprididae, Healdiidae and Bairdiidae) to end of Lower Toarcian (assemblages dominated by family Cytherellidae and the extinction of family Healdiidae). In the Algarve Basin the families Healdiidae and Bairdiidae were dominant previously to the Toarcian Oceanic Anoxic Event. Differences in the composition of ostracod assemblages between Traras Mountains and External Subbetic may be related to the fragmentation of these palaeomargins related to early stages of rifting in the Western Tethys. Moreover, the incidence of oxygen-depleted conditions was higher in External Subbetic than in Traras Mountains where some ostracod genera survive during the biotic crisis.     

Benthic foraminiferal bioevents in lower to upper Toarcian strata of Southern Beaujolais (SE France)

This paper documents changes in benthic foraminiferal assemblages compared with high resolution ammonite biozonation along the lower Toarcian to upper Toarcian marine succession of Southern Beaujolais in southeastern France. Eight ammonite and three benthic foraminiferal zones including five subzones are distinguished based on the occurrence of twelve foraminiferal events. Each benthic foraminiferal subzone is characterized by its taxonomic and morphogroup composition, which represents the paleoecological response of these taxa and morphotypes of benthic foraminifera in the Early Jurassic and early Middle Jurassic. Major changes in abundance and diversity occur at the end of the Toarcian Oceanic Anoxic Event (T-OAE) and near the Early-Middle Jurassic transition. The low-abundance foraminiferal assemblages recorded in the Serpentinus ammonite Zone are interpreted as reflecting adverse environmental conditions after the T-OAE. The later recovery and development of the foraminiferal assemblages is documented in the Bifrons up to the Aalensis zones and is attributed to improved bottom water oxygenation. Common occurrences of agglutinated foraminifera represented mostly by Trochammina pulchra Ziegler in the Dispensum Zone point to an influx of cooler water masses during the late Toarcian. The morphogroup analysis carried out on the foraminifera and their paleoecological interpretations shed light on the changes in the stratigraphic record at the end of the T-OAE up to the Toarcian/Aalenian boundary.     

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.     

The two Early Toarcian (Early Jurassic) extinction events in ammonoids

The Early Toarcian (Early Jurassic) biological crisis was one of the ‘minor’ mass extinctions. It is linked with an oceanic anoxic event. Fossil data from sections located in northwestern European (epicontinental platforms and basins) and Tethyan (distal, epioceanic) areas indicate that Late Pliensbachian–Early Toarcian ammonoids experienced two extinction events during the Early Toarcian. The older one is linked with disruption of the Tethyan–Boreal provinciality, whereas the younger event correlates with the onset of anoxia and corresponds with the Early Toarcian mass‐extinction event. These two extinctions cannot be interpreted as episodes of a single, stepwise, event. Values of the net diversification, more than the number of extinctions, allow the two extinction events to be clearly recognized and distinguished. Values of regional net diversification for northwestern European and Tethyan faunas point to greater evolutionary dynamics in the epioceanic areas. The inclusion of Mediterranean faunas in the database proves that the ammonite turnover at the Early Toarcian mass‐extinction event was more important than previously thought. Progenitor (evolute Neolioceratoides), survivor (Dactylioceras, Polyplectus pluricostatus) and Lazarus (Procliviceras) taxa have been recognized. Different selectivity patterns are shown for the two events. The first one, linked to the disruption of the Tethyan–Boreal provinciality, has mainly affected ammonites adapted to epicontinental platforms. In the mass‐extinction event, no selectivity is recognized, because also Phylloceratina and Lytoceratina were deeply affected at species level, although their wide biogeographical distribution at clade level was a significant buffer against extinction. In contrast to Palaeozoic mass extinctions, ammonoid survivors and Lazarus taxa are characterized by complex sutures: Phylloceratina (long‐ranging ammonoids) and Polyplectus (relatively long‐ranging compared to other Ammonitina).     

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.     

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.     

Ammonite-benthic Foraminifera turnovers across the Lower-Middle Jurassic transition in the Lusitanian Basin (Portugal)

This paper describes and characterises the co-occurrence of ammonite and benthic foraminiferal assemblages across the São Gião outcrop (Central Portugal), a reference section for the Lower-Middle Jurassic boundary in the Lusitanian Basin. The upper Toarcian-lower Aalenian marls and marly-limestones in this section provide a precise and detailed ammonite-based biostratigraphic zonation, with a mixed assemblage of northwest European and Mediterranean faunal elements, associated with benthic foraminifera assemblages with northern hemisphere affinities, both correlatable with the Aalenian GSSP at the Fuentelsaz section (Iberian Cordillera, Spain). A total of 447 well-preserved ammonite specimens and 13.116 foraminifera have been studied; no evidence was detected of any taphonomic processes that could have changed the original assemblages. From a biostratigraphic point of view, the ammonite record has enabled four biostratigraphic units to be recognised (the Mactra and Aalensis subzones of the Aalensis Biozone in the upper Toarcian, and the Opalinum and Comptum subzones of the Opalinum Biozone in the lower Aalenian). With regard to the benthic foraminifera, the taxa identified have enabled the Astacolus dorbignyi Zone and 11 bioevents to be identified, most of which representing local biostratigraphic proxies. However, the increase in the relative abundance of Lenticulina exgaleata Dieni from the upper part of the Opalinum Subzone to the lower part of the Comptum Subzone has a regional value. The constant and continuous ammonite record of northwest European taxa, together with typical Mediterranean taxa – namely Grammoceratinae – throughout the section, the high relative abundance of Miliolina representatives – generally interpreted as foraminifers typical of shallow waters – and the absence of foraminiferal forms typical of cool waters, do not support the inference of cool seawater temperatures attributed to the Early Aalenian, or the global character of the “Comptum cooling event”, at least with reference to the Lusitanian Basin.     

Global radiolarian zonation for the Pliensbachian, Toarcian and Aalenian

Jurassic radiolarians from 220 samples in Queen Charlotte Islands, B.C., Williston Lake, B.C., east-central Oregon, Baja California Sur, southern Spain, Austria, Slovenia, Turkey, Oman, Japan and Argentina were studied in order to construct global zonation for the Pliensbachian, Toarcian and Aalenian stages. Well-preserved faunas from continuous stratigraphic sections in Queen Charlotte Islands provide the most detailed record for this time interval, and all collections are tied to North American ammonite zones or assemblages. Collections from nearly all other areas lack independent dating except for early Toarcian carbon-isotope dating in Slovenia and late Aalenian ammonites in Spain.A database of 197 widely distributed updated taxonomic species was used to construct a Unitary Association (UA) zonation for the interval. A global sequence of 41 UAs was obtained for the top of the Sinemurian to the base of the Bajocian. The first and the last UAs represent the Late Sinemurian and the Early Bajocian respectively. The remaining 39 UAs were merged into nine zones (four Early Pliensbachian, one Late Pliensbachian, one Early Toarcian, one Middle-Late Toarcian, and two Aalenian) according to prominent radiolarian faunal breaks and ammonite data. The new zones are the Canutus tipperi - Katroma clara Zone (latest Sinemurian/earliest Pliensbachian); Zartus mostleri - Pseudoristola megaglobosa, Hsuum mulleri - Trillus elkhornensis and Gigi fustis - Lantus sixi zones (Early Pliensbachian); Eucyrtidiellum nagaiae - Praeparvicingula tlellensis Zone (Late Pliensbachian); Napora relica - Eucyrtidiellum disparile Zone (Early Toarcian); Elodium pessagnoi - Hexasaturnalis hexagonus Zone (Middle and Late Toarcian); Higumastra transversa - Napora nipponica Zone (early Aalenian); and Mirifusus proavus - Transhsuum hisuikyoense Zone (late Aalenian). These zones can be correlated worldwide and link previously established UA zonations for the Hettangian-Sinemurian and the Middle to Upper Jurassic. The new zonation allows high-resolution dating in the studied interval and provides a solid basis for analyzing faunal turnovers and the paleobiogeography of Jurassic radiolarians.     

The recovery of terrestrial vertebrate diversity in the South African Karoo Basin after the end-Permian extinction

The southern half of the main Karoo Basin in South Africa contains an almost continuous stratigraphic record of terrestrial sedimentation through the Permo-Triassic boundary (PTB). Detailed logging of multiple sections through the boundary sequence has defined the end-Permian mass extinction event using vertebrate fossils as well as a synchronous change in fluvial style reflecting a rapid aridification of climate. Field data demonstrates a 69% mass extinction of Late Permian terrestrial vertebrates lasting some 300 kyr terminating at the PTB, followed by a lesser extinction event (31%) approximately 160 kyr later involving four survivor taxa that crossed the PTB. The Early Triassic recovery fauna comprises proterosuchian archosauromorphs (Proterosuchus), small amphibians (Micropholis, Lydekkerina), small procolophonoids (‘Owenetta’ kitchingorum, Procolophon), medium-sized dicynodonts (Lystrosaurus) and small insectivorous cynodonts (Progalesaurus, Galesaurus, Thrinaxodon). Taphonomic bias towards preferential preservation of drought accumulations in the Early Triassic has probably over-emphasized the abundance and diversity of semi aquatic and burrowing animals. To cite this article: R. Smith, J. Botha, C. R. Palevol 4 (2005).     

Relationships between the early Toarcian anoxic event and organic-walled phytoplankton in central Italy

The integration of palynological and geochemical data from three lower Toarcian successions in central Italy reveals that the composition of organic-walled phytoplankton assemblages were strongly affected by palaeoecological conditions related to bituminous sedimentation which accompanied the global anoxic event. The marked compositional variations of dinoflagellate cysts and prasinophytes, together with geochemical variations, have been linked to changes in surface water habitats during the lower Toarcian transgression. On the basis of the relationships between total organic carbon (TOC) and marine palynomorph assemblage composition, the lower Toarcian evolution of the Umbria-Marche Basin, central Italy, has been divided into four phases. Total organic carbon values rose significantly during the early Toarcian (Lower-middle Dactylioceras tenuicostatum ammonite Zone), and this can be linked to certain dinoflagellate cyst datums, for example the temporary disappearance of Mancodinium semitabulatum and the extinction of Luehndea spinosa. The presence of Umbriadinium mediterraneense and Valvaeodinium spp. accompany these moderately high TOC values. Subsequently, TOC levels increased to over 2% and prasinophytes became abundant in the Middle-upper D. tenuicostatum ammonite Zone. Mancodinium semitabulatum reappeared when TOC values eventually decreased in the Upper D. tenuicostatum ammonite Zone. This analysis has allowed the different sunlight requirements and life strategies of the early Toarcian Tethyan dinoflagellates to be modelled. Due to the cosmopolitan nature of the early Toarcian anoxic event, the principal marine palynological signals observed have been interpreted as sequence stratigraphical and palaeoecological indices. The Transgressive Systems Tract (TST) is accompanied by an increase in dinoflagellate cyst species diversity and a decrease in abundance. The succeeding maximum flooding surface (mfs) corresponds with a prasinophyte acme. During the Highstand Systems Tract (HST), the phytoplankton shows an increase in abundance and a decrease in diversity. The range top of Luehndea spinosa appears to characterise the early Toarcian TST.     

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.     

Myths and legends in the correlation between the Boreal and Tethyan Realms. Implications on the dating of the Early Toarcian mass extinctions and the Oceanic Anoxic Event

This study constitutes a revision of the biostratigraphy of Early Toarcian age ammonites coming from different successions in the NW Europe and Mediterranean areas, complemented by new, important data from the Migiana di M. Malbe section (Perugia, Italy). The research has demonstrated that the problems of correlation between some Zones of the Standard and Mediterranean biostratigraphic scales in this period are doubtful. On the negative side, procedural problems and defects have emerged that greatly detract from the validity of some boundaries which were, almost unanimously, held to be synchronous. Undervaluation of the problems brought to light in this study has had repercussions both on the construction of the eustatic curves and on the dating of the Oceanic Anoxic Event (OAE), and consequently on the evolutionary models for the various areas of the Tethys and of NW Europe. The various datings obtained by different writers have led to the belief that the anoxic conditions became established diachronically in the two areas. However, ammonite dating of the black shale-like facies, made for the first time in the Umbria–Marche Apennines, shows instead that, at least between this area and Germany, the diachronism was only partial. In both areas, the establishment of “anoxic” conditions took place in the Dactylioceras semicelatum Subzone. Problems linked to ammonite faunal provinciality and evolution, complicated by tectono-eustatic phenomena that produce hiatuses in NW Europe, are at the root of the lack of continuity in tracing the Pliensbachian–Toarcian boundary between the NW Europe area and the rest of the world.     

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.     

Diverse earliest Triassic ostracod fauna of the non‐microbialite‐bearing shallow marine carbonates of the Yangou section,South China

Since diverse ostracod faunas in the immediate aftermath of the latest Permian mass extinction are mainly found within Permian–Triassic boundary microbialites (PTBMs), the idea of an ostracod ‘microbial‐related refuge’ has been proposed. Here, we report a diversified earliest Triassic ostracod fauna from the Yangou section in South China, where no PTBMs were deposited, providing evidence inconsistent with this ‘microbial‐related refuge’ hypothesis. In addition, a significant ostracod extinction is recorded, corresponding with the earliest Triassic mass extinction (ETME). This ETME of ostracods is associated with size increases and a length/height ratio (L/H) decrease, indicating varied evolutionary patterns of shape and size of ostracods through the Permian–Triassic (P‐Tr) extinction events. Although the nature of these biotic changes is somewhat unclear, the temporally varied ‘refuge zone’ scenario provides us with a window to reconstruct the environmental dynamics of ecosystem changes during the P‐Tr transition.     

The Early Jurassic palynostratigraphy of the Lusitanian Basin,western Portugal

A comprehensive investigation of the Early Jurassic stratigraphical palynology of the Lusitanian Basin in western Portugal was undertaken, with most emphasis placed on dinoflagellate cysts. A total of 214 samples from an upper Sinemurian to upper Toarcian composite section based on six successions were examined. The Sinemurian material examined was barren of dinoflagellate cysts; however, the Pliensbachian and Toarcian successions are characterised by relatively low diversities where Luehndea spinosa, Mancodinium semitabulatum, Mendicodinium microscabratum, Nannoceratopsis gracilis, Nannoceratopsis senex, and Scriniocassis priscus are relatively common and biostratigraphically significant. Luehndea spinosa dominates the lowermost Toarcian (Dactylioceras polymorphum ammonite Biozone), and is an index species. At the base of the Hildaites levisoni ammonite Biozone, the effects of the Toarcian Oceanic Anoxic Event (T-OAE) caused Luehndea spinosa to become extinct. At the same time, dinoflagellate cyst abundance and diversity markedly decreased. After the T-OAE, during the middle and late Toarcian, phytoplankton recovery was prolonged and slow in the Lusitanian Basin. The Luehndea spinosa and Mendicodinium microscabratum dinoflagellate cyst biozones are defined, both of which are subdivided into two dinoflagellate cyst subbiozones.     

Overgrowth on ammonite conchs: environmental implications for the Lower Toarcian Posidonia Shale

Ammonites of the south–west German Posidonia Shale (Early Toarcian) are occasionally overgrown by bivalves, brachiopods or serpulids. Attachment to both sides of the conchs in some specimens suggests attachment during the ammonite's lifetime, with a pseudoplanktic mode of life for the epizoans. However, dense overgrowth restricted to the upper surface of some ammonite shells indicates post–mortem colonization. Our study shows that apart from oxygen supply as a main factor controlling benthic colonization, substrate consistency also played an important part. Unfavourable living conditions prevailed during deposition of the organic–rich sediments, excluding macrofauna in the benthic environment. However, less than 0.1 per cent of the ammonite conchs found during the excavation show overgrowth, indicating that pseudoplanktonism was an infrequently adopted strategy for inhabiting surface waters.     

A new species of glypheid lobster,Glyphea pisuergae (Crustacea,Glypheoidea), from the Early Jurassic of Palencia,Basque-Cantabrian Basin,Spain

A new glypheid lobster (Crustacea, Glypheidae), Glyphea pisuergae sp. nov. is described from the Early Jurassic (late Pliensbachian–early Toarcian) of Salinas de Pisuerga, Palencia, Spain. This species represents the second record for the genus in the Early Jurassic.     

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.     

Key innovations in Mesozoic ammonoids: the multicuspidate radula and the calcified aptychus

A nearly complete radula with seven elements per row preserved inside of an isolated, bivalved, calcitic lower jaw (= aptychus) of the Late Jurassic ammonite Aspidoceras is described from the Fossillagerstätte Painten (Bavaria, southern Germany). It is the largest known ammonite radula and the first record for the Perisphinctoidea. The multicuspidate tooth elements (ctenodont type of radula) present short cusps. Owing to significant morphological differences between known aptychophoran ammonoid radulae, their possible function is discussed, partly in comparison with modern cephalopod and gastropod radulae. Analogies between the evolution of the pharyngeal jaws of cichlid fishes and the ammonoid buccal apparatus raise the possibility that the evolution of a multicuspidate radula allowed for a functional decoupling of the aptychophoran ammonoid jaw. The radula, therefore, represents a key innovation which allowed for the evolution of the calcified lower jaws in Jurassic and Cretaceous aptychophoran ammonites. Possible triggers for this morphological change during the early Toarcian are discussed. Finally, we hypothesize potential adaptations of ammonoids to different feeding niches based on radular tooth morphologies.